Periodic Table and Periodic Trends: The Complete Guide

Page purpose

The periodic table is not merely a chart to memorize.

It is a compact model connecting atomic number, electron structure, recurring properties, bonding, and reactivity.

This page teaches users how to read that model, make qualified predictions, and recognize when a simple trend arrow is inadequate.

It serves beginners, school and exam learners, introductory college students, teachers, and self-learners.

Every numerical comparison must link to a defined dataset and property convention.

Every safety statement must refer users to substance-specific safety data rather than treating an element name as a complete hazard description.

Learning objectives

• Define atomic number and explain why it determines elemental identity.
• State the modern periodic law.
• Distinguish group, period, and block.
• Connect table position with a ground-state electron configuration.
• Identify chemical families without assuming family members are identical.
• Explain effective nuclear charge, shielding, penetration, and shell effects.
• Predict qualified trends in neutral atomic radius.
• Compare cations and anions with parent atoms.
• Order isoelectronic species by size.
• Define first and successive ionization energies.
• Explain common ionization-energy exceptions using orbital occupancy.
• Distinguish electron affinity from electronegativity.
• State the sign convention when discussing electron affinity.
• Predict broad metallic and nonmetallic trends.
• Relate periodicity to common oxidation states and compound formulas.
• Explain why transition-metal patterns are less regular than simple main-group arrows.
• Describe lanthanoid contraction and consequences.
• Explain why hydrogen, helium, and group 3 require explicit editorial treatment.
• Use periodic evidence without overstating certainty.
• Verify exact values using IUPAC, PubChem, or NIST.

One-minute summary

• Elements are ordered by atomic number.
• Repeated valence-electron patterns produce recurring chemical behavior.
• A group is a vertical column; a period is a horizontal row.
• The s, p, d, and f blocks reflect the type of subshell receiving differentiating electrons.
• Atomic size generally decreases across a period and increases down a group.
• First ionization energy generally increases across a period and decreases down a group.
• Electronegativity generally increases across a period and decreases down a group.
• Cations are generally smaller than their parent atoms.
• Anions are generally larger than their parent atoms.
• In an isoelectronic series, the species with more protons is generally smaller.
• Trend arrows summarize patterns; electron configuration and measured data explain exceptions.
• Melting point, boiling point, density, and reactivity do not follow one universal arrow.

Prerequisites

• Proton, neutron, and electron.
• Atomic number and mass number.
• Isotope and ion notation.
• Shells, subshells, and orbitals.
• Basic ground-state electron configurations.
• Electrostatic attraction and repulsion.
• The difference between an observation, model, and law.

1. What the table represents

A chemical element is identified by its atomic number.

Atomic number equals the number of protons in each nucleus of that element.

Changing neutron count makes a different isotope.

Changing electron count makes an ion.

Changing proton count makes a different element.

The table places elements in increasing atomic-number order.

This order causes related valence configurations and properties to recur.

The table is therefore a structured index to chemical behavior.

It is also a map of electron filling.

It is not a map with perfectly sharp boundaries between every category.

Terms such as metal, nonmetal, and metalloid are useful but can be context-dependent near boundaries.

2. Modern periodic law

The modern periodic law states that elemental properties recur periodically when elements are arranged by atomic number.

Periodicity does not mean that every eighth or eighteenth element is identical.

It means that related electronic structures recur in an ordered way.

Those recurring structures create patterns in bonding, ion formation, oxidation state, size, and reactivity.

Atomic number, not atomic mass, is the organizing variable.

This distinction resolves historical reversals required by mass-based arrangements.

The law is supported by atomic spectra, electron configurations, chemical behavior, and nuclear charge.

3. Historical development

Early classifications grouped substances by visible or reactive similarities.

Johann Döbereiner identified selected triads with related properties.

John Newlands proposed an octave pattern when elements were ordered by mass.

Dmitri Mendeleev and Lothar Meyer developed influential periodic arrangements.

Mendeleev intentionally left gaps and predicted properties of missing elements.

Later discoveries including gallium, scandium, and germanium supported the predictive power of periodic organization.

Some elements did not fit cleanly in strict mass order.

Henry Moseley’s X-ray work linked ordering to nuclear charge.

Atomic-number order created the modern foundation.

Quantum mechanics later explained why electron configurations recur.

Historical credit should be presented as a network of evidence and contributors, not a single-person invention story.

4. Reading an element tile

A useful tile displays atomic number, standardized symbol, element name, and a sourced atomic-weight field.

It may also display family, standard state, electron configuration, and links to definitions.

Atomic number is exact for an element.

Atomic weight is not the same as mass number.

Mass number is the integer total of protons and neutrons in one nuclide.

Relative atomic mass can refer to a specified sample.

Standard atomic weight is an evaluated conventional value or interval for normal terrestrial material.

Natural isotopic variation means some atomic weights are not universal single constants.

Elements without characteristic natural isotopic abundance are often shown with a bracketed isotope mass number.

Every extra property must label units and conventions.

A blank value means unavailable or not conventionally assigned, not zero.

5. Groups

A group is a vertical column.

IUPAC numbers groups from 1 to 18.

Elements in a group often share related valence configurations.

That relationship supports family-level patterns.

Family names include alkali metals, alkaline-earth metals, halogens, and noble gases.

Similarity must be qualified by size, oxidation state, bonding environment, and period.

Hydrogen requires separate explanation even when drawn above group 1.

Helium is placed in group 18 because its shell is filled and behavior is noble-gas-like.

Transition groups require detailed d-electron reasoning.

The chosen display convention for group 3 must be stated rather than concealed.

6. Periods

A period is a horizontal row.

Moving across a period increases atomic number one proton at a time.

Electrons are added according to available quantum states.

Period 1 has two elements because the 1s subshell holds two electrons.

Periods 2 and 3 each show eight main-group positions.

Periods 4 and 5 include the d block.

Periods 6 and 7 include the f block.

The detached f-block rows are a layout choice, not separate chemical universes.

7. Blocks

The s block is associated mainly with filling an ns subshell.

The p block is associated mainly with filling an np subshell.

The d block is associated mainly with filling an (n−1)d subshell.

The f block is associated mainly with filling an (n−2)f subshell.

Block labels describe recurring electronic patterns.

They do not imply that only one subshell matters chemically.

Configuration exceptions occur because subshell energies are close and electron interactions matter.

For ions, electrons are not always removed in the same printed order used to build neutral configurations.

Transition-metal cations commonly lose ns electrons before (n−1)d electrons.

8. Valence electrons

Valence electrons are those treated as available for bonding and chemical change in a stated model.

For many main-group elements, group position gives a useful valence pattern.

Group 1 commonly has ns1.

Group 2 commonly has ns2.

Groups 13 through 18 commonly progress from ns2np1 through ns2np6.

Helium is 1s2 and is an exception to the np6 shorthand.

Transition-metal valence descriptions may include ns and (n−1)d electrons.

The idea becomes model-dependent for solids, hypervalent species, and transition-metal complexes.

Do not present valence count as an infallible rule for every oxidation state.

9. Effective nuclear charge

Every electron is attracted to the positively charged nucleus.

Other electrons repel it and alter spatial distribution.

Effective nuclear charge models the net positive attraction experienced by a selected electron.

Across a main-group period, proton number increases.

Added valence electrons enter the same general principal shell.

Shielding does not fully cancel the added nuclear charge.

Effective attraction therefore generally rises across the period.

Down a group, new shells place valence density farther from the nucleus.

Shielding and distance generally increase.

These ideas explain size and ionization patterns but do not replace quantum calculations or data.

10. Shielding and penetration

Shielding describes how electron density reduces nucleus–electron attraction for other electrons.

Inner electrons usually shield valence electrons substantially.

Electrons in the same shell shield one another less completely.

Penetration describes how much orbital density enters regions close to the nucleus.

For one principal shell, s generally penetrates more than p, p more than d, and d more than f.

Greater penetration can produce stronger attraction and lower orbital energy.

Poor shielding by d and f electrons contributes to contraction effects.

Simple shell-count diagrams are useful but incomplete.

A careful explanation should mention spatial electron density.

11. Atomic radius

An atom has no hard outer boundary.

Atomic radius is therefore operationally defined.

Covalent radius uses bonded internuclear distances.

Metallic radius uses distances in metallic structures.

van der Waals radius uses nonbonded contacts.

Different definitions yield different numbers.

Comparisons must use the same radius type and compatible conditions.

Across many main-group periods, neutral atomic radius generally decreases.

Increasing effective attraction pulls valence density inward.

Down a group, radius generally increases because extra shells are occupied.

Transition-series changes are smaller and can be irregular.

Noble-gas van der Waals values should not be mixed blindly with covalent radii.

12. Ionic radius

Ionic radius is an effective, model-dependent size.

A cation is generally smaller than its parent atom.

Electron removal reduces repulsion and may remove the outer shell.

An anion is generally larger than its parent atom.

Electron addition increases repulsion at unchanged proton number.

Charge magnitude often strengthens these effects.

Coordination number and spin state can alter quoted radii.

A comparison tool must display radius definition and context.

13. Isoelectronic comparisons

Isoelectronic species have the same electron count in the stated comparison.

Examples include O2−, F−, Ne, Na+, and Mg2+.

Their proton numbers differ.

With the same electron count, more protons generally pull electrons inward more strongly.

Radius therefore generally decreases as nuclear charge rises across an isoelectronic series.

Count electrons carefully before applying the rule.

Do not compare incompatible structural radius datasets.

14. First ionization energy

First ionization energy is the energy required for a gaseous atom to lose one electron.

The process is endothermic under the usual definition.

Across a period, first ionization energy generally increases.

Increasing effective attraction makes removal harder.

Down a group, first ionization energy generally decreases.

Greater distance and shielding make removal easier.

Filled and half-filled subshell patterns produce deviations.

Configuration-based explanation is better than memorizing an exception list.

15. Successive ionization energies

Second ionization removes an electron from a gaseous +1 ion.

Third ionization removes an electron from a gaseous +2 ion.

Successive ionization energies always rise for one element.

The ion becomes more positively charged after each removal.

A very large jump can indicate that readily removed valence electrons are gone.

This pattern can help infer a main-group valence count.

It does not prove that the element forms only one ion in every compound.

16. Ionization-energy exceptions

Boron can have a lower first ionization energy than beryllium.

The removed boron electron occupies a higher-energy 2p orbital.

Beryllium loses from a filled 2s subshell.

Oxygen can have a lower first ionization energy than nitrogen.

Oxygen has one paired 2p orbital, increasing electron–electron repulsion.

Nitrogen has a half-filled 2p arrangement.

Related effects occur in later periods with modifications.

The broad trend emerges from electronic structure and is allowed to show electronic-structure exceptions.

17. Electron affinity

Electron affinity concerns the energy change when a gaseous species gains an electron.

Sources may report the quantity with opposite sign conventions.

The page must state whether a positive value means energy released or whether thermodynamic energy change is shown.

Across a period, electron addition often becomes more favorable, but the pattern is irregular.

Filled subshells, half-filled subshells, small orbital size, and pairing matter.

Down a group, size and repulsion compete with nuclear attraction.

Chlorine and fluorine show why a size-only rule can fail.

Never label a missing or unfavorable value as zero without a source.

18. Electronegativity

Electronegativity describes an atom’s tendency to attract shared electron density in a bond.

It is not electron affinity.

It is not ionization energy.

It depends on a scale and chemical context.

The Pauling scale is common in introductory chemistry.

Across a period, electronegativity generally increases.

Down a group, it generally decreases.

Fluorine is highest on the usual Pauling scale.

Many tables omit conventional noble-gas values.

Electronegativity difference helps discuss bond polarity but does not create a perfect ionic–covalent boundary.

19. Metallic character

Metallic character is a qualitative combination of physical and chemical tendencies.

Metals commonly conduct, show metallic bonding, and form cations.

Across a period, metallic character generally decreases.

Down a group, it generally increases.

The boundary between metals and nonmetals is not perfectly sharp.

Metalloid boundaries vary among conventions.

Allotropes and pressure-dependent phases can complicate classification.

Bonding in a compound cannot be inferred solely from bulk element labels.

20. Reactivity trends

Reactivity is not one scalar property.

Metal reactivity may involve oxidation, kinetics, passivation, solvent, and products.

Nonmetal reactivity may involve oxidizing strength, bond energy, electron attachment, solvation, and kinetics.

Group-1 metals generally become easier to oxidize down the group.

Observed water reactions also involve melting, mixing, oxide layers, and heat transfer.

Halogen oxidizing strength generally decreases down group 17 under comparable aqueous conditions.

Fluorine cannot be explained by electronegativity alone.

Heavier noble gases can form compounds.

Always name reaction and conditions before ranking reactivity.

21. Oxidation-state patterns

Oxidation state is a formal electron-bookkeeping quantity.

Group-1 compounds commonly show +1.

Group-2 compounds commonly show +2.

Group 17 commonly shows −1, while heavier halogens also show positive states in oxycompounds.

Group 16 commonly shows −2 and positive states.

Group 15 spans −3 through positive states.

Group 14 often shows +4 and increasingly stable +2 for heavier members.

Transition metals frequently show multiple states.

The inert-pair effect helps describe heavier p-block preference for states two units below the group maximum.

Oxidation state must not be equated automatically with a free monatomic ion.

22. Oxide trends

Across a main-group period, oxide bonding often shifts from more ionic to more covalent.

Strongly basic oxides are common toward the metallic left.

Amphoteric oxides occur in intermediate regions.

Acidic oxides become common toward the nonmetallic right.

The pattern is useful for period-3 teaching.

Actual behavior depends on oxidation state, structure, solubility, and reaction medium.

An insoluble oxide can still show acid–base behavior under suitable conditions.

Avoid a single label for complex or mixed-valence oxides.

23. Transition elements

Transition chemistry reflects incomplete d subshells in atoms or common cations under the chosen definition.

Variable oxidation state is common.

Coordination compounds are common.

Ligands influence color, magnetism, and stability.

Catalytic activity is widespread but not universal.

Atomic radii change relatively little across a transition series.

The d-block label and transition-element definition are related but not identical.

Zinc, cadmium, and mercury are d-block elements, while their common d10 ions complicate a strict transition-element definition.

24. Lanthanoids

IUPAC collective naming uses lanthanoids for La through Lu.

The +3 oxidation state is common.

Differentiating 4f electrons are relatively shielded from bonding environments.

Chemical similarities make separation difficult.

Optical and magnetic differences make the series technologically important.

Uses include magnets, phosphors, lasers, catalysts, and medical materials.

The phrase rare earth does not mean uniformly scarce.

Supply depends on geology, concentration, separation, processing, economics, and environmental management.

25. Lanthanoid contraction

Atomic and ionic radii generally decrease across the lanthanoids.

Growing nuclear charge is incompletely shielded by added 4f electrons.

The contraction affects basicity, coordination, separation, and later-element sizes.

Zirconium and hafnium have unexpectedly similar sizes partly because of it.

The contraction illustrates imperfect d- and f-electron shielding.

Quantitative comparisons require consistent charge and coordination number.

26. Actinoids

IUPAC collective naming uses actinoids for Ac through Lr.

All actinoids are radioactive.

Early members show several oxidation states.

Later members often favor +3 in aqueous contexts.

5f orbitals can participate in bonding more than a simple lanthanoid analogy suggests.

Nuclear fuel, radioisotope power, medicine, research, and stewardship are major contexts.

Chemical toxicity and radiological hazard must be distinguished.

Do not publish operational instructions for acquisition, separation, or weapon-related activity.

27. Relativistic effects

Very heavy atoms require relativistic quantum treatment.

Relativity can contract and stabilize some orbitals.

Other orbitals can expand or shift indirectly.

These effects help explain gold’s color, mercury’s weak metal–metal bonding, and unusual superheavy predictions.

A beginner page should state the concept without pretending one sentence is a complete calculation.

Advanced links can connect to quantum chemistry and spectroscopy.

28. Hydrogen

Hydrogen has one electron and one proton in its most common isotope.

Its 1s1 configuration resembles the ns1 pattern of group 1.

It can form H+ in formal acid descriptions.

It can form H− in ionic hydrides.

It commonly forms covalent bonds and H2.

Its nonmetallic gas behavior differs radically from alkali metals.

The table should display it consistently while explaining its uniqueness.

29. Helium

Helium has a filled 1s2 shell.

Its configuration superficially resembles an ns2 count.

Its inertness, monatomic gas behavior, and closed shell align with noble gases.

That is why conventional tables place it in group 18.

Its extremely low boiling point reflects weak interatomic attractions.

It should not be taught as an ordinary group-2 element.

30. Group 3 conventions

Group 3 has appeared as Sc–Y–La–Ac or Sc–Y–Lu–Lr in different displays.

The site must choose a consistent visual convention.

The explanatory note must acknowledge alternatives.

Search, keyboard navigation, and compare features must work regardless of display placement.

Lanthanoid and actinoid membership must remain explicit.

A layout choice must not be presented as an unquestionable chemical fact.

31. Superheavy elements

Superheavy elements are synthesized in nuclear reactions and detected through decay chains and specialized measurements.

Many exist for fractions of a second.

Bulk properties cannot be measured directly when only a few atoms exist.

Chemical placements combine experiments with relativistic calculations.

Predictions must be labeled as predictions.

A blank density or boiling point is not zero.

Their scientific value lies in testing nuclear stability, relativistic structure, and the limits of periodic organization.

32. Properties without one universal arrow

Melting point depends on the structure and bonding of the elemental substance.

Boiling point depends on cohesive interactions and phase behavior.

Density depends on mass, packing, bonding, and crystal structure.

Conductivity depends on electronic structure, phase, defects, and temperature.

Magnetism depends on configuration and collective solid-state behavior.

Allotropy can give one element multiple structures and properties.

These properties need interactive maps and graphs rather than misleading universal arrows.

33. Reliable comparison workflow

• Define the exact species.
• Define the exact property.
• Keep units and measurement conventions consistent.
• Identify group, period, and block.
• Compare occupied shells.
• Compare proton number and likely effective attraction.
• Inspect configurations for subshell and pairing effects.
• Check whether the species are isoelectronic.
• Account for charge, coordination, spin, and phase.
• Make a qualitative prediction.
• Verify quantitative values in an authoritative dataset.
• Explain exceptions rather than deleting them.

34. Interactive table use

• Search by element name, symbol, or atomic number.
• Filter by family, block, state, and broad classification.
• Choose a property layer such as atomic radius or first ionization energy.
• Read the legend and unit before comparing colors.
• Open an element card for definitions and source metadata.
• Double-click or use an explicit Add to compare button to place an element in the compare tray.
• Keyboard users require an equivalent action.
• Mobile must provide all 118 elements and every comparison feature.
• Color must never be the only carrier of category or trend information.

35. Source and uncertainty discipline

• IUPAC and CIAAW anchor names, symbols, group numbering, and atomic-weight presentation.
• PubChem supplies a consistent broad element dataset and element links.
• NIST supports evaluated spectra, thermochemistry, ion energetics, and constants.
• Every property needs a unit, definition, source, and reviewed date.
• Predicted values must be distinguished from measured values.
• Ranges and uncertainties must not be flattened into fake precision.
• Missing values remain null with a human-readable explanation.
• Conflicting reputable values require definitions, conditions, and source notes.

36. Safety and responsible use

• The periodic table is educational and not a substitute for a Safety Data Sheet.
• Hazard depends on chemical form and exposure.
• Elemental sodium is not sodium chloride.
• Elemental chlorine is not chloride ion.
• Metallic chromium, Cr(III), and Cr(VI) have different hazard profiles.
• Insoluble barium sulfate is not equivalent to soluble barium salts.
• Radiological risk depends on isotope, activity, radiation type, route, and exposure.
• Do not publish unsupervised demonstrations involving reactive, toxic, corrosive, explosive, or radioactive materials.
• Use ACS RAMP: recognize hazards, assess risks, minimize risks, and prepare for emergencies.
• Use current GHS labels and substance-specific SDS information for real decisions.

Group-by-group guide

Group 1 — alkali metals, with hydrogen separate

One valence s electron commonly supports +1 ions. Reactivity comparisons must include solvent, temperature, passivation, and the special chemistry of hydrogen.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 2 — alkaline-earth metals

Two valence s electrons commonly support +2 ions. Size, hydration, lattice energy, oxide basicity, and solubility change substantially down the group.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 3 — convention-sensitive column

Mostly +3 chemistry is common. The site must label whether its display uses La/Ac or Lu/Lr and explain that alternative layouts exist.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 4 — titanium group

Common +4 chemistry, strong oxides, and passivation connect Ti, Zr, and Hf; superheavy Rf is known only from rapid experiments.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 5 — vanadium group

High oxidation states, oxo chemistry, refractory metals, alloys, and catalysis are major themes.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 6 — chromium group

Oxidation-state diversity, colored species, refractory metals, alloys, and catalysis occur across the group.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 7 — manganese group

Oxidation-state diversity is especially visible; Tc and Re add radioisotope and high-temperature contexts.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 8 — iron column

Redox, coordination, catalysis, magnetism, and alloy chemistry are central, but the members are not interchangeable.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 9 — cobalt column

Coordination chemistry and catalysis connect the group while oxidation-state stability differs substantially.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 10 — nickel column

Catalysis, noble-metal behavior, and strong metal–ligand chemistry are prominent.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 11 — coinage metals

High conductivity and relatively low bulk reactivity coexist with rich +1 and heavier-element oxidation chemistry.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 12 — zinc group

Filled d subshells distinguish common ions from many transition-metal ions; volatility and toxicity matter especially for Cd and Hg.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 13 — boron group

The ns2np1 pattern connects electron-deficient boron chemistry with metallic bonding and inert-pair effects down the group.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 14 — carbon group

Catenation, networks, semiconductors, +4/+2 states, and increasing metallic character create a broad progression.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 15 — pnictogens

States from −3 to +5, lone-pair behavior, multiple bonds, allotropy, and biological roles vary strongly down the group.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 16 — chalcogens

Oxides, sulfides, redox behavior, allotropy, catenation, and increasing metallic character make the family diverse.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 17 — halogens

High electronegativity and common −1 ions anchor the family, while oxidizing strength, bond energy, and physical state vary down the group.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Group 18 — noble gases

Closed shells explain low ordinary reactivity, but heavier members can form compounds and all have important physical applications.

• Teach the shared valence pattern.
• Show a meaningful down-group difference.
• Link compounds, redox, bonding, and applications.
• Keep hazards substance-specific.

Period-by-period guide

Period 1

Only H and He occupy the n = 1 shell. Their placements emphasize electronic structure, but both have exceptional chemistry.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 2

The 2s and 2p subshells fill from Li to Ne. Small size and inability to use low-energy valence-shell d orbitals produce strong second-period effects.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 3

The 3s and 3p subshells fill from Na to Ar. Changing oxide behavior and selected diagonal relationships are valuable teaching themes.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 4

The 4s, 3d, and 4p sequence creates the first transition series and broad oxidation, color, and magnetic behavior.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 5

The 5s, 4d, and 5p sequence resembles period 4 while showing heavier-element differences.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 6

Lanthanoids, 5d metals, and heavy p-block elements introduce lanthanoid contraction and strong relativistic effects.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Period 7

Actinoids and superheavy elements are radioactive; evidence often comes from tiny samples, decay chains, and rapid experiments.

• Show the block sequence.
• Connect shell filling to size, ionization, bonding, and state.
• Include useful exceptions.

Worked reasoning examples

Compare Na and Mg atomic radii

• Reasoning: Both are in period 3. Mg has one more proton while the principal valence shell is unchanged, so effective nuclear attraction is generally greater and Mg is smaller.
• Quality check: Use the same neutral-radius convention for both values.
• Practice move: require a prediction before revealing the answer.

Compare Na and Na+

• Reasoning: Removing the 3s electron eliminates the common outer shell and reduces repulsion, so Na+ is much smaller.
• Quality check: Do not mix neutral covalent radius and ionic radius without labels.
• Practice move: require a prediction before revealing the answer.

Compare F− and Na+

• Reasoning: Both have ten electrons. Na+ has more protons, so it is smaller.
• Quality check: Count electrons before using the isoelectronic rule.
• Practice move: require a prediction before revealing the answer.

Explain the Be/B ionization anomaly

• Reasoning: B loses a higher-energy 2p electron, while Be loses from a filled 2s subshell; B can have the lower first ionization energy.
• Quality check: Use orbital occupancy, not an exception list.
• Practice move: require a prediction before revealing the answer.

Explain the N/O ionization anomaly

• Reasoning: O has a paired 2p electron with additional repulsion; N has a half-filled 2p arrangement, so O can be easier to ionize.
• Quality check: The overall across-period increase remains a trend, not an absolute sequence.
• Practice move: require a prediction before revealing the answer.

Compare C and Si electronegativity

• Reasoning: Both are group 14; the smaller period-2 atom generally attracts shared electron density more strongly, so C is higher.
• Quality check: Electronegativity is a bonding scale.
• Practice move: require a prediction before revealing the answer.

Compare Mg and Al metallic character

• Reasoning: Metallic character generally decreases across period 3, so Mg is expected to be more metallic.
• Quality check: Amphoterism and covalency require compound-level evidence.
• Practice move: require a prediction before revealing the answer.

Order K+, Ca2+, and Sc3+ radii

• Reasoning: They are isoelectronic with Ar. More protons give stronger attraction, so K+ > Ca2+ > Sc3+.
• Quality check: Check charge and electron count.
• Practice move: require a prediction before revealing the answer.

Explain high group-1 metal reactivity

• Reasoning: Valence electrons become farther from the nucleus and more shielded down the group, easing oxidation.
• Quality check: Observed rate also depends on physical and experimental conditions.
• Practice move: require a prediction before revealing the answer.

Explain fluorine’s oxidizing strength

• Reasoning: Small size, strong attraction, fluoride formation, bond energetics, and solvation all contribute.
• Quality check: Electronegativity alone is incomplete.
• Practice move: require a prediction before revealing the answer.

Explain transition-metal radius irregularity

• Reasoning: Nuclear charge rises while added d electrons shield imperfectly, but electron interaction and configuration compete.
• Quality check: Do not draw a perfectly linear arrow.
• Practice move: require a prediction before revealing the answer.

Explain lanthanoid similarity

• Reasoning: Differentiating 4f electrons are relatively core-like and +3 is common.
• Quality check: Small differences remain vital for separation and uses.
• Practice move: require a prediction before revealing the answer.

Explain lanthanoid contraction

• Reasoning: Added 4f electrons shield poorly while nuclear charge rises, so radii generally decrease.
• Quality check: Use consistent charge and coordination for numeric comparison.
• Practice move: require a prediction before revealing the answer.

Explain hydrogen placement

• Reasoning: Its 1s1 configuration resembles group 1, but covalent H2 and nonmetallic behavior are exceptional.
• Quality check: Placement does not erase uniqueness.
• Practice move: require a prediction before revealing the answer.

Explain helium placement

• Reasoning: Its filled 1s shell and chemical inertness align with group 18 despite lacking np6.
• Quality check: Classification uses shell closure and behavior.
• Practice move: require a prediction before revealing the answer.

Predict period-3 oxide behavior

• Reasoning: The broad change is basic ionic oxides to amphoteric behavior to acidic covalent oxides.
• Quality check: Specify oxidation state and exact oxide.
• Practice move: require a prediction before revealing the answer.

Predict a common chlorine ion

• Reasoning: Gaining one electron completes a noble-gas-like shell, so Cl− is common.
• Quality check: Positive oxidation states occur in oxycompounds.
• Practice move: require a prediction before revealing the answer.

Explain transition-metal color

• Reasoning: Partially filled d levels can allow visible-energy transitions; ligands and oxidation state alter splitting.
• Quality check: d0 and d10 species can be colorless.
• Practice move: require a prediction before revealing the answer.

Explain variable transition-metal states

• Reasoning: ns and (n−1)d orbitals can be close in energy, allowing different electron counts in bonding.
• Quality check: Stability depends on ligand, solvent, pH, and redox environment.
• Practice move: require a prediction before revealing the answer.

Explain noble-gas compounds

• Reasoning: Heavier atoms are more polarizable and strong oxidants can access outer electrons, enabling xenon compounds.
• Quality check: Low reactivity does not mean zero reactivity.
• Practice move: require a prediction before revealing the answer.

Compare Cl and Cl− size

• Reasoning: Added electron repulsion at unchanged nuclear charge makes Cl− larger.
• Quality check: Use compatible radius definitions.
• Practice move: require a prediction before revealing the answer.

Compare Fe2+ and Fe3+ size

• Reasoning: Fe3+ has fewer electrons and stronger attraction per remaining electron, so it is generally smaller.
• Quality check: Spin and coordination affect quoted radii.
• Practice move: require a prediction before revealing the answer.

Explain Li–Mg diagonal similarity

• Reasoning: Opposing changes in charge and size give somewhat similar charge density and selected chemical similarities.
• Quality check: It is a limited analogy.
• Practice move: require a prediction before revealing the answer.

Rank Li, Na, and K first ionization energy

• Reasoning: Down group 1, shielding and distance increase, so Li > Na > K.
• Quality check: Verify numerical values in one dataset.
• Practice move: require a prediction before revealing the answer.

Rank N, O, and F electronegativity

• Reasoning: Across period 2 the usual Pauling-scale order is N < O < F.
• Quality check: Do not assign every noble gas a conventional value.
• Practice move: require a prediction before revealing the answer.

Explain irregular melting points

• Reasoning: Melting depends on elemental structure and bonding: metallic, molecular, covalent-network, and allotrope differences matter.
• Quality check: Use a property map, not a universal trend arrow.
• Practice move: require a prediction before revealing the answer.

Explain electron-affinity irregularity

• Reasoning: Subshell stability, pairing, size, and sign convention compete.
• Quality check: State the gas-phase process and sign convention.
• Practice move: require a prediction before revealing the answer.

Use successive ionization energies

• Reasoning: A large jump after two removals suggests two readily removed valence electrons before a core electron.
• Quality check: This does not prove every compound is +2.
• Practice move: require a prediction before revealing the answer.

Predict H–F bond polarity

• Reasoning: F attracts shared density more strongly, so F carries partial negative character.
• Quality check: Bond polarity alone does not determine whole-molecule polarity.
• Practice move: require a prediction before revealing the answer.

Connect trends to MgCl2

• Reasoning: Group patterns support Mg2+ and Cl−, so charge neutrality gives MgCl2.
• Quality check: Transition and heavier p-block metals require explicit oxidation-state information.
• Practice move: require a prediction before revealing the answer.

Misconceptions and repairs

Misconception: Same-group elements are chemically identical

• Repair: They share patterns, but size, bond strengths, oxidation states, phases, and relativistic effects can create major differences.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Atomic mass determines table order

• Repair: The modern table is ordered by atomic number.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Mendeleev made the exact modern table

• Repair: He developed a powerful mass-based arrangement and predictions; atomic-number ordering and quantum explanation came later.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Every property changes smoothly across a period

• Repair: Many trends are qualified, and structural properties can be highly irregular.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Atomic radius is a hard-sphere measurement

• Repair: Atoms have diffuse density; radius depends on an operational definition.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Cations are larger than their atoms

• Repair: Common cations are generally smaller.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Anions are smaller because electrons are attracted

• Repair: Common anions are generally larger because repulsion rises at unchanged proton number.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Electronegativity equals electron affinity

• Repair: Electronegativity concerns attraction in bonds; electron affinity concerns gas-phase electron attachment.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Electron affinity always becomes more favorable across a period

• Repair: Subshell, pairing, size, and sign-convention complications create exceptions.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Ionization energy is released on ion formation

• Repair: Ionization energy is required to remove an electron from a specified gaseous species.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Shielding cancels nuclear charge completely

• Repair: Shielding reduces attraction but is not simple one-for-one cancellation.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: d and f electrons shield well

• Repair: They often shield less effectively than simple shell counting suggests.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Hydrogen is an ordinary alkali metal

• Repair: Its electron count creates analogies, but its chemistry and physical state are unique.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Helium belongs in group 2

• Repair: Its filled shell and noble-gas behavior support group 18.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: All transition-metal compounds are colored

• Repair: d0 and d10 configurations and selection rules create common exceptions.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Noble gases never bond

• Repair: Compounds of heavier noble gases, especially xenon, are established.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: A trend arrow proves a reaction occurs

• Repair: Feasibility and rate depend on thermodynamics, kinetics, phase, solvent, and conditions.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Electronegativity difference perfectly separates ionic and covalent

• Repair: Bonding is a continuum affected by structure, polarization, and lattice or molecular context.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Metallic character guarantees conductivity

• Repair: Conductivity depends on structure, purity, phase, defects, and temperature.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Oxidation state is a real free-ion charge

• Repair: It is formal electron bookkeeping and need not describe an isolated ion.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: The most common oxidation state is the only one

• Repair: Many elements support several states.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Group number always equals valence count

• Repair: The shortcut works only in specified main-group contexts.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: The f block is unimportant

• Repair: It is central to magnets, lighting, medicine, catalysis, nuclear chemistry, and theory.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: All radioactive elements are synthetic

• Repair: Several occur naturally.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Element hazards apply unchanged to all compounds

• Repair: Hazard depends on form, dose, route, particle size, solubility, oxidation state, and exposure.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: A safe compound proves the element is safe

• Repair: Elemental and compound properties may differ radically.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: The table predicts exact numbers

• Repair: It supports qualitative prediction; datasets are needed quantitatively.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: IUPAC requires one visual layout

• Repair: IUPAC standardizes names and group numbering but does not mandate one unique form.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Group 3 placement is shown identically everywhere

• Repair: Alternative conventions appear and must be labeled.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Misconception: Blank property values mean zero

• Repair: Blank means unavailable, inapplicable, unassigned, or not reliably established.
• Teaching move: ask which definition, species, condition, or configuration was omitted.

Frequently asked questions

Why is the table periodic?

Valence-electron configurations recur in a structured way as atomic number rises, creating recurring chemical and physical patterns.

What determines element identity?

Atomic number, the proton count.

Why are isotopes one element?

They share proton number but differ in neutron number.

Why do group members resemble one another?

They often share related valence patterns, modified by size and energy differences.

What is a period?

A horizontal row associated broadly with related shell-filling structure.

What is a group?

A vertical column numbered 1 through 18.

What are blocks?

Regions associated with the subshell receiving differentiating electrons.

Why is the f block below?

The detached display keeps the chart narrow; these elements belong within periods 6 and 7.

What is effective nuclear charge?

A model for net positive attraction experienced by an electron after shielding and penetration are considered.

Why does size usually decrease across?

Nuclear charge rises while added electrons enter the same general shell, so effective attraction generally increases.

Why does size usually increase down?

Additional shells place valence density farther away and increase shielding.

Why are cations smaller?

Electron loss reduces repulsion and can remove the outer shell.

Why are anions larger?

Electron gain increases repulsion without increasing proton number.

What is first ionization energy?

Energy required to remove the first electron from a gaseous atom in the stated process.

What is electronegativity?

A scale for an atom’s attraction of shared electron density in a bond.

Why is fluorine highly electronegative?

Small size and strong effective attraction favor a strong pull on bonding density.

Does helium have the highest electronegativity?

Many common scales do not assign helium a conventional value because it rarely forms ordinary bonds.

What is metallic character?

A qualitative tendency toward electron loss, metallic bonding, and metallic physical behavior.

Why do transition metals have variable states?

Their ns and (n−1)d electrons can be close enough in energy for different counts to participate.

Why are some atomic weights bracketed?

For elements without characteristic natural isotopic abundance, tables often show a selected isotope mass number in brackets.

Are atomic weights constant everywhere?

Natural isotopic composition can vary, so some standard weights are intervals or abridged values.

What is the most reactive element?

The question needs a reaction and conditions; different families are ranked by different processes.

Can trends predict compounds?

They guide likely charge, bonding, and reactivity, but full context and evidence remain necessary.

Why is mercury liquid near room conditions?

Relativistic and bonding effects weaken cohesion compared with simple expectations.

Why is gold yellow?

Relativistic effects shift electronic-energy differences and visible-light absorption.

Why are Zr and Hf similar in size?

The lanthanoid contraction reduces the expected increase before Hf.

Are superheavy elements useful?

Their present value is scientific: testing nuclear stability, relativistic chemistry, and periodic limits.

How should two elements be compared?

Define species and property, hold convention constant, examine structure, predict, then verify data.

Which data sources should be trusted?

Use IUPAC/CIAAW for naming and weights, PubChem for broad element records, and NIST for evaluated properties and spectra.

How often should the page be reviewed?

Review annually and whenever a canonical source changes materially.

Retrieval-practice set

Question 1: Why is the table periodic?

• Try first: answer in one or two sentences.
• Answer: Valence-electron configurations recur in a structured way as atomic number rises, creating recurring chemical and physical patterns.
• Follow-up: give one qualification or data check.

Question 2: What determines element identity?

• Try first: answer in one or two sentences.
• Answer: Atomic number, the proton count.
• Follow-up: give one qualification or data check.

Question 3: Why are isotopes one element?

• Try first: answer in one or two sentences.
• Answer: They share proton number but differ in neutron number.
• Follow-up: give one qualification or data check.

Question 4: Why do group members resemble one another?

• Try first: answer in one or two sentences.
• Answer: They often share related valence patterns, modified by size and energy differences.
• Follow-up: give one qualification or data check.

Question 5: What is a period?

• Try first: answer in one or two sentences.
• Answer: A horizontal row associated broadly with related shell-filling structure.
• Follow-up: give one qualification or data check.

Question 6: What is a group?

• Try first: answer in one or two sentences.
• Answer: A vertical column numbered 1 through 18.
• Follow-up: give one qualification or data check.

Question 7: What are blocks?

• Try first: answer in one or two sentences.
• Answer: Regions associated with the subshell receiving differentiating electrons.
• Follow-up: give one qualification or data check.

Question 8: Why is the f block below?

• Try first: answer in one or two sentences.
• Answer: The detached display keeps the chart narrow; these elements belong within periods 6 and 7.
• Follow-up: give one qualification or data check.

Question 9: What is effective nuclear charge?

• Try first: answer in one or two sentences.
• Answer: A model for net positive attraction experienced by an electron after shielding and penetration are considered.
• Follow-up: give one qualification or data check.

Question 10: Why does size usually decrease across?

• Try first: answer in one or two sentences.
• Answer: Nuclear charge rises while added electrons enter the same general shell, so effective attraction generally increases.
• Follow-up: give one qualification or data check.

Question 11: Why does size usually increase down?

• Try first: answer in one or two sentences.
• Answer: Additional shells place valence density farther away and increase shielding.
• Follow-up: give one qualification or data check.

Question 12: Why are cations smaller?

• Try first: answer in one or two sentences.
• Answer: Electron loss reduces repulsion and can remove the outer shell.
• Follow-up: give one qualification or data check.

Question 13: Why are anions larger?

• Try first: answer in one or two sentences.
• Answer: Electron gain increases repulsion without increasing proton number.
• Follow-up: give one qualification or data check.

Question 14: What is first ionization energy?

• Try first: answer in one or two sentences.
• Answer: Energy required to remove the first electron from a gaseous atom in the stated process.
• Follow-up: give one qualification or data check.

Question 15: What is electronegativity?

• Try first: answer in one or two sentences.
• Answer: A scale for an atom’s attraction of shared electron density in a bond.
• Follow-up: give one qualification or data check.

Question 16: Why is fluorine highly electronegative?

• Try first: answer in one or two sentences.
• Answer: Small size and strong effective attraction favor a strong pull on bonding density.
• Follow-up: give one qualification or data check.

Question 17: Does helium have the highest electronegativity?

• Try first: answer in one or two sentences.
• Answer: Many common scales do not assign helium a conventional value because it rarely forms ordinary bonds.
• Follow-up: give one qualification or data check.

Question 18: What is metallic character?

• Try first: answer in one or two sentences.
• Answer: A qualitative tendency toward electron loss, metallic bonding, and metallic physical behavior.
• Follow-up: give one qualification or data check.

Question 19: Why do transition metals have variable states?

• Try first: answer in one or two sentences.
• Answer: Their ns and (n−1)d electrons can be close enough in energy for different counts to participate.
• Follow-up: give one qualification or data check.

Question 20: Why are some atomic weights bracketed?

• Try first: answer in one or two sentences.
• Answer: For elements without characteristic natural isotopic abundance, tables often show a selected isotope mass number in brackets.
• Follow-up: give one qualification or data check.

Question 21: Compare Na and Mg atomic radii

• Try first: state the species, property, and comparison rule.
• Answer: Both are in period 3. Mg has one more proton while the principal valence shell is unchanged, so effective nuclear attraction is generally greater and Mg is smaller.
• Check: Use the same neutral-radius convention for both values.

Question 22: Compare Na and Na+

• Try first: state the species, property, and comparison rule.
• Answer: Removing the 3s electron eliminates the common outer shell and reduces repulsion, so Na+ is much smaller.
• Check: Do not mix neutral covalent radius and ionic radius without labels.

Question 23: Compare F− and Na+

• Try first: state the species, property, and comparison rule.
• Answer: Both have ten electrons. Na+ has more protons, so it is smaller.
• Check: Count electrons before using the isoelectronic rule.

Question 24: Explain the Be/B ionization anomaly

• Try first: state the species, property, and comparison rule.
• Answer: B loses a higher-energy 2p electron, while Be loses from a filled 2s subshell; B can have the lower first ionization energy.
• Check: Use orbital occupancy, not an exception list.

Question 25: Explain the N/O ionization anomaly

• Try first: state the species, property, and comparison rule.
• Answer: O has a paired 2p electron with additional repulsion; N has a half-filled 2p arrangement, so O can be easier to ionize.
• Check: The overall across-period increase remains a trend, not an absolute sequence.

Question 26: Compare C and Si electronegativity

• Try first: state the species, property, and comparison rule.
• Answer: Both are group 14; the smaller period-2 atom generally attracts shared electron density more strongly, so C is higher.
• Check: Electronegativity is a bonding scale.

Question 27: Compare Mg and Al metallic character

• Try first: state the species, property, and comparison rule.
• Answer: Metallic character generally decreases across period 3, so Mg is expected to be more metallic.
• Check: Amphoterism and covalency require compound-level evidence.

Question 28: Order K+, Ca2+, and Sc3+ radii

• Try first: state the species, property, and comparison rule.
• Answer: They are isoelectronic with Ar. More protons give stronger attraction, so K+ > Ca2+ > Sc3+.
• Check: Check charge and electron count.

Question 29: Explain high group-1 metal reactivity

• Try first: state the species, property, and comparison rule.
• Answer: Valence electrons become farther from the nucleus and more shielded down the group, easing oxidation.
• Check: Observed rate also depends on physical and experimental conditions.

Question 30: Explain fluorine’s oxidizing strength

• Try first: state the species, property, and comparison rule.
• Answer: Small size, strong attraction, fluoride formation, bond energetics, and solvation all contribute.
• Check: Electronegativity alone is incomplete.

Question 31: Explain transition-metal radius irregularity

• Try first: state the species, property, and comparison rule.
• Answer: Nuclear charge rises while added d electrons shield imperfectly, but electron interaction and configuration compete.
• Check: Do not draw a perfectly linear arrow.

Question 32: Explain lanthanoid similarity

• Try first: state the species, property, and comparison rule.
• Answer: Differentiating 4f electrons are relatively core-like and +3 is common.
• Check: Small differences remain vital for separation and uses.

Question 33: Explain lanthanoid contraction

• Try first: state the species, property, and comparison rule.
• Answer: Added 4f electrons shield poorly while nuclear charge rises, so radii generally decrease.
• Check: Use consistent charge and coordination for numeric comparison.

Question 34: Explain hydrogen placement

• Try first: state the species, property, and comparison rule.
• Answer: Its 1s1 configuration resembles group 1, but covalent H2 and nonmetallic behavior are exceptional.
• Check: Placement does not erase uniqueness.

Question 35: Explain helium placement

• Try first: state the species, property, and comparison rule.
• Answer: Its filled 1s shell and chemical inertness align with group 18 despite lacking np6.
• Check: Classification uses shell closure and behavior.

Question 36: Predict period-3 oxide behavior

• Try first: state the species, property, and comparison rule.
• Answer: The broad change is basic ionic oxides to amphoteric behavior to acidic covalent oxides.
• Check: Specify oxidation state and exact oxide.

Question 37: Predict a common chlorine ion

• Try first: state the species, property, and comparison rule.
• Answer: Gaining one electron completes a noble-gas-like shell, so Cl− is common.
• Check: Positive oxidation states occur in oxycompounds.

Question 38: Explain transition-metal color

• Try first: state the species, property, and comparison rule.
• Answer: Partially filled d levels can allow visible-energy transitions; ligands and oxidation state alter splitting.
• Check: d0 and d10 species can be colorless.

Question 39: Explain variable transition-metal states

• Try first: state the species, property, and comparison rule.
• Answer: ns and (n−1)d orbitals can be close in energy, allowing different electron counts in bonding.
• Check: Stability depends on ligand, solvent, pH, and redox environment.

Question 40: Explain noble-gas compounds

• Try first: state the species, property, and comparison rule.
• Answer: Heavier atoms are more polarizable and strong oxidants can access outer electrons, enabling xenon compounds.
• Check: Low reactivity does not mean zero reactivity.

Glossary

actinoid

An element Ac through Lr under IUPAC collective naming.

alkali metal

A group-1 metal, with hydrogen normally discussed separately.

alkaline-earth metal

A group-2 metal.

allotrope

One of multiple structural forms of an element in the same physical state.

anion

A negatively charged ion.

atomic number

Number of protons in an atomic nucleus.

atomic orbital

A one-electron quantum state or its wavefunction representation, depending on context.

atomic radius

An operational estimate of atomic size using a defined structural context.

atomic weight

Relative atomic mass of an element in a stated sample or conventional context.

block

A region associated with filling an s, p, d, or f subshell.

cation

A positively charged ion.

chalcogen

A collective name for group-16 elements.

chemical element

Atoms characterized by one atomic number.

covalent radius

A radius derived from covalently bonded internuclear distances.

d-block

The region associated mainly with filling (n−1)d subshells.

effective nuclear charge

Net positive attraction modeled for an electron after shielding effects.

electron affinity

Energy change for electron attachment to a gaseous species, with sign convention stated.

electron configuration

Distribution of electrons among orbitals or subshells for a specified state.

electronegativity

A scale for attraction of shared electron density in a bond.

f-block

The region associated mainly with filling (n−2)f subshells.

group

A vertical column numbered 1 through 18.

halogen

A collective name for group-17 elements.

ion

An atom or molecular entity with net charge.

ionic radius

An effective radius assigned to an ion in a stated coordination and structural model.

ionization energy

Minimum energy required for a specified ionization process.

isoelectronic

Having the same electron count and compared arrangement context.

isotope

Atoms of one element with different neutron numbers.

lanthanoid

An element La through Lu under IUPAC collective naming.

main-group element

An s- or p-block element in common usage.

metalloid

A contextual classification near the metal–nonmetal boundary.

modern periodic law

Elemental properties recur periodically with atomic number.

noble gas

A group-18 element.

oxidation state

A formal charge assigned by agreed electron bookkeeping.

p-block

The region associated mainly with filling np subshells.

period

A horizontal row.

periodic trend

A recurring property pattern with qualifications and exceptions.

pnictogen

A collective name used for group-15 elements.

shielding

Reduction in nucleus–electron attraction due to other electron density.

s-block

The region associated mainly with filling ns subshells.

standard atomic weight

An evaluated value or interval for normal terrestrial materials under stated conventions.

subshell

Orbitals sharing principal and angular-momentum quantum numbers.

superheavy element

A very high-atomic-number synthetic element in common usage.

transition element

An element with an incomplete d subshell in the atom or a cation under a stated definition.

valence electron

An electron treated as able to participate in bonding or chemical change.

van der Waals radius

An effective size derived from defined nonbonded contact distances.

Source notes

• Use IUPAC for names, symbols, group numbering, table-release notes, and standard terminology.
• Use CIAAW for atomic weights, intervals, abridged values, and isotope-abundance updates.
• Use PubChem for canonical element records and broad property layers.
• Use NIST Chemistry WebBook for evaluated thermochemical, spectroscopic, and ion-energetic data.
• Use NIST CODATA for constants and display the adjustment year.
• Use ACS RAMP for safety framing.
• Use UNECE GHS for hazard-communication concepts and current pictograms.
• Do not reproduce source prose; keep this site’s explanations original.
• Record publishedOn and reviewedOn separately.
• Trigger re-review when a canonical source changes.
• Require a qualified chemistry reviewer before displaying a scientific-review badge.

Required internal links

• /periodic-table/
• /elements/
• /compare-elements/
• /learn/atomic-theory-and-quantum-structure/electron-configurations/
• /learn/chemical-bonding/
• /learn/formulas-compounds-and-nomenclature/oxidation-numbers/
• /learn/inorganic-and-coordination-chemistry/
• /learn/nuclear-chemistry/
• /tools/electron-configuration/
• /tools/molar-mass-calculator/
• /practice/periodic-trends/
• /glossary/atomic-number/
• /glossary/effective-nuclear-charge/
• /glossary/electronegativity/
• /editorial-policy/
• /sources-and-corrections/

Structured-data eligibility

• Article schema may describe the visible article.
• BreadcrumbList may describe visible navigation.
• FAQPage may include only visibly rendered questions and answers.
• Do not invent Course, Quiz, Dataset, author credentials, ratings, or reviews.
• Do not fabricate publication or review dates.

Review checklist

• All 118 element profiles exist exactly once.
• Every numerical property comes from the canonical dataset.
• Every value shows unit and source.
• Missing and not-applicable states are distinct.
• Electron-affinity sign convention is explicit.
• Radius definition is explicit.
• Predictions are labeled.
• Group-3 display convention is disclosed.
• Hydrogen and helium receive explanatory notes.
• Mobile exposes all elements and comparison features.
• Keyboard users can add and remove compare items.
• Color is never the only category signal.
• Safety language is substance-specific.
• The page has sources and a reviewed date.
• No source prose was copied.
• No thin placeholder routes were created.

Element-by-element learning index

These concise profiles connect the periodic lesson to all 118 element routes.

Numeric properties must come from the canonical IUPAC/PubChem-backed dataset rather than this prose.

Element 001 — Hydrogen (H)

• Identity: atomic number 1; symbol H.
• Learning focus: Water, acids, hydrides, biological molecules, and energy systems make hydrogen foundational across chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/hydrogen/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 002 — Helium (He)

• Identity: atomic number 2; symbol He.
• Learning focus: A filled 1s shell, exceptional inertness, and very weak attractions connect helium to cryogenics and noble-gas behavior.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/helium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 003 — Lithium (Li)

• Identity: atomic number 3; symbol Li.
• Learning focus: Small Li+ ions, polarizing power, organolithium reagents, ceramics, medicines, and rechargeable batteries make lithium unusually important.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/lithium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 004 — Beryllium (Be)

• Identity: atomic number 4; symbol Be.
• Learning focus: Small Be2+ gives strongly polarizing and often covalent chemistry; useful light alloys coexist with serious inhalation hazards.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/beryllium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 005 — Boron (B)

• Identity: atomic number 5; symbol B.
• Learning focus: Electron-deficient bonding, boranes, borates, glass, magnets, and neutron capture make boron structurally distinctive.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/boron/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 006 — Carbon (C)

• Identity: atomic number 6; symbol C.
• Learning focus: Catenation, strong C–C bonds, multiple bonding, and varied hybridization underpin organic chemistry, life, fuels, and advanced materials.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/carbon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 007 — Nitrogen (N)

• Identity: atomic number 7; symbol N.
• Learning focus: Stable N2, oxidation states from −3 to +5, fertilizers, proteins, and nucleic acids connect atmosphere, industry, and life.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/nitrogen/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 008 — Oxygen (O)

• Identity: atomic number 8; symbol O.
• Learning focus: High electronegativity, oxides, water, respiration, combustion, and redox chemistry make oxygen central to natural and industrial systems.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/oxygen/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 009 — Fluorine (F)

• Identity: atomic number 9; symbol F.
• Learning focus: Extreme electronegativity and strong fluoride bonding support fluoropolymers, medicines, minerals, and powerful oxidizing chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/fluorine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 010 — Neon (Ne)

• Identity: atomic number 10; symbol Ne.
• Learning focus: Closed-shell electronic structure explains low ordinary reactivity and applications in discharge lighting and lasers.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/neon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 011 — Sodium (Na)

• Identity: atomic number 11; symbol Na.
• Learning focus: Na+ dominates many salts and biological fluids, while elemental sodium illustrates how compound and element properties can differ radically.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/sodium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 012 — Magnesium (Mg)

• Identity: atomic number 12; symbol Mg.
• Learning focus: Mg2+, chlorophyll, light alloys, organomagnesium reagents, medicine, and pyrotechnics connect biology and synthesis.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/magnesium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 013 — Aluminum (Al)

• Identity: atomic number 13; symbol Al.
• Learning focus: A passivating oxide layer, amphoteric compounds, low density, and abundant ores underpin major structural and materials uses.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/aluminum/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 014 — Silicon (Si)

• Identity: atomic number 14; symbol Si.
• Learning focus: Silicate networks, glass, concrete, silicones, semiconductors, and photovoltaics link Earth’s crust to modern electronics.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/silicon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 015 — Phosphorus (P)

• Identity: atomic number 15; symbol P.
• Learning focus: Phosphates in DNA, ATP, bones, and fertilizers connect oxidation-state chemistry with biology and agriculture.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/phosphorus/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 016 — Sulfur (S)

• Identity: atomic number 16; symbol S.
• Learning focus: Allotropes, catenation, sulfides, sulfates, proteins, vulcanization, and sulfuric acid make sulfur chemically and industrially broad.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/sulfur/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 017 — Chlorine (Cl)

• Identity: atomic number 17; symbol Cl.
• Learning focus: Chloride, chlorine, oxyanions, disinfection, polymers, and synthesis demonstrate strongly form-dependent properties and hazards.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/chlorine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 018 — Argon (Ar)

• Identity: atomic number 18; symbol Ar.
• Learning focus: Low reactivity makes argon useful for welding, lighting, metallurgy, and controlled atmospheres.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/argon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 019 — Potassium (K)

• Identity: atomic number 19; symbol K.
• Learning focus: K+ is an essential electrolyte and plant nutrient; fertilizers, glass, and reactive-metal chemistry extend its importance.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/potassium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 020 — Calcium (Ca)

• Identity: atomic number 20; symbol Ca.
• Learning focus: Calcium compounds dominate bones, shells, limestone, cement, water hardness, and cellular signaling.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/calcium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 021 — Scandium (Sc)

• Identity: atomic number 21; symbol Sc.
• Learning focus: Mostly Sc3+ chemistry links selected light alloys, lamps, ceramics, and comparisons with rare-earth ions.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/scandium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 022 — Titanium (Ti)

• Identity: atomic number 22; symbol Ti.
• Learning focus: Strength, low density, passivation, TiO2 pigments, implants, and catalysis make titanium a major materials element.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/titanium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 023 — Vanadium (V)

• Identity: atomic number 23; symbol V.
• Learning focus: Multiple colored oxidation states, steel alloys, oxidation catalysts, and flow batteries illustrate transition-metal versatility.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/vanadium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 024 — Chromium (Cr)

• Identity: atomic number 24; symbol Cr.
• Learning focus: Stainless steel, passivation, pigments, and the sharp hazard contrast between Cr(III) and Cr(VI) make speciation essential.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/chromium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 025 — Manganese (Mn)

• Identity: atomic number 25; symbol Mn.
• Learning focus: Steelmaking, batteries, enzymes, pigments, and permanganate redox chemistry connect industry and biology.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/manganese/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 026 — Iron (Fe)

• Identity: atomic number 26; symbol Fe.
• Learning focus: Steel, hemoglobin, enzymes, magnets, ores, and Fe2+/Fe3+ redox chemistry make iron central to civilization and life.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/iron/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 027 — Cobalt (Co)

• Identity: atomic number 27; symbol Co.
• Learning focus: Vitamin B12, superalloys, pigments, catalysts, magnets, and batteries show cobalt’s biological and technological range.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/cobalt/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 028 — Nickel (Ni)

• Identity: atomic number 28; symbol Ni.
• Learning focus: Alloys, plating, batteries, coins, coordination chemistry, and hydrogenation catalysis make nickel widely encountered.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/nickel/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 029 — Copper (Cu)

• Identity: atomic number 29; symbol Cu.
• Learning focus: High conductivity, Cu+/Cu2+ redox, wiring, alloys, catalysts, and electron-transfer proteins connect materials and biology.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/copper/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 030 — Zinc (Zn)

• Identity: atomic number 30; symbol Zn.
• Learning focus: Galvanizing, brass, batteries, enzymes, Lewis acidity, and common Zn2+ chemistry make zinc broadly useful.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/zinc/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 031 — Gallium (Ga)

• Identity: atomic number 31; symbol Ga.
• Learning focus: Near-room-temperature melting and III–V semiconductors support LEDs, high-frequency electronics, solar cells, and medical isotopes.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/gallium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 032 — Germanium (Ge)

• Identity: atomic number 32; symbol Ge.
• Learning focus: Covalent semiconductor behavior supports fiber optics, infrared optics, electronics, catalysts, and specialized photovoltaics.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/germanium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 033 — Arsenic (As)

• Identity: atomic number 33; symbol As.
• Learning focus: Oxidation state and chemical form control its semiconductor uses, environmental behavior, and substantial toxicity.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/arsenic/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 034 — Selenium (Se)

• Identity: atomic number 34; symbol Se.
• Learning focus: Sulfur-like redox chemistry, essential trace biology, glass, pigments, photoconductivity, and photovoltaics define selenium.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/selenium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 035 — Bromine (Br)

• Identity: atomic number 35; symbol Br.
• Learning focus: A liquid nonmetal near room conditions, bromide chemistry, synthesis, medicines, and flame-control applications make bromine distinctive.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/bromine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 036 — Krypton (Kr)

• Identity: atomic number 36; symbol Kr.
• Learning focus: Mostly inert behavior supports lighting, lasers, insulated windows, and specialized measurement technologies.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/krypton/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 037 — Rubidium (Rb)

• Identity: atomic number 37; symbol Rb.
• Learning focus: Highly reactive Rb+ chemistry and atomic transitions make rubidium important mainly in spectroscopy, clocks, and quantum research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/rubidium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 038 — Strontium (Sr)

• Identity: atomic number 38; symbol Sr.
• Learning focus: Sr2+ chemistry, red flame emission, ferrites, ceramics, clocks, and medical isotopes connect periodic trends to applications.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/strontium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 039 — Yttrium (Y)

• Identity: atomic number 39; symbol Y.
• Learning focus: Y3+ chemistry and rare-earth-like behavior support phosphors, lasers, superconductors, ceramics, and high-temperature materials.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/yttrium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 040 — Zirconium (Zr)

• Identity: atomic number 40; symbol Zr.
• Learning focus: Passivation, stable ZrO2, nuclear cladding, ceramics, implants, and corrosion-resistant equipment make zirconium important.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/zirconium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 041 — Niobium (Nb)

• Identity: atomic number 41; symbol Nb.
• Learning focus: Superconducting compounds, strong steels, turbines, capacitors, and quantum devices showcase niobium.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/niobium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 042 — Molybdenum (Mo)

• Identity: atomic number 42; symbol Mo.
• Learning focus: High-strength alloys, hydrodesulfurization catalysts, lubricants, and molybdenum enzymes connect industry and biology.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/molybdenum/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 043 — Technetium (Tc)

• Identity: atomic number 43; symbol Tc.
• Learning focus: All isotopes are radioactive; technetium-99m makes this element especially important in diagnostic nuclear medicine.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/technetium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 044 — Ruthenium (Ru)

• Identity: atomic number 44; symbol Ru.
• Learning focus: Many oxidation states, coordination chemistry, catalysis, electronics, and emerging medicines make ruthenium versatile.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/ruthenium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 045 — Rhodium (Rh)

• Identity: atomic number 45; symbol Rh.
• Learning focus: Noble-metal corrosion resistance and catalytic activity support catalytic converters, synthesis, coatings, and thermocouples.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/rhodium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 046 — Palladium (Pd)

• Identity: atomic number 46; symbol Pd.
• Learning focus: Hydrogen interaction and bond-forming catalysis connect palladium to converters, electronics, synthesis, and hydrogen purification.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/palladium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 047 — Silver (Ag)

• Identity: atomic number 47; symbol Ag.
• Learning focus: Exceptional conductivity, Ag+ precipitation and complexation, light-sensitive salts, mirrors, electronics, and antimicrobial uses define silver.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/silver/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 048 — Cadmium (Cd)

• Identity: atomic number 48; symbol Cd.
• Learning focus: Cd2+ chemistry supports batteries, pigments, coatings, and semiconductors, but toxicity has driven strict controls and substitution.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/cadmium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 049 — Indium (In)

• Identity: atomic number 49; symbol In.
• Learning focus: Soft metal behavior and transparent conducting oxides support touchscreens, displays, solders, semiconductors, and solar cells.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/indium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 050 — Tin (Sn)

• Identity: atomic number 50; symbol Sn.
• Learning focus: Common +2 and +4 states connect solder, tinplate, bronze, glass coatings, catalysts, and organotin chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/tin/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 051 — Antimony (Sb)

• Identity: atomic number 51; symbol Sb.
• Learning focus: +3/+5 chemistry supports flame-control systems, alloys, batteries, semiconductors, pigments, and selected medicines.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/antimony/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 052 — Tellurium (Te)

• Identity: atomic number 52; symbol Te.
• Learning focus: Semiconducting and thermoelectric behavior supports solar cells, thermoelectrics, alloys, optics, and catalysts.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/tellurium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 053 — Iodine (I)

• Identity: atomic number 53; symbol I.
• Learning focus: Iodide, iodine, oxyanions, thyroid hormones, antiseptics, imaging, and analytical redox chemistry make iodine vital.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/iodine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 054 — Xenon (Xe)

• Identity: atomic number 54; symbol Xe.
• Learning focus: Real xenon fluorides and oxides disprove absolute noble-gas inertness; lighting, propulsion, imaging, and anesthesia add applications.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/xenon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 055 — Cesium (Cs)

• Identity: atomic number 55; symbol Cs.
• Learning focus: Extreme electropositivity and precise atomic transitions connect cesium to atomic time, drilling fluids, sensors, and research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/cesium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 056 — Barium (Ba)

• Identity: atomic number 56; symbol Ba.
• Learning focus: BaSO4 medical contrast and soluble-barium toxicity show why chemical form matters; drilling, ceramics, and pyrotechnics add uses.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/barium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 057 — Lanthanum (La)

• Identity: atomic number 57; symbol La.
• Learning focus: La3+ chemistry supports optical glass, catalysts, battery electrodes, hydrogen storage, and rare-earth comparisons.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/lanthanum/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 058 — Cerium (Ce)

• Identity: atomic number 58; symbol Ce.
• Learning focus: Accessible +3/+4 redox supports catalytic converters, glass polishing, lighter flints, and oxidation catalysts.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/cerium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 059 — Praseodymium (Pr)

• Identity: atomic number 59; symbol Pr.
• Learning focus: Pr3+ optical and magnetic behavior supports magnets, alloys, glass coloration, ceramics, and lasers.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/praseodymium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 060 — Neodymium (Nd)

• Identity: atomic number 60; symbol Nd.
• Learning focus: High-strength permanent magnets and useful optical transitions support motors, turbines, headphones, and lasers.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/neodymium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 061 — Promethium (Pm)

• Identity: atomic number 61; symbol Pm.
• Learning focus: All isotopes are radioactive; research, nuclear batteries, thickness gauges, and luminescent sources are specialized contexts.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/promethium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 062 — Samarium (Sm)

• Identity: atomic number 62; symbol Sm.
• Learning focus: Samarium–cobalt magnets, neutron absorption, catalysts, lasers, and medical isotopes connect magnetism and nuclear science.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/samarium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 063 — Europium (Eu)

• Identity: atomic number 63; symbol Eu.
• Learning focus: Eu2+/Eu3+ luminescence supports red and blue phosphors, displays, lighting, and anti-counterfeiting marks.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/europium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 064 — Gadolinium (Gd)

• Identity: atomic number 64; symbol Gd.
• Learning focus: Strong magnetism and neutron capture support MRI contrast compounds, shielding, phosphors, and materials research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/gadolinium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 065 — Terbium (Tb)

• Identity: atomic number 65; symbol Tb.
• Learning focus: Green luminescence and magnetostriction support phosphors, sensors, sonar materials, magnets, and displays.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/terbium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 066 — Dysprosium (Dy)

• Identity: atomic number 66; symbol Dy.
• Learning focus: Magnetic anisotropy helps high-performance magnets work at elevated temperature; control rods and lasers add uses.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/dysprosium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 067 — Holmium (Ho)

• Identity: atomic number 67; symbol Ho.
• Learning focus: Large magnetic moment and sharp optical transitions support lasers, calibration, magnetic research, and specialized medicine.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/holmium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 068 — Erbium (Er)

• Identity: atomic number 68; symbol Er.
• Learning focus: Er3+ emission near telecommunications wavelengths supports fiber amplifiers, lasers, photonics, and colored glass.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/erbium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 069 — Thulium (Tm)

• Identity: atomic number 69; symbol Tm.
• Learning focus: Rare-earth optical behavior and radioisotopes support lasers, portable X-ray sources, and specialized research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/thulium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 070 — Ytterbium (Yb)

• Identity: atomic number 70; symbol Yb.
• Learning focus: Yb2+/Yb3+ chemistry and narrow transitions support fiber lasers, atomic clocks, alloys, and quantum science.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/ytterbium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 071 — Lutetium (Lu)

• Identity: atomic number 71; symbol Lu.
• Learning focus: Small Lu3+, high density, and scintillating compounds support PET detectors, catalysts, glass, and medical research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/lutetium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 072 — Hafnium (Hf)

• Identity: atomic number 72; symbol Hf.
• Learning focus: Zirconium-like chemistry plus strong neutron absorption supports control rods, superalloys, high-k electronics, and ceramics.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/hafnium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 073 — Tantalum (Ta)

• Identity: atomic number 73; symbol Ta.
• Learning focus: Exceptional corrosion resistance and a stable dielectric oxide support capacitors, implants, reactors, and superalloys.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/tantalum/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 074 — Tungsten (W)

• Identity: atomic number 74; symbol W.
• Learning focus: The highest melting point among pure metals supports tools, hot components, electrodes, shielding, and dense alloys.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/tungsten/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 075 — Rhenium (Re)

• Identity: atomic number 75; symbol Re.
• Learning focus: High-temperature strength and catalytic behavior support jet-engine superalloys, reforming catalysts, and thermocouples.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/rhenium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 076 — Osmium (Os)

• Identity: atomic number 76; symbol Os.
• Learning focus: Extreme density and distinctive but highly hazardous OsO4 chemistry support limited alloy, catalysis, and microscopy uses.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/osmium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 077 — Iridium (Ir)

• Identity: atomic number 77; symbol Ir.
• Learning focus: Exceptional corrosion and heat resistance support spark plugs, crucibles, catalysts, electronics, and radioisotope systems.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/iridium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 078 — Platinum (Pt)

• Identity: atomic number 78; symbol Pt.
• Learning focus: Noble behavior and powerful catalysis support converters, fuel cells, laboratory ware, sensors, jewelry, and anticancer compounds.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/platinum/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 079 — Gold (Au)

• Identity: atomic number 79; symbol Au.
• Learning focus: Corrosion resistance, conductivity, relativistic color, nanochemistry, and biocompatible uses support electronics, medicine, and jewelry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/gold/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 080 — Mercury (Hg)

• Identity: atomic number 80; symbol Hg.
• Learning focus: Liquid-metal behavior, amalgams, volatility, and bioaccumulative compounds make mercury both distinctive and hazardous.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/mercury/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 081 — Thallium (Tl)

• Identity: atomic number 81; symbol Tl.
• Learning focus: +1 chemistry and severe toxicity coexist with limited electronics, optical, research, and medical radioisotope uses.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/thallium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 082 — Lead (Pb)

• Identity: atomic number 82; symbol Pb.
• Learning focus: High density, +2/+4 chemistry, batteries, shielding, and cumulative toxicity make exposure control essential.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/lead/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 083 — Bismuth (Bi)

• Identity: atomic number 83; symbol Bi.
• Learning focus: Bi3+ chemistry, low-melting alloys, medicines, cosmetics, catalysts, and lead-free substitutes make bismuth useful.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/bismuth/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 084 — Polonium (Po)

• Identity: atomic number 84; symbol Po.
• Learning focus: Rare and intensely radioactive, polonium belongs mainly to nuclear history, research, and tightly controlled heat-source contexts.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/polonium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 085 — Astatine (At)

• Identity: atomic number 85; symbol At.
• Learning focus: Extreme rarity and radioactivity limit chemistry to tiny-scale experiments and targeted alpha-therapy research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/astatine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 086 — Radon (Rn)

• Identity: atomic number 86; symbol Rn.
• Learning focus: A radioactive noble gas produced in decay chains, radon is chiefly important as an indoor-air hazard and geological tracer.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/radon/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 087 — Francium (Fr)

• Identity: atomic number 87; symbol Fr.
• Learning focus: Minute natural abundance and short lifetimes restrict francium to atomic-structure and spectroscopy research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/francium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 088 — Radium (Ra)

• Identity: atomic number 88; symbol Ra.
• Learning focus: Radioactive Ra2+ chemistry, historical luminous paints, medicine, decay chains, and legacy cleanup define radium’s significance.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/radium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 089 — Actinium (Ac)

• Identity: atomic number 89; symbol Ac.
• Learning focus: Radioactive, mainly trivalent chemistry and actinium-225 targeted alpha-therapy research make actinium important.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/actinium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 090 — Thorium (Th)

• Identity: atomic number 90; symbol Th.
• Learning focus: Naturally radioactive +4 chemistry, fertile nuclear behavior, and historical materials uses define thorium.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/thorium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 091 — Protactinium (Pa)

• Identity: atomic number 91; symbol Pa.
• Learning focus: Extreme rarity, radioactivity, and +4/+5 chemistry restrict protactinium mostly to actinoid and geochemical research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/protactinium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 092 — Uranium (U)

• Identity: atomic number 92; symbol U.
• Learning focus: Multiple oxidation states, uranyl chemistry, nuclear fuel, dating, and stewardship make uranium scientifically and socially important.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/uranium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 093 — Neptunium (Np)

• Identity: atomic number 93; symbol Np.
• Learning focus: The first transuranium element, multiple oxidation states, neutron detection, and plutonium-238 production connect neptunium to nuclear science.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/neptunium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 094 — Plutonium (Pu)

• Identity: atomic number 94; symbol Pu.
• Learning focus: Complex allotropy, multiple oxidation states, fuel and power uses, and weapons relevance require strict security and stewardship.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/plutonium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 095 — Americium (Am)

• Identity: atomic number 95; symbol Am.
• Learning focus: Americium-241 in ionization smoke detectors, gauges, and research is a familiar application of a synthetic actinoid.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/americium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 096 — Curium (Cm)

• Identity: atomic number 96; symbol Cm.
• Learning focus: Strong radioactivity and mainly +3 chemistry support research, alpha sources, and production pathways to heavier elements.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/curium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 097 — Berkelium (Bk)

• Identity: atomic number 97; symbol Bk.
• Learning focus: Tiny synthetic quantities are used to study actinoid chemistry and produce heavier nuclei.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/berkelium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 098 — Californium (Cf)

• Identity: atomic number 98; symbol Cf.
• Learning focus: Spontaneous-fission neutron emission supports analysis, reactor startup, well logging, research, and selected therapy.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/californium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 099 — Einsteinium (Es)

• Identity: atomic number 99; symbol Es.
• Learning focus: Discovered in thermonuclear-test debris, einsteinium is now limited to research and heavy-element synthesis.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/einsteinium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 100 — Fermium (Fm)

• Identity: atomic number 100; symbol Fm.
• Learning focus: Short-lived synthetic fermium is used only to investigate nuclear structure and heavy-element chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/fermium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 101 — Mendelevium (Md)

• Identity: atomic number 101; symbol Md.
• Learning focus: Atom-scale production limits mendelevium to nuclear-property and late-actinoid research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/mendelevium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 102 — Nobelium (No)

• Identity: atomic number 102; symbol No.
• Learning focus: Synthetic nobelium has an unusually accessible +2 state and tests actinoid trends.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/nobelium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 103 — Lawrencium (Lr)

• Identity: atomic number 103; symbol Lr.
• Learning focus: The last actinoid, lawrencium tests simple electron-configuration predictions and atom-at-a-time chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/lawrencium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 104 — Rutherfordium (Rf)

• Identity: atomic number 104; symbol Rf.
• Learning focus: A short-lived group-4 superheavy element used to test nuclear stability and periodic chemical behavior.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/rutherfordium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 105 — Dubnium (Db)

• Identity: atomic number 105; symbol Db.
• Learning focus: A synthetic group-5 element studied through rapid chemical separations of only a few atoms.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/dubnium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 106 — Seaborgium (Sg)

• Identity: atomic number 106; symbol Sg.
• Learning focus: Experiments broadly support group-6 placement while probing relativistic effects and nuclear models.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/seaborgium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 107 — Bohrium (Bh)

• Identity: atomic number 107; symbol Bh.
• Learning focus: A short-lived group-7 superheavy element used only in fundamental nuclear and chemical research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/bohrium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 108 — Hassium (Hs)

• Identity: atomic number 108; symbol Hs.
• Learning focus: Formation of volatile tetroxide-like species supports hassium’s group-8 placement.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/hassium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 109 — Meitnerium (Mt)

• Identity: atomic number 109; symbol Mt.
• Learning focus: A very short-lived group-9 element known only from nuclear-decay and superheavy research.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/meitnerium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 110 — Darmstadtium (Ds)

• Identity: atomic number 110; symbol Ds.
• Learning focus: A synthetic group-10 element with short-lived isotopes and no bulk applications.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/darmstadtium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 111 — Roentgenium (Rg)

• Identity: atomic number 111; symbol Rg.
• Learning focus: A synthetic group-11 element expected to show gold-like chemistry modified by relativity.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/roentgenium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 112 — Copernicium (Cn)

• Identity: atomic number 112; symbol Cn.
• Learning focus: Relativistic effects may give copernicium unusual volatility compared with lighter group-12 elements.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/copernicium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 113 — Nihonium (Nh)

• Identity: atomic number 113; symbol Nh.
• Learning focus: Named for Japan, nihonium is a synthetic group-13 element studied through decay chains and p-block predictions.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/nihonium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 114 — Flerovium (Fl)

• Identity: atomic number 114; symbol Fl.
• Learning focus: Relativity may alter flerovium’s volatility and bonding compared with simple lead analogies.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/flerovium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 115 — Moscovium (Mc)

• Identity: atomic number 115; symbol Mc.
• Learning focus: A synthetic group-15 element used to examine heavy-nucleus stability and p-block chemistry.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/moscovium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 116 — Livermorium (Lv)

• Identity: atomic number 116; symbol Lv.
• Learning focus: A synthetic group-16 element expected to show strong relativistic effects and very short lifetimes.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/livermorium/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 117 — Tennessine (Ts)

• Identity: atomic number 117; symbol Ts.
• Learning focus: A synthetic group-17 element whose predicted behavior may depart from lighter halogens.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/tennessine/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.

Element 118 — Oganesson (Og)

• Identity: atomic number 118; symbol Og.
• Learning focus: The heaviest named element may depart from simple noble-gas expectations because of extreme relativistic and polarizability effects.
• Periodic connection: relate the ground-state configuration to group, period, block, common bonding, and qualified trends.
• Compound rule: do not transfer the properties or hazards of the element unchanged to every compound containing it.
• Canonical route: /elements/oganesson/
• Data rule: load atomic weight, configuration, state, radius, ionization, affinity, electronegativity, melting, boiling, density, oxidation states, and discovery metadata from the canonical record.
• Evidence rule: distinguish measured, calculated, predicted, disputed, and unavailable values.
• Safety rule: specify substance, form, isotope, dose or exposure route before making a hazard claim.
• Compare rule: show units, property definition, source, and a meaningful null state.