Glossary
Definitions of key chemistry terms — concise and sourced. Each published term has a short definition, full explanation, notation, example, confused-with, sources, and related resources. 178 published terms.
Acid HA
A substance that donates protons (Brønsted–Lowry) or accepts electron pairs (Lewis).
Activation Energy Ea, kJ·mol⁻¹
Minimum energy needed for reactants to reach the transition state.
Actual Yield m_actual
Mass of product actually obtained from a reaction.
Adsorption —
Accumulation of molecules on a surface.
Alkali Metal Group 1
Group 1 metal, highly reactive, forms +1 ions.
Alkaline Earth Metal Group 2
Group 2 metal, forms +2 ions, less reactive than alkali metals.
Alkane C_nH_2n+2
Saturated hydrocarbon with single bonds only.
Alkene C_nH_2n
Hydrocarbon with at least one C=C double bond.
Alkyne C_nH_2n-2
Hydrocarbon with at least one C≡C triple bond.
Allotrope —
Different structural forms of the same element.
Amorphous Solid —
Solid without long-range order, e.g., glass.
Amphiprotic e.g., HCO3⁻
Species that can donate or accept a proton.
Anion X⁻
Negatively charged ion formed by gaining electrons.
Anode —
Electrode where oxidation occurs.
Aqueous (aq)
Dissolved in water; denoted (aq).
Aromatic Compound benzene C6H6
Cyclic conjugated compound with delocalized π electrons following Hückel rule.
Arrhenius Acid HA → H⁺ + A⁻
Acid that increases H⁺ in water.
Atom X
Smallest unit of an element retaining its chemical identity, with nucleus and electrons.
Atomic Mass m_atom
Mass of an atom, typically in atomic mass units (u).
Atomic Number Z
Number of protons in the nucleus; defines the element.
Avogadro Constant N_A
Number of entities per mole: exactly 6.02214076×10²³ mol⁻¹.
Azimuthal Quantum Number l
Quantum number l describing orbital shape.
Base B
Substance that accepts protons or donates electron pairs.
Basic Solution pH>7
Solution with pH >7; [OH⁻] > [H⁺].
Beers Law A=εlc
Linear relation A = ε l c between absorbance, path length, and concentration.
Beta Decay β⁻
Emission of a beta particle (electron) from a nucleus, increasing Z by one.
Bohr Model —
Model with electrons in quantized circular orbits around nucleus.
Boiling Point T_b
Temperature at which vapor pressure equals external pressure.
Bond Energy D, kJ·mol⁻¹
Energy needed to break one mole of a bond in the gas phase.
Bond Order —
Number of shared electron pairs between two atoms (1,2,3).
Brønsted-Lowry Acid HA
Proton donor.
Brønsted-Lowry Base B
Proton acceptor.
Buffer HA/A⁻
Solution resisting pH change when acid/base added, made of weak acid/base pair.
Calibration Curve —
Plot of instrument response vs known concentration for quantification.
Calorimetry q=mcΔT
Measurement of heat changes via temperature change of a known mass.
Catalyst —
Substance that speeds a reaction by lowering activation energy without being consumed.
Cathode —
Electrode where reduction occurs.
Cation M⁺
Positively charged ion formed by losing electrons.
Chemical Bond —
Attraction holding atoms together via shared or transferred electrons.
Chemical Equation —
Symbolic representation of a chemical reaction with formulas and coefficients.
Chromatography —
Separation technique based on partitioning between stationary and mobile phases.
Colligative Property —
Property depending on solute particle number, not identity.
Colloid —
Heterogeneous mixture with 1–1000 nm particles that scatter light.
Combined Gas Law P1V1/T1=P2V2/T2
Relation combining Boyle, Charles, and Gay-Lussac: P1V1/T1=P2V2/T2.
Common Ion Effect —
Reduced solubility due to presence of a common ion.
Compound —
Pure substance of two or more elements in fixed proportions.
Concentration c, M
Amount of solute per amount of solution or solvent.
Condensation —
Gas to liquid phase change, exothermic.
Conjugate Acid BH⁺
Acid formed when a base accepts a proton.
Conjugate Base A⁻
Base formed when an acid donates a proton.
Covalent Bond —
Bond formed by sharing electron pairs between atoms.
Crystal Lattice —
Regular repeating 3D arrangement of particles in a crystalline solid.
Decomposition Reaction AB→A+B
One compound breaks into two or more simpler substances.
Delocalized Electrons —
Electrons spread over several atoms, as in resonance or metals.
Density ρ=m/V
Mass per unit volume; intensive property for identification.
Desublimation —
Gas to solid without liquid intermediate.
Dilution c1V1=c2V2
Decreasing concentration by adding solvent; moles of solute conserved.
Dimensional Analysis —
Factor-label unit conversion checking dimensional consistency.
Dipole μ
Separation of positive and negative charge in a molecule.
Dipole-Dipole Force —
Intermolecular force between permanent dipoles.
Disproportionation —
Redox where same element is both oxidized and reduced.
Dissociation —
Separation of a compound into ions when dissolving.
Distillation —
Separation by differences in boiling points and vapor-pressure.
Double Bond =
Covalent bond with two shared pairs (one σ, one π).
Effective Nuclear Charge Z_eff
Net positive charge experienced by an electron after shielding.
Electrolyte —
Substance that conducts electricity when dissolved or molten.
Electromagnetic Radiation c=λν
Energy propagated as waves of electric and magnetic fields.
Electron e⁻
Negatively charged subatomic particle in the electron cloud.
Electron Affinity EA
Energy change when a gaseous atom gains an electron.
Electron Configuration 1s²2s²…
Distribution of electrons among orbitals.
Electronegativity χ
Ability of an atom in a bond to attract shared electrons.
Element X
Pure substance of only one kind of atom, defined by atomic number.
Empirical Formula —
Simplest whole-number ratio of atoms in a compound.
Emulsion —
Colloidal dispersion of liquid droplets in another immiscible liquid.
Endothermic ΔH>0
Process absorbing heat from surroundings (ΔH>0).
Enthalpy H
Heat content at constant pressure; state function H.
Entropy S
Measure of dispersal of energy and number of microstates.
Equilibrium ⇌
State where forward and reverse rates are equal and concentrations appear constant.
Exothermic ΔH<0
Process releasing heat to surroundings (ΔH<0).
Faraday Constant F
Charge per mole of electrons: 96485 C·mol⁻¹.
Filtration —
Separation of solids from liquids using a porous barrier.
First-Order Reaction rate=k[A]
Rate depends linearly on concentration of one reactant.
Formal Charge FC
Charge assigned assuming equal sharing to evaluate Lewis structures.
Frequency ν, Hz
Number of wave cycles per second, in hertz.
Functional Group —
Group of atoms giving organic molecules characteristic reactions.
Galvanic Cell —
Electrochemical cell producing electricity spontaneously.
Gas (g)
State of matter with no fixed shape or volume, highly compressible.
GHS Pictogram —
Standard hazard symbol on chemical labels per Globally Harmonized System.
Gibbs Energy G
Energy available for non-PV work at constant T,P; ΔG determines spontaneity.
Ground State —
Lowest energy electronic arrangement of an atom or molecule.
Half-Life t½
Time for half the nuclei or reactant to decay or be consumed (first-order).
Halogen Group 17
Group 17 element, highly reactive nonmetal forming -1 ions.
Heat q
Energy transferred due to temperature difference, not a property of a body.
Heating Curve —
Plot of temperature vs heat added showing phase changes and plateaus.
Henderson-Hasselbalch Equation pH=pKa+log([A⁻]/[HA])
pH = pKa + log([A⁻]/[HA]) for buffer calculations (limited applicability).
Hess Law ΔH_rxn=ΣΔH_steps
Total enthalpy change is independent of pathway; sums stepwise ΔH.
Heterogeneous Mixture —
Mixture with non-uniform composition and visible phases.
Homogeneous Mixture —
Uniform mixture with same composition throughout; a solution.
Hybridization sp, sp2, sp3
Mixing of atomic orbitals to form equivalent hybrid orbitals.
Hydrocarbon —
Compound of only carbon and hydrogen.
Hydrogen Bonding —
Strong dipole interaction involving H bound to N, O, or F.
Ideal Gas PV=nRT
Hypothetical gas following PV=nRT exactly with no intermolecular forces.
Indicator —
Substance changing color over a narrow pH range to signal titration endpoint.
Intermolecular Force —
Force between molecules affecting phase and properties, weaker than bonds.
Ion M⁺/X⁻
Atom or molecule with net positive or negative charge.
Ionic Bond —
Bond from electrostatic attraction between oppositely charged ions.
Ionization Energy IE
Energy to remove an electron from a gaseous atom.
Isomer —
Compounds with same formula but different connectivity or arrangement.
Isotope ^A_Z X
Atoms of same element with different neutron numbers and masses.
Joule J
SI unit of energy: 1 J = 1 kg·m²·s⁻².
Kelvin K
SI base unit of temperature; 0 K is absolute zero.
Kinetics —
Study of reaction rates and factors affecting them.
Law of Conservation of Mass Σm_react=Σm_prod
Mass is neither created nor destroyed in chemical reactions.
Lewis Acid —
Electron pair acceptor.
Lewis Base —
Electron pair donor.
Lewis Structure —
Diagram showing valence electrons and bonds as dots and lines.
Limiting Reactant —
Reactant that runs out first and limits product amount.
London Dispersion Force —
Weak intermolecular force from transient induced dipoles, present in all molecules.
Mass Number A
Total protons + neutrons in a nucleus.
Meniscus —
Curved surface of liquid in a narrow container; read at bottom.
Metallic Bond —
Bond from delocalized valence electrons among metal cations.
Molality m = mol·kg⁻¹
Moles solute per kilogram solvent.
Molarity M = mol·L⁻¹
Moles solute per liter solution.
Mole mol, n
SI unit for amount of substance; one mole contains exactly 6.02214076×10²³ entities.
Molecular Formula —
Actual number of atoms of each element in a molecule.
Molecule —
Two or more atoms bound together as a neutral unit.
Nernst Equation E=E°−RT/nF lnQ
E = E° − (RT/nF) ln Q for nonstandard cell potentials.
Net Ionic Equation —
Equation showing only species that actually change, omitting spectators.
Neutralization HA + BOH → BA + H2O
Acid-base reaction forming a salt and often water.
Neutron n
Neutral subatomic particle in the nucleus, mass ≈1 u.
Noble Gas Group 18
Group 18 element, generally inert with full valence shell.
Nonelectrolyte —
Substance that doesn't conduct electricity when dissolved.
Nuclear Fission —
Splitting of a heavy nucleus into smaller nuclei releasing energy.
Nuclear Fusion —
Combining light nuclei into heavier ones releasing energy.
Nucleus —
Dense central core of atom containing protons and neutrons.
Octet Rule —
Atoms tend to gain, lose, or share electrons to achieve eight valence electrons.
Orbital 1s, 2p, etc.
Region of high probability for finding an electron, defined by quantum numbers.
Oxidation —
Loss of electrons; increase in oxidation number.
Oxidation Number —
Assigned charge assuming ionic bonding, tracking electron transfer.
Oxidizing Agent —
Species reduced while causing oxidation of another.
Partial Pressure P_i
Pressure exerted by a single gas in a mixture.
Periodic Law —
Properties of elements are periodic functions of atomic number.
pH pH
Negative log of hydrogen ion concentration: pH=−log[H⁺].
Phase Diagram —
Map of stable phases vs temperature and pressure.
Photon E=hν
Quantum of electromagnetic radiation.
Physical Change —
Change altering physical properties without new substances formed.
Pi Bond π
Bond from side-on p orbital overlap, above/below internuclear axis.
Polyatomic Ion e.g., SO4²⁻
Covalently bonded group of atoms with net charge.
Precipitate (s)
Insoluble solid formed from solution.
Precision —
Reproducibility of measurements; closeness of results to each other.
Proton p⁺
Positively charged subatomic particle in nucleus.
RAMP RAMP
Recognize, Assess, Minimize, Prepare — lab risk assessment framework.
Rate Law rate=k[A]^m[B]^n
Mathematical expression linking rate to concentrations and rate constant.
Reactant —
Starting substance consumed in a reaction.
Redox Reaction —
Reaction involving electron transfer, with oxidation and reduction paired.
Reduction —
Gain of electrons; decrease in oxidation number.
Resonance ↔
Delocalization described by two or more valid Lewis structures.
Safety Data Sheet SDS
Document detailing hazards, handling, and first aid for a chemical.
Salt —
Ionic compound from neutralization of acid and base.
Shielding —
Reduction of effective nuclear attraction by inner electrons.
Sigma Bond σ
Bond from head-on orbital overlap along internuclear axis.
Significant Figures sig figs
Digits in a measurement known reliably plus one estimated digit.
Solubility —
Maximum amount of solute that can dissolve at equilibrium.
Solute —
Substance dissolved in a solvent.
Solution —
Homogeneous mixture of solute and solvent.
Solvent —
Medium that dissolves solute; larger proportion.
Spectroscopy —
Study of interaction of light and matter to infer structure.
Standard Enthalpy of Formation ΔH°f
Enthalpy to form one mole of compound from elements in standard states.
State Function —
Property depending only on current state, not path taken.
Stoichiometry —
Quantitative relations between amounts of reactants and products.
Sublimation —
Solid to gas transition without liquid phase.
Temperature T, K
Measure of average kinetic energy of particles; determines direction of heat flow.
Theoretical Yield m_theor
Maximum product mass predicted by stoichiometry.
Titration —
Technique adding titrant to analyte until equivalence point is reached.
Triple Bond ≡
Covalent bond with three shared pairs (one σ, two π).
Vapor Pressure P_vap
Pressure of vapor in equilibrium with its condensed phase.
VSEPR Theory AXmEn
Valence Shell Electron Pair Repulsion predicts molecular shapes.
Wavelength λ
Distance between successive peaks of a wave.
Published terms each include short definition, full explanation, notation, example, confused-with, sources, and related resources. Not a thin definition page.