Hydrogen(H)
Hydrogen is the lightest element — a single proton with one electron. It forms diatomic H₂ gas at standard conditions and is central to acids, water, and organic chemistry.
KEY PROPERTIES
Electron configuration & shells
1s¹
Shell model is schematic — see lesson on atomic structure for orbitals. Oxidation states: -1, +1. Common ions: H⁺, H⁻.
Measured properties
Values at stated conditions; missing data shown as “—”. Radii distinguished by type; densities at stated phase/temperature.
- Electronegativity (Pauling)
- 2.2
- First ionization energy
- 1312 kJ·mol⁻¹ (first ionization, gas phase)
- Atomic radius
- 53 pm (calculated)
- Covalent radius
- 31 pm (covalent)
- Van der Waals radius
- 120 pm (van der Waals)
- Density
- 0.00008988 g·cm⁻³ (gas at STP)
- Melting point
- 14.01 K (1 atm)
- Boiling point
- 20.28 K (1 atm)
Standard conditions: 1 atm unless noted. Values with null are not established or not comparable across allotropes. Radii are distinguished by type. Densities at stated phase/temperature.
| Mass number | Abundance | Stability | Half-life |
|---|---|---|---|
| 1 | 99.985% | stable | — |
| 2 | 0.015% | stable | — |
| 3 | trace/synthetic | radioactive | 12.32 y |
Isotopic abundances are natural terrestrial values where applicable; synthetic isotopes shown as trace.
Occurrence & uses
Most abundant element in the universe; ~75% of baryonic mass. On Earth mainly as water and organic compounds.
- Fuel
- Ammonia synthesis
- Hydrogenation
BIOLOGICAL ROLE
Biologically relevant — see uses/occurrence for context.
Discovery
Henry Cavendish — 1766
Safety
Compounds featuring H
Reactions involving H
RELATED CONTENT
SOURCES
- [nist-atomic-weights]nist-atomic-weights (2022)/#nist-atomic-weights
- [nist-2022]nist-2022 (2022)/#nist-2022
- [iupac-gold]iupac-gold (2022)/#iupac-gold
Last reviewed: 2026-08-22 • Reviewer: dr-example