Hydrogen,  1H
Appearancecolorless gas
Standard atomic weight Ar, std(H)[1.007841.00811] conventional: 1.008
Atomic number (Z)1
Group1: H and alkali metals
Periodperiod 1
Blocks-block
Element category  Reactive nonmetal
Electron configuration1s1
Electrons per shell
1
Phase at STPgas
Melting point(H2) 13.99 K ​(−259.16 °C, ​−434.49 °F)
Boiling point(H2) 20.271 K ​(−252.879 °C, ​−423.182 °F)
Density (at STP)0.08988 g/L
when liquid (at m.p.)0.07 g/cm3 (solid: 0.0763 g/cm3)
when liquid (at b.p.)0.07099 g/cm3
Triple point13.8033 K, ​7.041 kPa
Critical point32.938 K, 1.2858 MPa
Heat of fusion(H2) 0.117 kJ/mol
Heat of vaporization(H2) 0.904 kJ/mol
Molar heat capacity(H2) 28.836 J/(mol·K)
Oxidation states−1, +1 (an amphoteric oxide)
ElectronegativityPauling scale: 2.20
Ionization energies
  • 1st: 1312.0 kJ/mol
Covalent radius31±5 pm
Van der Waals radius120 pm
Natural occurrenceprimordial
Speed of sound1310 m/s (gas, 27 °C)
Thermal conductivity0.1805 W/(m·K)
Magnetic orderingdiamagnetic
Magnetic susceptibility−3.98·10−6 cm3/mol (298 K)
CAS Number12385-13-6
1333-74-0 (H2)
DiscoveryHenry Cavendish (1766)
Named byAntoine Lavoisier (1783)