Periodic table › Hydrogen
1. Identity and classification
Colorless, odorless, tasteless diatomic gas (H₂); glows violet-pink in plasma discharge
- Name and symbol
- Hydrogen (Hydrogen) · H
- Atomic number
- 1
- Relative atomic weight / mass number
- 1.0080 ± 0.0002
- Group · period · block
- 1 · 1 · s
- Category
- Reactive Nonmetal
- State near room temperature
- Gas
- Stable isotopes
- At least one stable isotope
- CAS number
- 1333-74-0
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 1 · 1
- Neutrons in reference isotope
- 0 (1H)
- Full electron configuration
1s¹- Abbreviated configuration
1s¹- Electrons per shell
- 1
- Listed oxidation states
- +1, -1
- Electronegativity
- 2.2 (Pauling)
- First ionization energy
- 1312.008 kJ/mol · 13.598 eV
- Electron affinity
- 72.75 kJ/mol
- Covalent radius
- 31 pm
- van der Waals radius
- 120 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 0.00008988 g/cm³
- Melting point
- 13.81 K · -259.34 °C · -434.812 °F
- Boiling point
- 20.28 K · -252.87 °C · -423.166 °F
- Crystal structure
- HCP
- Specific heat
- 14.304 J/(g·K)
- Thermal conductivity
- 0.1805 W/(m·K)
- Enthalpy of fusion
- 0.117 kJ/mol
- Enthalpy of vaporization
- 0.904 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- Not established
- Speed of sound
- 1310 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
Hydrogen forms covalent bonds; hydride ions occur with strongly electropositive metals.
Selected compounds: H₂O (Water) · NH₃ (Ammonia) · CH₄ (Methane) · HCl (Hydrochloric acid) · H₂SO₄ (Sulfuric acid)
Reaction reference
Hydrogen burns in oxygen to form water.
At ordinary conditions hydrogen has low solubility and does not react directly with liquid water.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 1H | Stable | 99.9855% | Stable |
| 2H | Stable | 0.0145% | Stable |
| 3H | Radioactive | — | 12.32 Y |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1766 CE
- Scientists associated with discovery
- Henry Cavendish
- Location
- United Kingdom (London)
- Origin
- Natural
- Listed crustal abundance
- 1400 ppm
Minerals and naming reference
Water (H₂O), hydrocarbons, clay minerals, organic matter
From Greek 'hydro' (water) + 'genes' (forming), named by Antoine Lavoisier in 1783
7. Applications
Hydrogenated vegetable oils, fuel cells in zero-emission vehicles, weather balloons
Detailed applications by field
- everyday
- Hydrogenated vegetable oils, fuel cells in zero-emission vehicles, weather balloons
- industry
- Haber-Bosch ammonia synthesis, hydrocracking and desulfurization of petroleum, methanol production
- electronics
- Annealing atmosphere for silicon wafers and semiconductor fabrication
- medicine
- Hydrogen peroxide antiseptics; deuterium-labeled metabolic tracers
- energy
- Liquid LH₂ rocket propellant, proton-exchange membrane (PEM) fuel cells, fusion energy research
- aerospace
- Cryogenic liquid rocket fuel (Space Launch System, Ariane, Centaur upper stages)
- agriculture
- Primary feedstock for nitrogen fertilizers via ammonia production
- science
- NMR spectroscopy reference (¹H), bubble chambers, cryogenics
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Fundamental constituent of water, proteins, nucleic acids (DNA/RNA), lipids, and carbohydrates; drives ATP synthesis via proton gradients across mitochondrial membranes.