Hydrogen
Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic mass of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass.
Identity
- Atomic mass
- [1.00784, 1.00811] u
- Category
- Reactive non-metals
- Group
- 1
- Period
- 1
- Block
- s
- Oxidation States
- −1, +1
- Phase (25 °C)
- Gas
- Appearance
- colorless gas
Electronic Structure
- Electron configuration
- 1s1
- Short configuration
- 1s1
- Electrons per shell
- 1
- Electronegativity (Pauling)
- 2.20
- Electron affinity
- 72.8 kJ/mol
- 1st ionization energy
- 1312 kJ/mol
Physical properties
- Melting point
- -259.2 °C (14 K)
- Boiling point
- -252.9 °C (20 K)
- Density
- 0.00008988 g/cm³
- Molar heat capacity
- 28.8 J/(mol·K)
Chemical behavior of hydrogen
Hydrogen has a single electron in the 1s orbital, which it can share, lose or complete with another electron. It therefore shows amphoteric chemistry: with nonmetals it forms covalent (sometimes polar) bonds acting formally as +1, while with strongly electropositive metals it gains an electron to form hydrides with the H- anion. Its intermediate electronegativity (2.20) sits between both behaviors, and its tiny size lets it form hydrogen bonds, decisive in water and biomolecules.
Reactivity
2 H₂ + O₂ → 2 H₂O
Hydrogen burns in oxygen with an almost invisible flame and great heat release. The mixture is explosive over a wide concentration range, so a single spark suffices for ignition.
H₂ + Cl₂ → 2 HCl
It reacts with chlorine, under light or heat, to give hydrogen chloride. It is a very fast exothermic chain reaction, the basis of industrial hydrochloric acid production.
N₂ + 3 H₂ → 2 NH₃
With nitrogen it forms ammonia in the Haber-Bosch process, at 400-500 C and 150-300 atm with an iron catalyst. It is one of the world's most important industrial reactions because of fertilizers.
2 K + H₂ → 2 KH
With alkali metals, when heated, it forms saline hydrides containing the hydride anion. These hydrides react with water releasing hydrogen, so they are reducers and H2 sources.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Water | H₂O | Universal solvent; basis of life and countless processes |
| Ammonia | NH₃ | Nitrogen fertilizers and industrial refrigeration |
| Methane | CH₄ | Main component of natural gas; fuel and feedstock |
| Hydrochloric acid | HCl | Gastric juices, cleaning and chemical processes |
| Hydrogen peroxide | H₂O₂ | Disinfectant, bleaching agent and propellant |
Electron shell distribution
View in the atom builderIsotopes
The dominant isotope is protium (1H), with no neutrons, at 99.98% abundance. Deuterium (2H or D), with one neutron, is stable at 0.016% and is used in heavy water, tracers and magnetic resonance. Tritium (3H or T), with two neutrons, is radioactive with a 12.3-year half-life and is used in fusion research and luminous marking.
Occurrence and production
It is the most abundant element in the universe (about 75% of baryonic mass) and the fuel of stars. On Earth it almost never appears free: it is found combined in water, hydrocarbons, hydrated minerals and biomass. Industrially it is obtained mainly by Steam reforming of methane, and with higher purity by water electrolysis.
History and origin of the name
Henry Cavendish described in 1766 the inflammable air obtained from acids and metals, and Antoine Antoine Lavoisier named it hydrogene, generator of water, after showing that its combustion produces water. Deuterium was discovered in 1931 by Harold Urey, a find that earned him the 1934 Nobel Prize in Chemistry.
Applications and relevance of hydrogen
Uses of the element
As H2 it is used in oil refining, in ammonia and methanol synthesis, as a reducing atmosphere in metallurgy and electronics, and as an energy carrier in fuel cells and rockets, where its combustion produces only water.
Uses of its compounds
Water is an essential solvent and resource; ammonia sustains nitrogen fertilization; methane is fuel and feedstock; hydrochloric acid cleans metals; peroxide disinfects and bleaches.
Health, safety and environment
Biological role
It has no toxicity as such, but as H2 it is an asphyxiant in confined spaces by displacing oxygen. Hydrogen bonds determine protein folding, DNA structure and the properties of water in living beings.
Safety
Its great hazard is flammability: mixtures with air from 4% to 75% are explosive and ignite with minimal energy. It requires ventilation, leak detectors and no ignition sources; liquid hydrogen causes cryogenic burns.
Environment
Its combustion only generates water, but it is not harmless: H2 leaks lengthen the lifetime of atmospheric methane and, if produced by gas reforming, each kilogram emits about 9 kg of CO2 (grey hydrogen).
Frequently asked questions about Hydrogen
Why is hydrogen in group 1 if it is not a metal?
Its 1s1 configuration matches the alkali metals: a single valence electron that can be lost to give H+. But its electronegativity is much higher and it can also gain an electron to give H-, like the group 17 halogens. So it heads group 1 by configuration, although some tables duplicate it over group 17.
What do green, grey and blue hydrogen mean?
They are labels for how it is produced, not different kinds of molecule. Grey comes from natural gas reforming and emits about 9 kg of CO2 per kg of H2. Blue uses the same process but captures much of the CO2. Green is made by water electrolysis with renewable electricity, with no direct emissions.
Why does hydrogen form H2 molecules and not lone atoms?
Because two hydrogen atoms share their electrons in a covalent bond and both complete their shell with two electrons, like helium. This union releases much energy, so the H2 molecule is far more stable than two separated atoms. Breaking it takes 436 kJ per mol.
What is the hydrogen bond and why is it so important?
It is the attraction between a hydrogen bound to a very electronegative atom (oxygen, nitrogen or fluorine) and an electron pair of another molecule. It is weak beside a chemical bond, but together it makes water liquid, keeps DNA's double helix and folds proteins. Without it, the chemistry of life would not exist.
Why is water liquid if hydrogen and oxygen are gases?
Because water molecules attract each other through hydrogen bonds: hydrogen bound to oxygen carries a partial positive charge and attracts the oxygen of its neighbor. This network demands much energy to break, raising the boiling point to 100 C. Without these bridges, water would boil well below 0 C.
How do protium, deuterium and tritium differ?
They are hydrogen's three isotopes: the same proton with different neutron numbers. Protium has no neutrons and forms 99.98% of natural hydrogen. Deuterium has one neutron, is stable and weighs twice as much. Tritium has two neutrons, is radioactive with a 12.3-year half-life and is used in fusion research.
Can hydrogen be the fuel of the future?
It has strong assets: its combustion only produces water and fuel cells generate electricity without emissions. But it poses large challenges: it must be compressed to 700 bar or liquefied at -253 C, leaks are easy and today almost all of it comes from natural gas. It will be key in industry and heavy transport if green hydrogen cost falls.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.