Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula H2. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter. Stars such as the Sun are mainly composed of hydrogen in the plasma state. Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds. For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.

In the early universe, the formation of protons, the nuclei of hydrogen, occurred during the first second after the Big Bang. The emergence of neutral hydrogen atoms throughout the universe occurred about 370,000 years later during the recombination epoch, when the plasma had cooled enough for electrons to remain bound to protons. Hydrogen is nonmetallic (except when it becomes metallic at extremely high pressures) and readily forms a single covalent bond with most nonmetallic elements, forming compounds such as water and nearly all organic compounds. Hydrogen plays a particularly important role in acid–base reactions because these reactions usually involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge (i. e. , anion) where it is known as a hydride, or as a positively charged (i. e. , cation) species denoted by the symbol H+. The H+ cation is simply a proton (symbol p) but its behavior in aqueous solutions and in ionic compounds involves screening of its electric charge by nearby polar molecules or anions. Because hydrogen is the only neutral atom for which the Schrödinger equation can be solved analytically, the study of its energetics and chemical bonding has played a key role in the development of quantum mechanics.

Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–1781, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means "water-former".

Industrial production is mainly from steam reforming of natural gas, oil reforming, or coal gasification. A small percentage is also produced using more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing (e. g. , hydrocracking) and ammonia production. Hydrogen can be deployed as an energy source in fuel cells to produce electricity, or via combustion to generate heat. When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapour. Combustion of hydrogen can lead to the thermal formation of nitrogen oxides. Hydrogen atoms may embrittle metals.

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Atomic properties

Standard atomic weight1.008 ±2·10-4 [1.00784 … 1.00811]
Atomic mass1.0081 u

Atomic radii

Radius (empirical)25 pm
Radius (calculated)53 pm
Covalent radius31 ±5 pm
Van der Waals radius120 pm

Atomic shell

Electron configuration1s1
Ionization energy(1st) 13.598434 eV
Shell model

Physical properties

Densitygaseous: 0.0899 kg·m−3 (273 K)
liquid: 0.0709 kg·l−1 (20.32 K)
Molar volumesolid: 1.142·10-5 m3·mol−1
Speed of sound1,270 m·s−1 (298.1 K)


Melting point13.99 K
Boiling point20.27 K
Liquid range6.281 K
Triple point13.8 K @ 7.041 kPa
Critical point32.93 K @ 1.285 MPa


Melting enthalpy0.558 kJ·mol-1
Enthalpy of vaporization0.452 kJ·mol-1
Binding energy218 kJ·mol-1

Heat and conductivity

Specific heat capacity14,300 J·kg−1·K−1
Thermal conductivity0.1805 W·m-1·K-1


Magnetic susceptibility-3.98·10-6 cm3·mol−1 (298 K)

Optical properties

Refractive indexgaseous: 1
liquid: 1.12

Chemical properties

Oxidation state+1, 0, -1
Standard potential0 V


Pauling scale2.2
Sanderson scale2.59
Allred-Rochow scale2.2
Mulliken scale2.25
Allen scale2.3
Ghosh-Gupta scale7.186 eV
Nagle scale2.27
Pearson absolute negativity7.18 eV

Other properties

Natural occurrenceprimordial
Crystal structureHexagonal
Goldschmidt Classificationatmophile
Superconductorwithout transition tempperature
Price/kg1.39 USD

Natural abundances

7.5·108 ppb ≈ 7.54·1018 M☉
7.5·108 ppb ≈ 1.49·1018 Mt
2.4·107 ppb ≈ 24 kg
Earth’s crust
1.5·106 ppb ≈ 4.15·107 Mt
1.078·108 ppb ≈ 147,000 Mt
Flowing water
1.15·108 ppb ≈ 1,840 Mt
Human body
1·108 ppb ≈ 7 kg