1 H 1.008

Hydrogen

Reactive nonmetal Hydrogenium gas

1s¹ · 1

He Helium →

The lightest element and the most abundant in the universe: hydrogen accounts for roughly three quarters of all baryonic mass. On Earth it is almost never free — too light to stay in the atmosphere — but bound into water and into every organic compound.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

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Atomic properties

Atomic number: 1
Atomic mass: 1.008 а.е.м.
Electron configuration: 1s¹
Electrons per shell: 1
Block: s-block
Group: Alkali metals
Period: 1
Electronegativity (Pauling): 2.2
Electronegativity (Allen): 2.3
Atomic radius: 53 pm
Covalent radius: 31 pm
Van der Waals radius: 120 pm
Ionisation energy 1: 1 312 kJ/mol
Electron affinity: 72.8 kJ/mol
Common oxidation states: +1
Oxidation states: -1, +1

Physical properties

State at 25 °C: gas
Density: 8.988·10⁻⁵ g/cm³
Melting point: 13.99 K · -259.2 °C
Boiling point: 20.27 K · -252.9 °C
Speed of sound: 1 310 m/s

Thermal properties

Heat of fusion: 0.117 kJ/mol
Heat of vaporisation: 0.904 kJ/mol
Specific heat capacity: 14.304 J/(g·K)
Thermal conductivity: 0.1805 W/(m·K)

Mechanical properties

Young's modulus: no data
Shear modulus: no data
Bulk modulus: no data
Poisson ratio: no data
Mohs hardness: no data
Brinell hardness: no data

Electrical and magnetic properties

Electrical resistivity: no data
Magnetic ordering: diamagnetic
Curie point: no data
Néel point: no data
Superconducting point: no data

Crystal structure

Crystal structure: hexagonal close-packed (hcp)
Lattice constants: a = 470 pm · c = 340 pm

Abundance

In the crust: 1 400 mg/kg
In the ocean: 108 000 mg/L
In the universe: 7.5·10⁸ mg/kg
In the human body: 100 000 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
¹H	99.986 %	stable	—
²H	0.0145 %	stable	—
³H	—	12.32 yr	β−

Deuterium (0.0145 %) is stable and gives heavy water; tritium is radioactive with a 12.3-year half-life and is made continuously in the atmosphere by cosmic rays.

Discovery

Year of discovery: 1766
Discovered by: Henry Cavendish
Where: England
Origin of the name: from Greek hydor (water) and genes (forming): burning it produces water

History

Henry Cavendish produced "inflammable air" in 1766 by dropping acids on metals, and showed that burning it made water. Lavoisier named the element in 1783 after taking water apart and putting it back together from hydrogen and oxygen — the experiment that finally buried phlogiston.

Where it occurs

In the crust hydrogen is locked into water, hydrocarbons and clay minerals. Free H₂ exists in the atmosphere only in traces: the molecule moves fast enough to leak steadily into space.

How it is produced

Industrially by steam reforming of methane, which yields "grey" hydrogen along with carbon dioxide. Electrolysis of water gives clean hydrogen when the electricity is carbon-free, but still costs more.

Role in living things

Hydrogen makes up about 62 % of all the atoms in a human body. The proton gradient across the mitochondrial membrane is the very mechanism by which a cell makes ATP.

Safety

Hydrogen–air mixtures are explosive across an unusually wide range, from 4 % to 75 %. The flame is nearly invisible in daylight, which makes leaks particularly treacherous.

Uses

Ammonia synthesis by the Haber–Bosch process: roughly half the nitrogen in humanity's protein has passed through it
Hydrocracking and desulfurisation in oil refining
Fuel cells and hydrogen transport
Reducing atmospheres in metallurgy and semiconductor manufacture
Liquid hydrogen as rocket fuel, paired with liquid oxygen

Curiosities

Hydrogen is the only element whose three isotopes have names of their own: protium, deuterium and tritium.
Metallic hydrogen, under millions of atmospheres, is thought to make up the cores of Jupiter and Saturn.
Atomic hydrogen in interstellar space radiates at 21 cm; that line is how our Galaxy was mapped.
The hydrogen atom is the only one the Schrödinger equation solves exactly — every other orbital in this section is an approximation built on its pattern.
The Hindenburg was filled with hydrogen because the United States refused to sell Germany helium.