Hydrogen connects water, living molecules, and industrial chemistry. Its gas reacts with oxygen to form water, and industry combines it with nitrogen to make ammonia. The same element can therefore appear in a fuel cell, a fertilizer plant, and the molecules inside a leaf.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
How does hydrogen supply electricity without sending protons through the external wire?
Hydrogen gas, H₂
FOLLOW THIS RELATIONSHIP
Separate charge paths
At a PEM fuel cell's anode, hydrogen oxidation supplies protons and electrons; the electrolyte and external circuit provide different paths.
Explore the model (optional)
HFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Early experimenters saw flammable bubbles when metals reacted with acids, but recognizing a distinct substance required further evidence. Henry Cavendish distinguished the gas in 1766; its formation of water helped overturn older ideas about water as an element. The later discovery of deuterium extended that history: hydrogen atoms could share an elemental identity while differing in their nuclear composition.
1766
Distinct gas recognized
Cavendish established that the collected gas differed from other known gases.
1931
Deuterium detected
Harold Urey and colleagues identified the heavier stable hydrogen isotope.
Hydrogen occurs throughout stars and in terrestrial water and organic matter. Much of the hydrogen around us is chemically bonded rather than present as free gas. Its occurrence therefore spans very different settings, from stellar nuclear reactions to the molecular ingredients of oceans, fuels, and living tissues.
Hydrogen gas consists of molecules that can react with oxygen to form water. The element also forms compounds with many other elements. Protium, deuterium, and tritium share one per ; differences in number distinguish these without turning them into different elements.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
CITE THIS PAGE
Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
gas
293.15 K, 101.325 kPa
Melting point in K
13.99 K
at standard pressure unless noted
Boiling point in K
20.27 K
101.325 kPa
Density in g/cm³
0.0000899 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
2.2 Pauling
Pauling scale
First ionization energy in kJ/mol
1312 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
31 pm
single-bond covalent radius
Common oxidation states
-1, +1
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust in mg/kg
1400 mg/kg
upper continental crust, mass fraction
Has stable isotopes
Yesderived
Discovery year in CE
1766 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity down group 1
H 2.2 Pauling · Hydrogen
Electronegativity down group 1. Exact values.
H
2.2
Li
0.98
Na
0.93
K
0.82
Rb
0.82
Cs
0.79
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
