hydrogen

Hydrogen is element 1: one Proton, one Electron, and in its commonest form no Neutron at all. It is the simplest thing that can be called an atom, the most abundant chemical element in the universe, and — because it is so light — almost absent from Earth's atmosphere, since anything that light escapes.

Properties

property	value
atomic number (Z)	1
standard atomic weight	[1.007 84, 1.008 11]
electron configuration	1s¹
group / period / block	1 / 1 / s
melting point (H₂)	13.99 K (−259.16 °C)
boiling point (H₂)	20.271 K (−252.879 °C)
density (gas, STP)	0.000082 g cm⁻³ — lowest of all gases
first ionisation energy	1312.0 kJ/mol
H–H bond dissociation energy	435.7 kJ/mol
electronegativity (Pauling)	2.20

The atomic weight is written as an interval, not a single number, and that is deliberate. Hydrogen's isotope mix varies by where you scoop it up: seawater sits near 1.007 98, while H₂ gas from a Kansas natural-gas well came in at 1.007 8507. CIAAW publishes the range rather than pretend to a precision the element does not have.

Three isotopes

isotope	name	neutrons	natural amount fraction	stability
¹H	protium	0	[0.999 72, 0.999 99]	stable
²H	deuterium	1	[0.000 01, 0.000 28]	stable
³H	tritium	2	trace	β⁻, half-life 12.32 y

Deuterium is the only isotope of any element with its own name and symbol in routine use (D, and T for tritium). It earned that because doubling the mass of the lightest atom actually changes how fast it reacts — the kinetic isotope effect — which almost no other Isotope pair does. Heavy water is D₂O.

Deuterium was found in 1931 by Harold Urey's group at Columbia, decades after the element itself, precisely because there is so little of it.

Why the universe is mostly hydrogen

Hydrogen-1 needs no assembly. A proton and an electron is a hydrogen atom, so when the early universe cooled enough for nuclei to hold electrons, hydrogen was what there was. Big Bang nucleosynthesis left roughly 75% ¹H and 25% ⁴He by mass, with a smear of deuterium and traces of lithium. Nothing heavier than boron came out of it.

Since then, stars have been burning it back down. In the Sun's core, at about 15 million °C, hydrogen fuses into helium; the outward pressure from that reaction is the only thing holding the Sun up against its own gravity. Fourteen billion years of this has not made much of a dent: hydrogen is still about 74% of ordinary matter by mass, against 23–25% for helium.

environment	hydrogen share
ordinary matter, by mass	~74%
ordinary matter, by atom count	~92%
Solar System, hydrogen-1 by mass	705,700 ppm

On Earth, it is always bonded to something

Free H₂ floats away, so terrestrial hydrogen is locked in compounds — water above all, then every hydrocarbon and nearly every molecule in your body. Getting it back out costs energy, and that cost is the entire economics of hydrogen fuel.

Global hydrogen demand reached 97 Mt in 2023, up 2.5% on 2022. Nearly all of it goes to oil refining and chemicals, and nearly all of it is made from unabated fossil fuels. Low-emissions hydrogen — electrolysis, or fossil production with carbon capture — came to less than 1 Mt of that.

• Ammonia via the Haber process, which is how roughly half the world's food gets its nitrogen.
• Refining — removing sulfur from fuels.
• Hydrogenation — turning oils into fats, e.g. margarine.
• Rocket fuel — liquid H₂ with liquid oxygen powered the Saturn V upper stages and the Space Shuttle main engines.
• Fuel cells, where H₂ and O₂ react to make electricity and water, with no carbon in the exhaust.

The catch is upstream. Hydrogen is only as clean as whatever made it, and today that is mostly methane.

Odd states

• Ortho and para. The two protons in H₂ can have aligned or opposed spins. At room temperature the equilibrium gas is about 75% ortho, 25% para, and the two forms have measurably different thermal properties. Ortho sits 1.455 kJ/mol higher in energy.
• Metallic hydrogen. Above roughly 400 GPa, hydrogen is predicted to conduct electricity like a metal. It is thought to exist deep inside Jupiter, and remains extremely hard to make in a laboratory.

Related: Isotope · chemical element · periodic table · uranium · Atom · Energy