Hydrogen

The simplest atom

A hydrogen atom is a single proton with a single electron around it. Nothing lighter exists: its atomic mass is 1.008, and its electron configuration, 1s¹, is the shortest that can be written. Because the atom is so bare, hydrogen is the one element whose spectrum could be calculated exactly, and the lines it emits — the Balmer series, whose pattern Johann Balmer found a formula for in 1885 — were what Niels Bohr explained with his model of the atom in 1913. Quantum mechanics learnt to walk on hydrogen.

On its own, hydrogen pairs up: the gas is made of H₂ molecules, colourless, odourless and tasteless, held together by one of the strongest single bonds in chemistry, about 436 kJ per mole. It sits alone at the top left of the periodic table, in group 1 above the alkali metals, but it is a nonmetal — it shares their single outer electron, not their behaviour. Under the enormous pressure inside Jupiter and Saturn it is thought to become a metallic liquid.

Hydrogen has three natural isotopes. Nearly all of it is protium, one proton and no neutron; about one atom in 6,400 is deuterium, with a neutron added; tritium, with two neutrons, is radioactive and exists only in traces, made by cosmic rays in the upper air.

Where it is and how it behaves

Hydrogen is the substance of the universe. About 74 % of ordinary matter by mass, and roughly nine atoms in ten, is hydrogen; nearly all of it was made in the first minutes after the Big Bang and has been burning in stars ever since. Interstellar clouds are mostly hydrogen, and it is from their collapse that new stars are born.

On Earth the picture is reversed. The planet's gravity is too weak to hold so light a gas, so free hydrogen is almost absent from the air — less than one part in a million. What hydrogen the Earth has is bound: in the oceans, in the minerals of the crust, in oil, coal and gas, and in every protein, sugar and strand of DNA. By number of atoms it is the commonest element in the human body.

The gas is placid at room temperature, because the H–H bond is so hard to break, and violent once it is lit. Mixed with air it burns or explodes at any concentration between about 4 % and 75 % by volume, and the flame is nearly invisible. It gives water with oxygen, hydrides with reactive metals, and hydrogen halides with the halogens; bonded to oxygen or nitrogen it forms the hydrogen bonds that make water liquid, ice float and the double helix hold together.

Discovery and name

An inflammable air, and the man who weighed it

Alchemists had long noticed that iron dropped into acid gives off a gas that burns; Paracelsus remarked on it in the sixteenth century, and Robert Boyle collected it in 1671. But nobody had shown it to be a substance in its own right. That was the work of Henry Cavendish, a wealthy and painfully shy Londoner who in 1766 sent the Royal Society his “Three Papers, Containing Experiments on Factitious Air”. Cavendish made the gas from zinc, iron and tin with dilute acids, showed it was the same gas whatever the metal, weighed it, and found it far lighter than common air. He called it “inflammable air” and believed it might be phlogiston itself, the supposed substance of fire.

In 1781 Cavendish burnt his inflammable air with ordinary air in a closed vessel and found that the product was pure water — a discovery that dissolved the ancient idea of water as an element. Antoine Lavoisier repeated the experiment in Paris, drew the modern conclusion that water is a compound of two gases, and in 1787, in the Méthode de nomenclature chimique, gave the lighter one its name: hydrogène, from the Greek hydro, water, and genes, forming. The name entered his Traité élémentaire de chimie in 1789 and has been the element's ever since.

Why it matters

Water, stars, fuel

Every molecule of water carries two atoms of hydrogen, and water is where life happens. The same atoms, in hydrocarbons, carry most of the energy the world burns; in the Sun they are the energy itself. At its core the Sun fuses about 600 million tonnes of hydrogen into helium every second, and the four million tonnes that vanish as mass come out as the light that reaches us eight minutes later. Every star that shines does so by burning hydrogen, and when a star runs out of it, it begins to die.

Industry makes tens of millions of tonnes of hydrogen a year, most of it from natural gas, and uses most of that for two things: turning nitrogen from the air into ammonia for fertiliser by the Haber–Bosch process, which feeds a large part of the world, and refining oil. Burnt with liquid oxygen it is the most energetic rocket fuel in common use, lifting the upper stages of the Saturn V and the main engines of the Space Shuttle. It is also the reason airships stopped flying: the Hindenburg, filled with it, burnt at Lakehurst in 1937.

Because its only exhaust is water, hydrogen is now proposed again as a fuel for the twenty-first century — in fuel cells, in steelmaking, as a store for wind and solar power. Whether that future arrives depends on making the gas without the carbon that today comes with it.