Hydrogen H

Discovery

People noticed a flammable gas long before anyone understood it. In 1671 Robert Boyle reported that iron filings dropped into dilute acid gave off a gas that burned readily, but he did not treat it as a separate substance.

The decisive step came in 1766, when the English scientist Henry Cavendish prepared the gas carefully from metals and acids, measured how light it was and described it as a distinct kind of air, which he called 'inflammable air'. Around 1781 he showed that burning it produced water, a result that helped overturn the old idea that water was an element.

Antoine Lavoisier repeated the water experiments and in 1783 gave the gas its modern name. That same year Jacques Charles flew the first hydrogen balloon in Paris. In 1931 Harold Urey detected a heavier form of hydrogen, deuterium, work that earned him the 1934 Nobel Prize in Chemistry.

Origin of the name

The name comes from the Greek hydro (water) and genes (forming), so hydrogen literally means 'water-former', a nod to Cavendish's and Lavoisier's discovery that it burns to produce water. The symbol H is simply the first letter of the name.

Main uses

• Ammonia production: the Haber-Bosch process, which combines hydrogen with nitrogen to make fertiliser, is the largest single use.
• Oil refining: hydrogen removes sulfur from fuels and breaks heavy oil into lighter products.
• Chemicals and food: it is a feedstock for methanol and is used to harden vegetable oils.
• Energy: fuel cells turn hydrogen and oxygen into electricity with water as the only exhaust, and liquid hydrogen has long been a rocket propellant.
• Industry: it serves as a coolant in large electric generators and as a reducing atmosphere in metalworking.

Most hydrogen today is made from natural gas; producing it by splitting water with renewable electricity, so-called green hydrogen, is a major goal of the energy transition.

Isotopes

Hydrogen is the only element whose isotopes have their own names. Ordinary hydrogen, protium (1H), has no neutron and makes up more than 99.98% of natural hydrogen. Deuterium (2H or D) carries one neutron and is also stable; roughly one hydrogen atom in 6,400 is deuterium. Heavy water (D2O) moderates neutrons in some nuclear reactors, and deuterated solvents are standard in NMR spectroscopy.

Tritium (3H) is radioactive, with a half-life of about 12.3 years. It lights self-powered exit signs and watch dials, serves as a tracer for dating groundwater, and is a planned fuel, together with deuterium, for fusion reactors.

In everyday life

Every water molecule contains two hydrogen atoms, and hydrogen makes up about a tenth of the human body by mass. It is present in fats, sugars, proteins and DNA, and hydrogen bonds hold the two strands of DNA together.

The gas itself is colourless, odourless and far lighter than air. It burns over a very wide range of concentrations in air, roughly 4 to 75 percent, and its flame is almost invisible in daylight, so leaks require careful handling. The 1937 Hindenburg disaster ended the era of hydrogen-filled passenger airships.