Hydrogen atom scheme.

A chemical element with atomic number 1. Hydrogen is the lightest and most abundant element, composed of only one proton and one electron. The universe is 75 percent hydrogen. It is a colourless, odourless gas lighter than air. When it burns with oxygen, water is formed. On Earth, hydrogen is bound mainly in water and organic compounds.

Hydrogen is mostly produced by thermochemical processes, e.g., steam reforming, but can also be generated by electrolysis. Hydrogen made through renewable energy can be used for energy storage because it can be easily transported and stored. It can be used as fuel in fuel cells, e.g., in cars.

In a nuclear power plant, hydrogen can be produced during a severe accident due to the contact of hot water vapor with the zircaloy fuel cladding. To reduce the risk of detonation, nuclear power plant containers are equipped with hydrogen recombiners.

Two isotopes of hydrogen, deuterium and tritium, are the most promising fuel for thermonuclear fusion, the ignition of which is being studied intensively. Thermonuclear fusion takes place in the cores of stars, burning mostly hydrogen during p-p fusion, or the CNO cycle.