A colourless odourless gaseous chemical element; a.n. 1; r.a.m. 1.008; d. 0.0899 g dm−3; m.p. –259.14°C; b.p. –252.87°C. It is the lightest element and the most abundant in the universe. It is present in water and in all organic compounds. There are three isotopes: naturally occurring hydrogen consists of the two stable isotopes hydrogen–1 (99.985%) and deuterium. The radioactive tritium is made artificially. The gas is diatomic and has two forms: orthohydrogen, in which the nuclear spins are parallel, and parahydrogen, in which they are antiparallel. At normal temperatures the gas is 25% parahydrogen. In the liquid it is 99.8% parahydrogen. The main source of hydrogen is steam reforming of natural gas. It can also be made by the Bosch process (see Haber process) and by electrolysis of water. The main use is in the Haber process for making ammonia. Hydrogen is also used in various other industrial processes, such as the reduction of oxide ores, the refining of petroleum, the production of hydrocarbons from coal, and the hydrogenation of vegetable oils. Considerable interest has also been shown in its potential use in a ‘hydrogen fuel economy’ in which primary energy sources not based on fossil fuels (e.g. nuclear, solar, or geothermal energy) are used to produce electricity, which is employed in electrolysing water. The hydrogen formed is stored as liquid hydrogen or as metal hydrides. Chemically, hydrogen reacts with most elements. It was discovered by Henry Cavendish in 1766.
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