Lightest chemical element, chemical symbol H, atomic number 1. A colourless, odourless, tasteless, flammable gas, it occurs as the diatomic molecule H2. Its atom consists of one proton (the nucleus) and one electron; the isotopes deuterium and tritium have an additional one and two nuclear neutrons, respectively. Though only the ninth most abundant element on Earth, it represents about 75% of all matter in the universe.
Hydrogen was formerly used to fill airships; nonflammable helium has replaced it. It is used to synthesize ammonia, ethanol, aniline, and methanol; to treat petroleum fuels; as a reducing agent (&see; reduction) and to supply a reducing atmosphere; to make hydrogen chloride (&see; hydrochloric acid) and hydrogen bromide; and in hydrogenation (e.g., of fats). Liquid hydrogen (boiling point −423 °F [−252.8 °C]) is used in scientific and commercial applications to produce extremely low temperatures and as a rocket propellant and a fuel for fuel cells. Combustion of hydrogen with oxygen gives water as the sole product. The properties of most acids, especially in water solutions, arise from the hydrogen ion (H+, also referred to as the hydronium ion, H3O+, the form in which H+ is found in a water environment). &Seealso; hydride; hydrocarbon.
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