n.
Lightest chemical element , chemical symbol H, atomic number 1.
A colourless, odourless, tasteless, flammable gas , it occurs as the diatomic molecule H 2 . Its atom consists of one proton (the nucleus) and one electron ; the isotope s deuterium and tritium have an additional one and two nuclear neutron s, 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 airship s; 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 fat s). 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 cell s. Combustion of hydrogen with oxygen gives water as the sole product. The properties of most acid s, especially in water solutions, arise from the hydrogen ion (H + , also referred to as the hydronium ion, H 3 O + , the form in which H + is found in a water environment). See also hydride ; hydrocarbon .