Symbol He. A colourless odourless gaseous nonmetallic element belonging to group 18 of the periodic table (see noble gases); a.n. 2; r.a.m. 4.0026; d. 0.178 g dm−3; m.p. –272.2°C (at 20 atm.); b.p. –268.93°C. The element has the lowest boiling point of all substances and can be solidified only under pressure. Natural helium is mostly helium–4, with a small amount of helium–3. There are also two short-lived radioactive isotopes: helium–5 and –6. It occurs in ores of uranium and thorium and in some natural-gas deposits. It has a variety of uses, including the provision of inert atmospheres for welding and semiconductor manufacture, as a refrigerant for superconductors, and as a diluent in breathing apparatus. It is also used in filling balloons. Chemically it is totally inert and has no known compounds. It was discovered in the solar spectrum in 1868 by Joseph Lockyer (1836–1920).
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