He; at. wt 4.0026; at. no. 2; The natural gas is 4He with trace of 3He. Three short-lived artificial isotopes are known: 5; 6; 8. Noble gas discovered in the sun by Lockyer and Frankland in 1868. Obtained by Hillebrand in 1890 by heating uranium minerals and identified by Ramsay in 1895. Found in natural gas from which it is extracted on a commercial scale. Produced in the decay of radiactive elements: 1 kg of uranium in its conversion into 865 g of lead forms 756 l of helium; also produced by the bombardment of beryllium, lithium, and other light elements with cosmic rays, x-rays and high-speed protons and deuterons. Such bombardments take place in nature and give a constant supply. Monograph: G. A. Cook, Argon, Helium and the Rare Gases (Interscience, New York, 1961). Review: Cockett, Smith, "The Monatomic Gases" in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 139-211; E. Cook, Science 206, 1141-1146 (1979).

Colorless, odorless, nonflammable, inert gas. d0 (gas) 0.17847 g/l. bp -268.9° (4.215°K). Crit temp -267.9°; crit press. 2.26 atm. Heat of vaporisation 19.6 cal/g-atom. Trouton's const 4.64. Two liq forms exist: He I above ~2.2°K; He II below ~2.2°K. d (liq; at bp): 0.1249 g/cc. He II is a superconducting liq; has very low viscosity; super-fluid. Helium cannot be frozen by lowering the temp at ordinary press.; no triple point. mp at 25.05 atm 1.00°K. Heat of fusion 4.35 cal/g-atom. Very slightly sol in water: at 0° = 0.97 ml/100 ml; at 50° = 1.08 ml/100 ml.

USE: Liquid helium (the most volatile liquid known) is used for the production of low temps. The inert, nonflammable gas is used for balloons and airships, lifting power is 0.93 if hydrogen is taken as 1.00. In nucleonics and rocket research; in lasers. Diluent for gases. Q-Gas, a mixture of 98.7% helium and 1.3% butane, has been used as a filling for gas-flow Geiger counters.