A colourless radioactive gaseous element belonging to group 18 of the periodic table (the noble gases); a.n. 86; r.a.m. 222; d. 9.73 g dm−3; m.p. –71°C; b.p. –61.8°C. At least 20 isotopes are known, the most stable being radon-222 (half-life 3.8 days). It is formed by decay of radium-226 and undergoes alpha decay. It is used in radiotherapy. Radon occurs naturally, particularly in areas underlain by granite, where it is thought to be a health hazard. As a noble gas, radon is practically inert, although a few compounds, e.g. radon fluoride, can be made. It was discovered by Friedrich Ernst Dorn (1848–1916) in 1900.
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