radon

radon , gaseous radioactive chemical element; symbol Rn; at. no. 86; mass no. of most stable isotope 222; m.p. about -71°C; b.p. -61.8°C; density 9.73 grams per liter at STP; valence usually 0. Radon is colorless and the most dense gas known. Chemically unreactive, it is classed as an inert gas in Group 18 of the periodic table . Synthesis of radon fluoride has been reported. Radon is highly radioactive and has a short half-life. The chief use of radon is in the treatment of cancer by radiotherapy. It has also found some use (mixed with beryllium) as a neutron source. All naturally occurring radon decays by the emission of alpha particles. The element is found in some spring waters, in streams, and to a very limited extent (about 1 part in 10 ²¹ ) in air. Radon is produced by the disintegration of its precursors in minerals, from which it diffuses in small amounts. In homes and other buildings in some areas of the U.S., radon produced by the radioactive decay of uranium-238 present in soil and rock can reach levels regarded as dangerous, but the seriousness of the problem is unclear. Twenty isotopes of radon are known, but only three occur naturally. Radon-222 (half-life 3.82 days) is produced by the decay of radium-226. Radon-220 (half-life 55 sec), also called thoron, is produced in the decay series of thorium -232. Radon-219 (half-life 4 sec), also called actinon, is produced in the decay series of uranium -235 (actinouranium). Ernest Rutherford discovered thoron in 1899. F. O. Dorn discovered radon-222 in 1900 and called it radium emanation. In about 1902, F. O. Giesel discovered actinon. In 1908 William Ramsay and R. W. Whytlaw-Gray isolated the element, which they called niton, and studied its physical properties. The name radon was adopted in the 1920s to refer to all the isotopes of the element, although the name emanation and symbol Em are sometimes used.

radon Radon, atomic number 86, is a natural α-radioactive inert gas produced within the radioactive decay chains of uranium and thorium. Its three natural isotopes, ²²²Rn, ²²⁰Rn, and ²¹⁹Rn have respective half-lives of 3.8 days, 55 seconds, and 4 seconds. Radon abundance is measured in terms of its radioactivity in becquerels per cubic metre of radon in air, written Bq m⁻³. ²²²Rn, commonly referred to simply as radon, is the most discussed isotope.

Since uranium is widely present as a trace element in the ground, radon is ubiquitous in the environment. Radon behaviour has been extensively investigated. Ground-surface radon emissions have been used in uranium exploration and earthquake prediction.

In the late 1980s, high levels of radon were found in domestic houses, and this resulted in substantial surveys of domestic radon levels worldwide. The UK and world average radon exposures are respectively 20 and 42 Bq m⁻³. Radon is known to cause lung cancer in highly exposed uranium miners. In the general population, radon has been suggested as a partial cause of lung cancer in adults, and of leukaemia in children.

Denis L. Henshaw

radon (ray-don) n. a radioactive gaseous element that is produced during the decay of radium. It emits alpha and gamma radiation. Symbol: Rn. r. seeds sealed capsules containing radon, used in radiotherapy for the treatment of cancer but now largely replaced by newer agents and techniques.