RADON

(Rn), chemical element, a heavy radioactive gas of Group 0 (noble gases) of the periodic table, generated by the radioactive decay of radium. Radon is a colourless, odourless, tasteless gas, 7.5 times heavier than air and more than 100 times heavier than hydrogen. The gas liquefies at -61.8 C (-80 F) and freezes at -71 C (-96 F). On further cooling, solid radon glows with a soft yellow light that becomes orange-red at the temperature of liquid air (-195 C ). Radon is rare in nature because its isotopes are all short-lived and because radium, its source, is a scarce element. The atmosphere contains traces of radon near the ground as a result of seepage from soil and rocks, all of which contain minute quantities of radium. (Radium occurs as a natural decay product of uranium present in various types of rocks.) By the late 1980s, naturally occurring radon gas had come to be recognized as a potentially serious health hazard. The gas, arising from soil and rocks, seeps through the foundations, basements, or piping of buildings and can accumulate in the air of houses that are poorly ventilated. Exposure to high concentrations of this radon over the course of many years can greatly increase the risk of developing lung cancer. Indeed, radon is now thought to be the single most important cause of lung cancer among nonsmokers in the United States. Radon levels are highest in homes built over geological formations that contain uranium mineral deposits. Concentrated samples of radon are prepared synthetically for medical and research purposes. Typically a supply of radium is kept in a glass vessel in aqueous solution or in the form of a porous solid from which the radon can readily flow. At intervals of a few days, the accumulated radon is pumped off, purified, and compressed into a small tube, which is then sealed and removed. The tube of gas is a source of penetrating gamma radiation, which comes mainly from one of radon's decay products, bismuth-214. Such tubes of radon have been used for radiotherapy and radiography. Natural radon consists of three isotopes, one from each of the three natural radioactive-disintegration series (uranium, thorium, and actinium series). The longest-lived isotope, radon-222 (3.823-day half-life), discovered (1900) by the German chemist Friedrich E. Dorn, arises in the uranium series. The name radon is sometimes reserved for this isotope to distinguish it from the other two natural isotopes, called thoron and actinon because they originate in the thorium and the actinium series, respectively. Radon-220 (thoron; 51.5-second half-life) was first observed (1899) by the British scientists R.B. Owens and Ernest Rutherford, who noticed that some of the radioactivity of thorium compounds could be blown away. Radon-219 (actinon; 3.92-second half-life) was found (1904), associated with actinium, independently by Friedrich O. Giesel and Andr-Louis Debierne. More than a dozen artificial radioactive isotopes of radon are known. Radon atoms possess a particularly stable electronic configuration of eight electrons in their outer shell, which accounts for the characteristic chemical inactivity of the element. Radon, however, is not completely inert chemically. The existence of a compound, radon difluoride, apparently more stable chemically than compounds of the other reactive noble gases, krypton and xenon, was established in 1962. The short lifetime of radon and the energy of its radioactivity make difficult the experimental investigation of radon compounds. atomic number 86 stablest isotope (222) melting point -71 C (-96 F) boiling point -62 C (-80 F) density (1 atm, 0 C) 9.73 g/litre valence 0, +2(?) electronic configuration 2-8-18-32-18-8 or (Xe)4f 145d106s26p6