Radon, symbol Rn, is a colorless, odorless radioactive gaseous element that is the heaviest of the noble gases of the periodic table. The atomic number of radon is 86.

Radon-222 was discovered in 1900 by the German chemist Friedrich Ernst Dorn.

Radon was believed to be chemically inert. Since 1962, however, chemists have been able to make radon compounds. Radon-222, which is the most abundant isotope of radon, is formed by the radioactive decay of radium-226. Radon-222 has a half-life of 3.8 days, decaying by the emission of alpha particles into an isotope of the element polonium.

Small quantities, formed by decay of uranium minerals, are found in rock and soil, and radon makes up most normal background radioactivity.

Concentrations of the gas, however, are believed to pose a serious health hazard. Because such concentrations have been found indoors in areas with substantial radon decay, several states in the northeastern United States have developed programs to determine whether the gas is present in amounts high enough to pose a risk of lung cancer.

Radon-222 is obtained by passing air through a solution of radium salt and collecting the air and the radon gas that was present in the solution.

This isotope can be used in the treatment of malignant tumors. The gas is enclosed in a tube, usually made of glass or gold, called a radon seed, which is inserted in the diseased tissue.

19 other isotopes of radon are known. The isotope of mass 220, discovered in 1899 by Ernest Rutherford, is a product of the radioactive decay of an isotope of thorium and is known as thoron; it has a half-life of 55 seconds.

The isotope of mass 219, with a half-life of 4 seconds, is a product of the radioactive decay of an isotope of actinium and is known as actinon.

Radon melts at about -71° C (about -96° F), boils at -62° C (-80° F), and has a density of 9.73 g/liter at 0° C (32° F) and 1 atmosphere pressure.

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