Radon was discovered while studying the natural radioactive decay of radium. Finding that radium decays to radon which, in turn, decays to polonium was a key element in the discovery that radioactive elements are transmuted into lighter elements.

Radon is at the bottom of the column of noble gases. It is a naturally radioactive, odorless, colorless gas. It can be detected by a Geiger counter. Radon is the heaviest of all gases with a density of almost 10 grams/L (air= 1.3 grams/L). Most of the radon is found in small, scattered pockets of air several inches underground in regions having radium- and uranium-bearing rocks.

Radon decays into radioactive polonium and alpha radiation. This radiation makes radon useful in cancer therapy.