Symbol Name Atomic Number Atomic Weight Group Number
Rn Radon 86 222 18


Standard State: gas at 298 K (the heaviest known mononuclear gas at 298 K)
Color: Colorless

Radon was discovered in 1900 by Dorn, who called it radium emanation.  In 1908 Ramsay and Gray, who named it niton, isolated the element and determined its density, finding it to be the heaviest known gas.  It is essentially inert and occupies the last place in the zero group of gases in the Periodic Table.  Since 1923, it has been called Radon. At ordinary temperatures radon is a colorless gas; when cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperatures is lowered and orange red at the temperatures of liquid air.  Radon is still produced for the therapeutic use by a few hospitals by pumping it from radium source and sealing it in minute tubes, called seeds for application to patients.  Exposure to Radon needs to be limited because of the lung cancer caused by its exposure.