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This is radon.

Symbol Rn
Atomic number 86
Atomic mass (222) amu

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.

Distinguishing properties

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.

See also

Periodic table of the elements