A gaseous radioactive chemical element; symbol Rn. Atomic number 86; mass number of most stable isotope 222; melting point about -71°C; boiling point -61.8°C. Radon was discovered in 1900 by the German chemist Friedrich Ernst Dorn and it was first isolated in 1910 by the Scottish chemist William Ramsay and the English chemist Robert Whytlaw-Gray. The longest half-life associated with this unstable element is 3.8 day.
The name indicates its origin from → radium. It had first been called radium emanation or just emanation (with chemical symbol Em) because it was a decay product of radium. Ramsay subsequently suggested the name "niton" (with chemical symbol Nt), which means "shining" in Latin. It was finally changed to radon in 1923.