Radon is a gaseous radioactive element arising from the atomic disintegration of radium and was discovered in 1900. The atomic symbol is Rn and the atomic number is 86. The three naturally occurring isotopes are Rn 219 (actinon), Rn 220 (thoron) and Rn 222 (radon.) It is this latter isotope, Rn 222 with a ‘half life’ of 3.82 days, which is of concern and which is usually meant when radon is referred to.

Actinon, half life 4 seconds, formed during the radioactive decay series of actinium (Ac 227).

Thoron, half life 55 seconds, formed during the radioactive decay series of thorium (Th 232).

Radon, half life 3.82 days, formed during the radioactive decay series of uranium (U 238)

There are a further 17 known Rn isotopes.