from
The Collaborative International Dictionary of English v.0.48
Radon \Ra"don\ (r[=a]"d[o^]n), n. (Chem.)
An intensely radioactive gaseous element produced by the
radioactive decay of radium-226, which is the main isotope of
radium found in pitchblende. Chemically it is an inert noble
gas. Its atomic symbol is Rn. It has an atomic number of 86.
The radon isotope produced by decay of radium has an atomic
weight of 222.017, and this isotope decays by alpha emission
with a half-life of 3.82 days. Numerous other isotopes have
been observed, all radioactive and all having half-lives
shorter than that of radon-222. Radon was discovered by M.
and Mme. Curie, of Paris, in their studies of the radioactive
substances in pitchblende. Radon was originally called
{radium emanation} or {exradio}.
[PJC]