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]