%radon%

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 ]