Pronunciation: (rā'don), [key]
a chemically inert, radioactive gaseous element produced by the decay of radium: emissions produced by outgassing of rock, brick, etc. are a health hazard. Symbol: Rn; at. no.: 86; at. wt.: 222.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.