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Rn,symbol for the element radonradon
, gaseous radioactive chemical element; symbol Rn; at. no. 86; mass no. of most stable isotope 222; m.p. about −71°C;; b.p. −61.8°C;; density 9.73 grams per liter at STP; valence usually 0. Radon is colorless and the most dense gas known.
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