- 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; atomic number: 86; atomic weight: 222.
- a colourless radioactive element of the rare gas group, the most stable isotope of which, radon-222, is a decay product of radium. It is used as an alpha particle source in radiotherapy. Symbol: Rn; atomic no: 86; half-life of 222 Rn: 3.82 days; valency: 0; density: 9.73 kg/m³; melting pt: –71°C; boiling pt: –61.7°C
