(From radium) A heavy, radioactive, gaseous chemical element, one of the noble gases, generated by the radioactive decay of radium. Radon decays into polonium and alpha rays, and this radiation makes it useful in radiotherapy. High levels of radon exposure are associated with areas of granite rock. It can penetrate buildings through floors and walls and accumulate in the lungs, causing respiratory damage, including lung cancer.

Symbol: Rn
Atomic number: 86
Atomic weight: 222 (isotope with the longest known half-life)
Density (at 0°C with 1 atmosphere of pressure): 9.73 g/L
Melting point: -71°C
Boiling point: -61.7°C
Most common valence: 0
Ground state electron configuration: [Xe]4f145d106s26p6