- noun an inert naturally radioactive gas formed by the radioactive decay of radium that occurs naturally in soil, in construction materials and ground water. It can seep into houses and cause radiation sickness.
- A colorless noble gas whose atomic number is 86. It is the densest known gas, and has over 30 known isotopes, all of which are unstable. It is used in the treatment of diseases, such as cancer, and as a neutron source. Its chemical symbol is Rn.
- chemical symbol Rn
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