Radon-222
Radon is the chemical element of symbol Rn and atomic number 86. It is a rare radioactive gas, belonging to the noble gas series, and is formed as part of three radioactive decay chains that begin with uranium or thorium. Thirty-six radioactive isotopes of radon, with mass number from 193 to 228, have been characterized. The most stable isotope is Radon-222 (half-life of 3.8 days); it is generated naturally by the decay of 238U and emits alpha particles. Because of its radioactivity and unreactivity as a chemical element, radon has few uses and is seldom used in academic research. Radon is responsible for the majority of the mean public exposure to ionizing radiations. (L1075)