High Quality Content by WIKIPEDIA articles! There are 39 known isotopes of radon (Rn) from 193Rn to 231Rn. The most stable isotope is 222Rn with a half-life of 3.823 days. Four isotopes, 218, 219, 220, 222Rn occur in trace quantities in nature as decay products of, respectively, 218At, 223Ra, 224Ra, and 226Ra. 218Rn and 222Rn are intermediate steps in the decay chain for 238U, 219Rn is an intermediate step in the decay chain for 235U and 220Rn occurs in the decay chain for 232Th.