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Isotopes

Radon has no stable isotopes. Thirty-nine radioactive isotopes have been characterized, with mass numbers ranging from 193 to 231. Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is Rn (half-life 3.82 days), which is a decay product of Ra, the latter being itself a decay product of U. A trace amount of the (highly unstable) isotope Rn (half-life about 35 milliseconds) is also among the daughters of Rn. The isotope Rn would be produced by the double beta decay of natural Po; while energetically possible, this process has however never been seen.

Three other radon isotopes have a half-life of over an hour: Rn (about 15 hours), Rn (2.4 hours) and Rn (about 1.8 hours). However, none of these three occur naturally. Rn, also called thoron, is a natural decay product of the most stable thorium isotope (Th). It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, Rn is derived from the most stable isotope of actinium (Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds. No radon isotopes occur significantly in the neptunium (Np) decay series, though trace amounts of the isotopes Rn (26 minutes) and Rn (0.5 millisecond) are produced in minor branches.