Allan B. Tanner reports that all radon isotopes are noble gases, which implies that they occur as non-polar, monatomic molecules and are inert for practical purposes. They are also radioactive, half of a collection of radon atoms disintegrate in 3.825 days, their mean life is 5.52 days, and 90% disintegrate in 12.7 days. Thoron decays more rapidly, it has a half life of about 54.5 seconds, a mean life of about 78.6 seconds, and 90% disintegrate in about 181 seconds.
Radon denotes both the chemical element containing 86 protons in its nucleus, and the specific isotope of radon whose nucleus also contains 136 neutrons, giving the isotope a mass number of 222. A reference to radon or Rn is usually a reference to the isotope 222Rn. Two other isotopes occur in nature, 220Rn, conventionally called thoron or Tn, and 219Rn, conventionally called actinon or An.
Actinon decays most rapidly: it has a half life of about 3.92 seconds, a mean life of about 5.66 seconds, and 90% disintegrate in about 13 seconds. Radon-222, 220Rn, and 219Rn are decay products of radium isotopes that are, in turn, members of radioactive series headed respectively by the radionuclides uranium-238, thorium-232, and uranium-235, which are so long lived that the present quantities of them are inferred to be the remainders of those nuclides that were present at the time of formation of the Earth. Over such a long period geologic processes have distributed both uranium and thorium widely in the crust.
The uranium isotopes are particularly mobile in surface and ground waters, soils, and rocks wherever there is free oxygen. As a result, the radon isotopes are present practically everywhere, although in exceedingly minute concentrations by weight, and actinon are likely to be smaller by factors of 10 and 105, respectively. Typical concentrations are 102-104 lower in soil gases, and 106 lower in outdoor air, than these estimated maximum concentrations.
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