Radioactivity is, and always has been a part of the earth. Radon-222 is one of the elements in the long radioactive decay chain from uranium-238, and the less common isotope radon-220 is part of the decay series from thorium-232.o Back. The decay of radon-222 with emission of an alpha particle is followed within about an hour by a series of four further decays, two of them accompanied by emission of alpha particles and the other two accompanied by other types of radiation.