Radon is formed as part of the normal radioactive decay chain of uranium.

These differences were considered to be an exception attributed to lead isotopes being products of the natural radioactive decay chains of uranium.

Radon is formed as one intermediate step in the normal radioactive decay chains, through which thorium and uranium slowly decay into lead.

Radon has a half-life of approximately 3.8 days, which means that it can be found only shortly after it has been produced in the radioactive decay chain.

For example, in a radioactive decay chain the concentrations of intermediate isotopes are constant because the rate of production is equal to the rate of decay.

These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U,U andTh respectively.

A more accurate model would consider the effects of precursors, since many isotopes follow several steps in their radioactive decay chain, and the decay of daughter products will have a greater effect longer after shutdown.

The existence of isotopes was first suggested in 1912 by the radiochemist Frederick Soddy, based on studies of radioactive decay chains that indicated about 40 different species described as radioelements (i.e. radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements from uranium to lead.