Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces. These are released as radioactive particles (there are many types). This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate. The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time. For geological purposes, this is taken as one year. Another way of expressing this is the half-life period (given the symbol T). The half-life is the time it takes for half of the parent atoms to decay. The relationship between the two is: T = 0.693 / λ