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.
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.
For an element to be useful for geochronology (measuring geological time), the isotope must be reasonably abundant and produce daughter isotopes at a good rate.
those that form during chemical reactions without breaking down).The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes.