These latter techniques are based on the principles of radioactive decay, the process by which radioactive isotopes break down into their decay products with a half-life specific to the isotope in question. Half life refers to the time taken for half the quantity of a radioactive isotope in a sample to decay. These methods rely on the decay of the naturally occurring radiogenic isotopes shown in the table to the right. Uranium, Thorium, and Potassium are by far the most common radiogenic isotopes in the Earth's crust. Radiocarbon is important in dating organic remains from the relatively recent past.
Radioactivity is the most commonly used method.
It is defined as the spontaneous adjustment of nuclei of unstable atoms to a more stable state.