Radiometric dating uses the measurement of radioactive decay of atoms to determine the ages of volcanic rocks and some organic materials. Radioactive elements are those with unstable nuclei: large atoms, or isotopes of smaller atoms. Atoms can decay through the loss of an alpha particle (consisting of two protons and two neutrons), loss of a beta particle (electron), or the capture of a beta particle (which turns a proton into a neutron).

Each of the radioactive elements decays at its own constant rate. After a certain amount of time, half of the radioactive atoms will decay. This time is called the half-life of the element. Beginning with 100 percent of a sample of radioactive atoms, 50 percent will remain after one half-life, 25 percent after two half-lives, 12.5 percent after three half-lives, and so on. Each of the radioactive isotopes is most useful for a different span of time. If it decays very slowly, it cannot be used to determine the age of geologically young material, as an immeasurably small amount of decay will have occurred. If it decays rapidly, it cannot be used to determine the age of geologically old material, as an unmeasurably small amount of the original material remains.

The ratio of product to original isotope indicates the time since the formation of the material that contains them. The assumption must be made that the element that is the product of decay was not originally present. This assumption is frequently cited by creationists as an invalidation of radiometric dating, but geologists make this assumption only when it is valid. . .

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