Radioactive dating is a technique used to date specimens of igneous rock. Elements vary in form based on the difference in the number of neutrons in the atom’s nucleus. These various forms are called isotopes. Some isotopes are radioactive and can be used to date rocks using their known half-life, the amount of time for the original number of atoms to fall to half its original value. Different minerals contain different radioactive isotopes. Once the rock has cooled, radioactive decay begins, and the original, or parent, atoms begin to decay and form daughter atoms. The parent-to-daughter proportion of atoms in a substance can tell us how many half-lives have occurred. The number of half-lives is multiplied by the length of one half-life, and the age of the substance is revealed. The chart below shows the parent isotopes, daughter isotopes, and the half-life of the parent isotopes.