Half-Life

Period of time required for 50% (½) of the atoms of a radioactive nuclideA nuclear species characterized by Z protons and N neutrons. in a sample to decay. After two  half-lives, 25% ( ½ x ½ = 1/4) of the original radioactive nuclide will remain. After three half-lives, 12.5% ( ½ x ½ x ½ = 1/8) of the original radioactive nuclide will remain.