* This is the actual age of a substance. * This is measured by using the half life of radioactive elements. * Radioactive elements - These are isotopes (varieties of certain elements containing different masses) of elements whose nucleus is decaying (breaking down, releasing energy and particles at a constant rate).
* EX. Uranium 238 Carbon 14 * - The number stands for the mass of the isotope * - Uranium decays until it reaches lead 206. When an element stops decaying it is considered stable.
* This is the time it takes half the mass of a radioactive isotope to decay into its stable form.