Radioactive One of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More – Atomic A nuclear species characterized by Z protons and N neutrons. Click on Term to Read More that decays radioactively .