Isotope is a term that refers to the different individual atomic masses that a particular element exhibits. The masses can be different due to the number of neutrons that the nucleus contains. For example, the element carbon most stably occurs in nature as carbon-12, which contains 6 neutrons, indicating an atomic mass of 12 (6 protons and 6 neutrons). However, an common unstable isotope, carbon-14, which contains 8 neutrons and is popularly used by scientist to date artifacts, has an atomic mass of 14 (6 protons and 8 neutrons). Unstable isotopes are known as radioactive isotopes, and decay over time to more stable isotopes.