Each atom contains protons and neutrons in its nucleus, in which the number of protons defines the element (i.e. hydrogen, carbon, nitrogen, etc.) and the sum of the number of protons and neutrons defines the atomic mass. The number of neutrons (neutral particles) can vary and these are different isotopes of one element. For example, most carbon atoms have six protons and six neutrons with an atomic mass of 12, however some are “heavy” and have seven neutrons, with an atomic mass of 13. Stable isotopes do not decay into other elements – they are “stable” through time. In contrast, radioactive isotopes (e.g., 14C) are unstable and will decay into other elements.