One of two or more atoms with the same number of protons, but different numbers of neutrons in their nuclei. Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element, carbon, the numbers denoting the mass number of each isotope. Isotopes have very nearly the same chemical properties, but often have different physical properties. For example, carbon-12 and carbon-13 are stable; carbon-14 is unstable, that is, it is radioactive.