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Isotope

One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons. Isotopes have the same atomic number but different mass numbers. Isotopes of an element are denoted by indicating their mass numbers as superscripts to the element symbol, e.g., 233U or 239Pu, or as numbers following the name or symbol of the element, e.g., uranium-233 or Pu-239. Some isotopes are unstable to the extent that their decay needs to be considered for nuclear material accountancy purposes (e.g., 241Pu has a half-life of 14.35 years).