Definition: One of two or more nuclides that are chemically identical, having the same number of protons, yet differ in mass number, because their nuclei contain different numbers of neutrons; individual isotopes are names with the inclusion of their mass number in the superscript position (12C) and the atomic number (nuclear protons in the subscript position (6C). In former usage, the mass numbers follow the chemical symbol (C-12).
[iso- + G. topos, part, place]
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