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mass number


Definition: the mass of the atom of a particular isotope relative to hydrogen-1 (or to 1/12 the mass of carbon 12); generally very close to the whole number represented by the sum of the protons and neutrons in the atomic nucleus of the isotope (indicated in the name or symbol of the isotope, oxygen-16, 16O); not to be confused with the atomic weight of an element, which may include a number of isotopes in their natural proportion.

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© Copyright 2018 Wolters Kluwer. All Rights Reserved. Review date: Sep 19, 2016.