Article
Article
Mass number
Article By:
Duckworth, Henry E. Formerly, Department of Physics, University of Manitoba, Winnipeg, Canada.
Last reviewed:January 2020
DOI:https://doi.org/10.1036/1097-8542.408600
The mass number A of an atom is the total number of its nuclear constituents, or nucleons, as the protons and neutrons are collectively called. The mass number is placed before and above the elemental symbol, as in 238U. Because of the approximate equality of the proton and neutron masses, and the relative insignificance of that of the electron, the mass number gives a useful rough figure for the atomic mass; for example, 1H = 1.00783 atomic mass units (amu), 238U = 238.051 amu, and so on. The mass number is reduced by four during alpha emission, but is not altered during beta decay or electron capture. See also: Atomic number
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