- the integer nearest in value to the atomic weight of an atom and equal to the number of nucleons in the nucleus of the atom. Symbol: A
Origin of mass number
First recorded in 1920–25
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- the total number of neutrons and protons in the nucleus of a particular atomSymbol: A Also called: nucleon number
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
- The total number of protons and neutrons in the nucleus of an atom. For example, nitrogen has 7 protons and 7 neutrons in its nucleus, giving it a mass number of 14. Isotopes of elements are distinguished by their mass number; for example, carbon-12 and carbon-14 have mass numbers of 12 and 14 respectively. Also called nucleon number Compare atomic mass atomic weight.
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