The sum of the number of neutrons and protons in an atomic nucleus.
The number of neutrons and protons in the nucleus of an atom: the approximate mass of a given nucleus is obtained by multiplying the mass number by the fundamental unit of mass, 1.6605 × 10-24 grams (112 the mass of C12 atom)
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.