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 grams ( the mass of C atom)
"mass number." YourDictionary, n.d. Web. 18 December 2018. <https://www.yourdictionary.com/mass-number>.
mass number. (n.d.). Retrieved December 18th, 2018, from https://www.yourdictionary.com/mass-number
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