Definitions for "Mass number"
The mass number (M) of an atom is equal to the number of protons and neutrons of the atom. For example carbon-12 has 6 protons and 6 neutrons, thus, its mass number M = p + n, M = 6 + 6 = 12. The chlorine isotope Cl-35 has 17 protons and 18 neutrons. Its mass number MCl-35 = 17 + 18 = 35. The Cl-37 isotope has 17 protons and 20 neutrons, MCl-37 = 17 + 20 = 37. However, the atomic weight of naturally occuring chlorine is (35 x 0.7553) + (37 x 0.2447) = 35.5 amu , where 75.53% and 24.47% are the natural abundances of the Cl-35 and Cl-37 isotopes.
The number of protons plus neutrons in the nucleus of a particular isotope of an element.
total mass of an atom; the sum of neutrons and protons.