Mass number is the average number of nucleons found in a large sample of an element Naturally occurring chlorine exists in the form of two isotopes Cl37 (25%) and Cl35 (75%) Therefore the 'average' mass of a chlorine atom is 35 5 Chemists use mass number because they deal with large quantities of atoms Physicists prefer the term nucleon number and deal with individual isotopes rather than the naturally occurring mixture of them
The sum of the number of neutrons and protons in an atomic nucleus. Also called nucleon number
Mass number (A) is a characteristic property of a specific isotope of an element, equal to the sum of the number of protons and neutrons in the nucleus
The whole number nearest the value of the atomic mass of an element as expressed in atomic mass units