What is meant by the term mass number?

The term mass number refers to the total number of protons and neutrons present in the nucleus of an atom. Since both of these particles contribute to the overall mass of the atom, the mass number provides a crucial understanding of the atom's stability and isotopic identity.

Protons are positively charged and contribute to the atomic number, which defines the element, while neutrons have no charge and add to the mass. Therefore, calculating the mass number involves simply adding these two quantities together. This is fundamental in distinguishing between different isotopes of the same element, which have the same number of protons but different numbers of neutrons, resulting in varying mass numbers.

The other interpretations do not capture this comprehensive definition, as they focus solely on one type of subatomic particle rather than considering the total contribution of both protons and neutrons.