How is the mass number related to the number of protons and neutrons an atom has?

The mass number of an atom is defined as the total count of protons and neutrons present in its nucleus. Since protons and neutrons are collectively referred to as nucleons, adding these two quantities together gives a clear representation of the mass number. Each proton carries a positive charge, while neutrons are neutral, and together they contribute to the overall mass of the atom.

The other options do not accurately describe the relationship between mass number and nucleons. For example, stating that the number of neutrons plus the mass number equals protons is not correct because it misconstrues the definition of mass number. Similarly, the mass number is not defined by the number of electrons, as electrons do not significantly contribute to the mass of an atom. Finally, the mass number cannot be equated to the difference between neutrons and protons, as this also fails to represent the actual relation, which specifically involves the total count.