What do mass numbers represent in terms of an atom?

Mass numbers are defined as the total count of protons and neutrons in an atom's nucleus. This total reflects the atomic structure quite simply: protons contribute a mass of approximately one atomic mass unit (amu), and neutrons also contribute nearly one amu, while electrons have negligible mass in comparison.

While the mass number provides an important measure, it does not equate to the average atomic weight, which accounts for the isotopic distribution of an element and is influenced by the relative abundances of isotopes. The mass number should also not be confused with solely the number of neutrons or protons, as it represents the combined total of both types of particles in the nucleus. Therefore, the choice indicating the total mass of the atom corresponds to the mass number's function, as this number gives insight into the overall mass contribution of the nucleus where most of an atom's mass is concentrated.