The mass number of an atom is determined by adding which two components?

The mass number of an atom is determined by adding the number of protons to the number of neutrons. This is because the mass number reflects the total count of nucleons (protons and neutrons) in the nucleus of an atom. Protons have a significant mass and contribute positively to the atom's overall mass, while neutrons also contribute mass but are neutral in charge.

Electrons, on the other hand, have negligible mass compared to protons and neutrons and therefore do not factor into the calculation of the mass number. Options that mention electrons or atomic weight do not accurately represent how mass number is calculated. The emphasis on protons and neutrons in the correct choice highlights their roles as the primary contributors to an atom's mass, establishing the key understanding of mass number in the context of atomic structure.