What is the combined number of protons and neutrons in an atom called?

The combined number of protons and neutrons in an atom is known as the mass number. This term is significant in atomic science because it reflects the total count of these two subatomic particles, which together determine the atomic mass and identity of an element. Protons define the atomic number and thus the element itself, while neutrons contribute to the atom's overall mass without affecting its chemical properties directly.

In contrast, atomic weight refers to the average mass of an element's isotopes and takes into account the relative abundance of each isotope. Molecular weight is a distinct term that pertains to the total mass of a molecule calculated by summing the atomic weights of all its constituent atoms. The term electromagnetic number is not commonly used in the context of atomic structure and does not accurately represent any concept related to protons and neutrons. Thus, the mass number is the correct choice as it directly describes the total number of protons and neutrons in an atom.