What is referred to as the sum of protons and neutrons in the nucleus?

The sum of protons and neutrons in the nucleus of an atom is referred to as the atomic mass. This value is significant because it represents the total number of these subatomic particles, which together contribute to the mass of the atom. Protons, which carry a positive charge, and neutrons, which are neutral, are both located in the nucleus and their combined total defines the mass number of an element.

The atomic number, on the other hand, refers specifically to the number of protons in the nucleus and uniquely identifies an element. The net charge discusses the overall charge of the atom after accounting for electrons, and isotopes are variations of elements with the same number of protons but different numbers of neutrons. Therefore, atomic mass accurately describes the sum of protons and neutrons, making it the correct choice.