What is atomic mass a sum of?

Atomic mass is primarily a measure of the total mass of an atom, which includes the collective mass of the protons and neutrons in the nucleus. Protons have a positive charge and are located in the nucleus, while neutrons are neutral particles that also reside in the nucleus. The mass of electrons, which orbit the nucleus, is negligible compared to that of protons and neutrons. Thus, the atomic mass effectively represents the sum of the masses of these two types of nucleons (protons and neutrons), emphasizing their contribution to the overall mass of the atom.

Since the overall charge of the atom does not directly relate to its atomic mass, it is irrelevant in this context. The atomic mass provides important information about the element's properties and behavior in chemical reactions, but it does not depend on the atom's charge or the number of electrons.