What makes up the atomic mass of an atom?

The atomic mass of an atom is primarily determined by the combined mass of protons and neutrons, which are collectively known as nucleons. Protons are positively charged particles found in the nucleus of the atom, while neutrons have no charge and also reside in the nucleus. The mass of electrons is negligible compared to that of protons and neutrons, so they do not significantly contribute to the overall atomic mass.

By accounting for only protons and neutrons, we accurately reflect the majority of the mass of an atom, as these particles carry most of the mass due to their larger size compared to electrons. Thus, identifying protons and neutrons as the components that make up the atomic mass provides a clear understanding of how atomic weight is calculated in chemistry and atomic physics.