Atomic mass represents the weighted average of the masses of all naturally occurring isotopes of an element, providing the characteristic atomic weight listed on the periodic table. Fundamentally, the mass of an atom is derived primarily from the total count of protons and neutrons within the nucleus, as the contribution of orbiting electrons is generally negligible. This measurement is typically expressed in atomic mass units (amu), which is derived from the unified atomic mass unit (u). The variation in atomic mass across elements is a result of the differing number of neutrons and protons found in the various stable and unstable isotopes belonging to that element.