The atomic number is the number of protons in an atomic nucleus. The atomic mass of an atom—usually measured in atomic mass units—is the total mass of the atom, or the combined mass of its protons and neutrons (the mass of the electrons is negligible). The importance of atomic numbers and mass is simple: The atoms of each element has a specific atomic number and mass—each determined by “adding” or “subtracting” protons and neutrons within the atom.