Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
Atomic mass indicates the size of an atom. Although technically the mass is the sum of the mass of all the protons, neutrons, and electrons in an atom, the mass of an electron is so much less than that of the other particles, that mass is simply that of the nucleus (protons and neutrons).
