Mean (Weighted average) of the Masses of all the naturally occurring Isotopes of a chemical Element; the Atomic Mass is the Mass of any individual isotope. Atomic weight is usually expressed in atomic mass
units (amu); the atomic mass unit is defined as exactly 1/12 the mass of a Carbon-12 atom. Each proton or neutron weighs about 1 amu, and thus the atomic mass
is always very close to the Mass Number (total number of Protons and Neutrons in the nucleus). Because most naturally occurring elements have one
principal isotope and only insignificant amounts of other isotopes, most atomic weights
are also very nearly whole numbers. For the atomic weight of individual
elements, see Chemical Elements.