What is atomic mass?

Atomic mass is the total mass of protons, neutrons, and electrons in a given element, commonly expressed in unified atomic mass units. The International Union of Pure and Applied Chemistry (IUPAC) has defined a unified atomic mass unit as 1/12 of the mass of a single carbon-12 atom.

Atomic mass should not be confused with atomic weight, also termed "relative atomic mass," which is included in the standard periodic tables of chemical elements. The weight or relative mass of an atom of a given chemical element takes into account all of the atoms of that element that are present on earth and in the atmosphere.