Relative atomic mass, also known as atomic weight, is the weighted average mass of the isotopes of an element relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity that reflects the mass of an atom compared to the standard reference mass. Each element has multiple isotopes, which are atoms that have the same number of protons but different numbers of neutrons.
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For a chemical element in a specified population, its relative atomic mass or atomic weight is the dimensionless ratio , with the mean neutral atomic mass weighted by isotopic abundance. For the illustrative neon population, , whereas has mass units.[ref]