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Relative atomic mass
The relative atomic mass of an element is a weighted average of the masses of its isotopes, based on their natural abundances, compared to 1/12th the mass of a carbon-12 atom. Since it is a ratio, $A_r$ has no units.
Relative formula mass
The relative formula mass ($M_r$) is the sum of the relative atomic masses ($A_r$) of all the atoms in a chemical formula.