Relative atomic mass ( symbol : Ar ) is a dimensionless physical quantity , the ratio of the average mass of atoms of an element ( from a single given sample or source ) to 1\/12 of the mass of an atom of carbon-12 ( known as the unified atomic mass unit ) .
A relative atomic mass ( also called atomic weight ; symbol : Ar ) is the ratio of the average mass per atom of an element from a given sample to 1\/12 the mass of a carbon-12 atom .