The relative atomic masses of elements are proportional measures. For example, the Ar for helium is 4.0, and the A r for carbon is 12.0. This means that carbon atoms have three times the mass of ...

The M r of a substance is the mean mass of one of its units compared to 1/12 th the mass of a carbon-12 atom. To calculate the M r for a substance: work out how many atoms of each element there are in ...