Atomic mass
Like relative isotopic mass, a relative atomic mass (symbol: Ar) is a ratio with no units. A relative atomic mass 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. To find the relative atomic mass of a sample of an element, someone would work out the abundance-weighted mean of the relative isotopic masses. For example, to continue the chlorine example from above, if there is 75% of chlorine-35 and 25% of chlorine-37 in a sample of chlorine,