<P> Relative atomic mass (symbol: A), or atomic weight, is a dimensionless (number only) physical quantity . In its modern definition, it is the ratio of the average mass of atoms of an element (in a given sample) to one unified atomic mass unit . </P> <P> The unified atomic mass unit, symbol u, is defined being ​ ⁄ of the mass of a carbon - 12 atom . Since both values in the ratio are expressed in the same unit (u), the resulting value is dimensionless; hence the value is relative . </P>

The average of the masses of all the atoms in a sample of an element is called