relative molecular mass
- The ratio of the average mass per molecule or other formula unit, to one-twelfth the mass of a carbon-12 atom, expressed in atomic mass units. It is the same as the sum of the atomic weights of the atoms comprising said molecule. For example, the molecular weight of sodium chloride is 58.44 grams, as the combined atomic weights of sodium and chlorine are approximately 58.44 atomic mass units. Also known as molecular weight.