Atomic mass
A relative isotopic mass is the mass of an isotope relative to 1/12 of the mass of a carbon-12 atom. In other words, a relative isotopic mass tells you the number of times an isotope of an element is heavier than one-twelfth of an atom of carbon-12. The word relative in relative isotopic mass refers to this scaling relative to carbon-12. Relative isotopic mass is similar to isotopic mass and has exactly the same numerical value as isotopic mass, whenever isotopic mass is expressed in atomic mass units. However, unlike isotopic mass, relative isotopic mass values have no units.