Atomic Mass is the mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundances of different isotopes.Relative atomic mass is the ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12.Relative isotopic mass is the mass of an atom of an isotope of an element compared with one-twelfth the mass of an atom of carbon-12.Thus an element will have ONE Relative Atomic Mass but there may be MANY individual Relative Isotopic Masses for an Element (depending on how many Isotopes it has).