atomic mass and molecular mass
Atoms have such a small mass that it's conventional to state atomic mass in terms of a standard atom. Carbon is been adopted as the standard atom, one atomic mass unit (amu) being defined as one-twelfth of the mass of a single atom of the isotope carbon-12. This is equivalent to 1.6605 × 10-27 kilogram, or approximately 931 MeV.
This definition of the atomic mass unit was agreed upon by the International Union of Pure and Applied Physics in 1960 and the International Union of Pure and Applied Chemistry (in 1961), resolving a long-standing difference between chemists and physicists. The current atomic mass unit, also called the unified atomic mass unit, replaced the atomic mass unit (chemical scale) and the atomic mass unit (physical scale), both having the symbol amu. The amu (physical scale) was one-sixteenth of the mass of an atom of oxygen-16. The amu (chemical scale) was one-sixteenth of the average mass of oxygen atoms as found in nature. 1 amu (unified) = 1.000 317 9 amu (physical scale) = 1.000 043 amu (chemical scale).
Relative atomic mass
Relative atomic mass is the mean mass of the atoms of an element weighted to the relative abundance of its naturally occurring isotopes and measured relative to the standard atomic mass unit. On this scale relative atomic masses for the naturally occurring elements range from 1.008 (hydrogen) to 238.03 (uranium). The relative atomic mass of chlorine, for example, is about 35.5. This is because naturally occurring chlorine is a mixture of two isotopes, 35Cl, with an abundance of about 75%, and 37Cl with an abundance of about 25%. The concept of relative atomic mass is closely related to that of the mole, which is that standard measure of the amount of substance.
Relative molecular mass
Relative molecular mass is the ratio of the average mass per molecule or specified entity of a substance to 1/12 of the mass of a carbon-12 atom. Relative molecular mass is equal to the sum of the relative atomic masses of all the atoms that comprise a molecule.