Atomic mass
The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The dimensionless standard atomic weight instead refers to the average (mathematical mean) of atomic mass values of a typical naturally-occurring mixture of isotopes for a sample of an element. Atomic mass values are thus commonly reported to many more significant figures than atomic weights. Standard atomic weight is related to atomic mass by the abundance ranking of isotopes for each element. It is usually "about" the same value as the atomic mass of the most abundant isotope, other than what looks like (but is not actually) a rounding difference.
Semi-empirical mass formula
The mass of an atomic nucleus is given by
where formula_2 and formula_3 are the rest mass of a proton and a neutron, respectively, and formula_4 is the binding energy of the nucleus. The semi-empirical mass formula states that the binding energy will take the following form: