Atomic weight vs. number of periodic table elements
The atomic weight or relative atomic mass, Ar(iE), of an atom (the unbound neutral nuclide) iE of element E is defined as the "ratio of the mass of the atom to the unified atomic mass unit". The atomic mass constant mu is equal to the dalton, Da, or the unified atomic mass unit, u, and is defined in terms of the mass of the carbon-12 atom: mu = 1 u = 1 Da = ma(12C)/12 [12]. Thus, the atomic weight is a quantity of dimension 1 (dimensionless quantity).
The atomic weight of an element E, in a substance P, Ar(E, P), is the weighted average of the atomic weights Ar(iE) of the isotopes (nuclides) iE of this element in substance P.