Atomic Weight

[2016. Scaled to an atomic weight of 12 for carbon-12 (¹²C), where ¹²C is a neutral atom in its nuclear and electronic ground state, having the result that atomic weight values are dimensionless.]

The atomic weights of many elements are not invariant, but depend on the origin and treatment of the material. The standard atomic weights apply to elements of natural terrestrial origin. Although the atomic weights of some elements in some naturally occurring materials may differ from given values because of a variation in the mole fractions of an element’s stable isotopes, the last significant figure of each tabulated value is considered reliable to ±1 except for zinc, which is ±2. For twelve of these elements, both a conventional atomic weight and an atomic weight interval is given with the symbol [a, b] to denote the set of atomic weight values in normal materials; thus, a ≤ atomic weight ≤ b. For lithium, the conventional atomic weight is only three digits because of the large variation found in lithium-bearing reagents.