The standard atomic weight of a chemical element (symbol Ar°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope 63Cu (Ar = 62.929) constitutes 69% of the copper on Earth, the rest being 65Cu (Ar = 64.927), so A r ° ( 29 Cu ) = 0.69 × 62.929 + 0.31 × 64.927 = 63.55.
The standard atomic weight of a chemical element (symbol Ar°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope 63Cu (Ar = 62.929) constitutes 69% of the copper on Earth, the rest being 65Cu (Ar = 64.927), so A r ° ( 29 Cu ) = 0.69 × 62.929 + 0.31 × 64.927 = 63.55.
Source: Wikipedia "Standard atomic weight" · CC BY-SA 4.0
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