Average Atomic Mass

The atomic mass (weight) of an element is the average of the atomic masses, weighted according to the naturally occuring abundances of the isotopes of the element.

Example: What is the atomic weight of Mg ? The element consists of 78.70 % atoms (mass = 23.99 u), 10.13 % atoms (mass = 24.99 u) and 11.17 % atoms (mass = 25.98 u).

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