Atomic weight

The atomic weight or standard atomic weight (A_{r, standard}(E)) of a chemical element 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 (A_r = 62.929) constitutes 69% of the copper on Earth, the rest being ⁶⁵Cu (A_r = 64.927):

A_{r, standard}(₂₉Cu) = 0.69 x 62.929 + 0.31 x 64.927 = 63.55

Because relative isotopic masses are dimensionless quantities, this weighted mean is also dimensionless. It can be converted into a measure of mass (with dimension M) by multiplying it with the dalton, also known as the atomic mass constant.

References

Relevant Sciencemadness threads

• Atomic weights to be revised