Calculations in Chemistry:. Average Atomic Mass. Isotopes. An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.
Average Atomic Mass
For example, there are two different types (isotopes) of copper atoms. One type of copper atoms weighs in at 62.93 amu, the other has a mass of 64.94 amu.
Average atomic mass =
Σ (mass of isotope × relative abundance)
Σ = sum or to add them all together
Average atomic mass of copper =
(62.93 amu × 0.6909)
[the mass and abundance of isotope #1]
(64.94 amu × 0.3091)
[the mass and abundance of isotope #2]