Why is atomic mass an average?
Average atomic mass = (% as dec.)(mass of isotope 1) + (% as dec.)(mass of isotope 2) + ....
As shown above, the atomic mass of an element is the average of the isotopes of that element.
For example, the average atomic mass of lithium will be calculated by using lithium's two isotopes, Li-6 and Li-7:
Average atomic mass = (0.9258)(7.015) + (0.0742)(6.015)
= 6.940 u