The Average Atomic mass of an element is the weighted average of the atomic masses of all naturally occurring isotopes of an element. It is also known as the atomic weight and is expressed in amu.
The weighted average gives a more accurate value than the normal average by considering all isotopic forms of an element.
For example, the atomic mass of a 12C isotope is 12 amu with a natural isotopic abundance of 98.89%. The 13C isotope has an atomic mass of 13 amu, and its natural isotopic abundance is 1.11%.
Therefore, the average atomic mass or atomic weight of a Carbon atom is
= (12 x 98.89/100) + (13 x 1.11/100) = 11.866 + 0.1443
= 12.01 amu.