Question

explain why atomic mass of an atom is expressed as average atomic mass.

Answer 1

As it is impossible to calculate the original mass as the mass is so small

Answer 2

Answer: An atom contains various isotopes.

Explanation:

An atom exists in various isotopic forms each having different mass. Therefore, an average of the isotopes is taken otherwise an atom would have two different masses, which is not possible

Average atomic mass is the sum of mass of isotopes of an atom each multiplies by its natural fractional abundance.

Mathematically,

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$

Hence, atomic mass of an atom is always expressed in the form of average atomic mass.