Question

Why do atomic masses of most of the elements in atomic mass units involve fraction?

Answer 1

Atomic mass unit is defined as $\frac{1}{{12}^{th}}$ of mass of an atom of carbon-12.

Many naturally occurring elements exist in the form of one or more isotopes.Isotopes are the atoms of the same element having the same atomic number but different mass numbers.

While taking the weighted average of all naturally occurring isotopes, the result may be a fractional number.

For example, carbon occurs naturally in three isotopic forms $C-12,C-13,C-14$. Its average atomic mass is $12.01u$.