Explain why the atomic masses of many ele­ments are in fractions and not in whole numbers ?

Atomic mass:

1. The atomic mass of an element is the average relative mass of all the natural isotopes of that element.
2. The atomic masses of some elements are in fraction instead of whole numbers.
3. This is due to the existence of isotopes having different mass number.
4. The isotopes of an element have different atomic mass and the percentage of these isotopes vary in nature.
5. $\mathrm{Average}\mathrm{atomic}\mathrm{mass}=\frac{\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{isotope}1\times \%\mathrm{of}\mathrm{occurence}+\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{isotope}2\times \%\mathrm{of}\mathrm{occurence}}{100}$
6. The elements such as copper, and chlorine have atomic masses in fraction.