The relative atomic masses of many elements are not whole numbers because of the presence of:

Relative atomic mass is determined by taking the average of the atomic masses of isotopes of an element which can sometimes not be a whole number. 

Isotopes of the elements are defined as elements that contain the same number of protons but a different number of a neutron. 

For example, the isotope of hydrogen is hydrogen, deuterium, and tritium.

The relative atomic masses of many elements are not whole numbers because of the presence of isotopes. Relative atomic mass is given by

$\sum _{ } isotope mass \times isotope abundance / 100.$