Atomic masses of elements are not usually whole numbers due to the existence of isotopes. Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons.

For example, the element carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 has six protons and six neutrons, carbon-13 has six protons and seven neutrons, and carbon-14 has six protons and eight neutrons. The atomic mass of carbon is not a whole number because it is a weighted average of the masses of its isotopes, taking into account their relative abundances. In the case of carbon, the atomic mass is approximately 12.01 amu.

Therefore, the weighted average of the isotopic masses, accounting for their relative abundances, results in non-whole-number atomic masses for most elements.