Final answer:
Many elements have non-integral atomic masses due to the presence of isotopes with different numbers of neutrons, which leads to a weighted average mass for the element when all naturally occurring isotopes are considered. The correct option is A.
Step-by-step explanation:
Many elements have non-integral atomic masses because they have isotopes. This is the correct option to explain why elements might have atomic masses that are not whole numbers. Isotopes are atoms with the same number of protons but different numbers of neutrons. Consequently, they have different mass numbers, which affects the atomic mass of the element.
When calculating an element's atomic mass, it is essential to consider all of its natural isotopes and their relative abundances. The atomic mass listed in the periodic table is the weighted average of these isotopes. This explains why the atomic mass of most elements is not a whole number, as it is a mean value of all the isotope masses based on their occurrence in nature.
For instance, chlorine (Cl) has an atomic mass of 35.45 because it is made up of isotopes with mass numbers of 35 and 37, but in different proportions. Similarly, carbon has an atomic mass of 12.011 because the great majority is Carbon-12, with some contribution from Carbon-13 and a negligible amount from Carbon-14.
Therefore, the correct option in this case is A. They have isotopes, as it is the presence of isotopes with different numbers of neutrons affecting the average atomic mass of elements.