The atomic masses listed on the periodic table are not whole numbers because they take into account the average mass of all the naturally occurring isotopes of an element, weighted by their relative abundance.
Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons in their atomic nuclei. Since the atomic mass is determined by the sum of protons and neutrons in an atom, the presence of different isotopes with varying numbers of neutrons affects the overall atomic mass.
Naturally occurring elements often have multiple isotopes, and the abundance of each isotope in nature can vary. For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope, followed by carbon-13 and carbon-14, which is present in trace amounts. The atomic mass of carbon on the periodic table is listed as approximately 12.01 because it takes into account the average mass of all these isotopes, weighted by their abundance.
To calculate the atomic mass, the mass of each isotope is multiplied by its relative abundance (expressed as a decimal) and then summed. This average atomic mass is a more accurate representation of the typical mass of atoms of that element found in nature, rather than using a whole number that would only apply to a single isotope.
It's important to note that some elements, especially those with only one stable isotope, may have atomic masses that are very close to whole numbers. However, the majority of elements have atomic masses that are decimal values due to the presence of multiple isotopes.