Why are atomic masses on the periodic table not whole numbers?

Atomic masses on the periodic table are not whole numbers because they represent the average mass of an atom's isotopes.

The atomic mass of an element on the periodic table is not a whole number because it is a weighted average of the masses of the different isotopes of that element. Isotopes are atoms of the same element that have different numbers of neutrons, and therefore different masses. For example, carbon has two stable isotopes, carbon-12 and carbon-13, which have different numbers of neutrons and thus different masses.

The atomic mass is calculated by multiplying the mass of each isotope by its natural abundance (the percentage of that isotope found in nature), and then adding these values together. This results in a weighted average, which is often not a whole number.

For instance, the atomic mass of carbon is 12.01, not 12. This is because about 99% of carbon in nature is carbon-12, which has a mass of 12 atomic mass units, and about 1% is carbon-13, which has a mass of 13 atomic mass units. The weighted average of these masses, taking into account their natural abundances, is 12.01.

So, the atomic masses on the periodic table represent the average mass of an atom's isotopes, taking into account their natural abundances. This is why they are usually not whole numbers.