The atomic mass of an element listed on the periodic table is typically not a whole number because it takes into account the natural abundance of different isotopes of that element.
Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their nuclei. Since the atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, and their abundances can be fractional values, the resulting atomic mass can also be a decimal or fractional value.
To calculate the atomic mass, the masses of each isotope are multiplied by their respective abundances (expressed as decimals or percentages), and then these values are summed up. The resulting value represents the weighted average mass of all the isotopes present in nature.
For example, carbon has two stable isotopes: carbon-12 (with a mass of approximately 12 atomic mass units) and carbon-13 (with a mass of approximately 13 atomic mass units). The natural abundance of carbon-12 is about 98.9%, while carbon-13 has an abundance of about 1.1%. When you calculate the weighted average, taking into account these abundances, you get an atomic mass for carbon close to 12.01 atomic mass units.
So, the decimal or fractional atomic mass of an element reflects the average mass of its isotopes, considering their relative abundance in nature.