Atomic masses are often expressed in decimal form because they take into account the average mass of the naturally occurring isotopes of an element. Isotopes are variants of an element that have the same number of protons but differ in the number of neutrons. Since isotopes of an element have different masses, their contributions to the overall atomic mass of the element can vary.
The atomic mass listed on the periodic table is the weighted average of the masses of all the naturally occurring isotopes of that element, taking into account their relative abundances. The average atomic mass is calculated by multiplying the mass of each isotope by its abundance, summing up these values, and then dividing by the total abundance of all isotopes.
The resulting atomic mass is typically not a whole number because it is a weighted average. For example, carbon has two main isotopes: carbon-12 (which has a mass of approximately 12 atomic mass units) and carbon-13 (which has a mass of approximately 13 atomic mass units). Since carbon-12 is more abundant in nature, the atomic mass of carbon is closer to 12 than 13, but not exactly 12.
By expressing atomic masses in decimal form, it provides a more accurate representation of the overall mass of the element, accounting for the presence and relative abundance of its isotopes.