The atomic masses of elements are often expressed as decimal fractions because they take into account the average masses of the different isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei, resulting in slightly different atomic masses.
Most elements in nature exist as a mixture of isotopes, with varying abundances. The atomic mass listed on the periodic table is a weighted average of the masses of these isotopes, taking into account their relative abundances. Since the atomic masses of individual isotopes are usually close to whole numbers, their weighted average will typically be a decimal fraction.
For example, let's consider the element carbon. Carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope, followed by carbon-13 and carbon-14. The atomic mass of carbon is approximately 12.01 atomic mass units (u), which is closer to the mass of carbon-12 (12.00 u) due to its high abundance.
So, the atomic mass of an element is an average value that takes into account the masses and relative abundances of its isotopes, resulting in a decimal fraction rather than a whole number.