The atomic masses of elements are typically expressed in decimal places because they take into account the average masses of different isotopes of the element.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the number of neutrons can vary among isotopes, the mass of the atom can also vary.
The atomic mass of an element is a weighted average of the masses of its naturally occurring isotopes, taking into account their relative abundances. The atomic mass is calculated by multiplying the mass of each isotope by its abundance, summing up these values, and expressing the result in atomic mass units (amu).
The atomic masses of isotopes are not whole numbers because they include fractional contributions from the various isotopes. As a result, the atomic mass of an element is also typically expressed as a decimal number. This allows for a more accurate representation of the actual average mass of the atoms found in nature.
It's worth noting that some elements have atomic masses that are very close to whole numbers because they have a single dominant isotope with a high abundance. However, for elements with multiple isotopes, the atomic mass will generally be a decimal value.