The term "atomic weight" or "atomic mass" refers to the average mass of an atom of a particular element. However, it can sometimes be referred to as "fractional" or "frictional" because the atomic weight listed on the periodic table is an average value based on the naturally occurring isotopes of an element.
Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei, resulting in slightly different masses. Since isotopes have different abundances in nature, the atomic weight is calculated as a weighted average of the masses of each isotope, taking into account their relative abundances.
The fractional or decimal values in the atomic weight occur due to the contribution of different isotopes with different masses. For example, carbon has two stable isotopes, carbon-12 and carbon-13, with atomic masses of approximately 12 and 13 atomic mass units (amu), respectively. The atomic weight of carbon on the periodic table is listed as 12.01 amu, which is a weighted average based on the abundance of each isotope in nature.
So, the fractional nature of atomic weights arises from the presence of multiple isotopes with different masses and their respective abundances in a given element.