When calculating the average atomic mass or atomic weight of an element, the individual atoms are not counted directly because their masses are extremely small. Instead, a weighted average is used, taking into account the relative abundance of each isotope of the element.
Isotopes of an element have different masses due to the varying number of neutrons in their nuclei. These isotopes occur naturally in different proportions, meaning some isotopes are more abundant than others. The average atomic mass considers both the mass of each isotope and its abundance.
The calculation of average atomic mass involves multiplying the mass of each isotope by its relative abundance (expressed as a decimal), and then summing these values for all the isotopes of the element. This weighted sum represents the average mass of the isotopes present in a naturally occurring sample of the element.
By using the average atomic mass, scientists can represent the typical mass of an atom of a specific element, accounting for the different isotopes and their relative abundances found in nature.