Final answer:
The average atomic mass of an element is calculated using the relative abundances of its naturally occurring isotopes and their mass numbers, resulting in a weighted average value listed in the periodic table.
Step-by-step explanation:
The average atomic mass of an element is determined by two main factors: the relative abundances of the element's naturally occurring isotopes and the mass numbers of those isotopes. When calculating the average atomic mass, isotopes are represented by mass numbers. These mass numbers, combined with the relative abundances of each isotope, make up the weighted average that reflects the average atomic mass.
To calculate this, you use the formula: average atomic mass = f1M1 + f2M2 + f3M3..., where f represents the fractional abundance of each isotope, and M denotes the mass number of that isotope. Since isotopes of elements vary in the number of neutrons, this calculation takes into account the different weights of each isotope according to how common they are in nature.
The periodic table gives the average atomic mass for each element which is essential when determining the most abundant isotopic form of an element or performing chemical calculations.