Final answer:
The average atomic mass of an element is the weighted average of all its naturally occurring isotopes, taking into account their masses and natural abundance.
Step-by-step explanation:
The average atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. This mass takes into account the mass of each isotope and its percent abundance in nature. For instance, hydrogen has three isotopes: H-1, H-2, and H-3. The weighted average is primarily based on H-1 as it is closest to the average mass of hydrogen on the periodic table and is the most abundant isotope.
In chemistry, we rarely deal with a single isotope but rather a mixture since isotopes of an element generally react similarly. Thus, the weighted average mass is a critical concept reflecting the masses of isotopes in a natural sample of the element. The atomic mass or atomic weight listed in the periodic table reflects this weighted average and is typically reported in atomic mass units (amu).