The average mass of isotopes of an element found in nature is called the atomic mass or atomic weight.

Here's how it works:

* Isotopes: Atoms of the same element can have different numbers of neutrons, which are called isotopes. For example, carbon has two common isotopes: carbon-12 and carbon-14.

* Abundance: Each isotope occurs in a specific proportion in nature, called its natural abundance.

* Weighted Average: The atomic mass is calculated as a weighted average of the masses of all the isotopes of an element, taking into account their natural abundances.

Example:

Let's say an element has two isotopes:

* Isotope A: Mass = 10 amu, Abundance = 75%

* Isotope B: Mass = 12 amu, Abundance = 25%

The atomic mass would be:

(10 amu * 0.75) + (12 amu * 0.25) = 10.5 amu

You can find the atomic masses of elements on the periodic table. Keep in mind that these are average masses, and the actual mass of an individual atom may vary slightly.