Hello!
Isotopes are elements where they have the same number of electrons and protons, and they have the same chemical reactions. But, they have a different amount of neutron, which affect their overall atomic mass.
ALL elements have a variety of isotopes. Some can have 1, 2 or 3. To calculate the average atomic mass of an element, we would use this:
Average Mass = (mass of isotope #1)(percent abundance in nature) + ...
The "..." means if there is another isotope. You would use the same format, but add: (mass of isotope #2)(percent abundance in nature) + ... if there is another isotope.
Therefore, the answer is choice D, a weighted average of the masses of an element's isotopes.
