The atomic mass of an element isa) the sum of the protons any electron in one atom of the elementb) twice the number of protons in one atom of the elementc)a ratio based on the mass of carbon 12 atomd) a weighted average of the masses of an element's isotopes

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.