To determine the atomic mass of an element, you would take a(n) average of the isotopes that make up that element.

The answer to this question would be : Neutrons

The molecular weight of an atom is determined by the number of protons and neutrons. An isotope can have a different count of neutrons, but they should have the same count of protons. In fact, the number of protons is the one that determines which element the atom was.

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