To determine the atomic mass of an element, the masses of all the naturally occurring isotopes of that element are averaged together. why are some isotopes more heavily weighted than others? a. they occur more frequently in nature. b. they have a higher mass. c. they have a higher number of orbitals. d. they have more protons.

The answer to this question would be: B. They have a higher mass

The mass of an atom is determined by their mass number which was the sum of their protons and neutrons. Protons of an element will be same. So any isotope of an element will have the same number of proton, no isotope should have more proton. The mass is increased because some isotope has more neutrons than others, resulting in a higher mass.