Answer:
atoms of isotopes of an element have different numbers of neutrons.
Explanation:
isotopes have the same number of protons, but different numbers of neutrons.
Well, all of this we owe it to Bohr who analyzed the atomic emission spectrum of hydrogen and he could probe matematically that it was a result of movement of e- from an especific energy level to a lower one. The understanding of levels of energy took to the development of the atomic theory
Pangea. :) Hope it helped!