Answer:
The difference is the number of neutrons they have.
Step-by-step explanation:
The atomic number is the number of protons. The number of protons is what differentiates elements. The isotope number is the total weight of the atom, that is the protons plus the neutrons. Therefore the number of neutrons is equal to the atomic mass minus the atomic number. Electrons are generally equal to the number of protons, to balance the charge of the atom.
In short: the difference between the two isotopes is that one has more neutrons.