Answer is: neutrons. Isotopes have same number of protons and different number of neutrons. For example hydrogen has three isotopes: protium(one proton, no neutron), deuterijuma (one proton, one neutron), tritiumt(one proton, two neutrons).
Isotopes are basically from the same element. In terms of subatomic particles of the element, the isotopes will then basically have the same number of protons. The electrons also have to be same in number given that the elements are in neutral charges.
We are then left with neutrons, which is one of the subatomic particles residing in the nucleus of an atom.
