Isotopes of an atom differ by what?

The element is characterized by $Z$, the number of massive, positively charged nucular particles. There are 100 or so elements. And each element can have several isotopes, characterized by differing numbers of neutrons, massive, neutrally charged nuclear particles. The nuclear particles engage in the strong nuclear force, which at short ranges, is strong enuff to overcome the electrostatic force of repulsion. But this is fyziks and not chemistry.