Isotopes of the same element are atoms that differ only in their number of:

1. An element is defined by a fixed number of protons, so that number cannot vary between isotopes of the same element. 2. The text states: "isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess." 3. Protons stay fixed (that is what makes them the same element), and while electron count can change in ions, isotopes are specifically about neutral atoms differing in neutrons. 4. So isotopes of an element differ only in their neutron count. _Source: OpenStax Biology Ch 2 "The Chemical Foundation of Life", p.34 §2.1 | Atoms, Isotopes, Ions, and Molecules: The Building Blocks_