Question

What is an isotope
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Answer 1

each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.

Answer 2

Isotopes have the same number of protons and electrons thus they have the same atomic number and the same chemical reactions. But they have different numbers of neutrons giving them different atomic masses. Some isotopes are heavier or lighter than others. That is the only difference between them. Some are stable (not radioactive) and some will break down through radioactive decay.