What is a isotopes?

Isotopes are different versions of an element or different versions of a certain kind of atom. In order for an atom to be an isotope, it must have the same number of protons because the protons determine what kind of element it is. Also, there must be a different number of neutrons in the core the atom.

Isotope - 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.