Question

two versions of an element with different numbers of neutrons are called molecules. ions. re - runs. electron pairs. isotopes.

Explanation:

Isotopes are defined as two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei. Molecules are groups of atoms bonded together, ions are atoms or molecules with a net electric charge, and electron - pairs are two electrons associated with an atom or molecule. Re - runs is not relevant in this context.

Answer:

isotopes.