Isomers are molecules or molecular compounds that are similar in that they have the same molecular formula, however have different arrangements of the atoms or groups of atoms (functional groups) involved.
Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus and therefore different atomic weights .
Isotopes may be stable or radioactive. Most elements have two or more isotopes. The isotopes most commonly used in chemistry include: carbon (12C,13C,14C), hydrogen (1H, 2H, 3H), oxygen (16O,18O), sulfur (32S, 34S) and nitrogen (14N, 15N).