An Isotopes is a term used to describe atoms of a given chemical element having the same atomic number but different atomic mass numbers. Isotopes have the same number of protons. Since the sum of protons + neutrons= Atomic Number, the difference is the Atomic Mass. Isotopes can be stable or unstable. A stable Isotope does not undergo radioactive or (nuclear decay). An Unstable Isotope undergoes radioactive or (nuclear decay). See atoms, Proton, and Neutron.