Isotopes have the same number of protons, but a different number of neutrons. This means that they have different masses, but retain the same chemical name. For this problem, Pb-206 and Pb-208 are both lead, but have different masses (206 and 208), so they are isotopes.
C-14 and N-14 have a different number of protons (they are not the same element).O2 and O3 have a different number of molecules.S and S(2-) are the same element and have the same number of neutrons, but S(2-) just has 2 more electrons that give it the charge.
