By definition isotopes are elements with the same number of protons by different number of neutrons. Elements X-18 & X-20 have 17 protons and represent Chlorine isotopes Cl-18 & Cl-20 each with 17 protons and 18 electrons to give the isotopes a -1 oxidation state. Both isotope of chlorine have 7 electron in its valence shell and 10 electrons in its core structure. Gaining 1 electron fills the valence octet and establishes a -1 oxidation state.
