Relation ISOTOPE-OF

Used to denote the form of element having the same atomic number (i.e., the same number of protons), but differing in atomic weight or mass due to the presence
of one or more additional neutrons. Both for stable and radioactive isotopes.
Arity: 2
Domain: Element-ion-or-isotope
Range: Chemical-element
Subrelation-Of: Chemical-relation
Axioms:
(=> (Isotope-Of ?A ?B)
    (Exists (?Z ?W)
            (And (Value-Filler ?Z)
                 (Value-Filler ?W)
                 (Atomic-Mass-Value ?A ?Z)
                 (Atomic-Mass-Value ?B ?W)
                 (Different ?Z ?W))))