Following the Born modelBorn model for ion solvation energies in 1920 there was a laps in model development until Libby attempted to explain electron exchange reactions by invoking Born–OppenheimerBorn-Oppenheimer bond vibrational potentialsVibrational potential and the Franck–Condon factor for electron transferElectron transfer in a way that did not conserve energy. It is shown how Libby might have succeeded had he invoked electrochemistry by using a half-cellHalf-cell reaction path for the electron exchange. Marcus noted Libby’s failure to conserve energy and developed a model for energies of electron transfer transition statesTransition state where there are two kinds of polarizationsPolarization of the dielectric, one dependent on the electric fieldElectric field the ion experiences from the other ion and a structural one that does not. Marcus introduced the ion solvation shell and used the breathing mode model of George and Griffiths satisfy the Franck–Condon factor. ThePolarization in Marcus theory Marcus theoryMarcus theory identified “inverted regions” where a reaction would be activated though the product is more stable than the reactant. Hush developed a theory for electron transferElectron transfer at electrode surfaces where at the transition state the electron is delocalized over the reactant and transition stateTransition state. Both the Marcus and Hush theories gave electron transfer parameters for use in the Butler-Volmer[aut]Butler-Volmer equations.