If more molecules of the same species are added to the solvent, the effect is to enlarge the probability distributions for the two states, as indicated in the figure to the left. The reactants in this example are Fe2+ and Fe3+, termed a redox couple. For the ferrous (Fe2+)/ferric (Fe3+) redox couple the two energy states have significant overlap. Overlap of the two distributions signifies that the energy states of the oxidized state and the reduced state are closely spaced. Both species are in the solvent and the reactions correspond to “outer sphere” reactions, which means there is little or no interaction between the metal and the reactants. When the metal electrode in introduced into the reactant solution a small amount of charge (usually considered to be negligible) is exchanged between the redox couple and the electrode so that density of states in the metal is shifted to a level that the Fermi level of the metal corresponds to the boundary between the occupied and unoccupied states in the redox couple are aligned. In electrochemistry terms, the potential that the electrode assumes is the “standard potential” of the redox couple, E0, when the concentration of the species in the oxidized state and reduced state are equal.