The voltage or potential difference between an oxidation and reduction reaction arises from the different electrochemical potentials of the reduction and oxidation reactions in the battery. The electrochemical potential is a measure of the difference between the average energy of the outer most electrons of the molecule or element in its two valence states. (For those familiar with semiconductor theory, the electrochemical potential difference between an oxidation and reduction reaction is analogous to the Fermi level difference between two sides diode). As the electrochemical potential is a measure of the energy of the outer most electrons, examination of the electronic configuration of the outer shell of the material will give an indication of the magnitude and sign of the electrochemical potential between the reactants and products of a reduction or oxidation reaction. The lowest energy configuration for materials is for their outer shell to be fully occupied by electrons. Hence, an element (say lithium, Li) with one electron in its outer shell will have a higher energy than the element with the electron removed. Thus in the reaction

the lithium metal has a higher energy than Li+, and the oxidation reaction has a large positive electrochemical potential, with a value of 3.04V. The positive sign is defined such that the reaction proceeds spontaneously, if the electron is used by another reaction.

Figure 4: Outer electron configuration showing an element with one electron in its outer shell. When it looses its electrons, the outer shell has eight electrons, and hence is a stable, low energy configuration. The core electrons are not shown.

By convention half reactions (a half reaction is either the oxidation or reduction reaction of a oxidation/reduction reaction, ie half of the overall redox reaction) are written as reduction reactions, and hence the above reaction is typically tabulated as:

which has a negative standard potential. For all half reactions, changing the direction of the reaction changes the sign of the standard potential, such that the reduction reaction of lithium above has a standard potential of E⁰ = -3.04 V.

Similarly, an element with 7 electrons in its outer shell (say Cl), will have a higher electrochemical potential than Cl which has gained an electron and hence has a full outer shell.

Thus, in the reaction

the chlorine ion Cl^- has a lower energy than the neutral Cl, and the reduction reaction has a large positive electrochemical potential of 1.36V.