Why does a voltage develop at an electrode surface?

Think of the metal plates not as a lot of metal atoms packed together, but as a lot of metal ions floating in a sea of electrons. The boundary between the metal and a solution containing metal cations is pictured below.

Notice that each of the metal ions has a different 'atmosphere' of charges around it. The cations in the metal are surrounded by electrons and other cations; the cations in the solution are surrounded mostly by water molecules (with other ions farther off). Atoms right on the boundary line will 'feel' different charges on the solution side and on the metal side. This is where the voltage comes from.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why does a voltage develop at an electrode surface?