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 email@example.com
Copyright © 1997-2010 by Fred Senese
Comments & questions to firstname.lastname@example.org
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/redox/faq/print-why-electrode-voltages.shtml