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The elements involved in an electrochemical reaction are characterized by the number of electrons associate with each atom of the element. This number of electrons is often expressed in terms relative to the number of electrons such an atom would have when electrically neutral. The term used for such an expression is the [oxidation state]? of the atom.
For example, oxygen has 6 positively charged protons, and thus in the neutral state would also have 6 (negatively charged) electrons. In dihydrogen monoxide (ie, water), each oxygen atom can be viewed as surrendering two electrons, one to each of two hydrogen atoms. The oxidation state of each oxygen atom in water then is -2, and of each hydrogen atom in water is +1.
This is a simple example, because oxygen and hydrogen few oxidation states as commonly found on Earth. However, many of the transition metal elements each have a rich variety of commonly occuring oxidation states.