By convention, the element or compound that gained these electrons was said to undergo reduction . In this case, O 2 molecules were said to be reduced to form O 2- ions. Chemists eventually recognized that oxidation-reduction reactions don't always involve the transfer of electrons. The most powerful model of oxidation-reduction reactions is based on the following definitions. Oxidation involves an increase in the oxidation number of an atom.
Reduction occurs when the oxidation number of an atom decreases .
We find examples of oxidation-reduction or redox reactions almost every time we analyze the reactions used as sources of either heat or work. Silver metal, for example, is oxidized when it comes in contact with trace quantities of H 2 S or SO 2 in the atmosphere or foods, such as eggs, that are rich in sulfur compounds. Fortunately, the film of Ag 2 S that collects on the metal surface forms a protective coating that slows down further oxidation of the silver metal. The tarnishing of silver is just one example of a broad class of oxidation-reduction reactions that fall under the general heading of corrosion . Another example is the series of reactions that occur when iron or steel rusts.