Oxidation Numbers Keeping Tabs on Electrons

If electrons move between reactants during redox reactions, then it should be easy to recognize those reactions just by noticing changes in charge, right Sometimes. The following two reactions are both redox reactions  

In the first reaction, it s obvious that electrons cire transferred from mc nesium to hydrogen. There are only two reactants, and magnesium is neutral as a reactant but positively charged as a product. The charges can change in this way only through electron transfer. 

In the second reaction, electrons are transferred from hydrogen to oxygen, but the transfer isn t as obvious. Because you ll encounter both types of reactions, you need a simple way to keep tabs on electrons as they transfer between reactants. In short, you need oxidation numbers. 

Here are some basic rules for figuring out an atom s oxidation number  

An atom in elemental form (no charge, bonded to nothing or itself) has an oxidation number of 0. Therefore, the oxidation number of both Mg(s) and 02(g) is 0. 

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