Oxidation numbers and redox reactions
Oxidation numbers are a bookkeeping system for tracking electrons in reactions. A few rules assign them: an element on its own, like O2 or Fe, has an oxidation number of 0; a simple ion's oxidation number equals its charge, so Na+ is +1 and Cl- is -1; oxygen in compounds is usually -2 and hydrogen usually +1; and the oxidation numbers in a neutral compound add up to zero, while in a polyatomic ion they add up to the ion's charge. In CO2, for example, each oxygen is -2, so carbon must be +4. In a redox reaction, the element whose oxidation number increases is oxidized (it loses electrons) and the element whose oxidation number decreases is reduced (it gains electrons). The substance that is reduced is called the oxidizing agent, because it causes the other substance's oxidation, and the substance that is oxidized is the reducing agent. When zinc metal is placed in copper(II) sulfate solution, zinc goes from 0 to +2 (oxidized) and copper goes from +2 to 0 (reduced). Balancing a redox equation requires that the electrons lost equal the electrons gained.
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