An oxidation-reduction process involves the simultaneous loss of electrons by one specie(s) and gain of electrons by another specie(s). The specie that loses electrons is said to be oxidized. The specie that gains electrons is said to be reduced. A mnemonic commonly used to remember these facts is "LEO says GER." In words, "Loss of Electrons is Oxidation," and "Gain of Electrons is Reduction."
In a redox process, the oxidized specie is called a reducing agent because it is the specie that donates electrons to allow the reduction to take place. Similarly, the reduced specie is called a oxidizing agent.
When balancing redox reactions, the movement of all the electrons must be traced. The movements are traced through the assignment of oxidation numbers to each atom, ion, or molecule involved in the reaction. The oxidation number describes the state of oxidation of a specie. If the number changes during a reaction, electrons have moved, indicating that the reaction is a redox process. Note that in a single redox reaction, more than one specie may be oxidized or reduced simultaneously. When assigning oxidation numbers, check the oxidation numbers of all atoms and account for all electrons gained or lost.
A series of rules helps determine the oxidation number of an element.
