Oxidation and Reduction
Rust is oxidized metal.
When the term oxidation was coined, it was used to describe chemical reactions in which something reacted with oxygen to form a new compound. Reduction was the opposite process, or a reaction that took an oxidized compound and turned it back into the original element. For example, magnesium metal reacts with oxygen to create magnesium oxide. This is called the oxidation of magnesium. The reduction of magnesium oxide involves a reaction between magnesium oxide and carbon, which forms magnesium metal and carbon monoxide gas.
Finding the True Definition of Oxidation
Over time the definitions of oxidation and reduction were simplified. When chemists discovered electrons, they theorized that oxidation and reduction reactions involved the transfer of electrons. However, further experimentation led to the understanding that electrons are not always transferred in oxidation and reduction reactions. For instance, when carbon monoxide reacts with water to yield carbon dioxide and hydrogen gas, no electrons are transferred, yet this is considered an oxidation-reduction reaction, because the carbon monoxide reacts with oxygen (oxidation) and the hydrogen reverts back to its original pure state (reduction).
Oxidation Numbers
Soon, chemists introduced the idea of oxidation numbers. These numbers represent the charge an atom in a particular compound would have if all electrons were transferred to the atom that had the greater attraction in the compound.
Chemists now use oxidation numbers to define oxidation and reduction reactions. Formally speaking, an oxidation reaction occurs when the reaction causes the oxidation number of a particular atom to increase. Conversely, a reduction reaction occurs when the reaction causes the oxidation number of a particular atom to decrease. The two reactions seem to be tied together, as oxidation cannot occur unless reduction of another element in the chemical equation also occurs.