Chemical reactions are often divided into two categories:
oxidation-reduction or metathesis reactions. Metathesis reactions
include acid-base reactions that involve the transfer of an H+
ion from a Brønsted acid to a Brønsted base.
| CH3CO2H(aq) | + | OH-(aq) |  |
CH3CO2-(aq) | + | H2O(l) |
| Brønsted acid | Brønsted base | Brønsted base | Brønsted acid |
They can also involve the sharing of a pair of electrons by an electron-pair donor (Lewis base) and an electron-pair acceptor (Lewis acid).
| Co3+(aq) | + | 6 NO2-(aq) | |
Co(NO2)63-(aq) | |
| Lewis acid | Lewis base |
Oxidation-reduction reactions or redox reactions can involve the transfer of one or more electrons.
Cu(s) + 2 Ag+(aq)
Cu2+(aq) + 2 Ag(s)
They can also occur by the transfer of oxygen, hydrogen, or halogen atoms.
| CO2(g) + H2(g)
CO(g) + H2O(g) |
SF4(g) + F2(g) 
SF6(g) |
Fortunately, there is an almost foolproof method of distinguishing between metathesis and redox reactions. Reactions in which none of the atoms undergoes a change in oxidation number are metathesis reactions. There is no change in the oxidation number of any atom in either of the metathesis reactions, for example.
The word metathesis literally means "interchange" or "transposition," and it is used to describe changes that occur in the order of letters or sounds in a word as a language develops. Metathesis occurred, for example, when the Old English word brid became bird. In chemistry, metathesis is used to describe reactions that interchange atoms or groups of atoms between molecules.
When at least one atom undergoes a change in oxidation state, the reaction is an oxidation-reduction reaction. Each of the reactions in the figure below is therefore an example of an oxidation-reduction reaction.
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