The oxidation number of an atom refers to the number of electrons that that atom has gained or lost. In covalent compounds, where the bonds between the nuclei are formed from electrons which are being shared much more evenly, the oxidation number bears no relationship to the charge that resides on any particular atom. Inorganic chemistry may be divided into four types of reaction: acid/base, precipitation, complexation and redox. It is when considering redox reactions that oxidation numbers are used to greatest advantage. The new working definition for calculating the oxidation number of an atom in species that contain covalent bonds may be stated simply. In organic compounds which contain oxygen, the oxidation number for each atom may be calculated in a similar manner. The sum of the oxidation numbers is zero for a neutral molecule, or equal to the charge that the species bears if it is charged.