What Are Oxidation Numbers?

Oxidation numbers (also called oxidation states) help us keep track of electrons in chemical reactions. Historically, they were used to indicate how many oxygen atoms were bonded to an atom. For example, in carbon monoxide (CO) there is one oxygen atom per carbon atom, while in CO₂, there are two oxygen atoms per carbon. We say that the carbon atom in CO₂ has a higher oxidation number than the carbon atom in CO.

This way of thinking about oxidation numbers is still very useful, especially for analyzing many biochemical reactions. However, chemists have expanded the concept to include compounds that do not have any oxygen atoms in them. A more modern definition of oxidation numbers is:

Oxidation number - The charge on a monoatomic ion, or a pretend charge assigned to an atom.

The next two pages will explain the concept of oxidation numbers in more detail.