Oxidation state (or oxidation number) refers to the number of electrons added to or removed from an element when it forms a chemical compound. Sodium metal, for example, has an oxidation state of 0 in the elemental state. But when it gives up its one valence (outer) electron (symbolized by e), it becomes a sodium ion Na with an oxidation state of +1. Iron is another example. It can lose two electrons, forming an oxidation state of +2; or it can lose three electrons, forming an oxidation state of +3.