Oxidation number,also called Oxidation State, the total number of e- that an atom either gains or loses in order to form a chemical bond with another atom. Each atom that participates in an oxidation-reduction reaction is assigned an oxidation number that reflects its ability to acquire, donate, or share electrons. The iron ion Fe3+, for example, has an oxidation number of +3 because it can donate three electrons to form a chemical bond, while the oxygen ion O2- has an oxidation
 number of -2 because it can acquire two electrons. In an electronically 
neutral substance, the sum of the oxidation numbers is zero; for 
example, in hametite(Fe2O3) the oxidation number of the two iron atoms (+6 in total) balances the oxidation number of the three oxygen atoms (-6).Thus when elements participate in a reaction,assigning oxidation number clearly indicates how the e- are changing between them which helps in studying an particular reaction.
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Aarti Gupta
askiitians Faculty