Q. Define Oxidation state of an atom?

Oxidation State: Oxidation state of an atom is defined as an apparent or formal charge on that particular atom in a molecule. In majority of the cases it is easily identified. For example, a complex [Cu (NH₃)₄] Cl2 has the copper ion in the oxidation state of +2 since NH₃ molecules are neutral or do not carry any net charge. Note that the valence is not the same as oxidation state. We can say that in the above complex, copper shows a covalence of four and electrovalence of two. In some cases, however, it may not be easy to find out the oxidation state of the metal ion from the knowledge of the formula of the complex only. Suppose we consider a complex like [Fe(H₅O)2NO]^2+. We also know that NO can exist as a neutral entity having no charge or as NO^- or NO⁺. Now if in the above compound NO is acting as a neutral molecule, the oxidation state of Fe would be +2. If NO is present as NO^-, Fe would be in +3 oxidation state. Finally if it is present in the form of NO^+, Fe would have +I oxidation state. Thus more information is required before the Oxidation State of the metal ion can be decided.