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Oxidation Number

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↪ The residual charge to have appeared on the atom after the electron transfer process.
                                                                 OR
↪ Oxidation number of an atom may be defined as the residual charge appeared on a single atom when other atoms are removed from the molecules. 

For example, 
↪ In HCl, the oxidation number of Cl is -1 because when hydrogen is removed as ion from the molecule the residual charge on chlorine atom becomes -1.

 Rules for Determining Oxidation number

1. O.N. is usually calculated on the basis of number of electrons lost, gained or shared by an atom in a molecule.
If atom loses electrons, there is development of positive charge. Hence, O.N. becomes positive.
If an atom gains electrons, there is development of negative charge. Hence, O.N. becomes negative.

2. In covalent molecules, the electrons are shared between two same atoms equally. Therefore, there is no charge on each atom and O.N. is zero
For eg: In H2, O.N. of hydrogen is zero
Similarly, In Cl2, O.N. of Chlorine is zero

3. Electrons shared between two dissimilar atoms are attracted more towards the electronegative atom. Therefore, there is development of -ve charge on electronegative atom and the charge on atom bonded it.
For eg: in HF, fluorine is more E.N. than hydrogen and has -1 charge while hydrogen +1 charge.

4. O.N. of element in free or uncombined state is zero.
for eg; O.N. if H2, O2, Cl2, Na, Mg, etc. is zero.

5. O.N. of hydrogen is +1 in compounds except in metallic hydrides. It has O.N. of -1.
for eg: O.N. of hydrogen in H2O, HCl, etc. is +1 whereas O.N. of hydrogen in NaH, CaH2, etc is -1.

6. O.N. of oxygen is -2 in compounds except in peroxides, super peroxides and OF2.
for eg: O.N. of oxygen in compounds H2O, CaO, etc. is -2. But in peroxide like H2O, Ba2O, Na2O, etc. is -1. 
In super peroxides, O.N. of oxygen is -1/2.
for eg: KO2, RbO2, etc.
In OF2, O.N. of oxygen is +2.

7. O.N. of alkali metals ( li, Na, K, Rb, Cs, Fr ) is +1.

8. O.N. of alkaline earth metals ( Be, Mg, Ca, Sr, Ba, Ra ) is +2.

9. O.N. of ion or radical is the total charge on it.
For eg: O.N. of Na+, NH4+, Al3+ is +1, +1, +3 respectively.
Similarly, O.N. of Cl-, SO42-, PO43-  is -1, -2, -3 respectively.

10. Algebraic sum of O.N. of all elements in a compound is zero.
for eg: In SO2, the sum of O.N. of Sulphur atom and two oxygen atoms is equal to zero. 

11. O.N. of metals in alloy is zero. 
for eg: O.N. of Na in NaHg is zero.

12. The O.N. of fluorine in its compound is always -1 because it is the most electronegative element, while the O.N. of other halogens is -1 except in the compounds where they are bonded to more electronegative element.
for eg: in HCl, the O.N. of chlorine is -1 but in Cl2O7 the O.N. of Cl is +7.