What is oxidation state?

What is oxidation state? How to find or calculate it for an atom?

If you want to learn What is oxidation state? and How to find the oxidation state , you should learn the following rules to assign the oxidation number:

1.    The oxidation state of group 1 elements is always +1. 

Li, Na, K, Rb, Cs, Fr have +1 OS in all the compounds.

                  For example: OS of Na in Na2O  is +1.

2.    The OS of group 2 elements is always +2. 

Mg, Ca, Ba etc will always have +2 OS.

For Example: Oxidation number of Mg in MgO is +2.

3.    The free element or a free molecule will always have OS equal to zero.

For example: Fe and Owill have oxidation number equal to zero.

4.    Fluorine is the most electronegative element so it always has -1 oxidation number.

5.    Other halogens generally have -1 oxidation number.

For example: Cl has -1 oxidation number in NaCl.

In Interhalogen compounds like IF7,  Iodine has +7 Oxidation number because 

Fluorine always have -1 oxidation number.

6.    Oxygen generally have -2 OS in all oxides. 

For example: OS of O in CO2  is -2.

In peroxides ,OS of O is -1.

For example: OS of O in H2O2( Hydrogen peroxide) is -1.

In superoxides, OS of O is -1/2. 

For example; OS of O in NaO( Sodium superoxide) is -1/2.

Exceptionally oxygen has +2 OS in OF2( Oxygen fluoride) because F 

has -1 OS always.

7.    The OS of all the ions ( simple ions or polyatomic ion) is always equal 

to the charge on them. 

For example: Oxidation states of Aluminium and phosphate ions is equal to the 

charge on them.   Al3+and (PO4)3-  

8.    The sum of all the oxidation states of all atoms in compounds is always zero.

For example: The sum of all Oxidation states in H2O=0

H=+1 so +1×2 = +2

O= -2  so the sum is +2 -2= 0

9.    The oxidation states of transition elements isto be determined by using above 

rules because they have variable oxidation states.

The OS of Mn in KMnO is to be calculated.

K= +1 because it is in group 1.


O=-2 x4= -8

+1+Mn -8= 0

Mn = +7


Determine the following oxidation states.

1.    N in NO3–       

2.    S in H2SO4

3.    Cr in K2Cr2O7

4.    Fe in Fe2O3

5.    Fe in Fe3O4

6.    Mn in MnO2      

7.    S in SO32-      

8.    C in CH4      

9.    S in HSO4    

10. N in NO2        

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