How do you find oxidation number of an atom?

How do you find oxidation number of an atom?

If you want to learn about oxidation numbers and the method to assign the oxidation number, you should read the following points.

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

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

                  For example: Oxidation state of Na in Na₂O  is +1.

2.    The oxidation state of group 2 elements is always +2. 

Mg, Ca, Ba etc will always have +2 Oxidation state.

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

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

For example: Fe and O₂ will have oxidation state equal to zero.

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

5.    Other halogens generally have -1 oxidation state.

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

In Interhalogen compounds like IF_7,  Iodine has +7 Oxidation state because 

Fluorine always have -1 oxidation state.

6.    Oxygen generally have -2 oxidation state in all oxides. 

For example: Oxidation state of O in CO₂  is -2.

In peroxides ,oxidation state of O is -1.

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

In superoxides, Oxidation state of O is -1/2. 

For example; oxidation state of O in NaO₂ ( Sodium superoxide) is -1/2.

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

has -1 oxidation state always.

7.    The oxidation state 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 +1x2 = +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 Oxidation state of Mn in KMnO₄  is to be calculated.

K= +1 because it is in group 1.

Mn=?

O=-2 x4= -8

+1+Mn -8= 0

Mn = +7

Worksheet:

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 SO₃^2-      

8.    C in CH₄      

9.    S in HSO₄^-    

10. N in NO₂^-