 # 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.

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 SO32-

8.    C in CH4

9.    S in HSO4

10. N in NO2

Find `Chemistry data booklet`