According to this concept it is proposed that when the cations and Anions are at close 

vicinity the electron cloud of anion gets pulled towards cation.Thus deformation of electron

 clouds of the anion in the presence of attractive electrical field of the cation is referred to as

polarization of the anion. And  the cation is turned as polarizing agent.In the similar way,it 

can be argued that the electron cloud of the cation will get deformed by the anion but as the 

electron in the cation are much more lightly bound.Due to the excess positive charge, the 

polarization of cation is negligibly small. 


Due to such polarization some electron clouds are being transported towards the cation to 

produce overlapping zone which remains under the control of the nuclei of both cation and 

anion.  

             Thus the polarization introduces covalency in the bond. When there is no 

polarization, no over lapping zone exist and the bond is purely ionic.In this connection

it is worth remembering that if the degree of polarization exist a certain limit then a portion

of the electron cloud of the anion gave completely robbed by the cation. Under this

situation  oxidation no. of the cation decreases and reduction of cation occurs.

Factor's  governing the degree of Polarization(Pulling capacity by the cation)

1. Charge & size of the Cation: 

To measure the pulling intensity new parameter ionic potential =electrical charge/radius

has been considered with increasing  ionic potential the polarization power  of the cation

increases. It is evident that the polarizing power of third period element Na^+<Mg^2+

<Al^3+<Si^4+  

                        With increasing size of the cation polarizing power decreases because 

the distance between nucleus of cation and electron cloud of anion increases .So 

Li^+>Na^+>K^+>Rb^+

2. Size and Charge of the anion:

When size of the anion increases then the distance between nucleus and outer most electron

shell increases.Under this situation the electron cloud is not in the control  of nucleus and

pulling capacity by the nucleus of anion decreases the electron cloud of the anion attracted 

by the cation more strongly and covalent zone arises between cation & anion -covalent 

nature is generated in the compound.In case of I^- the electron cloud is present at a greater 

distance from the nucleus with respect to the outer most electron cloud of cl^- and 

covalency arises in iodide compound.

                    Charge of anion plays an important role on covalency with increasing charge 

of anion pulling capacity by the cation is greatly enhance. -Covalency increases in the 

molecule. For this reason the compound with -ve^-2 anion is more covalent with respect 

to mono -ve anion.

3.  Presence of d and f electron in the cation:

If  d and f electron is present in the cation the pulling capacity bt the cation nucleus 

increases highly because d and f electron has less shielding capacity.

The nucleus charge is not shielded strongly in case of transition metal so transition metal 

cation generates  greater covalency with in the molecule with respect to the cation of non 

transition element.