​what is polarising power? how it varies in modern periodic table.explain with example?

Dear student,
Polarizing power can be defined as the property of a cation to attract the electron cloud of a nearby anion.

Polarizing power varies both in the period and also in the group.Polarizing power of a cation depends upon the size of the cation. As the size of the cation decreases, the effective nuclear charge on the ion increases which results in the increase in the power to attract the electron cloud.
As we go from left to right, across the period, the size of the atoms decreases. So, the size of the cations of the corresponding atom also decreases going from left to right. Due to this decrease in size, the polarizing power of the cations increases.
When we talk about period 3, the polarizing power increases in the following way:
$Na <Mg<Al<Si<P <S<Cl$ 

As we go down the group, the atomic size increases due to the addition of an extra shell. So, the size of the cation also increases down the group. Due to this increase in the size on going down the group, the polarizing power decreases.
If we talk about group 1 , the polarizing power decreases in the following fashion;
$Fr<Cs<Rb<K<Na<Li$

Regards