Question

The charge/size ratio of a cation determines its polarising power. Which one of the following sequences represent the power. Which one of the following sequences represent the increasing order of the polarising power of the cationic species? ${K^2},C{a^{2 + }},M{g^{2 + }},B{e^{2 + }}$
(A) ${K^ + } < C{a^{2 + }} < M{g^{2 + }} < B{e^{2 + }}$
(B) $B{e^{2 + }} < {K^{1 + }} < C{a^{2 + }} < M{g^{2 + }}$
(C) $M{g^{2 + }} < B{e^{2 + }} < {K^{1 + }} < C{a^{2 + }}$
(D) $C{a^{2 + }} < M{g^{2 + }} < B{e^{2 + }} < {K^{1 + }}$

Answer 1

Hint : By knowing polarising power students can predict the nature of bond and for this they have to know Fajan’s rule. According to Fajan’s Rule, smaller the size of the cation and higher the charge on cation leads to more polarisation capacity of the cation.

Complete step by step solution:
The charge to size ratio determines the polarising power of cation.
According to Fajan’s rule, the power of cation to cause distortion in the electron cloud of negative ion (anion) is referred to as its Polarising Power and the ability of anion to undergo distortion is called its Polarisability.
Down the group, the size of an atom increases and the same applies to the cation. Also, along the period the size of atoms decreases generally.
Therefore, $B{e^{2 + }} < M{g^{2 + }} < C{a^{2 + }}$(Size of ion increases)
And size of ${K^ + }$is also greater than that of $C{a^{2 + }}$
Thus, the increasing order of the polarising power of the cationic species is ${K^ + } < C{a^{2 + }} < M{g^{2 + }} < B{e^{2 + }}$

Hence, the correct option is (A)

Additional information: We use Fajan’s Rule to determine the covalent character in ionic compounds as no bond is purely ionic or purely covalent.

Note: Polarising power is the ability of cation to attract the electron cloud towards itself. Polarising power is directly proportional to charge to size ratio of the cation and the charge to size ratio decreases down the group. Hence, the polarising power also decreases down the group and vice-versa.