Question

The polarization power is maximum for which of the following ions?
(A) $M{{g}^{2+}}$
(B) ${{K}^{+}}$
(C) $C{{s}^{+}}$
(D) $A{{l}^{3+}}$

Answer 1

This question is a direct application of Fajan's rule which relates the polarization power and size or charge of a cation. Thus, by knowing which of the cations among given options satisfies the Fajan's rule we could determine the ion with maximum polarization power.

Complete step by step solution:
-Let's start with the concept of Fajan's rule. The rule is used to predict whether a chemical bond is ionic or covalent and also depends on the relative charges and sizes of anion and cation.
-It’s observed that when two oppositely charged ions are brought together, the nature of the bond between the oppositely charged ions depends upon the effect of one ion on each other. According to Fajan's rule, the charge of cation, the covalent character of the ionic bond, would be greater. Also, the covalent character of an ionic bond would be greater with a smaller size of cation and the larger size of the anion.
-The polarising power of a cation can be related to the nation's ability to attract outermost electrons from a nearby anion in an ionic compound. As we mentioned above, for cations with the same radius, the charge is higher and thus the polarizing power would be high. Since the positive charge is distributed on a comparatively smaller area, the smaller cations which have greater positive charges would have higher polarizing power.
-Among the given options $A{{l}^{3+}}$ ion would be the smallest ion because three electrons are lost from it and it would have higher effective nuclear charge compared to the other options. Therefore $A{{l}^{3+}}$ is the smallest cation and thus it would be having maximum polarization power.

Therefore, the answer is option (D)- $A{{l}^{3+}}$.

Note: Do not confuse polarization power with polarizability. As we know polarization power is the ability of a cation to distort an anion whereas the tendency of an anion to become polarized by the cation can be described as polarizability. It’s the ability to form instantaneous dipoles and it provides the insight into a molecule's internal structure.