Question

Correct order of polarizing power is:
A. $Cs^{ + }$ < $K^{ + }$ < $Mg^{ 2+ }$ < $Al^{ 3+ }$
B. $Al^{ 3+ }$ < $Mg^{ 2+ }$ < $K^{ + }$< $Cs^{ + }$
C. $Mg^{ 2+ }$ < $Al^{ 3+ }$ < $K^{ + }$< $Cs^{ + }$
D. $K^{ + }$ < $Cs^{ + }$ < $Mg^{ 2+ }$ < $Al^{ 3+ }$

Answer 1

Hint: We should know that the polarizing power of a cation is the ability to distort an anion. Polarising power is dependent on the charge of cation and the radius of the cation.
Now, try to find out the answer accordingly.

Complete step by step answer:
Let’s discuss the stepwise answer -
According to Fajan’s rule, “the higher the charge on the cation, the more is its polarizing power.” The next factor which is considered is the size of the cation, “smaller the size of the cation, higher is its polarizing power.”
Hence, the order of polarizing power is,

Polarising power $\alpha \quad \dfrac { Charge\quad on\quad atom }{ Radius\quad of\quad atom }$

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Now, between $K^{ + }$ and $Cs^{ + }$ that they have the same charge and in radius,
Cs+ > K+ . So, We can conclude K+ has more polarising power.
Now, $Mg^{ 2+ }$ and $Al^{ 3+ }$ have more charge than $Cs^{ + }$ and $K^{ + }$ and less radius. $Al^{ 3+ }$ has more charge than $Mg^{ 2+ }$. So, we can conclude that the polarising power order is $Al^{ 3+ }$ >$Mg^{ 2+ }$.

Hence, the overall order is,
$Cs^{ + }$ < $K^{ + }$ < $Mg^{ 2+ }$ < $Al^{ 3+ }$

Therefore, we can conclude that the final answer to this question is option A.

Note: We should also know that Polarizability represents the ability of an anion to become polarized. In order to do this, its electron cloud must be distorted. This implies that the easier it is for an anion's electron cloud to be distorted, the more polarizable that anions are.