Question

Select the correct order of polarizing power of cations when their sizes are almost the same.
(The question has multiple correct options)
A) $Na^+$ > $Cu^+$
B) $Cu^+$ > $Na^+$
C) $K^+$ < $Ag^+$
D) $Ag^+$ < $K^+$

Answer 1

Hint: Polarizing-power of a cation can be defined as its ability to attract the electrons of an anion and hence distort the electron cloud.

Complete answer:
Polarizing-power of a cation is its ability to attract the electrons of an anion. Generally, the anion is bigger in size. Hence, the outermost electrons of an anion are away from the nucleus. These electrons experience very less force of attraction from the nucleus of an anion. When a cation which is smaller in size comes near an anion, its nucleus begins to pull the electrons of the anion. Hence there is a shifting of the electrons of anion towards the cation. This is said to be distorted or polarized.
The polarizing power is given by the formula:
$\varphi = \dfrac{charge\; on\; cation}{size\; of\; cation}$
The polarizing capacity of a cation depends upon the following facts:
a) Size of cation- Smaller the size of cation larger its polarizing capacity. This is because its nucleus can apply more force of attraction towards the electrons of anion.
b) Charge on cation- Greater the charge on cation higher the value of polarizing power and it can apply more force of attraction towards the electrons of the nucleus.
c) Stable electronic configuration- If a cation has noble gas like stable electronic configuration or totally filled shells than it has more polarizing power.
Now, in the given question we know that $Na^+$ has the smallest size but it does not have the highest polarizing power here. This is due to the fact that both $Cu^+$ and $Ag^+$ has completely filled outer electronic configuration. We can see the electronic configuration of both the ions below:
$Cu^+$(29) = [Ar] $4s^1$,${3d^{10}}$
$Ag^+$(47) = [Kr] ${4d^{10}}$,$5s^1$
Therefore, Both $Cu^+$ and $Ag^+$ have the highest polarizing power here.

Thus the correct options are (B) and (C).

Note: The polarizing power of cation depends upon the size of cation, charge on cation and on the electronic configuration. Smaller cations have more polarizing power and if a cation has more charge on it, it is more polarizing.