What is inert pair effect?

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Hint: Inert pair takes into account the poor screening/shielding effect of ‘d’ and ‘f’ orbital electrons. This effect is easily observed in p- block elements (group 4 and group 5 elements) where the stability of lower oxidation state increases down the group.

Complete Step by Step Solution:
Inert pair effect is the reluctance of ‘s’ electrons to take part in bonding. This is due to the poor screening effect of ‘d’ and ‘f’ orbitals.
The inert pair theory was proposed by Sidgwick.
Sidwick along with Powell accounted for the shapes of several molecules and correlated the shapes with some of their physical properties.
Now, we will look at some examples of inert pair effects.
The inert pair effect can be easily observed among group 4 and group 5 elements.
In group 4 , the electronic configuration is ${ ns }^{ 2 }{ np }^{ 2 }$. Here, we can see that the group oxidation state is +3 and the other oxidation state is +1.
The oxidation state is two units less than the group oxidation state became more stable for heavier elements in each group and down the group the stability of lower oxidation state increases, this is due to inert pair effect.
Because of this in group 4, on moving down the group only ‘p’ electrons are involved in bonding.
In the case of Ga, Tl and In , they can show both +1 and +3 oxidation state. But the stability of +1 oxidation state increases down the group.
In Tl, +1 oxidation state is more predominant whereas +3 oxidation state is highly oxidising in nature.

Additional Information:
Shielding effect is a different concept when compared with inert pair effect.
Shielding effect is basically , the outermost electrons will have an attractive pull from the nucleus and at the same time, they will be experiencing a repulsive force from the inner electrons. And thereby it results in a decrease in the effective nuclear charge.
We can have look at the equation of effective nuclear charge:
${ Z }^{ \ast }={ Z }-\sigma $
Where ${ Z }^{ \ast }$ is the effective nuclear charge
Z is the number of protons.
$\sigma $ is the screening constant or shielding constant.

The difference between shielding effect and inert pair effect is given below:

Inert pair EffectShielding effect
The effect explains the reluctance of s-electrons in the valence shell to participate in the bonding.This effect decreases the effective nuclear charge on the outer shell electrons , this is due to the repulsion between the inner electrons and the outer electrons.
The inert pair effect helps in understanding the stability of a particular oxidation state for a particular element.Shielding effect explains the ease of removal of valence electrons from an atom.

Note: Screening effect is additionally referred to as shielding effect. This effect occurs when there is an imbalance between charges. The outermost electrons experience an attractive pull from the nucleus but at the same time, they experience a repulsion from the inner electrons as well, and thus forms the effective nuclear charge.