Question

Which is likely to show inert pair effect?
(A)- K
(B)- Mg
(C)- Al
(D)- Pb

Answer 1

Hint: Inert pair effect is observed in the elements of group 13 up to group 17. It can be defined as the inability of s electrons in heavier p-block elements to participate in bonding. Consequently, the stability of oxidation states two units less than the maximum oxidation of a group (i.e. group number-10) increases down the group.

Complete step by step solution:
Let us try to understand the inert pair effect in group 13. General electronic configuration for group 13 is $n{{s}^{2}}n{{p}^{1}}$ and the highest oxidation state shown by elements of this group is +3. Down the group number of shells increases and electrons are filled into d and f orbitals. Shapes of d and f orbitals are quite diffused and thus, they have very poor shielding effect. Due to the improper and poor shielding of outer electrons by inner d and f orbitals, effective nuclear charge experienced by outer electrons is very large. As a result, s electrons are not able to participate in bonding. Stable oxidation state of B, Al and Ga is +3 whereas In and Tl mostly exhibit +1 oxidation state.

Now let us see which one of the above is mostly likely to show inert pair effect:
-K and Mg are s-block elements and since only elements of p-block show this effect, this option cannot be correct.
-Inert pair effect is a result of poor shielding of d and f orbitals and filling of electrons in Al does not involve any d or f orbital. Thus, Al cannot be the correct answer.
-Pb (82) is heavier group 14 element and has electronic configuration $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{10}}4{{s}^{2}}4{{p}^{6}}4{{d}^{10}}4{{f}^{15}}5{{s}^{2}}5{{p}^{6}}5{{d}^{10}}6{{s}^{2}}6{{p}^{2}}$.
Clearly, the element which is most likely to show inert pair effect is Pb. Option (D) is the correct answer.

Additional Information:
In group 14, the most stable state shown by C, Si, Ge is +4 whereas compounds of Sn and Pb are most stable in +2 states. Reducing property of oxides decreases down the group due to inert pair effect.

Note: Do not get confused between Al and Pb. Both are p-block elements but the effect is more pronounced in heavier elements of p-block.