Question

Assertion (A): Pb compounds are stable in $+ 4$ oxidation state.
Reason (R): Pb exhibits inert pair effect.
A.Both Assertion and Reason are true and the Reason is a correct explanation of Assertion.
B.Both Reason and Assertion are true and Reason is not a correct explanation of Assertion.
C.Assertion is true but the Reason is false.
D.Assertion is false but the Reason is true.

Answer 1

To answer this question, knowledge of the group 14 is required along with the knowledge of the “inert pair effect”. Heavier elements have more tendency to show inert pair effect than lighter elements. Group 14 in the periodic table consists of elements with electronic configuration ${\text{n}}{{\text{s}}^2}{\text{n}}{{\text{p}}^2}$.

Complete step by step answer:
Inert pair effect can be defined as the tendency to the outermost s-subshell electrons to stay away from bonding in post-transition elements. This is mainly due to the high amount of energy required to unpair them for bond formation. This effect is mainly seen in the heavy elements of group 13, group 14, and group 15. So thallium, lead and bismuth are the elements that show this behaviour.
The inert pair effect is born from the poor shielding effect of the “d” and “f” orbital electrons. The diffuse structure of these orbitals is mainly responsible for the effect. Due to the poor shielding effect and the greater penetration of the “s” orbitals, the outer s-orbitals penetrate deeper into the atom, thereby they are more tightly bound to the nucleus and hence are not lost easily.
Therefore, thallium, lead and bismuth, instead to showing their common group valance $+ 3$ , $+ 4$ , and $+ 5$ , show $+ 1$ , $+ 2$ and $+ 3$ valence, respectively, due to the non-involvement of the “s”-electrons in bonding.
Therefore the assertion that “Pb compounds are stable in $+ 4$ oxidation state” is wrong, but the reason, “Pb exhibits inert pair effect” is right.

Hence, option D is the correct answer.

Note:
The shielding effect can be defined as the decrease in the effective nuclear charge on the electron cloud due to the difference in the attractive forces on the electrons in the atom. d and f orbitals show poor shielding as compared to s and p orbitals.