Question

why does sn and pb show lower valency​

Answer 1

Explanation:

Because due to inert pair effect stable oxidation state of pb is +2, this means pb in +4 oxidation state wants to be reduced in stable oxidation state (+2) that's why pbo2 is a strong oxidizing agent. whereas in case of sn, stable oxidation state is +4 so it can't act as oxidizing agent.

Answer 2

That is the so-called Inert pair effect. In the p block, the post-transition metals (Sn and Pb as per your words, but also In, Tl, Bi) show such a characteristic behaviour due to increasing inertness (i.e. lack of chemical activity) on the ns pair of outermost electrons: the 5s pair of electrons in In, Sn (and Sb, too, although it is not a metal) and the 6s pair of electrons in Tl, Pb, Bi, and Po.

Basically, such pairs of electrons become less and less involved in chemical bonding as one goes down each p block Group that contains metals. The reasons why this happens are complex and can be ascribed to the fact that such metals of the 4th, 5th and 6th Period come after the d block, and d electrons do not screen efficiently the nuclear charges.

Such an effect worsens in Periods 5 and 6 because of the concomitant presence of the f electrons (whose shielding on their nuclei is even poorer). As a result, the ns electrons in the outermost shell remain more tightly held by the nucleus and therefore participate less in bond formation. Hence, the name inert pair.

If you work out the oxidation states of post-transition metals in the p block, you will easily see that they can indeed display values such as +1 or +3 (In, Tl), +2 or +4 (Sn, Pb), and +3 or +5 (Bi).

With the inert pair effect in mind, it is straightforward to understand that the inertness of the ns electrons increases as n increases. Therefore, in each pair of p block metals within the same Group, the lower-lying one will be more stable in its lower oxidation state; on the other hand, the higher-lying one will be more stable in its higher oxidation state. So,

Group 13: In(III) is more stable than In(I) and Tl(I) is more stable than Tl(III);

Group 14: Sn(IV) is more stable than Pb(IV) and Pb(II) is more stable than Sn(II);

Group 15: Sb(V) is more stable than Bi(V) and Bi(III) is more stable than Sb(V).

These are the reasons why, for example, Sn(II) compound show a conspicuous covalent character whereas Pb(IV) and Bi(V) compounds are very strong oxidisers.

The beauty of Chemistry!

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