Question

PbCl2 is stable but PbCl4 is unstable. Why?

Answer 1

This is due to inert pair effect.Pb has four electrons in its outermost shell,two are in s orbital & two  in the p orbital.Due to d-block contraction(or we can say,poor shielding effect of d & f orbital)the s orbital electrons are more strongly held than s electrons in upper periods in the same group.The s electrons are inert & are not that easily removed to give the group valency of 4.Therefore,Pb tends to form 2+ ion instead of expected 4+ ion.

Answer 2

Explanation:

Elements of d-block and f-block have poor shielding effect due to inert pair effect. As Pb (lead) has four valence electrons in its outermost shell as 2 electrons are held in s-orbital and other two are held in p-orbital.

So, due to contraction in d-block elements, electrons present in s-orbital are held more tightly in upper elements of the same group. Hence, s-orbital electrons become inert in nature and they are not easily removed.

As a result, Pb will attain a charge of +2 by losing only two electrons and it does not lose 4 electrons.

That is why, $PbCl_{2}$ is stable and $PbCl_{4}$ ias not stable enough.