Question

PbCl2 is more stable than PbCl4. Why?

Answer 1

This is due to inert pair effect. Pb has four electrons in its outermost shell, two are in s orbital and two in the p orbital. Due to d-block contraction{ or we can say, poor shielding effect of d and 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 and 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

due to inert pair effect Pb show it's valency as +2 so PbCl2 is more stable.