Question

Which of the following is the least stable?

• A. $Pb{I}_4$
• B. $C{I}_4$
• C. $Ge{I}_4$
• D. $Sn{I}_4$

Answer 1

The correct option is A $Pb{I}_4$

$Pb{I}_4$ disproportionates easily: $Pb{I}_4\to Pb{I}_2+I_2$

It can be explained by inert pair effect.
The inert-pair effect is the tendency of the two electrons in the outermost atomic $s-$orbital to remain unshared in compounds of post-transition metals.
Group $14$ elements commonly exsists in $+4$ and $+2$ oxidation state. As $Pb$ will show inert pair effect, $Pb$ remains in $+2$ oxidation state rather than $+4$ oxidation state. So, $Pb{I}_2$ is more stable than $Pb{I}_4$.