Nearly every compound of silicon has the element in the $\mathbf{+}\mathbf{4}$oxidation state. In contrast, most compounds of lead have the element in the $\mathbf{+}\mathbf{2}$state.

(a) What general observation do these facts illustrate?

(b) Explain in terms of atomic and molecular properties.

(c) Give an analogous example from Group$\mathbf{3}\mathit{A}\mathbf{\left(}\mathbf{13}\mathbf{\right)}$.

Down the group, the atomic size is increased. Silicon lies above lead. So lead has larger size than silicon.

When the size is increased, bonding capacity is decreased. Moreover, when bonds are formed, the energy gained is not sufficient to balance the additional ionization energy.

Both Silicon $\left(\text{Si}\right)$and lead $\left(\text{Pb}\right)$are Group $\text{4A}\left(14\right)$element. Lead is down to the silicon in the group.

In general, the lower oxidation state of an element becomes more stable down the group.

So, the oxidation state of the elements in a group decreases down the group. Hence, $+4$oxidation state for silicon and $+2$oxidation state for lead are common.

Element with low ionization energy and low electronegativity show higher oxidation states. The ionization energy and electronegativity of an atom decrease as electronegativity decrease moving down the group.

In the case Group $\text{4A}\left(14\right)$, the atomic size increases but ionization energy and electronegativity values do not decreases moving down the group because of the interference of transition and inner transition elements. So, the ionization energy of the lead is larger than the ionization energy of silicon.

Hence, silicon shows higher oxidation state in its compounds whereas lead shows lower state in most of its compounds.

Analogous example from Group $\text{3A}\left(\text{13}\right)$are $\text{Ga};\text{ln;Tl}$. $\text{Ga}$shows $+3$oxidation states while $\text{Tl}$shows $+1$stable oxidation state.

Hence, ${\text{TlCl}}_{\text{3}}$is so unstable that it decomposes readily to $\text{TICl}$and ${\text{Cl}}_{\text{2}}$gas.