Due to inert pair effect, $\mathrm{Tl}$ is more stable in $+1$ oxidation state than that of $+3$ oxidation state. Therefore, $\mathrm{Tl}\left(\mathrm{NO}_{3}\right)_{3}$ acts as strong oxidizing agent.

Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called catenation. This is because $\mathrm{C}-\mathrm{C}$ bonds are very strong. Down the group the size increases and electronegativity decreases and there by, tendency to show catenation decreases. This can be clearly seen from bond enthalpies values. The order of catenation is $\mathrm{C} \gg \mathrm{Si}>\mathrm{Ge} \approx \mathrm{Sn}$. Lead does not show catenation.