Give chemical properties of elements of group $-13$.

Due to small size of boron, the sum of its first three ionization enthalpies is very high. This prevents it to form $+3$ ions and forces it to form only covalent compounds.

But as we move from $\mathrm{B}$ to $\mathrm{Al}$, the sum of the first three ionisation enthalpies of $\mathrm{Al}$

considerably decreases and is therefore able to form $\mathrm{Al}^{3+}$ ions. In fact, aluminium is a highly

electro-positive metal.

However, down the group, due to poor shielding effect of intervening $d$ and $f$ orbitals, the

increased effective nuclear charge holds $n s$ electrons tightly (responsible for inert pair effect) and there by, restricting their participation in bonding.

As a result of this, only $p$-orbital electron may be involved in bonding. In fact in $Ga$, $In$ and $\mathrm{Tl}$, both $+1$ and $+3$ oxidation states are observed.

The relative stability of $+1$ oxidation state progressively increases for heavier elements : $Al < Ga < In < Tl.$

In thallium $+1$ oxidation state is predominant where as the $+3$ oxidation state is highly oxidising in character.

The compounds in $+1$ oxidation state, as expected from energy considerations, are more ionic than those in $+3$ oxidation state.

In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements (e.g., boron in $\mathrm{BF}_{3}$ ) will be only six. Such electron deficient

molecules have tendency to accept a pair of electrons to achieve stable electronic configuration and thus, behave as Lewis acids.

The tendency to behave as Lewis acid decreases with the increase in the size down the group.

$\mathrm{BCl}_{3}$ easily accepts a lone pair of electrons from ammonia to form $\mathrm{BCl}_{3} \cdot \mathrm{NH}_{3}$.