Chemical Properties

Chemical Properties of Group 13

Oxidation states and the inert-pair effect. Boron is so small that the sum of its first three ionisation enthalpies is enormous — far too costly for boron to simply lose three electrons and form a bare $B^{3+}$ ion. Boron therefore forms only covalent compounds. Moving to aluminium, that combined ionisation cost drops sharply, so aluminium readily loses three electrons to give $Al^{3+}$ and behaves as a strongly electropositive metal.

Further down the group, though, the poorly-shielding d and f electrons in Ga, In and Tl hold the outer $ns^2$ electron pair unusually tightly against the nucleus — this reluctance of the $ns^2$ pair to take part in bonding is called the inert-pair effect. It leaves mainly the single $np$ electron free to bond, so Ga, In and Tl show BOTH +1 and +3 states, and the +1 state becomes progressively more stable in the order:

For thallium, +1 is actually the more stable state, while $Tl^{3+}$ is a strong oxidising agent. As a general rule, +1-state compounds are more ionic than the corresponding +3-state compounds.

Electron deficiency and Lewis acidity. In the +3 state, these elements form compounds (e.g. $BF_3$) with only six electrons around the central atom — an incomplete octet. Such "electron-deficient" species readily accept a lone pair from a donor to complete their octet, i.e. they act as Lewis acids. Boron trichloride is the classic example: it accepts the lone pair on ammonia's nitrogen to form an adduct, going from planar $BCl_3$ to a tetrahedral adduct:

Because boron has no d orbitals available, its covalence is capped at 4 — it can never exceed four bonds. Aluminium and the heavier members DO have accessible d orbitals, so their covalence can expand past 4. This is why most other Group 13 trihalides dimerise: $AlCl_3$ exists as $Al_2Cl_6$, with each aluminium centre becoming tetrahedral by accepting a lone pair from a bridging chlorine on the neighbouring molecule.

Hydrolysis. Being covalent, most +3 compounds hydrolyse in water. The trichlorides give tetrahedral $[M(OH)_4]^-$ (M is $sp^3$-hybridised). Aluminium chloride in acidified water instead forms the octahedral complex $[Al(H_2O)_6]^{3+}$, in which aluminium's 3d orbitals participate and the hybridisation is $sp^3d^2$.

Reactivity toward air. Boron in its crystalline form is unreactive. Aluminium develops a thin, tightly-adherent oxide film that protects the metal underneath. On strong heating in air, both form the trioxide, and with nitrogen they form nitrides:

The acid–base character of these oxides changes down the group: $B_2O_3$ is acidic (reacts with basic oxides to form borates); $Al_2O_3$ and $Ga_2O_3$ are amphoteric; $In_2O_3$ and $Tl_2O_3$ are basic.

Reactivity toward acids and alkalis. Boron resists both acids and alkalis even on heating. Aluminium is amphoteric — it dissolves in dilute HCl liberating hydrogen: …