Trends in chemical properties and uses of group 13 elements

Group 13 is the first group of p-block elements and is termed as the Boron family. The electronic configuration of group 13 elements is ns2np1. Boron is a non-metal, while aluminium, gallium, indium, and thallium are metals. Boron is a rare element found as Borax, kernite, and orthoboric acid in nature. Aluminium is an abundant metal found in the form of Bauxite in the earth’s crust. 

Chemical properties of group 13 elements

Oxidation state and chemical reactivity

Boron atom is a small-sized atom, so it requires high three ionization enthalpies, therefore it cannot form cation (+3 ions), thus forming covalent compounds.

On moving down the group, the elements like Aluminium require less ionization enthalpy, thus forming Al3+ ion.

While going down the group, intervening d and f orbitals have poor shielding, thus increased effective nuclear charge which holds ns electrons tightly. So, they cannot form bonds easily, and only the p-orbital is involved in bonding.

Indium and Thallium have +1 and +3 oxidation states.

Compounds in the +1 oxidation state are more ionic than those in the +3 oxidation state.

The relative stability of +1 oxidation state: Al<Ga<In<Tl.

Electron deficient molecules of group 13 elements can accept a pair of electrons to achieve noble gas configuration, thus acting as Lewis acids. While going down the group, the tendency to form Lewis acid increases. It is a result of an increase in the size of the atom.

BCl3 accepts a lone pair of electrons from NH3 to form BCl3.NH3.  

To become stable, AlCl3 forms a dimer.

The trivalent state of the compounds is covalent, thus hydrolyzing in water. For example, Aluminium chloride forms octahedral [Al(H2O)6]3- in acidified aqueous solution having SP3d2 on Al.

Reactivity towards air

In its crystalline form, Boron is unreactive, while Aluminium forms a thin layer of oxide on its surface to prevent the further attack of oxygen on it. 

On heating in air, amorphous forms of Boron and Aluminium form B2O3 and Al2O3 and in reaction with dinitrogen they form Nitrides.

2E (s) + 3O2 ⎯⎯→ 2E2O3 (s)

2E (s) + N2 (g) ⎯⎯→ 2EN (s) 

[Where E = Element]

Nature of oxide: 

Boron trioxide = acidic

                          Aluminium and gallium oxide = amphoteric

                         Indium and thallium oxide = basic

Reactivity towards acids and alkalis

• Boron doesn’t react with acids and alkalis at moderate temperatures.

• Aluminium is amphoteric, hence dissolves in mineral acids and liberates hydrogen.

2Al(s) + 6HCl (aq) → 2Al3+ (aq) + 6Cl– (aq) + 3H2 (g)

• Aluminium also reacts with alkali to form Sodium tetrahydroxoaluminate(III)

and liberate hydrogen gas.

2Al (s) + 2NaOH(aq) + 6H2O(l) → 2 Na+ [Al(OH)4] – (aq) + 3H2(g) 

• Concentrated HNO3 renders aluminium and gallium passive due to the formation of a protective layer of oxide on its surface.

Reactivity towards halogens

Group 13 elements form trihalides on reaction with halogens (except TlI3).

2E(s) + 3 X2 (g) → 2EX3 (s) 

[where X = F, Cl, Br, I]

Some trends and anomalous behavior of group 13 elements:

• Tri-chlorides, Bromides, and iodides of group 13 elements hydrolyse in water due to covalent nature. Tetrahedral [M(OH)4]‑ and octahedral [M(H2O)6]3+ compounds exist in an aqueous medium (except Boron).
• Monomeric trihalides are strong Lewis acids.

Boron trifluoride reacts with Lewis bases like NH3.