`color{green}(★)` 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.

`color{green}(★)` But as we move from `color{red}(B)` to `color{red}(Al)`, the sum of the first three ionisation enthalpies of `color{red}(Al)` considerably decreases, and is therefore able to form `color{red}(Al^(3+))` ions.

`color{green}(★)` In fact, aluminium is a highly electropositive metal.


`color{green}(★)` However, down the group, due to poor shielding effect of intervening `color{red}(d)` and `color{red}(f)` orbitals, the increased effective nuclear charge holds `color{red}(ns)` electrons tightly (responsible for inert pair effect) and thereby, restricting their participation in bonding. As a result of this, only `color{red}(p)`-orbital electron may be involved in bonding. In fact in `color{red}(Ga, In)` and `color{red}(Tl)`, both `+1` and `+3` oxidation states are observed.

`color{green}(★)` The relative stability of `+1` oxidation state progressively increases for heavier elements: `color{red}(Al < Ga < In < Tl )`. In thallium `+1` oxidation state is predominant whereas the `+3` oxidation state is highly oxidising in character.

`color{green}(★)` The compounds in `+1` oxidation state, as expected from energy considerations, are more ionic than those in `+3` oxidation state.

`color{green}(★)` In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements (e.g., boron in `color{red}(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.

`color{green}(★)` The tendency to behave as Lewis acid decreases with the increase in the size down the group. `color{red}(BCl_3)` easily accepts a lone pair of electrons from ammonia to form `color{red}(BCl_3⋅NH_3).`


`color{green}(★)` In trivalent state most of the compounds being covalent are hydrolysed in water. For example, the trichlorides on hyrolysis in water form tetrahedral `color{red}([M(OH)_4]^(-))` species; the hybridisation state of element `color{red}(M)` is `color{red}(sp^3)`. Aluminium chloride in acidified aqueous solution forms octahedral `color{red}([Al(H_2O)_6]^(3+))` ion. In this complex ion, the `color{red}(3d)` orbitals of `color{red}(Al)` are involved and the hybridisation state of `color{red}(Al)` is `color{red}(sp^3d^2).`

`color{green}(★)` Boron is unreactive in crystalline form.

`color{green}(★)` Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack.

`color{green}(★)` Amorphous boron and aluminium metal on heating in air form `color{red}(B_2O_3)` and `color{red}(Al_2O_3)` respectively. With dinitrogen at high temperature they form nitrides.

`color{red}(2E (s) + 3O_2 (g) overset(Delta)→ 2E_2O_3 (s))`


`color{red}(2E(s) +N_2 (g) overset(Delta)→ 2EN (s))` (E = element)

`color{green}(★)` The nature of these oxides varies down the group.

`color{green}(★)` Boron trioxide is acidic and reacts with basic (metallic) oxides forming metal borates


`color{green}(★)` Aluminium and gallium oxides are amphoteric and those of indium and thallium are basic in their properties.

`color{green}(★)` Boron does not react with acids and alkalies even at moderate temperature; but aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.

`color{green}(★)` Aluminium dissolves in dilute `color{red}(HCl)` and liberates dihydrogen.

`color{red}(2Al (s) + 6HCl (aq) →2Al^(3+) (aq) + 6 Cl^(-) (aq) + 3H_2(g))`

`color{green}(★)` However, concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.

`color{green}(★)` Aluminium also reacts with aqueous alkali and liberates dihydrogen.

`color{green}(★)` These elements react with halogens to form trihalides (except `color{red}(Tl I_3)`).

`color{red}(2E(s) +3X_2 (g) →2EX_3(s) \ \ \ \ \ \ \ (X = F , Cl , Br , I))`