Hydrides

(i) Elements of group 13 do not react directly with hydrogen but a number of polymeric hydrides are known to exist.

(ii) Boron forms a large no. of volatile covalent hydrides, known as boranes e.g. B₂H₆,B₄H₁₀,B₅H₁₁,B₆H₁₀ Two series of borones with general formula B_nH_n+4 and B_nH_n+6 are more important.

(iii) Boranes are electron deficient compounds. It is important to note that although BX₃ are well known, BH₃ is not known. This is due of the fact that hydrogen atoms in BH₃ have no free electrons to form pπ–pπ back bonding and thus boron has incomplete octet and hence BH₃ molecules dimerise to form B₆H₆ having covalent and three centre bonds.

(iv) Al forms only one polymeric hydride (AlH₃)_n commonly known as alane It contains Al…..H……Al bridges.

(v) Al and Ga forms anionic hydrides e.g. LiAlH₄ and LiGa H₄,

Oxides

(i) The members of boron family form oxide and hydroxides of the general formula M₂O₃ and M (OH)₃ respectively.

(ii) The acidic nature of oxides and hydroxides changes from acidic to basic through amphoteric from B to Tl.

B₂O₃ and B(OH)₃> Al₂O₃ and Al(OH)₃ >

(acidic) (amphoteric)

Ga₂O₃ and Ga(OH)₃> In₂O₃ In (OH)₃> Tl₂O₃ Tl(OH)₃

(amphoteric) (basic) (strong basic)

B(OH)₃ or H₃BO₃ is weak monobasic Lewis acid.

(iii) Boric acid, B(OH)₃ is soluble in water as it accepts lone pair of electron to act as Lewis acid. Rest all hydroxides of group 13 are insoluble in water and form a gelatinous precipitate.

B(OH)₃ + H₂O → B(OH)₄^1–+H⁺

(iv) Al₂O₃ being amphoteric dissolves in acid and alkalies both.

Al₂O₃ + 3H₂SO₄ → Al₂ (SO₄)₃ + 3H₂O

(v) One of the crystalline form of alumina (Al₂O₃) is called corrundum. It is very hard and used as abrasive. It is prepared by heating amorphous form of Al₂O₃ to 2000 K.