Anomalous Properties of Boron

Boron ,the first member of group 13 elements, shows anomalous behaviour and differ from rest of the members of its family. The main reason for this difference are :

1) exceptionally small atomic and ionic size.

2) high ionization enthalpy.

3)absence of d orbital in its valence shell.

Important properties in which boron differ from the rest of the members of its group are as follow:

1)Boron being small is harder than the other elements of its group.

2)It has higher melting and boiling point than those of the other members of its group.

3) Boron forms only covalent compounds while all other members form both ionic and covalent compounds.

4)The oxides of boron and its hydroxides are weakly acidic and dissolves in alkalis forming metaborates. The oxides and hydroxides of Al and Ga are amphoteric while those of In and Tl are basic.

B₂O₃ + 2 NaOH ——> 2 NaBO₂ + H₂O

B(OH)₃ + NaOH ——> 2 NaBO₂ + H₂O

Al(OH)₃ + NaOH ——> 2 NaAlO₂ + 2 H₂O

Al(OH)₃ + 3HCl ——> AlCl₃ + 3 H₂O

5) The trihalides of group 13 being covalent are hydrolysed by water. Whereas boron trihalides ,due to the absence of d-orbitals ,form tetrahedral species [B(OH)₄]‾ ,trihalides of aluminium and other elements ,due to the presence of d orbitals, form octahedral [Al(H₂O)₆]‾ species.

6) The trihalides of boron are monomeric. The reason being that due to its small size, it cannot accommodate 4 large sized halogens atoms around it. The monomeric trihalide being electron deficient, are strong Lewis acid and thus form complexes with ammonia ,alcohol ,ether.

The trihalides of aluminium and other elements of group 13 have halogen bridged dimeric structure in which the metal completes its obtained by accepting an electron pair from in halogen atom of the other molecules.