The Elements of Group IIIA - Group 3 elements in Periodic Table

 The Elements of Group IIIA

Electronic Configuration of Group IIIA Elements

Element	symbol	electron configuration
Boron	B	[He]2s2 2p1
Aluminium	Al	[Ne]3s22p1
Gallium	Ga	[Ar]4s2 2p1
Indium	In	[Kr]5s2 2p1
Thalium	Tl	[Xe]6s2 2p1

§  In passing from boron to thallium we encounter a change from semi metallic to metallic properties, from acidic to amphoteric to basic oxides, and from halides in which the bonding is distinctly covalent to halides in which bonding is nearly ionic.

    Abundance:

§  The principal natural source of boron is the deposits of borax, Na₂B₄O.10H₂O, and recovery of the pure element from this compound is difficult.

 

§  On method that is used is the conversion of borax to the oxide B₂O₃, by reduction with magnesium.

 

§  Aluminum is the most abundant metal in the earth’s crust and is recovered pure and in quantity from its oxide by electrolytic reduction.

 

§  In contrast, gallium indium, and thallium are quite rare.

Properties of group IIIA

The trends in hardness, boiling temperature, and H_f are higher than those in group A and IIA.

Boron and Boron Compounds:

§  Although boron is formally in the +3 oxidation state in its oxide and in halides, there no chemistry associated with a free B⁺³ ion.

 

§  The energy required to remove three electrons from the boron atom is very large; moreover, the ion once formed would be extremely small and has large polarizing power. So, boron form covalent compounds (tend to share electrons)

Boron Halides:

Boron Halides conduct electricity, and their boiling points are all very low compared with those of the halides of the group IA and IIA.

The bond in Boron-Halide is covalent bond, and the boiling points of the boron halides increase as the atomic number of the halogen increases.

All the boron halides act as electron acceptors, as for example.

 In these reactions BF₃ accepts a pair of electrons donated by NH₃ or F^-. Thus, BF₃ and the other boron halides are Lewis acids.

Boron Oxides:

The oxide of boron is acidic. Boron trioxide, B₂O₃, when hydrated forms boric acid, B(OH)₃.Despite its formula, boric acid is a monobasic acid and is quite weak.

 

In this reaction boron accept electrons, and act as Lewis acid and accepts electrons from OH^-. In this case the hybridization of  B is converted from Sp² to Sp³.

When boric acid is heated, it loses water and gives metaboric acid and further heating produce boron oxide.

[B(OH)₄]^-  and H₃BO₃  act as strong monobasic acid with dihydroxy organic compounds such as glycerol or glycol as follow.

Boranes:

Boron forms a series of volatile compounds with hydrogen called boranes.

Diborane (B₂H₆ ) is the simplest of these hydrides and can be prepared by reacting lithium hydride with boron trifluoride:

When diborane is heated to temperatures between 100°C and 250^oC, it converts to a number of other boranes.

The Structure of Boranes (Diboranes)

The molecular structure and bonding in the borane series is shown in the figure. The boron atoms and four of the six hydrogen atoms lie in the same plane, with the remaining two hydrogens occupying “bridge” positions between the boron atoms. The B-H-B system is a three - center electron pair bond.

The two electrons in each bridge bond visit both boron atoms and the hydrogen nucleus. This type of bond called banana bond