Group 13 (Boron group) elements

Group 13 (Boron group) elements:

Occurrence:

The boron occurs mostly as borates and its important ores are borax - Na₂[B₄O₅(OH)₄].8H₂O and kernite - Na₂[B₄O₅(OH)₄].2H₂O.. Aluminium is the most abundant metal and occurs as oxides and also found in aluminosilicate rocks. Commercially it is extracted from its chief ore, bauxite (Al₂O₃.2H₂O). The other elements of this group occur only in trace amounts. The other elements Ga, In and Tl occur as their sulphides.

Physical properties:

Some of the physical properties of the group 13 elements are listed below

Table 2.3 Physical properties of group 13 elements

Chemical properties of boron:

Boron is the only nonmetal in this group and is less reactive. However, it shows reactivity at higher temperatures. Many of its compounds are electron deficient and has unusual type of covalent bonding which is due to its small size, high ionisation energy and similarity in electronegativity with carbon and hydrogen.

Formation of metal borides:

Many metals except alkali metals form borides with a general formula M_xB_y (x ranging upto 11 and y ranging upto 66 or higher)

Direct combination of metals with boron:

Reduction of borontrihalides:

Reduction of borontrichloride with a metal assisted by dihydrogen gives metal borides.

Formation of hydrides:

Boron does not react directly with hydrogen. However, it forms a variety of hydrides called boranes. The simplest borane is diborane - B₂H₆. Other larger boranes can be prepared from diborane. Treatment of gaseous boron trifluoride with sodium hydride around 450 K gives diborane. To prevent subsequent pyrolysis, the product diborane is trapped immediately.

Formation of boron trihalides:

Boron combines with halogen to form boron trihalides at high temperatures.

Formation of boron nitride:

Boron burns with dinitrogen at high temperatures to form boron nitride.

Formation of oxides:

When boron is heated with oxygen around 900 K, it forms its oxide.

Reaction with acids and alkali:

Halo acids have no reaction with boron. However, boron reacts with oxidising acids such as sulphuric acid and nitric acids and forms boric acid.

2B + 3H₂SO₄ → 2H₃BO₃ + 3SO₂

B + 3HNO₃  → H₃BO₃ + 3NO₂

Boron reacts with fused sodium hydroxide and forms sodium borate.

2B + 6NaOH  → 2Na₃BO₃ + 3H₂

Uses of boron:

·           Boron has the capacity to absorb neutrons. Hence, its isotope 10B₅ is used as moderator in nuclear reactors.

·           Amorphous boron is used as a rocket fuel igniter.

·           Boron is essential for the cell walls of plants.

·           Compounds of boron have many applications. For example eye drops, antiseptics, washing powders etc.. contains boric acid and borax. In the manufacture of Pyrex glass , boric oxide is used.