Boron

Boron is the first member of group 13 of the periodic table. It is the only non−metal of this group. Therefore, the chemistry of boron is quite different from rest of the members of this group. The electronic configuration of boron (Z = 5) is 1s²2s² and thus has three valence electrons.

Occurrence

Boron does not occur in the free state in nature. In the combined state also, its abundance in the earth’s crust is very low (0.001%). Boron mainly occurs as borates and orthoboric acid. Some important minerals of boron are

(i) Orthoboric acid, H₃BO₃

(ii) Borax, Na₂[B₄O₅(OH)₄].8H₂O or Na₂B₄O₇.10H₂O

(iii) Kernite, Na₂[B₄O₅(OH)₄]

(iv) Colemanite, Ca₂[B₃O₄(OH)₃]₂.2H₂O

or Ca₂B₆O₁₁.5H₂O

Isolation

Elemental boron is obtained by any one of the following methods.

(i) By reduction of boric oxide with highly electropositive metals like potassium and magnesium in absence of air.

B2O₃(s) + 6K(s) 2B(s) + 3K₂O(s)

B₂O₃(s) + 3Mg(s) 2B(s) + 3MgO(s)

The product thus obtained is boiled with hydrochloric acid and filtered when K₂O or MgO dissolves leaving behind elemental boron. The powdered boron thus obtained is thoroughly washed with water to free it from hydrochloric acid and is finally dried.

(ii) By reduction of volatile boron halides with dihydrogen at high temperature (1270 K)

2BCl₃(g) + 3H₂(g) 2B(s) + 6HCl(g)

2BBr₃(g) + 3H₂(g) 2B(s) + 6HBr(g)

(iii) By electrolytic reduction of fused borates or other compounds such as KBF₄ (potassium tetrafluoroborate) at 1000 K or above.

(iv) By thermal decomposition of boron tri−iodide over red hot tungsten filament (van Arkel method)

2BI₃(g) 2B(s) + 3I2(g)

(v) By thermal decomposition of boron hydrides

B2H₆(g) 2B(s) + 3H₂(g)

Diborane