Boron

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Boron (B) is the first member of group 13 of the periodic table and is the only non-metal of this group. It has an electronic configuration of 1s² 2s² 2p¹ with only three electrons in its valence shell.

Dissimilarities of boron with other elements of this group are:

• Boron is a non-metal while all other elements of this group are metals.

• Boron forms only covalent compounds, while other elements of this group form both covalent and ionic compounds.

• Boron shows a maximum covalency of four, while other elements of this group show a maximum valency of six.

• Boron forms electron-deficient compounds.

These dissimilarities are due to:

• Small atomic size

• High ionization energy

• Low electronegativity.

Boron occurs in the Earth's crust to an extent of about 0.001% mainly as borates and orthoboric acid. The chief minerals of boron are:

• Borax or tincal Na₂[B₄O₅(OH)₄.8H₂O

• Colemanite Ca₂[B₃O₄(OH)₃]₂.2H₂O

• Kernite Na₂[B₄O₅(OH)₄]

Minerals rich in borates are found in California (U.S.A.) and Turkey.

Preparation of Boron

From boric oxide

Boric oxide can be reduced to boron by highly electropositive metals.

From boron trichloride

By reducing volatile boron compound (boron trichloride) by dihydrogen at high temperature, boron is obtained.

Crystalline boron may be obtained from BCl₃, by heating it with zinc.

By electrolytic reduction of boron compounds

Boron can be obtained by the electrolysis of fused mixture containing boric anhydride (B₂O₃), magnesium oxide (MgO) and magnesium fluoride at 1100°C. The reactions taking place during electrolysis are,

By thermal decomposition of boron hydrides and halides

Boron hydrides / halides on heating decompose to give amorphous boron.

Physical properties of boron

• Boron is an extremely hard solid with a high melting point (2450 K), and boiling point (3970 K).

• It has low electrical and thermal conductivities.

• Two allotropic forms of boron are, amorphous boron (dark brown), and crystalline boron (black metallic luster).

• Boron occurs in two isotropic forms,

Chemical properties of boron

Boron is unreactive to most chemical reagents at ordinary temperature. It reacts only with strong oxidizing agents such as fluorine and concentrated nitric acid at room temperature. However, it combines with metals at very high temperatures to form borides, which are generally hard and high melting solids. Some reactions of boron are:

With oxygen

With nitrogen

With halogens

With acids

With alkalies

With metals

(except Cu, Ag, Au)

With steam

s

Uses of boron

• As a deoxidizer in the form of its salts such as calcium boride.

• As a moderator in nuclear reactors.

• As an abrasive and a refractory material.

Problem

1. Explain the following a) Boron is trivalent b) the action of boron on mineral acids.

Solution

(i) Boron's atomic number is 5. Its electronic configuration is 1s² 2s² 2p¹. Hence it can lose 3 electrons from the second orbit to form bonds, as the new configuration is more stable.

(ii) Boron reacts with strong oxidizing acids like concentrated sulphuric or concentrated nitric acid to give boric acid.

But boron shows no action with hydrochloric acid as it is less reactive and needs strong oxidizing atmosphere.