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Chemical Elements

Boron

H3BO3, kernite, boron oxide, colemanite, ulexite

Boron, symbol B, hard, brittle semimetallic element with an atomic number of 5. Boron is in group 13 (or IIIa) of the periodic table.

Compounds of boron, notably borax, have been known since early times, but the pure element was first prepared in 1808 by the French chemists Joseph Gay-Lussac and Baron Louis Thenard, and independently by the British chemist Sir Humphry Davy. It is a trace element needed for plant growth, but toxic in excess. Research suggests that it is also nutritionally important for bone health in humans and other vertebrates.

Pure boron, as usually prepared, is an amorphous powder. A crystalline form can be prepared, however, by dissolving boron in molten aluminum and cooling slowly. The atomic weight of boron is 10.81; the element melts at 2075C (3767F), boils at 4000C (7232F), and has a specific gravity of 2.35.

Boron does not react with water or hydrochloric acid and is unaffected by air at ordinary temperatures. At red heat it combines directly with nitrogen to form boron nitride (BN), and with oxygen to form boron oxide (B2O3). With metals it forms borides, such as magnesium boride (Mg3B2). The original sources of boron compounds were the minerals borax and boric acid. More recently, important boron ores have been, in the United States, ulexite (NaCaB5O9 8H2O), colemanite (Ca2B6O11 5H2O), and kernite (Na2B4O7 4H2O); and in Germany, boracite (Mg7 Cl2B16O30). Boron ranks about 38th in natural abundance among the elements in Earth's crust.

Although boron has a valence of 3 and its position in the periodic table would indicate a close relationship to aluminum, it is actually much more like carbon and silicon in chemical properties. In its compounds, boron acts like a nonmetal, but unlike most nonmetals, pure boron is an electrical conductor, like the metals and like carbon (graphite). Crystalline boron is similar to diamond in appearance and optical properties, and is almost as hard as diamond. Most extraordinary in their anomalous similarity to the compounds of silicon and carbon are the boron hydrides. The boron compounds of industrial importance include borax (Na2 B4O7 10H20), boric acid (H3BO3), and boron carbide (B4C). Borax is used in cleaning compounds, glass and ceramics, fertilizers, paper and paints, and fire retardants. Boric acid is used medically for its astringent and antiseptic properties. Boron carbide is used as an abrasive and alloying agent.

Boron has several important applications in the field of atomic energy. It is used in instruments designed to detect and count slow neutrons. Because of its high absorption of neutrons, it is employed as a control absorber in nuclear reactors and as a constituent material of neutron shields.



Article key phrases:

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