Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Boron

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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 Thénard, 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 about 2180°C (about 3956°F), boils at about 3650°C (about 6602°F), 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, and with oxygen to form boron oxide. With metals it forms borides, such as magnesium boride. The original sources of boron compounds were the minerals borax and boric acid. Boron ranks about 38th in natural abundance among the elements in the 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, boric acid, and boron carbide. 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 (see Particle Detectors). 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. See Nuclear Energy.