Boron ( ) is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent nonmetallic element which occurs abundantly in the evaporite ores borax and ulexite. Boron is never found as a free element on Earth.
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Boron ( ) is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent nonmetallic element which occurs abundantly in the evaporite ores borax and ulexite. Boron is never found as a free element on Earth.
Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, hard (9.3 on Mohs' scale), and a weak conductor at room temperature (22-28 °C, 72-82 °F). Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, nontoxic insecticides and preservatives, and reagents for chemical synthesis.
Boron is an essential plant nutrient, although higher soil concentrations of boron may also be toxic to plants. As an ultratrace element, boron is necessary for the optimal health of rats and presumably other mammals, though its physiological role in animals is poorly understood.
Characteristics
Brown amorphous boron is a product of certain chemical reactions. It contains boron atoms randomly bonded to each other without long range order.
Crystalline boron, a very hard, black material with a high melting point, exists in many polymorphs. Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the three most characterised crystalline forms.
Optical characteristics of crystalline/elemental boron include the transmittance of infrared light. At standard temperatures, elemental boron is a poor electrical conductor, but is a good electrical conductor at high temperatures.
Chemically boron is electron-deficient, possessing a vacant p-orbital. It is an electrophile. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances to compensate for boron's electron deficiency. The reactions of boron are dominated by such requirement for electrons. Also, boron is the least electronegative non-metal, meaning that it is usually oxidized (loses electrons) in reactions.
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks. Boron is also used for heat resistant alloys. Boron can form compounds whose formal oxidation state is not three, such as B(II) in B2F4.
History
Compounds of boron (Arabic Buraq from Persian Burah from Turkish Bor) have been known of for thousands of years. In early Egypt, mummification depended upon an ore known as natron, which contained borates as well as some other common salts. Borax glazes were used in China from CE 300, and boron compounds were used in glassmaking in ancient Rome.
The sources for borax and boric acid were at first dry lakes in Tibet and from 1818 on the geysers in the tuscan village Larderello Italy. During the 19th century the deposits in south America, California and Turkey were discovered.
