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boron (bōrˈŏn) [New Gr. from borax], chemical element; symbol B; at. no. 5; interval in which at. wt. ranges 10.806–10.821; m.p. about 2,300℃; sublimation point about 2,550℃; sp. gr. 2.3 at 25℃; valence +3. Boron is a nonmetallic element existing as a dark brown to black amorphous powder or as an extremely hard, usually jet-black to silver-gray, brittle, lustrous, metallike crystalline solid (see allotropy). One tetragonal and two rhombohedral forms of crystalline boron are known.
The chemistry of boron more closely resembles the chemistry of silicon than that of the other elements in Group 13 of the periodic table, of which it is a member. The chemical reactivity of boron depends on its form; generally, the crystalline form is far less reactive than the amorphous form. For example, the amorphous powder is oxidized slowly in air at room temperature and ignites spontaneously at high temperatures to form an oxide; the crystalline form is oxidized only very slowly, even at higher temperatures. Boron forms compounds with oxygen, hydrogen, the halogens, nitrogen, phosphorus, and carbon (only diamond is harder than boron carbide). It also forms organic compounds.
Boron is most commonly used in its compounds, especially borax and boric acid. Boron is used as a deoxidizer and degasifier in metallurgy. Because it absorbs neutrons, it is used in the shielding material and in some control rods of nuclear reactors. Boron fibers, which have a very high tensile strength, can be added to plastics to make a material that is stronger than steel yet lighter than aluminum.
Boron does not occur free in nature. Large deposits of borax, kermite, colemanite, and other boron minerals are found in the arid regions of the W United States. It occurs also in the mineral tourmaline. The simplest method of preparing boron is the reduction of boron trioxide by heating with magnesium; this yields the amorphous powder. Boron was first isolated in England in 1807 by Sir Humphry Davy and then in France in 1808 by Joseph Louis Gay-Lussac and Louis Jacques Thénard.
B, a chemical element in Group III of the Mendeleev periodic system. Atomic number, 5; atomic weight, 10.81. Grayish-black crystals. (Very pure boron is colorless.) Natural boron consists of two stable isotopes: 10B (19 percent) and 11B (81 percent).
Sodium tetraborate, a compound of boron that was known earlier than other compounds, was mentioned in the works of alchemists under the Arabic name bawraq and the Latin borax, from which the name “boron” was derived. Free (impure) boron was first obtained by the French chemists J. Gay-Lussac and L. Thénard in 1808 by heating boric anhydride, B2O3, with metallic potassium.
The general content of boron in the earth’s crust is 3 × 10-4 percent by weight. Free boron is not found in nature, but many boron compounds are widespread, especially in small concentrations. Boron is a component of many igneous and sedimentary rocks in the form of borosilicates, borates, and boron aluminom silicates, and also as an isomorphic admixture in other minerals. Boron compounds are found in petroleum waters, sea water, salt lakes, hot springs, volcanic and lava mud, and many soils.
Physical and chemical properties. Several crystalline modifications of boron are known. It has been possible by X-ray structural analysis to fully determine the crystalline structure for two of them; in both cases this structure is extremely complex. In these structures, boron atoms form a three-dimensional framework similar to the carbon atoms in diamond; this explains the great hardness of boron. However, the construction of the boron framework is much more complicated than that of diamond. The basic structural unit in boron crystals is formed by icosahedrons (20-sided figures), with 12 boron atoms in the apex of each one (see Figure 1, a). The icosahedrons are joined both directly (Figure 1, b) and by means of intermediate boron atoms that are not part ofanyicosahedron (Figure 1, c). In such a structure, the boron atoms in the crystals are found to have various coordination numbers: 4, 5, 6, and 5 + 2 (five close “neighbors” and two more distant ones). Since there are only three electrons in the outer shell of the boron atom (the electron configuration is 2s22p), there are essentially fewer than two electrons in each bond in crystalline boron. According to contemporary theories, there is a special type of covalent bond in boron crystals—a multicenter bond with an electron deficit. In ion-type compounds boron has a valence of 3. So-called amorphous boron, obtained by reducing B2O3 with metallic sodium or potassium, has a density of 1.73 g/cm3. Pure crystalline boron has a density of 2.3 g/mc3, a melting point of 2075° C, and a boiling point of 3860° C; its hardness on the mineralogical scale is 9, and its microhardness is 34 giganewtons per sq m (3,400 kg-force per sq mm). Crystalline boron is a semiconductor. Under ordinary conditions it is a poor conductor of electricity; however, when it is heated to 800° C, its electrical conductivity increases by several orders of magnitude, and the sign of conductivity changes (electron conductivity at low temperature, p-type conductivity at high temperatures).
Boron is chemically rather inert under ordinary conditions (it reacts actively only with fluorine), and crystalline boron is less active than the amorphous form. The activity of boron increases with an increase in temperature, and it combines with oxygen, sulfur, and the halogens. When heated in air to 700° C, boron burns with a reddish flame, forming boric anhydride, B2O3—a colorless, glasslike mass. When heated higher than 900° C with nitrogen, boron forms boron nitride, BN; with carbon, boron carbide, B4C; and with metals, it forms borides. Boron does not react noticeably with hydrogen; its hydrides (boranes) are obtained by an indirect method. When red-hot, boron reacts with water vapor: 2B + 3H20 = B2O3 + 3H2. At ordinary temperatures boron does not dissolve in acids, except in concentrated nitric acid, which oxidizes it to boric acid, H3BO3. Boron dissolves slowly in concentrated alkali solutions, forming borates.
In the fluoride, BF3, and in other halides, boron is bonded to the halogens by three covalent bonds. Since boron in the halide BX3 lacks a pair of electrons to complete the stable eight-electron shell of the atom, molecules of the halide— especially BF3—join molecules of other substances with free electron pairs—for example, ammonia:
In such complex compounds the boron atom is surrounded by four atoms (or groups of atoms), which corresponds to the coordination number 4 characteristic of boron and its compounds. Boric hydrides—for example, Na[BH,], and fluoroboric acid, H[BF4], which is formed from BF3 and HF—are important complex boron compounds. Most of the salts of fluoroboric acid (fluoroborates) are soluble in water (except the salts of K, Rb, and Cs).
A general peculiarity of boron and its compounds is their similarity to silicon and its compounds. Thus, boric acid, like silicic acid, has weak acidic properties and dissolves in HF to form gaseous BF3. (Silicic acid yields SiF4.) Boranes are similar to silanes, boron carbide is similar to silicon carbide, and so forth. The special similarity of the modifications of the nitride BN with graphite or diamond is of interest. This phenomenon is associated with the fact that atoms of B and N together imitate two atoms of C in electron configuration. (Boron has three valence electrons, nitrogen has five, and two atoms of carbon have four apiece.) This analogy is also charcteristic for other compounds containing boron and nitrogen simultaneously. Thus, borazane, BH3,—NH3, is similar to ethane, CH3—CH3; borazene, BH2=NH2, and the simplest borazine, BH≡NH, are similar to ethylene, CH2=CH2, and acetylene, CH≡CH, respectively. When the trimeriza-tion of acetylene, C2H2, yields benzene, C6H6, the analogous process yields borazol, B3N3H6, from borazine, BHNH.
Production and application Elementary boron is obtained from natural raw materials in several stages. The decomposition of borates by hot water or sulfuric acid (depending on their solubility) yields boric acid, whose dehydration yields boric anhydride. The reduction of B^ by metallic magnesium yields boron in the form of a dark-brown powder. It is cleansed of impurities by treatment with nitric and hydrofluoric acids. Extremely pure boron is indispensable for the production of semiconductors. It is obtained from its halides: BCl3 is reduced by hydrogen at 1200° C or BBr 3vapors are decomposed on a tantalum wire heated to 1500° C. Pure boron is also obtained by the thermal decomposition of boranes.
Boron in small quantities (fractions of a percent) is introduced into steel and some alloys in order to improve their mechanical properties; the addition of even 0.001–0.003 percent of boron to steel increases its strength. (Usually boron is added to steel in the form of ferroboron—that is, an alloy of iron with 10–20 percent boron.) The surface saturation of steel parts with boron (to a depth of 0.1–0.5 mm) improves not only the mechanical properties of steel but also its resistance to corrosion. Because of the ability of the isotope 10B to absorb thermal neutrons, it is used for making the control rods of nuclear reactors, which stop or slow nuclear fission. Boron in the form of gaseous BF3 is used in neutron counters. (When nuclei of I0B react with neutrons, charged alpha particles are formed which are easily recorded; the number of these alpha particles equals the number of neutrons entering the counter: ) Boron itself and its compounds—such as the nitride BN, the carbide B4C, and the phosphide BP—are used as dielectrics and semiconducting materials. Boric acid, its salts (especially borax), and the borides are also widely used. The compound BF3 is a catalyst for some organic reactions.
REFERENCESNekrasov, B. V. Osnovy obshchei khimii, vol. 2. Moscow, 1967.
Shchukarev, S. A. Letskii po kursu obshchei khimii, vol. 2. Leningrad, 1964.
Bor, ego soedineniia i splavy. Kiev, 1960.
The most important symptom of boron deficiency is the atrophy of the growth point of the main stem and then of the axillary buds. Simultaneously, graftings and leaves become brittle and flowers do not appear or fruits do not form; for this reason, the seed harvest drops when there is a boron deficiency. Many diseases are known to be linked to a boron deficiency, including heart rot of sugar beets, black spot of table beets, browning of rutabaga and cauliflower hearts, the drying-out of flax apexes, alfalfa top yellows, brown patch of apricots, and suberization of apples. When there is insufficient boron, the oxidation of sugars, amination of the products of hydrocarbon metabolism, and synthesis of cell proteins are retarded; however, the enzymes for which boron is essential are still unknown. According to the data of M. Ia. Shkol’nik, the content of adenosine triphosphate is decreased and the process of oxidizing phosphorylation is damaged in cases of boron deficiency in plants. As a result, the energy produced during respiration cannot be used for the synthesis of necessary substances. When boron is not present in sufficiently large quantities in the soil, boron fertilizers are applied. In biogeochemical provinces with a surplus of boron in the soil (for instance northwestern Kazakhstan), morphological changes and diseases are caused in plants by accumulations of boron; these pathological conditions include giantism, dwarfism, and destruction of the growth points. In soils with intensive boron salting there are sections without vegetation, or “bald patches,” which is one of the indications used in locating boron deposits.
The significance of boron in animal organisms is still unknown. The feeding of humans and animals (sheep, camels) with plants containing an excess of boron (60–600 mg/kg of dry weight and more) harms metabolism (especially the action of proteolithic enzymes) and produces endemic disease of the gastrointestinal tract, or boric enteritis.
REFERENCESSkok, J. “Funktsiia bora v rastitel’noi kletke.” In Mikroelementy. Moscow, 1962. (Translated from English.)
Koval’skii, V. V., A. V. Ananichev, and I. K. Shakhova. “Bornaia biogeokhimicheskaia provintsiia Severo-Zapadnogo Kazakhstana.” Agrokhimiia, 1965, no. 11.
V. V. KOVAL’SKII