History and Preparation of Boron
– Boron was coined from borax, the mineral from which it was isolated, by analogy with carbon.
– Borax in its mineral form first saw use as a glaze in China circa 300 AD.
– Boron compounds were relatively rarely used until the late 1800s when they were produced in volume at low cost.
– Boron was not recognized as an element until it was isolated by Sir Humphry Davy and by Joseph Louis Gay-Lussac and Louis Jacques Thénard.
– Pure boron was first produced by the American chemist Ezekiel Weintraub in 1909.
– Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures.
– Ultrapure boron for use in the semiconductor industry is produced by the decomposition of diborane at high temperatures.
– The earliest routes to elemental boron involved the reduction of boric oxide with metals such as magnesium or aluminium.
– The product is almost always contaminated with borides of those metals.
– The production of boron compounds does not involve the formation of elemental boron, but exploits the convenient availability of borates.
Characteristics and Allotropes of Boron
– Boron is similar to carbon in its capability to form stable covalently bonded molecular networks.
– Crystalline boron is a very hard, black material with a melting point above 2000°C.
– It forms four major allotropes: α-rhombohedral, β-rhombohedral, γ-orthorhombic, and β-tetragonal.
– All four phases are stable at ambient conditions, with β-rhombohedral being the most common and stable.
– Borospherene (fullerene-like B molecules) and borophene (proposed graphene-like structure) were described in 2014.
– Even nominally disordered boron contains regular boron icosahedra which are bonded randomly to each other without long-range order.
– Crystalline boron forms four major allotropes: α-rhombohedral, β-rhombohedral, γ-orthorhombic, and β-tetragonal.
– The phases are based on boron icosahedra, with the γ phase having a rocksalt-type arrangement of the icosahedra and boron atomic pairs.
– The β-T phase is produced at higher temperatures compared to other phases.
– Compressing boron above 160GPa produces a boron phase with an unknown structure, which is a superconductor at low temperatures.
Chemistry and Compounds of Boron
– Elemental boron is rare and poorly soluble in water.
– Boron forms a wide range of compounds with various oxidation states.
– Boron compounds are used in many industries, including fiberglass production, polymers, ceramics, and semiconductors.
– Boron is also used in the production of borosilicate glass and as a bleach.
– Boron is an essential plant nutrient and has low toxicity in mammals but is more toxic to arthropods.
– Boron resembles silicon more than aluminum in terms of its chemical behavior.
– Crystalline boron is chemically inert and resistant to attack by certain acids.
– Finely divided boron is slowly attacked by hot concentrated hydrogen peroxide, nitric acid, sulfuric acid, or a mixture of sulfuric and chromic acids.
– The rate of oxidation of boron depends on factors like crystallinity, particle size, purity, and temperature.
– Boron burns to form boron trioxide at higher temperatures.
– Boron compounds with formal oxidation state III are the most familiar.
– Boron trihalides adopt a planar trigonal structure and readily form adducts with Lewis bases.
– Boron trifluoride is commonly used as a catalyst in the petrochemical industry.
– Boron compounds are found in nature as various oxides, sulfides, nitrides, and halides.
– Borate minerals contain boron in oxidation state +3 and often have a tetrahedral coordination with oxygen.
– Many organoboron compounds are known and useful in organic synthesis.
– Organoboron(III) compounds are usually tetrahedral or trigonal planar in structure.
– Multiple boron atoms can form novel dodecahedral and icosahedral structures.
– Organoboron chemicals have diverse applications, from boron carbide to boron neutron capture therapy for cancer.
– Carboranes, a type of organoboron compound, can be halogenated to form reactive structures.
– Metal borides contain boron in negative oxidation states.
– Magnesium diboride (MgB2) is a metal boride that is a high-temperature superconductor.
– Metal borides like calcium hexaboride (CaB6) are used as hard materials for cutting tools.
– Boron centers in metal borides are trigonal planar with extra double bonds, forming sheets similar to graphite.
– Delocalized electrons in magnesium diboride allow it to conduct electricity.
– Boron nitrides exhibit a variety of structures, including those analogous to various allotropes of carbon.
– Cubic boron nitride (c-BN) has a diamond-like structure and is used as an abrasive.
– Hexagonal boron nitride (h-BN) has a structure similar to graphite and is a lubricant.
– h-BN is a relatively poor electrical and thermal conductor in the planar directions.
– Boron nitrides are used in various applications, including high-temperature materials and lubricants.
Boron Isotopes and Applications
– Boron has two naturally occurring and stable isotopes: B-10 (80.1%) and B-11 (19.9%).
– There are 13 known isotopes of boron, with the shortest-lived isotope being B-7.
– Boron isotopes are fractionated during mineral Source: https://en.wikipedia.org/wiki/Boron
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride.
 boron (β-rhombohedral) | |||||||||||||||||||||
| Boron | |||||||||||||||||||||
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| Pronunciation | /ˈbɔːrɒn/ | ||||||||||||||||||||
| Allotropes | α-, β-rhombohedral, β-tetragonal (and more) | ||||||||||||||||||||
| Appearance | black-brown | ||||||||||||||||||||
| Standard atomic weightAr°(B) | |||||||||||||||||||||
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| Boron in the periodic table | |||||||||||||||||||||
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| Atomic number (Z) | 5 | ||||||||||||||||||||
| Group | group 13 (boron group) | ||||||||||||||||||||
| Period | period 2 | ||||||||||||||||||||
| Block | p-block | ||||||||||||||||||||
| Electron configuration | [He] 2s2 2p1 | ||||||||||||||||||||
| Electrons per shell | 2, 3 | ||||||||||||||||||||
| Physical properties | |||||||||||||||||||||
| Phaseat STP | solid | ||||||||||||||||||||
| Melting point | 2349 K (2076 °C, 3769 °F) | ||||||||||||||||||||
| Boiling point | 4200 K (3927 °C, 7101 °F) | ||||||||||||||||||||
| Density when liquid (at m.p.) | 2.08 g/cm3 | ||||||||||||||||||||
| Heat of fusion | 50.2 kJ/mol | ||||||||||||||||||||
| Heat of vaporization | 508 kJ/mol | ||||||||||||||||||||
| Molar heat capacity | 11.087 J/(mol·K) | ||||||||||||||||||||
Vapor pressure
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| Atomic properties | |||||||||||||||||||||
| Oxidation states | −5, −1, 0, +1, +2, +3 (a mildly acidic oxide) | ||||||||||||||||||||
| Electronegativity | Pauling scale: 2.04 | ||||||||||||||||||||
| Ionization energies |
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| Atomic radius | empirical: 90 pm | ||||||||||||||||||||
| Covalent radius | 84±3 pm | ||||||||||||||||||||
| Van der Waals radius | 192 pm | ||||||||||||||||||||
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| Other properties | |||||||||||||||||||||
| Natural occurrence | primordial | ||||||||||||||||||||
| Crystal structure | rhombohedral | ||||||||||||||||||||
| Speed of sound thin rod | 16,200 m/s (at 20 °C) | ||||||||||||||||||||
| Thermal expansion | β form: 5–7 µm/(m⋅K) (at 25 °C) | ||||||||||||||||||||
| Thermal conductivity | 27.4 W/(m⋅K) | ||||||||||||||||||||
| Electrical resistivity | ~106 Ω⋅m (at 20 °C) | ||||||||||||||||||||
| Magnetic ordering | diamagnetic | ||||||||||||||||||||
| Molar magnetic susceptibility | −6.7×10−6 cm3/mol | ||||||||||||||||||||
| Mohs hardness | ~9.5 | ||||||||||||||||||||
| CAS Number | 7440-42-8 | ||||||||||||||||||||
| History | |||||||||||||||||||||
| Discovery | Joseph Louis Gay-Lussac and Louis Jacques Thénard (30 June 1808) | ||||||||||||||||||||
| First isolation | Humphry Davy (9 July 1808) | ||||||||||||||||||||
| Isotopes of boron | |||||||||||||||||||||
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Boron is synthesized entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, so it is a low-abundance element in the Solar System and in the Earth's crust. It constitutes about 0.001 percent by weight of Earth's crust. It is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax and kernite. The largest known deposits are in Turkey, the largest producer of boron minerals.
Elemental boron is a metalloid that is found in small amounts in meteoroids, but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, the very pure element is produced with difficulty because of contamination by carbon or other elements that resist removal. Several allotropes exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature. The primary use of the element itself is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all production consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and heat-resistant materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. As sodium perborate, it is used as a bleach. A small amount is used as a dopant in semiconductors, and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which (boron-10) has a number of uses as a neutron-capturing agent.
The intersection of boron with biology is very small. Consensus on it as essential for mammalian life is lacking. Borates have low toxicity in mammals (similar to table salt) but are more toxic to arthropods and are occasionally used as insecticides. Boron-containing organic antibiotics are known. Although only traces are required, it is an essential plant nutrient.
