Boron: Wikis


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General properties
Name, symbol, number boron, B, 5
Element category metalloid
Group, period, block 132, p
Standard atomic weight 10.811(7)g·mol−1
Electron configuration [He] 2s2 2p1
Electrons per shell 2, 3 (Image)
Physical properties
Phase solid
Liquid density at m.p. 2.08 g·cm−3
Melting point 2349 K, 2076 °C, 3769 °F
Boiling point 4200 K, 3927 °C, 7101 °F
Heat of fusion 50.2 kJ·mol−1
Heat of vaporization 480 kJ·mol−1
Specific heat capacity (25 °C) 11.087 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 2348 2562 2822 3141 3545 4072
Atomic properties
Oxidation states 4,[1] 3, 2, 1[2]
(mildly acidic oxide)
Electronegativity 2.04 (Pauling scale)
Ionization energies
1st: 800.6 kJ·mol−1
2nd: 2427.1 kJ·mol−1
3rd: 3659.7 kJ·mol−1
Atomic radius 90 pm
Covalent radius 84±3 pm
Van der Waals radius 192 pm
Magnetic ordering diamagnetic[3]
Electrical resistivity (20 °C) ~106Ω·m
Thermal conductivity (300 K) 27.4 W·m−1·K−1
Thermal expansion (25 °C) (ß form) 5–7 [4] µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 16,200 m/s
Mohs hardness ~9.5
CAS registry number 7440-42-8
Most stable isotopes
Main article: Isotopes of boron
iso NA half-life DM DE (MeV) DP
10B 19.9(7)%* 10B is stable with 5 neutrons[5]
11B 80.1(7)%* 11B is stable with 6 neutrons[5]
*Boron-10 content may be as low as 19.1% and as
high as 20.3% in natural samples. Boron-11 is
the remainder in such cases.

Boron (pronounced /ˈbɔrɒn/) is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite.

Several allotropes of boron exist: amorphous boron is a brown powder; whereas crystalline boron is black, extremely hard (about 9.5 on Mohs' scale), and a poor conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, insecticides and preservatives, and reagents for chemical synthesis.

Boron is an essential plant nutrient. Whereas lack of boron results in boron deficiency disorder, high 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 not yet fully understood.




boron chunks

Boron is similar to carbon in its capability to form stable covalently bonded molecular networks. Even nominally disordered (amorphous) boron contains regular boron icosahedra which are, however, bonded randomly to each other without long-range order.[7][8] Crystalline boron is a very hard, black material with a high melting point of above 2000 °C. It exists in four major polymorphs: α, β, γ and T. Whereas α, β and T phases are based on B12 icosahedra, the γ-phase can be described as a rocksalt-type arrangement of the icosahedra and B2 atomic pairs.[9] It can be produced by compressing other boron phases to 12-20 GPa and heating to 1500-1800 °C; it remains stable after releasing the temperature and pressure. The T phase is produced at similar pressures, but higher temperatures of 1800-2200 °C. As to the α and β phases, they might both coexist at ambient conditions with the β phase being more stable.[9][10] Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures 6-12 K.[11]

Boron phase α β γ T
Symmetry Rhombohedral Rhombohedral Orthorhombic Tetragonal
Atoms/unit cell[9] 12 ~105 28
Density (g/cm3)[12][13][14][15] 2.46 2.35 2.52 2.36
Vickers hardness (GPa)[16][17] 42 45 50-58
Bulk modulus (GPa)[17][18] 185 224 227
Bandgap (eV)[17][19] 2 1.6 2.1


Chemically, boron is closer to silicon than to aluminium. Crystalline boron is chemically inert and resistant to attack by boiling hydrofluoric or hydrochloric acid. When finely divided, it is attacked slowly by hot concentrated hydrogen peroxide, hot concentrated nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids.[20][21]

Oxidation of boron depends upon the crystallinity, particle size, purity and temperature. Boron does not react with air at room temperature, but at higher temperatures it burns to form boron trioxide:

4 B + 3 O2 (g) → 2 B2O3 (s)

Boron reacts with sulfur to produce boron sulfide:

2 B + 3 S (g) → B2S3 (s)

The first synthesis was performed by Jöns Jakob Berzelius in 1824. Another reaction, starting from boron and hydrogen sulfide, was conducted by Friedrich Wöhler and Henri Etienne Sainte-Claire Deville and published in 1858.[22][23]

2 B + 3 H2S → B2S3 (s) + 3 H2

Wöhler and Deville also documented vigorous reactions between boron and the halogens resulting in boron trichloride, boron trifluoride and boron tribromide.[23] For example:

2 B + 3 Br2 → 2 BBr3

Boron can form compounds whose formal oxidation state is not three, such as B(IV) in boron carbide BC,[1] B(II) in B2F4,[24] and B(I) in boron fluoride BF.[2] Boron compounds such as BCl3 behave as electrophiles or Lewis acids in their reactions.[25] Boron is the least electronegative non-metal.[26]


Boron has two naturally occurring and stable isotopes, 11B (80.1%) and 10B (19.9%). The mass difference results in a wide range of δ11B values, which are defined as a fractional difference between the 11B and 10B and traditionally expressed in parts per thousand, in natural waters ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is 7B which decays through proton emission and alpha decay. It has a half-life of 3.5×10−22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)3 and B(OH)4. Boron isotopes are also fractionated during mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect results in preferential removal of the 10B(OH)4 ion onto clays. It results in solutions enriched in 11B(OH)3 and therefore may be responsible for the large 11B enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.[27] The exotic 17B exhibits a nuclear halo, i.e. its radius is appreciably larger than that predicted by the liquid drop model.[28]

Enriched boron (boron-10)

Neutron cross section of boron (top curve is for 10B and bottom curve for 11B)

The 10B isotope is good at capturing thermal neutrons. Natural boron is about 20% 10B and 80%11B. The nuclear industry enriches natural boron to nearly pure 10B. The waste product, or depleted boron, is nearly pure 11B. 11B is a candidate as a fuel for aneutronic fusion and is used in the semiconductor industry. Enriched boron or 10B is used in both radiation shielding and in boron neutron capture therapy. In the latter, a compound containing 10B is attached to a muscle near a tumor. The patient is then treated with a relatively low dose of thermal neutrons. This causes energetic and short range alpha radiation from the boron to bombard the tumor.[29][30][31]

In nuclear reactors, 10B is used for reactivity control and in emergency shutdown systems. It can serve either function in the form of borosilicate control rods or as boric acid. In pressurized water reactors, boric acid is added to the reactor coolant when the plant is shut down for refueling. It is then slowly filtered out over many months as fissile material is used up and the fuel becomes less reactive.[32]

In future manned interplanetary spacecraft, 10B has a theoretical role as structural material (as boron fibers or BN nanotube material) which would also serve a special role in the radiation shield. One of the difficulties in dealing with cosmic rays, which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft materials is high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in the shielding. Among light elements that absorb thermal neutrons, 6Li and 10B appear as potential spacecraft structural materials which serve both for mechanical reinforcement and radiation protection.[33]

Depleted boron (boron-11)

Cosmic radiation will produce secondary neutrons if it hits spacecraft structures; and neutrons cause fission in 10B if it is present in the spacecraft's semiconductors, producing a gamma ray, an alpha particle, and a lithium ion. The resultant fission products may then dump charge into nearby semiconductor 'chip' structures, causing data loss (bit flipping, or single event upset). In radiation hardened semiconductor designs, one countermeasure is to use depleted boron which is greatly enriched in 11B and contains almost no 10B. 11B is largely immune to radiation damage. Depleted boron is a by-product of the nuclear industry.[32]

11B is also a candidate as a fuel for aneutronic fusion. When struck by a proton with energy of about 500 keV, it produces three alpha particles and 8.7 MeV of energy. Most other fusion reactions involving hydrogen and helium produce penetrating neutron radiation, which weakens reactor structures and induces long term radioactivity thereby endangering operating personnel. Whereas, the alpha particles from 11B fusion can be turned directly into electric power, and all radiation stops as soon as the reactor is turned off.[34]

NMR spectroscopy

Both 10B and 11B possess nuclear spin. The nuclear spin of 10B is 3 and that of 11B is 3/2. These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nuclei are available commercially. The 10B and 11B nuclei also cause splitting in the resonances of attached nuclei.[35]


A fragment of ulexite
Borax crystals

Boron is a relatively rare element in the Earth's crust, representing only 0.001%. The worldwide commercial borate deposits are estimated as 10 million tonnes.[36][37] Turkey and the United States are the world's largest producers of boron.[38][39] Turkey has almost 72% of the world’s boron reserves.[40] Boron does not appear on Earth in elemental form but is found combined in borax, boric acid, colemanite, kernite, ulexite and borates. Boric acid is sometimes found in volcanic spring waters. Ulexite is a borate mineral; it is a fibrous crystal where individual fibers can guide light like optical fibers.[41]

Economically important sources of boron are rasorite (kernite) and tincal (borax ore). They are both found in the Mojave Desert of California, but the largest borax deposits are in Central and Western Turkey including the provinces of Eskişehir, Kütahya and Balıkesir.[42][43][44]

History and etymology


The name boron originates from the Arabic word buraq or the Persian word burah;[45] which are names for the mineral borax.[46]

Boron compounds were known thousands of years ago. Borax was known from the deserts of western Tibet, where it received the name of tincal, derived from the Sanskrit. Borax glazes were used in China from AD300, and some tincal even reached the West, where the Arabic alchemist Geber seems to mention it in 700. Marco Polo brought some glazes back to Italy in the 13th century. Agricola, around 1600, reports its use as a flux in metallurgy. In 1777, boric acid was recognized in the hot springs (soffioni) near Florence, Italy, and became known as sal sedativum, with mainly medical uses. The rare mineral is called sassolite, which is found at Sasso, Italy. This was the main source of European borax from 1827 to 1872, at which date American sources replaced it.[47][48]

Boron was not recognized as an element until it was isolated by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808 through the reaction of boric acid and potassium. Davy called the element boracium.[49] Jöns Jakob Berzelius identified boron as an element in 1824. The first pure boron was arguably produced by the American chemist W. Weintraub in 1909.[21][50]


Pure elemental boron is difficult to extract. The earliest methods involved reduction of boric oxide with metals such as magnesium or aluminium. However the product is almost always contaminated with metal borides. Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures. Ultrapure boron, for the use in semiconductor industry, is produced by the decomposition of diborane at high temperatures and then further purified with the zone melting or Czochralski processes.[51]

Isotope enrichment

Because of its high neutron cross-section, boron-10 is often used to control fission in nuclear reactors as a neutron-capturing substance.[52] Several industrial-scale enrichment processes have been developed, however only the fractionated vacuum distillation of the dimethyl ether adduct of boron trifluoride (DME-BF3) and column chromatography of borates are being used.[53]

Market trend

Estimated global consumption of boron rose to a record 1.8 million tonnes of B2O3 in 2005, following a period of strong growth in demand from Asia, Europe and North America. Boron mining and refining capacities are considered to be adequate to meet expected levels of growth through the next decade. The form in which boron is consumed has changed in recent years. The use of ores like colemanite has declined following concerns over arsenic content. Consumers have moved towards the use of refined borates and boric acid that have a lower pollutant content. The average cost of crystalline boron is $5/g.[54]

Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine Company of Turkey opened a new boric acid plant with the production capacity of 100,000 tonnes per year at Emet in 2003. Rio Tinto Group increased the capacity of its boron plant from 260,000 tonnes per year in 2003 to 310,000 tonnes per year by May 2005, with plans to grow this to 366,000 tonnes per year in 2006. Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. This has led to imports of disodium tetraborate growing by a hundredfold between 2000 and 2005 and boric acid imports increasing by 28% per year over the same period.[55][56]

The rise in global demand has been driven by high growth rates in fiberglass and borosilicate production. A rapid increase in the manufacture of reinforcement-grade fiberglass in Asia with a consequent increase in demand for borates has offset the development of boron-free reinforcement-grade fiberglass in Europe and the USA. The recent rises in energy prices may lead to greater use of insulation-grade fiberglass, with consequent growth in the boron consumption. Roskill Consulting Group forecasts that world demand for boron will grow by 3.4% per year to reach 21 million tonnes by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.[55][57]


Glass and ceramics

Borosilicate glassware. Displayed are two beakers and a test tube.

Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate. In the United States, 70% of the boron is used for the production of glass and ceramics.[58] Borosilicate glass, which is typically 12%-15% B2O3, 80% SiO2, and 2% Al2O3, has a low coefficient of thermal expansion giving it a good resistance to thermal shock. Duran and Pyrex are two major brand names for this glass.[59]

Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures as a component of composite materials, as well as limited production consumer and sporting goods such as golf clubs and fishing rods.[60][61] The fibers can be produced by chemical vapor deposition of boron on a tungsten filament.[38][62]

Boron fibers and sub-millimeter sized crystalline boron springs are produced by laser-assisted chemical vapor deposition. Translation of the focused laser beam allows to produce even complex helical structures. Such structures show good mechanical properties (elastic modulus 450 GPa, fracture strain 3.7 %, fracture stress 17 GPa) and can be applied as reinforcement of ceramics or in micromechanical systems.[63]

Semiconductor industry

Boron is an important technological dopant for such important semiconductors as silicon, germanium and silicon carbide. Having one less valence electron than the host atom, it donates a hole resulting in p-type conductivity. Traditional method of introducing boron into semiconductors is via its atomic diffusion at high temperatures. This process uses either solid (B2O3), liquid (BBr3) or gaseous boron sources (B2H6 or BF3). However, after 1970s, it was mostly replaced by ion implantation, which relies mostly on BF3 as a boron source.[64] Boron trichloride gas is also an important chemical in semiconductor industry, however not for doping but rather for plasma etching of metals and their oxides.[65]

Engineering materials

Boron carbide is used for inner plates of ballistic vests

Boron carbide, a ceramic material which is obtained by decomposing B2O3 with carbon in the electric furnace:

2 B2O3 + 7 C → B4C + 6 CO

It is used in tank armor, bulletproof vests, and numerous other structural applications. Its ability to absorb neutrons without forming long lived radionuclides makes the material attractive as an absorbent for neutron radiation arising in nuclear power plants. Nuclear applications of boron carbide include shielding, control rod and shut down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.[66]

Magnesium diboride is an important superconducting material with the transition temperature of 39 K. MgB2 wires are produced with the powder-in-tube process and applied in superconducting magnets.[67][68]

Boron is a part of neodymium magnet (Nd2Fe14B), which is the strongest type of permanent magnet. It is found in all kinds of domestic and professional electromechanical and electronic devices, such as magnetic resonance imaging (MRI), various motors and actuators, computer HDDs, CD and DVD players, mobile phones, timer switches, speakers, etc.[3]

High-hardness compounds

Mechanical properties of BCN solids [69] and ReB2[70]
Material Diamond cubic-BC2N cubic-BC5 cubic-BN B4C ReB2
Vickers hardness (GPa) 115 76 71 62 38 22
Fracture toughness (MPa m1/2) 5.3 4.5 9.5 6.8 3.5

Several boron compounds are known for their extreme hardness and toughness, including

  • Heterodiamond (also called BCN);
  • Boron nitride. This material is isoelectronic to carbon. Similar to carbon, it has both hexagonal (soft graphite-like h-BN) and cubic (hard, diamond-like c-BN) forms. h-BN is used as a high temperature component and lubricant. c-BN, also known under commercial name borazon,[71] is a superior abrasive. Its hardness is only slightly smaller, but chemical stability is superior to that of diamond.
  • Rhenium diboride can be produced at ambient pressures, but is rather expensive because of rhenium. The hardness of ReB2 exhibits considerable anisotropy because of its hexagonal layered structure. Its value is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.[72]
  • AlMgB14 + TiB2 composites possess high hardness and wear resistance and are used in either bulk form or as coatings for components exposed to high temperatures and wear loads.[73]

Boron carbide and cubic boron nitride powders are widely used as abrasives. Metal borides are used for coating tools through chemical vapor deposition or physical vapor deposition. Implantation of boron ions into metals and alloys, through ion implantation or ion beam deposition, results in a spectacular increase in surface resistance and microhardness. Laser alloying has also been successfully used for the same purpose. These borides are an alternative to diamond coated tools, and their (treated) surfaces have similar properties to those of the bulk boride.[74]

In chemistry

Navy emergency flare
Apollo 15 launch.ogg
Launch of Apollo 15
  • Sodium tetraborate pentahydrate (Na2B4O7 • 5H2O) is used in large amounts in making insulating fiberglass and sodium perborate bleach.[75]
  • Boric acid (also known as orthoboric acid) H3BO3 is used in the production of textile fiberglass and flat panel displays [79] and in many PVAc and PVOH based adhesives. It also has antiseptic, antifungal, and antiviral properties and for this reasons is applied as a water clarifier in swimming pool water treatment.[80] Boric acid is also traditionally used as an insecticide, notably against ants, fleas, and cockroaches.[81]
  • Boron compounds show promise in treating arthritis.[85] Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.[86] It is also used as a melting point depressant in nickel-chromium braze alloys.[87]

Biological role

There is a boron-containing natural antibiotic, boromycin, isolated from streptomyces.[88][89] Boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. Conversely, high soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron content in the soil is greater than 1.8 ppm. When this content exceeds 2.0 ppm, few plants will perform well and some may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear.[90][91][92]

As an ultratrace element, boron is necessary for the optimal health of rats, although it is necessary in such small amounts that ultrapurified foods and dust filtration of air is necessary to show the effects of boron deficiency, which manifest as poor coat/hair quality. Presumably, boron is necessary to other mammals. No deficiency syndrome in humans has been described. Small amounts of boron occur widely in the diet, and the amounts needed in the diet would, by analogy with rodent studies, be very small. The exact physiological role of boron in the animal kingdom is poorly understood.[93]

Boron occurs in all foods produced from plants. Since 1989 its nutritional value has been argued. It is thought that boron plays several biochemical roles in animals, including humans.[94] The U.S. Department of agriculture conducted an experiment in which postmenopausal women took 3 mg of boron a day. The results showed that supplemental boron reduced excretion of calcium by 44%, and activated estrogen and vitamin D. However, whether these effects were conventionally nutritional, or medicinal, could not be determined. The US National Institutes of Health quotes this source:

Total daily boron intake in normal human diets ranges from 2.1–4.3 mg boron/day.[95][96]

Analytical quantification

For determination of boron content in food or materials the colorimetric curcumin method is used. Boron has to be transferred to boric acid or borates and on reaction with curcumin in acidic solution, a red colored boron-chelate complex, rosocyanine, is formed.[97]

Health issues

Elemental boron and borates are non-toxic to humans and animals (approximately similar to table salt). The LD50 (dose at which there is 50% mortality) for animals is about 6 g per kg of body weight. Substances with LD50 above 2 g are considered non-toxic. The minimum lethal dose for humans has not been established, but an intake of 4 g/day was reported without incidents, and medical dosages of 20 g of boric acid for neutron capture therapy caused no problems. Fish have survived for 30 min in a saturated boric acid solution and can survive longer in strong borax solutions.[98] Borates are more toxic to insects than to mammals. The boranes and similar gaseous compounds are quite poisonous. As usual, it is not an element that is intrinsically poisonous, but toxicity depends on structure.[47][48]

The boranes (boron hydrogen compounds) are toxic as well as highly flammable and require special care when handling. Sodium borohydride presents a fire hazard due to its reducing nature, and the liberation of hydrogen on contact with acid. Boron halides are corrosive.[99]

Congenital endothelial dystrophy type 2, a rare form of corneal dystrophy, is linked to mutations in SLC4A11 gene that encodes a transporter reportedly regulating the intracellular concentration of boron.[100]

See also


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External links

Study guide

Up to date as of January 14, 2010

From Wikiversity

Boron (B) is the fifth element on the periodic table.

Physical Constants

Molecular Weight = 10.81 g·mol-1
Melting Point = 2180°C
Boiling Point = 3650°C[1]

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BORON (symbol B, atomic weight ii), one of the non-metallic elements, occurring in nature in the form of boracic (boric) acid, and in various borates such as borax, tincal,. boronatrocalcite and boracite. It was isolated by J. Gay Lussac and L. Thenard in 1808 by heating boron trioxide with potassium, in an iron tube. It was also isolated at about the same time by Sir H. Davy, from boracic acid. It may be obtained as a dark brown amorphous powder by placing a mixture of io parts of the roughly powdered oxide with 6 parts of metallic sodium in a red-hot crucible, and covering the mixture with a layer of well-dried common salt. After the vigorous reaction has ceased and all the sodium has been used up, the mass is thrown into dilute hydrochloric acid, when the soluble sodium salts go into solution, and the insoluble boron remains as a brown powder, which may by filtered off and dried. H. Moissan (Ann. Chico. Phys., 1895, 6, p. 296) heats three parts of the oxide with one part of magnesium powder. The dark product obtained is washed with water, hydrochloric acid and hydrofluoric acid, and finally calcined again with the oxide or with borax, being protected from air during the operation by a layer of charcoal. Pure amorphous boron is a chestnut-coloured powder of specific gravity 2.45; it sublimes in the electric arc, is totally unaffected by air at ordinary temperatures, and burns on strong ignition with production of the oxide B 2 0 3 and the nitride BN. It combines directly with fluorine at Ordinary temperature, and with chlorine, bromine and sulphur on heating. It does not react with the alkali metals, but combines with magnesium at a low red heat to form a boride, and with other metals at more or less elevated temperatures. It reduces many metallic oxides, such as lead monoxide and cupric oxide, and decomposes water at a red heat. Heated with sulphuric acid and with nitric acid it is oxidized to boric acid, whilst on fusion with alkaline carbonates and hydroxides it gives a borate of the alkali metal. Like silicon and carbon, very varying values had been given for its specific heat, until H. F. Weber showed that the specific heat increases rapidly with increasing temperature. By strongly heating a mixture of boron trioxide and aluminium, protected from the air by a layer of charcoal, F. WOhler and H. Sainte-Claire Deville obtained a grey product, from which, on dissolving out the aluminium with sodium hydroxide, they obtained a crystalline product, which they thought to be a modification of boron, but which was shown later to be a mixture of aluminium borides with more or less carbon. Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond. In 1904 K. A. Kiihne (D.R.P. 147,871) described a process in which external heating is not necessary, a mixture of aluminium turnings, sulphur and boric acid being ignited by a hot iron rod, the resulting aluminium sulphide, formed as a by-product, being decomposed by water.

Boron hydride has probably never been isolated in the pure condition; on heating boron trioxide with magnesium filings, a magnesium boride Mg 3 B 2 is obtained, and if this be decomposed with dilute hydrochloric acid a very evil-smelling gas, consisting of a mixture of hydrogen and boron hydride, is obtained. This mixture burns with a green flame forming boron trioxide; whilst boron is deposited on passing the gas mixture through a hot tube, or on depressing a cold surface in the gas flame. By cooling it with liquid air Sir W. Ramsay and H. S. Hatfield obtained from it a gas of composition B3H3. The mixture probably contained also some BH 3 (W. Ramsay and H. S. Hatfield, Proc. Chem. Soc., 17, p. 152). Boron fluoride BF 3 was first prepared in 1808 by Gay Lussac and L. Thenard and is best obtained by heating a mixture of the trioxide and fluorspar with concentrated sulphuric acid. It is a colourless pungent gas which is exceedingly soluble in water. It fumes strongly in air, and does not attack glass. It rapidly absorbs the elements of water wherever possible, so that a strip of paper plunged into the gas is rapidly charred. It does not burn, neither does it support combustion. A saturated solution of the gas, in water, is a colourless, oily, strongly fuming liquid which after a time decomposes, with separation of metaboric acid, leaving hydrofluoboric acid HF BF3 in solution. This acid cannot be isolated in the free condition, but many of its salts are known. Boron fluoride also combines with ammonia gas, equal volumes of the two gases giving a white crystalline solid of composition BF 3 NH 3 i with excess of ammonia gas, colourless liquids BF 3.2NH 3 and BF 3.3NH 3 are produced, which on heating lose ammonia and are converted into the solid form.

Boron chloride BC1 3 results when amorphous boron is heated in chlorine gas, or more readily, on passing a stream of chlorine over a heated mixture of boron trioxide and charcoal, the volatile product being condensed in a tube surrounded by a freezing mixture. It is a colourless fuming liquid boiling at 17-18° C., and is readily decomposed by water with formation of boric and hydrochloric acids. It unites readily with ammonia gas forming a white crystalline solid of composition 2BC13.3NH3.

Boron bromide BBr 3 can be formed by direct union of the two elements, but is best obtained by the method used for the preparation of the chloride. It is a colourless fuming liquid boiling at 90.5° C. With water and with ammonia it undergoes the same reactions as the chloride. Boron and iodine do not combine directly, but gaseous hydriodic acid reacts with amorphous boron to form the iodide, BI 31 which can also be obtained by passing boron chloride and hydriodic acid through a red-hot porcelain tube. It is a white crystalline solid of melting point 43° C.; it boils at 210° C., and it can be distilled without decomposition. It is decomposed by water, and with a solution of yellow phosphorus in carbon bisulphide it gives a red powder of composition PBI 2, which sublimes in vacuo at 210° C. to red crystals, and when heated in a current of hydrogen loses its iodine and leaves a residue of boron phosphide PB.

Boron nitride BN is formed when boron is burned either in air or in nitrogen, but can be obtained more readily by heating to redness in a platinum crucible a mixture of one part of anhydrous borax with two parts of dry ammonium chloride. After fusion, the melt is well washed with dilute hydrochloric acid and then with water, the nitride remaining as a white powder. It can also be prepared by heating borimide B2(NH)31 or by heating boron trioxide with a metallic cyanide. It is insoluble in water and unaffected by most reagents, but when heated in a current of steam or boiled for some time with a caustic alkali, slowly decomposes with evolution of ammonia and the formation of boron trioxide or an alkaline borate; it dissolves slowly in hydrofluoric acid.

Borimide B 2 (NH) 3 is obtained on long heating of the compound B 2 S 3.6NH 3 in a stream of hydrogen, or ammonia gas at 115-120° C. It is a white solid which decomposes on heating into boron nitride and ammonia. Long-continued heating with water also decomposes it slowly.

Boron sulphide B 2 S 3 can be obtained by the direct union of the two elements at a white heat or from the tri-iodide and sulphur at 44 0 ° C., but is most conveniently prepared by heating a mixture of the trioxide and carbon in a stream of carbon bisulphide vapour. It forms slightly coloured small crystals possessing a strong disagreeable smell, and is rapidly decomposed by water with the formation of boric acid and sulphuretted hydrogen. A pentasulphide B2S5 is prepared, in an impure condition, by heating a solution of sulphur in carbon bisulphide with boron iodide, and forms a white crystalline powder which decomposes under the influence of water into sulphur, sulphuretted hydrogen and boric acid.

Boron trioxide B203 is the only known oxide of boron; and may be prepared by heating amorphous boron in oxygen, or better, by strongly igniting boric acid. After fusion the mass solidifies to a transparent vitreous solid which dissolves readily in water to form boric acid (q.v.); it is exceedingly hygroscopic and even on standing in moist air becomes opaque through absorption of water and formation of boric acid. Its specific gravity is 1.83 (J. Dumas). It is not volatile below a white heat, and consequently, if heated with salts of more volatile acids, it expels the acid forming oxide from such salts; for example, if potassium sulphate be heated with boron trioxide, sulphur trioxide is liberated and potassium borate formed. It also possesses the power of combining with most metallic oxides at high temperatures, forming borates, which in many cases show characteristic colours. Many organic compounds of boron are known; thus, from the action of the trichloride on ethyl alcohol or on methyl alcohol, ethyl borate B(OC2H5)3 and methyl borate B(OCH 3) 3 are obtained. These are colourless liquids boiling at 119° C. and 72° C. respectively, and both are readily decomposed by water. By the action of zinc methyl on ethyl borate, in the requisite proportions, boron trimethyl is obtained, thus :-2B(OC2H5)2+ 6Zn(CH 3) 2 =2B(CH 3) 3 +6Zn< OC2H5 as a colourless spontaneously inflammable gas of unbearable smell. Boron triethyl B(C 2 H 5) 3 is obtained in the same manner, by using zinc ethyl. It is a colourless spontaneously inflammable liquid of boiling point 95° C. By the action of one molecule of ethyl borate on two molecules of zinc ethyl, the compound B(C2H5)2.002H5 diethylboron ethoxide is obtained as a colourless liquid boiling at 102° C. By the action of water it is converted into B(C2H5)2. OH, and this latter compound on exposure to air takes up oxygen slowly, forming the compound B C 2 H 5.00 2 H 5. OH, which, with water, gives B (C2 H 5) (OH) 2. From the condensation of two molecules of ethyl borate with one molecule of zinc ethyl the compound B2 C2H5. (0C2H5)5 is obtained as a colourless liquid of boiling point 112° C. Boron triethyl and boron trimethyl both combine with ammonia.

The atomic weight of boron has been determined by estimating the water content of pure borax (J. Berzelius), also by conversion of anhydrous borax into sodium chloride (W. Ramsay and E. Aston) and from analysis of the bromide and chloride(Sainte-Claire Deville); the values obtained ranging from 10.73 to II 04. Boron can be estimated by precipitation as potassium fluoborate, which is insoluble in a mixture of potassium acetate and alcohol, For this purpose only boric acid or its potassium salt must be present; and to ensure this, the borate can be distilled with sulphuric acid and methyl alcohol and the volatile ester absorbed in potash.

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Simple English

Boron in a tube

Boron is a chemical element. It has the chemical symbol B. It has the atomic number 5. It is a metalloid (it has properties of a metal and a non-metal). Much boron is found in chemical compounds in its ore borax. Boron is never found free in nature.

Two types of boron are found (allotropes). Amorphous boron is a brown powder and metallic (crystalline) boron is black and hard and a weak conductor at room temperature.

Pure boron is used as a dopant (a substance added to semiconductors to change how it behaves with electricity) in the semiconductor industry. Chemical compounds of boron are important as to make strong materials not weigh very much, as nontoxic insecticides and preservatives, and for chemical synthesis.

Plants need boron in them to live. Very small amounts of boron are needed in animal's bodies so that they are very healthy. How it keeps animals healthy is not known in a lot of detail.

Boron was discovered by Sir Humphry Davy, an English chemist, in 1808.

Boron melts at 2075 °C (3767 °F), and boils at 4000 °C (7232 °F).

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