   #copyright

Boron

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                 5               beryllium ← boron → carbon
                 -
                ↑
                B
                ↓
                Al

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                          Name, Symbol, Number boron, B, 5
                                                Chemical series metalloids
                                             Group, Period, Block 13, 2, p
                                                    Appearance black/brown
                                              Atomic mass 10.811 (7) g/mol
                                     Electron configuration 1s^2 2s^2 2p^1
                                                  Electrons per shell 2, 3
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 2.34 g·cm^−3
                                    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
                         Heat capacity (25 °C) 11.087 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2348 2562 2822 3141 3545 4072

                                                         Atomic properties
                                            Crystal structure rhombohedral
                                                        Oxidation states 3
                                                     (mildly acidic oxide)
                                    Electronegativity 2.04 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 800.6 kJ·mol^−1
                                                  2nd: 2427.1 kJ·mol^−1
                                                  3rd: 3659.7 kJ·mol^−1
                                                       Atomic radius 85 pm
                                               Atomic radius (calc.) 87 pm
                                                     Covalent radius 82 pm
                                                             Miscellaneous
                                             Magnetic ordering nonmagnetic
                           Electrical resistivity (20 °C) 1.5×10^4 Ω·m
                       Thermal conductivity (300 K) 27.4 W·m^−1·K^−1
                      Thermal expansion (25 °C) 5–7 µm·m^−1·K^−1
                              Speed of sound (thin rod) (20 °C) 16200 m/s
                                            Bulk modulus (β form) 185 GPa
                                                         Mohs hardness 9.3
                                                Vickers hardness 49000 MPa
                                             CAS registry number 7440-42-8
                                                         Selected isotopes

                  CAPTION: Main article: Isotopes of boron

                           iso    NA   half-life DM DE ( MeV)     DP
                           ^10B 19.9%* B is stable with 5 neutrons
                           ^11B 80.1%* B is stable with 6 neutrons
                           *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.

                                                                References

   Boron ( IPA: /ˈbɔːrɒn/) is a chemical element with atomic number 5 and
   the chemical symbol B. A trivalent metalloid element, boron occurs
   abundantly in the ore borax. Boron is never found free in nature.

   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.

   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, and as an ultratrace mineral is
   necessary for the optimal health of animals, though its physiological
   role in animals is poorly understood.

Characteristics of the element and boron nitride

   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 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/metallic boron include the
   transmittance of infrared light. At standard temperatures, metallic
   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 nitride is a material in which the extra electron of nitrogen
   (with respect to carbon) in some ways compensates for boron's
   deficiency of an electron. Boron nitride can be used to make crystals
   that are extremely hard, second in hardness only to diamond, and the
   similarity of this compound to diamond extends to other applications.
   Like diamond, boron nitride acts as an electrical insulator but is an
   excellent conductor of heat.

   Like carbon, boron nitride exists in a second form that has structural
   and lubricating qualities similar to graphite. This form of boron
   nitride is composed of layers of fused hexagonal sheets (analogous to
   graphite). These sheets (unlike those in graphite) are in registry.
   This means that layers are placed directly upon one another such that a
   viewer looking down onto the structure would view only the top layer.
   The polar B-N bonds interfere with electron transfer so that boron
   nitride in this form is not an electrical conductor (in contrast to
   graphite which is a semimetal that conducts electricity through a
   network of pi bonds in the plane of its hexagonal sheets).

   Boron nitride nanotubes can be constructed analogously to carbon
   nanotubes.

   Boron is also similar to carbon with its capability to form stable
   covalently bonded molecular networks.

Boron Compounds

   The most economically important compounds of boron are:
     * Sodium tetraborate pentahydrate (Na[2]B[4]O[7] · 5 H[2]O), which is
       used in large amounts in making insulating fibreglass and sodium
       perborate bleach,
     * Orthoboric acid (H[3]BO[3]) or boric acid, used in the production
       of textile fibreglass and flat panel displays or eye drops, among
       many uses, and
     * Sodium tetraborate decahydrate (Na[2]B[4]O[7] · 10 H[2]O) or borax,
       used in the production of adhesives, in anti-corrosion systems and
       many other uses.

Uses for Boron

   Of the several hundred uses of boron compounds, especially notable uses
   include:
     * Boron is an essential plant micronutrient, notably playing a role
       in plant fertilization and in the building of cell wall structures;
       as such, borates are used in agriculture.
     * Because of its distinctive green flame, amorphous boron is used in
       pyrotechnic flares.
     * Boric acid is an important compound used in textile products. For
       example, boron compounds are used as nontoxic flame retardants used
       to treat cotton fibre.
     * Boric acid is also traditionally used as an insecticide, notably
       against ants or cockroaches.
     * Compounds of boron are used extensively in organic synthesis and in
       the manufacture of borosilicate and borophosphosilicate glasses.
     * Other compounds are used as wood preservatives, and are
       particularly attractive in this regard because they possess low
       toxicity.
     * Borax is sometimes found in laundry detergent.
     * ^10B is used to assist control of nuclear reactors, a shield
       against radiation and in neutron detection.
     * Purified ^11B (depleted boron) is used for borosilicate glasses in
       rad-hard electronics.
     * Research is being conducted into the production of hydrogen fuel
       through the interaction of water and a borohydride (such as
       NaBH[4]). The engine would work by mixing borohydride with water to
       produce hydrogen as needed, thus solving some present issues of
       safely transporting hydrogen gas. The research is being conducted
       at the University of Minessota, United States by Abu-Hamed and at
       the Weizmann Institute of Science in Rehovot, Israel. To succeed,
       the rate of hydrogen production by the small engine needs only to
       meet the energy demands of the engine. Five kilograms of hydrogen
       (corresponding to 40 kg of NaBH[4]) has the same amount of energy
       as twenty gallons (60 kg) of fuel.
     * Sodium borohydride (NaBH[4]), the same chemical as used in the
       experimental car, is a popular chemical reducing agent, used (for
       example) for reducing aldehydes and ketones to alcohols. (citation
       needed)
     * 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.
     * Boron in trace amounts is used as dopant for P-type semiconductors.
     * Boron is used as a melting point depressant in nickel-chromium
       braze alloys. Diffusion of the boron from the braze alloy into the
       parent metal at brazing temperature increases the melting
       temperature promoting solidification of the joint. Subsequent
       remelting occurs at a much higher temperature.

   Boron compounds are being investigated for use in a broad range of
   applications, including as components in sugar-permeable membranes,
   carbohydrate sensors and bioconjugates.

   Medicinal applications being investigated include boron neutron capture
   therapy and drug delivery. Other boron compounds show promise in
   treating arthritis.

   Hydrides of boron are oxidized easily and liberate a considerable
   amount of energy. They have therefore been studied for use as possible
   rocket fuels, along with elemental boron. However, issues of cost,
   incomplete combustion, and boric oxide deposits have so far made this
   use infeasible.

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 300 AD, and boron compounds were used in glassmaking in
   ancient Rome.

   The element was not isolated until 1808 by Sir Humphry Davy, Joseph
   Louis Gay-Lussac, and Louis Jacques Thénard, to about 50 percent
   purity, by the reduction of boric acid with sodium or magnesium. These
   men did not recognize the substance as an element. It was Jöns Jakob
   Berzelius in 1824 that identified boron as an element. The first pure
   boron was produced by the American chemist W. Weintraub in 1909,
   although this is disputed by some researchers.

   Boron was not believed to be useful to the human body until 1989
   research suggested its signficance.

Occurrence

   The United States and Turkey are the world's largest producers of
   boron. Turkey has almost 63% of the world’s boron potential and boron
   reserves. Boron does not appear in nature 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 that naturally has properties of fibre
   optics.
   Borax crystals
   Enlarge
   Borax crystals

   Economically important sources are from the ore rasorite (kernite) and
   tincal (borax ore) which are both found in the Mojave Desert of
   California, with borax being the most important source there. Turkey is
   another place where extensive borax deposits are found.

   Even a boron-containing natural antibiotic, boromycin, isolated from
   streptomyces, is known.

   Pure elemental boron is not easy to prepare. The earliest methods used
   involve reduction of boric oxide with metals such as magnesium or
   aluminium. However the product is almost always contaminated with metal
   borides. (The reaction is quite spectacular though). Pure boron can be
   prepared by reducing volatile boron halogenides with hydrogen at high
   temperatures. The highly pure boron, for the use in semiconductor
   industry, is produced by the decomposition of diborane at high
   temperatures and than further purified with the Czochralski process.

   In 1997 crystalline boron (99% pure) cost about US$5 per gram and
   amorphous boron cost about US$2 per gram.

Food

   Boron occurs in all foods produced by plants. Since 1989 its
   nutritional value has been argued. The U.S. Department of agriculture
   conducted an experiment in which postmenopausal women took 3 mg of
   boron a day. The results showed that boron can drop excretion of
   calcium by 44%, and activate estrogen and vitamin D.

   The US National Institute of Health quotes this source:

          Total daily boron intake in normal human diets ranges from
          2.1-4.3 mg boron/kg body weight (bw)/day. /Total boron/ Zook EG
          and Lehman J; J. Assoc. Off Agric. Chem. 48: 850-5 (1965)

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.

Market trend

   Estimated global consumption of boron rose to a record 1.8 million
   tonnes of B[2]O[3] 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 beneficiated ores like colemanite has declined following
   concerns over arsenic content. Consumers have moved towards the use of
   refined borates or boric acid that have a lower pollutant content.

   Increasing demand for boric acid has led a number of producers to
   invest in additional capacity. Eti Mine opened a new 100,000 tonnes per
   year capacity boric acid plant at Emet in 2003. Rio Tinto 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.

   The rise in global demand has been driven by high rates of growth in
   fibreglass and borosilicate production. A rapid increase in the
   manufacture of reinforcement-grade fibreglass in Asia with a consequent
   increase in demand for borates has offset the development of boron-free
   reinforcement-grade fibreglass in Europe and the USA. The recent rises
   in energy prices can be expected to lead to greater use of
   insulation-grade fibreglass, with consequent growth in the use of
   boron.

   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.

Isotopes

   Boron has two naturally-occurring and stable isotopes, ^11B (80.1%) and
   ^10B (19.9%). The mass difference results in a wide range of δ^11B
   values 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
   3.26500x10^-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
   H[2]O phase changes in hydrothermal systems, and during hydrothermal
   alteration of rock. The latter effect species preferential removal of
   the ^10B(OH)[4] ion onto clays results in solutions enriched in
   ^11B(OH)[3] 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.

   The exotic ^17B exhibits a Nuclear halo.

Depleted boron

   The ^10B isotope is good at capturing thermal neutrons from cosmic
   radiation. It then undergoes fission - producing a gamma ray, an alpha
   particle, and a lithium ion. When this happens inside of an integrated
   circuit, the fission products may then dump charge into nearby chip
   structures, causing data loss (bit flipping, or single event upset). In
   critical semiconductor designs, depleted boron -- consisting almost
   entirely of ^11B -- is used to avoid this effect, as one of radiation
   hardening measures. ^11B is a by-product of the nuclear industry.

B-10 enriched boron

   The ^10B isotope is good at capturing thermal neutrons, and this
   quality has been used in both radiation shielding and in neutron
   capture medical therapy where a tumor is treated with a compound
   containing ^10B is attached to a tissue, and the patient treated with a
   relatively low dose of thermal neutrons which go on to cause energetic
   and short range alpha radiation in the tissue treated with the boron
   isotope.

   In nuclear reactors, ^10B is used for reactivity control and in
   emergency shutdown systems. It can serve either function in the form of
   borosilicate 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.

   In future manned interplanetary spacecraft, ^10B has a theoretical role
   as structural material (as boron fibers or BN nanotube material) which
   also would 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 structural materials, is high energy spallation
   neutrons. Such neutrons can be moderated by materials high in light
   elements such as structural polyethylene, but the moderated neutrons
   continue to be a radiation hazard unless actively absorbed in a way
   which dumps the absorption energy in the shielding, far away from
   biological systems. Among light elements that absorb thermal neutrons,
   ^6Li and ^10B appear as potential spacecraft structural materials able
   to do double duty in this regard.

Precautions

   Elemental boron is nontoxic and common boron compounds such as borates
   and boric acid have low toxicity (approximately similar to table salt
   with the lethal dose being 2 to 3 grams per kg) and therefore do not
   require special precautions while handling. Some of the more exotic
   boron hydrogen compounds, however, are toxic as well as highly
   flammable and do require special handling care.

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