5
  B  
10.811000
Boron

Name: Boron
Symbol: B
Atomic Number: 5
Atomic Weight: 10.811000
Family: Boron Family
CAS RN: 7440-42-8
Description:  A black semiconductor & yellow-brown non-metallic crystal.
State (25C): Solid
Oxidation states: +3

Molar Volume: 4.68 cm3/mole
Valence Electrons: 2p1

Boiling Point:  4275K, 4002C, 7236F
Melting Point:
2573K, 2300C, 4172F
Electrons Energy Level: 2, 3
Isotopes: 9 + 2 Stable
Heat of Vaporization: 489.7 kJ/mol
Heat of Fusion: 50.2 kJ/mol
Density: 2.34 g/cm3 @ 300K
Specific Heat: 1.02 J/gK
Atomic Radius: 1.17
Ionic Radius: 0.23
Electronegativity: 2.04 (Pauling); 2.01 (Allrod Rochow)
Vapor Pressure: 0.0000143 Pa @ 961C
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, Na2B4O7, glazes were used in China from 300 AD, and boron compounds were used in glassmaking in ancient Rome.

Joseph Louis Gay-Lussac and Louis Jacques Thenard of France concentrated it to about 50 percent purity by the reduction of Boric Acid, H3BO3, with potassium.  These men did not recognize the substance as an element.  The element was not isolated until 1808 by the English chemist Sir Humphry Davy.  It was Jons 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.  Today, boron is obtained by heating borax (Na2B4O710H2O) with carbon, although other methods are used if high-purity boron is required.

It is thought that boron plays several biochemical roles in animals, including humans.

1s2 2s2p1 

5
B
10.81
13
Al
26.98
31
Ga
69.72
49
In
114.8
81
Tl
204.3

Characteristics

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.

1s2
2s2 2p1

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, a-boron and -boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the three most characterized crystalline forms.

The element can be prepared by the reduction of borax (Na2B4O7) with carbon.  High-purity boron can be produced by electrolysis of molten Potassium Fluoroborate.

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.  Boron filaments are used in the aerospace industry because of their high-strength and light-weight.

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 used in pyrotechnics and flares to produce a green color. Boron has also been used in some rockets as an ignition source. Boron-10, one of the naturally occurring isotopes of boron, is a good absorber of neutrons and is used in the control rods of nuclear reactors, as a radiation shield and as a neutron detector.

Occurrence

Boron exists in the earth's crust to the extent of only about 10 ppm (about the same abundance as lead).  This behavior as well as many of its other properties earn it the classification of a metalloid.

Turkey and the United States 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 fiber optics.

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. The largest borax deposits are found in Central and Western Turkey including the provinces of Eskisehir, Kutahya and Balikesir.

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 Axid with metals such as Magnesium or Aluminum.  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 Hydrogn at high temperatures. The highly pure boron, for the use in semiconductor industry, is produced by the decomposition of Diborane at high temperatures and then further purified with the Czochralski Process.

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 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 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 tons per year capacity boric acid plant at Emet in 2003.  Rio Tinto increased the capacity of its Boron plant from 260,000 tons per year in 2003 to 310,000 tons per year by May 2005, with plans to grow this to 366,000 tons 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 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 can be expected to lead to greater use of insulation-grade fiberglass, 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 tons by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.

Applications

Of the several hundred uses of boron compounds, especially notable uses include:

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.

Compounds

The most economically important compounds of boron are:

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 d11B 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 seconds.  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 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 crist and continental crust; this difference may act as an isotopic signature.

The exotic 17B exhibits a Nuclear halo.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
B7 7.0299 1.4M eV
B8 8.0246 770 ms
B9 9.0133 0.54 keV
B10 10.0129 Stable
B11 11.0093 Stable
B12 12.0144 20.2 ms
B13 13.0178 17.36 ms
B14 14.0254 13.8 ms
B15 15.0311 10.5 ms
B16 16.0398 <200 ps
B17 17.047 5.08 ms
B18 18.056  
B19 19.064  

10B and 11B NMR Spectroscopy

Both 10B (18.8 percent) and 11B (81.2 percent) possess nuclear spin; that of boron-10 has a value of 3 and that of boron-11, 3/2.  These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nucleus are available commercially.  The boron-10 and boron-11 nuclei also cause splitting in the resonances of attached nuclei.

B-11 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

80px-Flammable.jpg (2186 bytes) 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.

atom.gif (700 bytes)

Boron Data

Atomic Radius (): 1.17
Atomic Volume cm3/mol : 4.6cm3/mol
Covalent Radius: 0.82
Crystal Structure: Rhombohedral
Ionic Radius: 0.23

Chemical Properties

Electrochemical Equivalents: 0.1344 g/amp-hr
Electron Work Function: 4.45eV
Electronegativity: 2.04 (Pauling); 2.01 (Allrod Rochow)
Heat of Fusion: 50.2 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 8.298
Second Ionization Potential: 25.154
Third Ionization Potential: 37.93
Valence Electron Potential: 190
Ionization Energy (eV): 8.298 eV

Physical Properties

Atomic Mass Average: 10.811
Boiling Point: 4275K, 4002C, 7236F
Melting Point: 2573K, 2300C, 4172F
Heat of Vaporization: 489.7 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 5E-6
Electrical Conductivity: 1.0E-12 106/cm
Thermal Conductivity: 0.274 W/cmK
Density: 2.34 g/cm3 @ 300K
Enthalpy of Atomization: 573.2 kJ/mole @ 25C
Enthalpy of Fusion: 22.18 kJ/mole
Enthalpy of Vaporization: 480 kJ/mole
Flammability Class: unknown
Molar Volume: 4.68 cm3/mole
Optical Reflectivity: unknown
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 1.02 J/gK
Vapor Pressure: 0.348  Pa@ 2300C
Estimated Crustal Abundance: 1.0101 milligrams per kilogram
Estimated Oceanic Abundance: 4.44 milligrams per liter


(Ar. Buraq, Pers. Burah) Boron compounds have been known for thousands of years, but the element was not discovered until 1808 by Sir Humphry Davy and by Gay-Lussac and Thenard. The element is not found free in nature, but occurs as orthoboric acid usually in certain volcanic spring waters and as borates in boron and colemanite. Ulexite, another boron mineral, is interesting as it is nature's own version of "fiber optics." Important sources of boron are the ore rasorite (kernite) and tincal (borax ore). Both of these ores are found in the Mohave Desert. Tincal is the most important source of boron from the Mohave. Extensive borax deposits are also found in Turkey. Boron exists naturally as 19.78% 10B isotope and 80.22% 11B isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. Boron of 99.9999% purity has been produced and is available commercially. Elemental boron has an energy band gap of 1.50 to 1.56 eV, which is higher than that of either silicon or germanium. It has interesting optical characteristics, transmitting portions of the infrared, and is a poor conductor of electricity at room temperature but a good conductor at high temperature. Amorphous boron is used in pyrotechnic flares to provide a distinctive green color, and in rockets as an igniter. By far the most commercially important boron compound in terms of dollar sales is Na2B4O7.5H2O. This pentahydrate is used in very large quantities in the manufacture of insulation fiberglass and sodium perborate bleach. Boric acid is also an important boron compound with major markets in textile products. Use of borax as a mild antiseptic is minor in terms of dollars and tons. Boron compounds are also extensively used in the manufacture of borosilicate glasses. Other boron compounds show promise in treating arthritis. The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. It also has lubricating properties similar to graphite. The hydrides are easily oxidized with considerable energy liberation, and have been studied for use as rocket fuels. Demand is increasing for boron filaments, a high-strength, lightweight material chiefly employed for advanced aerospace structures. Boron is similar to carbon in that it has a capacity to form stable covalently bonded molecular networks. Carbonates, metalloboranes, phosphacarboranes, and other families comprise thousands of compounds. Crystalline boron (99%) costs about $5/g. Amorphous boron costs about $2/g. Elemental boron and the borates are not considered to be toxic, and they do not require special care in handling. However, some of the more exotic boron hydrogen compounds are definitely toxic and do require care.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond