| Boron - B
CAS: 7440-42-8 Description: Yellow-brown, non-metallic crystals Classification: Metalloid Date of Discovery: 1808 Discoverer: Sir Humphry Davy, J.L. Gay-Lussac Name Origin: Arabic Buraq, Persian Burah |
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Atomic Number: 5
Number of Neutrons: 6 Atomic Mass: 10.81(5) amu Melting Point: 2075 °C Boiling Point: 4000 °C Density (293 K): 2.34 g/cm3 (crystals) 2.37 g/cm3 (amorphous variety) Atomic volume: 4.6 cm3/mol Electrical resistivity: 1.0e-12 10-6/cm 
Thermal conductivity: 0.270 W/cmK Enthalpy of atomization: 573.21 kJ/mol Enthalpy of vaporization: 489.70 kJ/mol Enthalpy of fusion: 50.20 kJ/mol Specific heat capacity: 1.02 J/gK |
Energy levels: 2-3
Electron configuration: [He]2s22p1 Crystal Structure: Rhombohedral Atomic radius: 1.17 Å Covalent radius: 0.82 Å Oxidation States: +3 Electronegativity, Pauling: 2.04 Electron affinity: 0.277 eV First ionization energy: 8.298 eV 2nd ionization energy: 25.154 eV 3rd ionization energy: 37.93 eV Polarizability: 3.03 10-24cm3 |
| Isotope | Natural Abundance | Atomic Mass | Half-life | Decay Mode | Spin |
| 7B | 7.0299 | 4 x 10-22 s | p | ||
| 8B | 8.024607 | 0.770 s |  +, 2 | 2+ | |
| 9B | 9.013329 | 8 x 10-19 s | p 2 |
3/2- | |
| 10B | 19.9(2) | 10.012937 | Stable | 3+ | |
| 11B | 80.1(2) | 11.009306 | Stable | 3/2- | |
| 12B | 12.014352 | 0.0202 s | -, - |
1+ | |
| 13B | 13.017780 | 0.0174 s | -, - n |
3/2- | |
| 14B | 14.02540 | 14 ms | - |
2- | |
| 15B | 15.03110 | 10.4 ms | -, (n) |
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| 17B | 17.0469 | 5.1 ms | - |
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| 19B | 19.0637 | - |
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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 borax (Na2B4O7 · 10H2O) and colemanite [CaB3O4(OH)3 · H2O]. Ulexite [NaCaB5O6(OH)6 · 5H2O], another boron mineral, is interesting as it is nature's own version of "fiber optics." Important sources of boron are the ores rasorite or kernite, [Na2B4O6(OH)2 · 3H2O] and tincalconite (Na2B4O5(OH)4 · 3H2O, borax ore). Both of these ores are found in the Mojave Desert. Tincalconite is the most important source of boron from the Mojave. Extensive borax deposits are also found in Turkey. Boron exists naturally as 19.9% 10B isotope and 80.1% 11B isotope. Eleven isotopes of boron are known. 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 (H3BO3) is also an important boron compound with major markets in textile fiberglass and in cellulose insulation as a flame retardant. Next in order of importance is borax which is used principally in laundry 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.
Carboranes, metalloboranes, phosphacarboranes, and other families comprise thousands of compounds. 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.
LINKS: Toxicology and Carcinogenesis Studies of Boric Acid Boron and boron-rich solids Boron Biologicals, Inc. Boron Neutron Capture Therapy Project at LBNL Boron Phenols and Health Laboratory for the Study of Novel Carbon Materials - Washington University US Borax: The World's Largest Borate Mine Synthesis of Alcohols Return
Sources for the information on this website include: Lide, David R., ed. CRC Handbook of Chemistry and Physics, 78th Ed., 1997-1998. |
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