Boron (Wiktionary:boron)is a chemical element in the periodic table that has the symbol B and atomic number 5. A trivalent metalloid element, boron occurs abundantly in the ore borax. There are two allotropes of boron; amorphous boron is a brown powder, but metallic boron is black. The metallic form is hard (9.3 on Mohs' scale) and a bad conductor in room temperatures. It is never found free in nature.
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| General | |||||
|---|---|---|---|---|---|
| Name, Symbol, Number | Boron, B, 5 | ||||
| Series | Metalloids | ||||
| Group, Period, Block | 13 (IIIA), 2, p | ||||
| Density, Hardness | 2460 kg/m3, 9.3 | ||||
| Appearance | Black |
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| Atomic properties | |||||
| Atomic weight | 10.811 amu | ||||
| Atomic radius (calc.) | 85 (87)pm | ||||
| Covalent radius | 82 pm | ||||
| van der Waals radius | no data | ||||
| Electron configuration | [He]2s22p1 | ||||
| e- 's per energy level | 2, 3 | ||||
| Oxidation states (Oxide) | 3 (mildly acidic) | ||||
| Crystal structure | Rhombohedral | ||||
| Physical properties | |||||
| State of matter | Solid (nonmagnetic) | ||||
| Melting point | 2349 K (3769 °F) | ||||
| Boiling point | 4200 K (7101 °F) | ||||
| Molar volume | 4.39 ×10-6 m3/mol | ||||
| Heat of vaporization | 489.7 kJ/mol | ||||
| Heat of fusion | 50.2 kJ/mol | ||||
| Vapor pressure | 0.348 Pa at 2573 K | ||||
| Speed of sound | 16200 m/s at 293.15 K | ||||
| Miscellaneous | |||||
| Electronegativity | 2.04 (Pauling scale) | ||||
| Specific heat capacity | 1026 J/(kg*K) | ||||
| Electrical conductivity | 1.0 10-4/m ohm | ||||
| Thermal conductivity | 27.4 W/(m*K) | ||||
| 1st ionization potential | 800.6 kJ/mol | ||||
| 2nd ionization potential | 2427.1 kJ/mol | ||||
| 3rd ionization potential | 3659.7 kJ/mol | ||||
| 4th ionization potential | 25025.8 kJ/mol | ||||
| 5th ionization potential | 32826.7 kJ/mol | ||||
| SI units & STP are used except where noted. | |||||
Boron is electron-deficient, possessing a vacant p-orbital. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances in an attempt to quench boron's insatiable hunger for electrons.
Optical characteristics of this element include the transmittance of infrared light. At standard temperatures boron is a poor electrical conductor but is a good conductor at high temperatures.
Boron has the highest tensile strength of any known element.
Boron nitride can be used to make materials that are as hard as diamond. The nitride also acts as an electrical insulator but conducts heat similar to a metal. This element also has lubricating qualities that are similar to graphite. Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks.
Applications
The most economically important compound of boron is sodium tetraborate decahydrate Na2B4O7 · 10H2O, or borax, which is used in large amounts in making insulating fiberglass and sodium perborate bleach. Other uses:
- Because of its distinctive green color, amorphous boron is used in pyrotechnic flares.
- Boric acid is an important compound used in textile products.
- Compounds of boron are used extensively in organic synthesis and in the manufacture of borosilicate glasses.
- Other compounds are used as wood preservatives, and are particularly attractive in this regard because they possess low toxicity.
- Boron-10 is used to assist control of nuclear reactors, a shield against radiation and in neutron detection.
- Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures.
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.
History
Compounds of boron (Arabic Buraq, Persian Burah) 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 AD 300, and boron compounds were used in glassmaking in ancient Rome.
The element was not isolated until 1808 by Sir Humphry Davy, Gay-Lussac and L. J. Thenard, to about 50% purity. These men did not recognize the substance as an element. It was Jöns Jacob Berzelius in 1824 who identified boron as an element. The first pure boron was produced by the American chemist W. Weintraub in 1909.
Occurrence
The United States and Turkey are the world's largest producers of boron. 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). Turkey is another place where extensive borax deposits are found.
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 reducing volatile boron halogenides with hydrogen at high temperatures.
In 1997 crystalline boron (99% pure) cost about US$5 per gram and amorphous boron cost about US$2 per gram.
Isotopes
Boron has two naturally-occurring stable isotopes, B-11 (80.1%) and B-10 (19.9%). The mass difference results in a wide range of ´B-11 values in natural waters, ranging from -16 to +59. 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 crust and continental crust.
Precautions
Elemental boron and borates are not toxic and therefore do not require special precautions while handling. Some of the more exotic boron hydrogen compounds, however, are toxic and do require special handling care.