Boron
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| General | |||||||||||||||||||||||||
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| Name, Symbol, Number | boron, B, 5 | ||||||||||||||||||||||||
| Chemical series | metalloids | ||||||||||||||||||||||||
| Group, Period, Block | 13, 2, p | ||||||||||||||||||||||||
| Appearance | black/brown |
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| Atomic mass | 10.811 (7) g/mol | ||||||||||||||||||||||||
| Electron configuration | 1s2 2s2 2p1 | ||||||||||||||||||||||||
| Electrons per shell | 2, 3 | ||||||||||||||||||||||||
| Physical properties | |||||||||||||||||||||||||
| Phase | solid | ||||||||||||||||||||||||
| Density (near r.t.) | 2.34 g/cm³ | ||||||||||||||||||||||||
| Liquid density at m.p. | 2.08 g/cm³ | ||||||||||||||||||||||||
| Melting point | 2349 K (2076 ° C, 3769 ° F) |
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| Boiling point | 4200 K (3927 ° C, 7101 ° F) |
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| Heat of fusion | 50.2 kJ/mol | ||||||||||||||||||||||||
| Heat of vaporization | 480 kJ/mol | ||||||||||||||||||||||||
| Heat capacity | (25 °C) 11.087 J/(mol·K) | ||||||||||||||||||||||||
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| Atomic properties | |||||||||||||||||||||||||
| Crystal structure | rhombohedral | ||||||||||||||||||||||||
| Oxidation states | 3 (mildly acidic oxide) |
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| Electronegativity | 2.04 ( Pauling scale) | ||||||||||||||||||||||||
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Ionization energies ( more) |
1st: 800.6 kJ/mol | ||||||||||||||||||||||||
| 2nd: 2427.1 kJ/mol | |||||||||||||||||||||||||
| 3rd: 3659.7 kJ/mol | |||||||||||||||||||||||||
| Atomic radius | 85 pm | ||||||||||||||||||||||||
| Atomic radius (calc.) | 87 pm | ||||||||||||||||||||||||
| Covalent radius | 82 pm | ||||||||||||||||||||||||
| Miscellaneous | |||||||||||||||||||||||||
| Magnetic ordering | nonmagnetic | ||||||||||||||||||||||||
| Electrical resistivity | (20 °C) 150 µΩ·m | ||||||||||||||||||||||||
| Thermal conductivity | (300 K) 27.4 W/(m·K) | ||||||||||||||||||||||||
| Thermal expansion | (25 °C) 5–7 µm/(m·K) | ||||||||||||||||||||||||
| 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 | ||||||||||||||||||||||||
| Notable isotopes | |||||||||||||||||||||||||
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| References | |||||||||||||||||||||||||
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 several 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 poor conductor at room temperature. It is never found free in nature. Crystalline boron 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.
Notable characteristics
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 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 nitride can be used to make materials that are almost 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 compounds of boron are:
- Sodium tetraborate pentahydrate (Na2B4O7 · 5 H2O), which is used in large amounts in making insulating fiberglass and sodium perborate bleach,
- Orthoboric acid (H3BO3) or boric acid, used in the production of textile fiberglass and flat panel displays or eye drops, among many uses, and
- Sodium tetraborate decahydrate (Na2B4O7 · 10 H2O) or borax, used in the production of adhesives, in anti-corrosion systems and many other uses.
Of the several hundred uses of boron compounds, one can cite the following ones:
- Boron being an essential micronutrient, playing notably a role in plant fertilisation and in the building of cell wall structures, it is 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.
- 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 borophophosilicate 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.
- Purified boron-11 (depleted boron) is used for borosilicate glasses in rad-hard electronics.
- Research is being conducted into fusion power by interaction of hydrogen and boron. Potential benefits include relatively small and uncomplicated reactors and supposedly greater safety.
- Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures as a component of composite materials.
- Sodium borohydride (NaBH4), is a popular chemical reducing agent, used (for example) for reducing aldehydes and ketones to alcohols.
- Boron in trace amounts is used as dopant for P-type semiconductors.
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 from 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 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. These men did not recognize the substance as an element. It was Jöns Jakob 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 by 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.
- see also category:Borate minerals
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 δB-11 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 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; this difference may act as an isotopic signature.
Depleted boron
The boron-10 isotope is good at capturing thermal neutrons from cosmic radiation or in PWRs (Pressurized Water Reactor, a type of nuclear power reactor). 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 boron-11 - is used, to avoid this effect, as one of radiation hardening measures. Boron-11 is a by-product of the nuclear industry.
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 as well as highly flammable and do require special handling care.