Boron

5 beryllium ← boron → carbon - ↑ B ↓ Al 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	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)
Boiling point	4200 K (3927 °C, 7101 °F)
Heat of fusion	50.2 kJ/mol
Heat of vaporization	480 kJ/mol
Heat capacity	(25 °C) 11.087 J/(mol·K)
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
	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) 1.5×104 Ω·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
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.

Boron (chemical symbol B, atomic number 5)

It is classified as a metalloid—some of its properties resemble those of metals, and others resemble those of nonmetals. . A trivalent element, Boron is not found free in nature but occurs abundantly in the ore borax.

Several allotropes of boron exist; amorphous boron is a brown powder, though metallic (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 materals, 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.

Occurrence

In nature, boron does not appear in elemental form but is found combined in borax, boric acid, colemanite, kernite (rasorite), ulexite, and borates. Boric acid is sometimes found in volcanic spring waters. Also, a boron-containing natural antibiotic named boromycin has been isolated from Streptomyces.

Borax crystals.

Economically important sources are from kernite and borax ores, both of which are found in the Mojave Desert of California, with borax being the main source there. Extensive borax deposits are also found in Turkey. The United States and Turkey are the world's largest producers of boron.

Pure elemental boron is not easy to prepare. The earliest methods involved reduction of boric oxide with metals such as magnesium or aluminum. The product, however, is almost always contaminated with metal borides. Pure boron can be prepared by reducing volatile boron halogenides with hydrogen at high temperatures. Highly pure boron, for use in the semiconductor industry, is produced by the high-temperature decomposition of diborane, followed by what is called the Czochralski process.

• Ulexite is a borate mineral that has fiber optic properties.

History

Compounds of boron (Arabic Buraq, from Persian Burah, from Turkish Bor) have been known of for thousands of years. In early Egypt, the process of mummification depended on an ore known as natron, which contained borates as well as some other common salts. Borax glazes were used in China from CE 300, and boron compounds were used in glassmaking in ancient Rome.

The element was first isolated in 1808 by Sir Humphry Davy, Joseph Louis Gay-Lussac, and Louis Jacques Thénard. By reducing boric acid with sodium or magnesium, they obtained boron at about 50 percent purity. These men, however, did not recognize the substance as an element. In 1824, Jöns Jakob Berzelius identified boron as an element. It is generally thought that the first pure boron was produced by American chemist W. Weintraub in 1909, but some researchers have cast doubt on the accuracy of that view.

• Boron was not believed to be useful to the human body until 1989 research suggested its signficance.^{[citation needed]}

Notable characteristics

In the periodic table, boron is situated at the top of group 13 (former group 3A), just above aluminum, and in period 2, between beryllium and carbon. It is classified as a metalloid.

Boron exists in different allotropic forms. Amorphous boron is a brown powder and is produced by certain chemical reactions. It contains boron atoms randomly bonded to one another, without long-range order. By contrast, crystalline (metallic) boron is black in color and extremely hard (9.3 on Mohs' scale). It has a high melting point and exists in many polymorphs—forms of boron that differ in their crystal lattice structure.

The best characterized crystalline forms are: two rhombohedral forms, α-boron and β-boron, containing 12 and 106.7 atoms in each rhombohedral unit cell, respectively; and a tetragonal, 50-atom form. These forms are somewhat analogous to diamond crystals, but unlike diamond, boron has various possible structures because each boron atom can form three bonds, forcing the atoms to be asymmetrically bonded in three-dimensional space.

Crystalline boron can transmit infrared light. At standard temperatures, it is a poor conductor of electricity, but it becomes is a good conductor at high temperatures.

Boron is similar to carbon in terms of being able to form stable, covalently bonded molecular networks. On the other hand, each boron atom has one less electron than a carbon atom, and boron behaves as an electrophile—it seeks to form a covalent bond with an element that can provide a pair of electrons to form the bond. The reactions of boron are dominated by this requirement for electrons. Based on this property, boron is called a Lewis acid.

Isotopes

There are 13 known isotopes of boron. Two stable isotopes occur in nature: ¹¹B (about 80%) and ¹⁰B (about 20%). Also, ¹¹B is a by-product of the nuclear industry. The shortest-lived isotope is ⁷B which has a half-life of 3.26500x10^-22 seconds and decays through proton emission and alpha decay.

The ¹⁰B isotope is good at capturing thermal neutrons, such as from cosmic rays. It then undergoes nuclear fission, producing a gamma ray, an alpha particle, and a lithium ion. When this happens within an integrated circuit, the fission products may dump charge into nearby chip structures, resulting in data loss. To avoid this effect in critical semiconductor designs, "depleted boron" (consisting almost entirely of ¹¹B) is used.

The ability of ¹⁰B to capture thermal neutrons has been used in radiation shielding and medical therapy. 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 medical treatment for a tumor, a compound containing ¹⁰B may be attached to the tumor tissue, and the patient is treated with a relatively low dose of thermal neutrons. The ¹⁰B absorbs neutrons and releases short-range alpha radiation. This radiation kills the tumor cells.

Boron nitride

Boron nitride is a material in which the extra electron of the nitrogen atom (compared to a carbon atom) in some ways compensates for the boron atom's deficiency of an electron (compared to carbon). Boron nitride can be used to make crystals that are extremely hard, second in hardness only to diamond. Also, like diamond, boron nitride acts as an electrical insulator, but it is an excellent conductor of heat.

Like carbon, boron nitride exists in a second form that has structural and lubricating qualities similar to those of graphite. Like graphite, this form of boron nitride is composed of layers of fused hexagonal sheets. Yet, unlike graphite, these sheets are "in registry." This means that the layers are placed directly above 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. (By contrast, graphite conducts electricity through a network of pi bonds in the plane of its hexagonal sheets.)

Boron nitride nanotubes can be constructed in a form analogous to carbon nanotubes.

Applications

There are several hundred uses of boron and its compounds. Some of the notable ones are highlighted below.

• As boron is an essential plant micronutrient, borates are used in agriculture.
• Because it produces a distinctive green flame, amorphous boron is used in pyrotechnic flares.
• Sodium tetraborate pentahydrate (Na₂B₄O₇ · 5H₂O) is used in large amounts in making insulating fiberglass and sodium perborate bleach.
• Sodium tetraborate decahydrate (Na₂B₄O₇ · 10H₂O, or borax) is used in the production of adhesives and anti-corrosion systems, among its many uses.
• Boric acid (or orthoboric acid, H₃BO₃)is an important compound used in textile products, as a nontoxic flame retardant. [1]
• Boric acid is also traditionally used as an insecticide, notably against ants and 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 have low toxicity.
• ¹⁰B is used to control reactions in nuclear reactors, as a shield against radiation, and in neutron detection.
• Purified ¹¹B ("depleted boron," separated from ¹⁰B) is used for borosilicate glasses in "rad-hard" (radiation hardened) electronics.
• Sodium borohydride (NaBH₄) is a popular chemical reducing agent. For example, it is used for converting aldehydes and ketones to alcohols.
• Boron filaments are high-strength, lightweight materials that are chiefly used in composites for advanced aerospace structures. They are also used in such items as golf clubs and fishing rods.
• In trace amounts, boron is used as a dopant for P-type semiconductors.

Potential uses

• 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.

Boron in food

Boron occurs in all foods produced by plants. Its nutritional value has been argued since at least 1989. 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.

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.

See also

• Chemical element
• Periodic table

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Boron

External links

• Boron
• Computational Chemistry Wiki
• Environmental Health Criteria 204: Boron (1998) by the International Programme on Chemical Safety.
  • A summary of the above report by GreenFacts.
• It's Elemental – Boron
• National Pollutant Inventory - Boron and compounds
• WebElements.com – Boron

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