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Boron (pronounced /bɔrɒn/) is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite.

Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, extremely hard (9.3 on Mohs' scale), and a poor conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, insecticides and preservatives, and reagents for chemical synthesis.

Boron is an essential plant nutrient. Whereas lack of boron results in boron deficiency disorder, high soil concentrations of boron may also be toxic to plants. As an ultratrace element, boron is necessary for the optimal health of rats and presumably other mammals, though its physiological role in animals is yet poorly understood.

Structure and physical properties
See also: Allotropes of boron
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks. Brown amorphous boron is a product of certain chemical reactions. It contains boron icosahedra that are randomly bonded to each other without long range order. Crystalline boron is a very hard, black material with a high melting point. It exists in four major polymorphs: α, ß, γ and T. The tetragonal (T) boron phase contains 192 atoms per unit cell and has density 2.364 g/cm3. The γ-phase can be described as a NaCl-type arrangement of two types of clusters, B12 icosahedra and B2 pairs. It can be produced by compressing other boron phases to 12-20 GPa, heating to 1500-1800 0C and is quenchable to ambient conditions. Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures 6-12 K.

Chemical properties
Chemically, boron is closer to silicon than to aluminium. Crystalline boron is chemically inert and resistant to attack by boiling HF or HCl. When finely divided it is attacked slowly by hot concentrated hydrogen peroxide, hot concentrated nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids.

The behavior of boron to air depends upon the crystallinity of the sample, temperature, particle size, and purity. Boron does not react with air at room temperature. At higher temperatures, boron burns to form boron trioxide:

4B + 3O2(g) → 2B2O3(s)
Boron reacts with sulfur to produce boron sulfide

2B + 3S(g) → B2S3(s)
The first synthesis was performed by Jöns Jakob Berzelius in 1824. Another synthesis was favoured by Friedrich Wöhler and Henri Etienne Sainte-Claire Deville first published in 1858, starting from boron and hydrogen sulfide.

2B + 3H2S → B2S3 + 3H2
Wöhler and Deville also documented vigorous reactions between boron and halogens resulting in boron trichloride, Boron trifluoride and boron tribromide, e.g.,

2 B + 3 Br2 → 2 BBr3
Boron can form compounds whose formal oxidation state is not three, such as B(II) in B2F4. Boron compounds such as BCl3 behave as electrophiles or Lewis acids in their reactions. Boron is the least electronegative non-metal.

History and etymology
The name boron comes from the Arabic word buraq and the Persian word burah; which are names for the mineral borax.

Boron compounds were known thousands of years ago. Borax was known from the deserts of western Tibet, where it received the name of tincal, derived from the Sanskrit. Borax glazes were used in China from AD300, and some tincal even reached the West, where the Arabic alchemist Geber seems to mention it in 700. Marco Polo brought some back to Italy in the 13th century. Agricola, around 1600, reports its use as a flux in metallurgy. In 1777, boric acid was recognized in the hot springs or soffioni near Florence, Italy, and became known as sal sedativum, with mainly medical uses. The rare mineral is called sassolite, which is found at Sasso, Italy. This was the main source of European borax from 1827 to 1872, at which date American sources replaced it.

Boron was never recognized as an element until it was isolated by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808 through the reaction of boric acid and potassium. Davy called the element boracium. Jöns Jakob Berzelius identified boron as an element in 1824. The first pure boron was arguably produced by the American chemist W. Weintraub in 1909.
Boron is a relatively rare element in the Earth's crust, representing only 0.001%. The worldwide commercial borate deposits are estimated as 1010 kg. Turkey and the United States are the world's largest producers of boron. Turkey has almost 72% of the world’s boron reserves. Boron does not appear on Earth 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 fiber-optics properties.

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 Eskişehir, Kütahya and Balıkesir

Production
Pure elemental boron is difficult to extract. The earliest methods involved reduction of boric oxide with metals such as magnesium or aluminum. However the product is almost always contaminated with metal borides. Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures. Very pure boron, for the use in semiconductor industry, is produced by the decomposition of diborane at high temperatures and then further purified with the zone melting or Czochralski processes