Boron

Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride.
Boron is synthesized entirely by cosmic ray spallation and supernovas and not by stellar nucleosynthesis, so it is a low-abundance element in the Solar System and in the Earth's crust. It constitutes about 0.001 percent by weight of Earth's crust. It is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax and kernite. The largest known deposits are in Turkey, the largest producer of boron minerals.
Elemental boron is found in small amounts in meteoroids, but chemically uncombined boron is not otherwise found naturally on Earth.
Several allotropes exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard, and a poor electrical conductor at room temperature. The primary use of the element itself is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all production consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and heat-resistant materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. As sodium perborate, it is used as a bleach. A small amount is used as a dopant in semiconductors and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which has a number of uses as a neutron-capturing agent.
Borates have low toxicity in mammals but are more toxic to arthropods and are occasionally used as insecticides. Boron-containing organic antibiotics are known. Although only traces are required, boron is an essential plant nutrient.

History

The word boron was coined from borax, the mineral from which it was isolated, by analogy with carbon, which boron resembles chemically.
Borax in its mineral form first saw use as a glaze, beginning in China circa 300 AD. Some crude borax traveled westward, and was apparently mentioned by the alchemist Jabir ibn Hayyan around 700 AD. Marco Polo brought some glazes back to Italy in the 13th century. Georgius Agricola, in around 1600, reported the use of borax as a flux in metallurgy. In 1777, boric acid was recognized in the hot springs near Florence, Italy, at which point it became known as sal sedativum, with ostensible medical benefits. The mineral was named sassolite after Sasso Pisano in Italy. Sasso was the main source of European borax from 1827 to 1872, when American sources replaced it. Boron compounds were rarely used until the late 1800s when Francis Marion Smith's Pacific Coast Borax Company first popularized and produced them in volume at low cost.
Boron was not recognized as an element until it was isolated by Sir Humphry Davy and by Joseph Louis Gay-Lussac and Louis Jacques Thénard. In 1808, Davy observed that electric current sent through a solution of borates produced a brown precipitate on one of the electrodes. In his subsequent experiments, he used potassium to reduce boric acid instead of electrolysis. He produced enough boron to confirm a new element and named it boracium. Gay-Lussac and Thénard used iron to reduce boric acid at high temperatures. By oxidizing boron with air, they showed that boric acid is its oxidation product. Jöns Jacob Berzelius identified it as an element in 1824. Pure boron was arguably first produced by the American chemist Ezekiel Weintraub in 1909.

Characteristics of the element

Isotopes

Boron has two naturally occurring and stable isotopes, ¹¹B and ¹⁰B. The mass difference results in a wide range of δ¹¹B values, which are defined as a fractional difference between the ¹¹B and ¹⁰B and traditionally expressed in parts per thousand, 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 with a half-life of 3.5×10⁻²² s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B₃ and tetrahydroxyborate|⁻. Boron isotopes are also fractionated during mineral crystallization, during H₂O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect results in preferential removal of the ⁻ ion onto clays. It results in solutions enriched in ¹¹B₃ and therefore 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.
The exotic ¹⁷B exhibits a nuclear halo, i.e. its radius is appreciably larger than that predicted by the liquid drop model.

NMR spectroscopy

Both ¹⁰B and ¹¹B possess nuclear spin. The nuclear spin of ¹⁰B is 3 and that of ¹¹B is. These isotopes are therefore of use in nuclear magnetic resonance spectroscopy, and spectrometers specially adapted to detecting the boron-11 nuclei are available commercially. The ¹⁰B and ¹¹B nuclei also cause splitting in the resonances of attached nuclei.

Allotropes

Boron forms four major allotropes: α-rhombohedral and β-rhombohedral, γ-orthorhombic, and β-tetragonal. All four phases are stable at ambient conditions. β-rhombohedral is the most stable and the most common. An α-tetragonal phase also exists but is very difficult to produce without significant contamination. Most of the phases are based on B₁₂ icosahedra, but the γ phase can be described as a rocksalt-type arrangement of the icosahedra and B₂ atomic pairs. It can be produced by compressing other boron phases to 12–20 GPa and heating to 1500–1800 °C; it remains stable after releasing the temperature and pressure. The β-T phase is produced at similar pressures, but higher temperatures of 1800–2200 °C. The α-T and β-T phases might coexist at ambient conditions, with the β-T phase being the more stable. Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures below 6–12 K.

Boron phase	α-R	β-R	γ	β-T
Symmetry	Rhombohedral	Rhombohedral	Orthorhombic	Tetragonal
Atoms/unit cell	12	~105	28
Density	2.46	2.35	2.52	2.36
Vickers hardness	42	45	50–58
Bulk modulus	185	224	227
Bandgap	2	1.6	2.1

Atomic structure

Atomic boron is the lightest element having an electron in a p-orbital in its ground state. Its first three ionization energies are higher than those for heavier group III elements, reflecting its electropositive character.

Chemistry of the element

Preparation

Elemental boron is rare and poorly studied because the pure material is extremely difficult to prepare. Most studies of "boron" involve samples that contain small amounts of carbon. Very pure boron is produced with difficulty because of contamination by carbon or other elements that resist removal.
Some early routes to elemental boron involved the reduction of boric oxide with metals such as magnesium or aluminium. However, the product was often contaminated with borides of those metals. Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures. Ultrapure boron for use in the semiconductor industry is produced by the decomposition of diborane at high temperatures and then further purified by the zone melting or Czochralski processes.

Reactions of the element

boron is a hard, black material with a melting point of above 2000 °C. Crystalline boron is chemically inert and resistant to attack by boiling hydrofluoric or hydrochloric acid. 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.
Since elemental boron is very rare, its chemical reactions are of little significance. The elemental form is not typically used as a precursor to compounds. Instead, boron compounds are produced from borates.
When exposed to air, under normal conditions, a protective oxide or hydroxide layer forms on the surface of boron, which prevents further corrosion. The rate of oxidation of boron depends on the crystallinity, particle size, purity and temperature. At higher temperatures boron burns to form boron trioxide:
File:Tetraborate-xtal-3D-balls.png|thumb|right|Ball-and-stick model of tetraborate anion, ²⁻, as it occurs in crystalline borax, Na₂·8H₂O. Boron atoms are pink, with bridging oxygens in red, and four hydroxyl hydrogens in white. Note two borons are trigonally bonded sp² with no formal charge, while the other two borons are tetrahedrally bonded sp³, each carrying a formal charge of −1. The oxidation state of all borons is III. This mixture of boron coordination numbers and formal charges is characteristic of natural boron minerals.