Boron is a chemical element with the 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.
|Allotropes||α-, β-rhombohedral, β-tetragonal (and more)|
|Standard atomic weight Ar°(B)|
|Boron in the periodic table|
|Atomic number (Z)||5|
|Group||group 13 (boron group)|
|Electron configuration||[He] 2s2 2p1|
|Electrons per shell||2, 3|
|Phase at STP||solid|
|Melting point||2349 K (2076 °C, 3769 °F)|
|Boiling point||4200 K (3927 °C, 7101 °F)|
|Density when liquid (at m.p.)||2.08 g/cm3|
|Heat of fusion||50.2 kJ/mol|
|Heat of vaporization||508 kJ/mol|
|Molar heat capacity||11.087 J/(mol·K)|
|Oxidation states||−5, −1, 0, +1, +2, +3 (a mildly acidic oxide)|
|Electronegativity||Pauling scale: 2.04|
|Atomic radius||empirical: 90 pm|
|Covalent radius||84±3 pm|
|Van der Waals radius||192 pm|
|Spectral lines of boron|
|Speed of sound thin rod||16,200 m/s (at 20 °C)|
|Thermal expansion||β form: 5–7 µm/(m⋅K) (at 25 °C)|
|Thermal conductivity||27.4 W/(m⋅K)|
|Electrical resistivity||~106 Ω⋅m (at 20 °C)|
|Molar magnetic susceptibility||−6.7×10−6 cm3/mol|
|Discovery||Joseph Louis Gay-Lussac and Louis Jacques Thénard (30 June 1808)|
|First isolation||Humphry Davy (9 July 1808)|
|Main isotopes of boron|
|10B content is 19.1–20.3% in natural samples, with the remainder being 11B.|
Boron is synthesized entirely by cosmic ray spallation and supernovae 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 a metalloid that is found in small amounts in meteoroids but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, the very pure element is produced with difficulty because of contamination by carbon or other elements that resist removal. Several allotropes exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), 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 (boron-10) has a number of uses as a neutron-capturing agent.
The intersection of boron with biology is very small. Consensus on it as essential for mammalian life is lacking. Borates have low toxicity in mammals (similar to table salt) but are more toxic to arthropods and are occasionally used as insecticides. Boron-containing organic antibiotics are known. Although only traces are required, it is an essential plant nutrient.