Boron

Boron, ₅B
	boron (β-rhombohedral)
General properties
Pronunciation	/ˈbɔːrɒn/ (BOHR-on)
Allotropes	α-, β-rhombohedral, β-tetragonal (and more)
Appearance	black-brown
Standard atomic weight (Ar, standard)	[10.806, 10.821]
Boron in the periodic table
	beryllium ← boron → carbon
Atomic number (Z)	5
Group	group 13 (boron group)
Period	period 2
Block	p-block
Element category	metalloid
Electron configuration	[He] 2s2 2p1
Electrons per shell	2, 3
Physical properties
Phase at STP	B: 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)
Atomic properties
Oxidation states	−5, −1, +1, +2, +3 (a mildly acidic oxide)
Electronegativity	Pauling scale: 2.04
Ionization energies	1st: 800.6 kJ/mol ; 2nd: 2427.1 kJ/mol ; 3rd: 3659.7 kJ/mol ; (more) ;
Atomic radius	empirical: 90 pm
Covalent radius	84±3 pm
Van der Waals radius	192 pm
	Spectral lines of boron
Other properties
Natural occurrence	B: Primordial
Crystal structure	rhombohedral
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)
Magnetic ordering	diamagnetic
Magnetic susceptibility	−6.7·10−6 cm3/mol
Mohs hardness	~9.5
CAS Number	7440-42-8
History
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 may be as low as 19.1% and as high as 20.3% in natural samples. 11B is the remainder in such cases.
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Boron is a chemical element. It has the chemical symbol B. It has the atomic number 5. It is a metalloid (it has properties of a metal and a non-metal). Much boron is found in chemical compounds in its ore borax. Boron is never found free in nature.

Boron in a tube

Electrofluorochromic Properties of Aza Boron dipyrromethene

Two types of boron are found (allotropes). Amorphous boron is a brown powder and metallic (crystalline) boron is black and hard and a weak conductor at room temperature. Boron is the 5th element in the periodic table and is part of the Earth's surface. Pure boron is used as a dopant (a substance added to semiconductors to change how it behaves with electricity) in the semiconductor industry. Chemical compounds of boron are important as to make strong materials not weigh very much, as nontoxic insecticides and preservatives, and for chemical synthesis.

Plants need boron in them to live. Very small amounts of boron are needed in animal's bodies so that they are very healthy. How it keeps animals healthy is not known in detail.

Boron was discovered by Sir Humphry Davy, an English chemist, in 1808.

Boron melts at 2075 °C (3767 °F), and boils at 4000 °C (7232 °F).

Boron Media

Yellow sassolite from Vulcano island, Italy. Photograph taken at the Natural History Museum, London.
Bor
Boron (III) trifluoride structure, showing "empty" boron p orbital in pi-type coordinate covalent bonds
Ball-and-stick models showing the structures of the boron skeletons of borane clusters. The structures can be rationalised by polyhedral skeletal electron pair theory.
Ball-and-stick model of superconductor magnesium diboride. Boron atoms lie in hexagonal aromatic graphite-like layers, with a charge of −1 on each boron atom. Magnesium(II) ions lie between layers
Neutron cross section of boron (top curve is for 10B and bottom curve for 11B)
Photograph of mineral ulexite from California, USA, displaying its characteristic optic property. Photographed by Andres Boni.
Borax crystals from Kramer, California, USA. Photograph taken at the Natural History Museum, London.
Borosilicate glassware. Displayed are two beakers and a test tube.
Launch of Apollo 15 Saturn V rocket, using triethylborane ignitor