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.

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.

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Atomic properties

Standard atomic weight10.81 ±0.02 [10.806 … 10.821]
Atomic mass10.811 u

Atomic radii

Radius (empirical)90 pm
Radius (calculated)87 pm
Covalent radius84 ±3 pm
Van der Waals radius192 pm

Atomic shell

Electron configurationHe 2s2 2p1
Ionization energy(1st) 8.298019 eV
(2nd) 25.15483 eV
(3rd) 37.93058 eV
(4th) 259.3715 eV
(5th) 340.22602 eV
Shell model

Physical properties

Density2.35 g·cm−3
Molar volume4.39·10-6 m3·mol−1
Speed of sound16,200 m·s−1 (293.1 K)


Melting point2,349 K
Boiling point4,200 K
Liquid range1,851 K
Transition temperature11.2 K


Melting enthalpy50 kJ·mol-1
Enthalpy of vaporization507 kJ·mol-1
Binding energy563 kJ·mol-1

Heat and conductivity

Specific heat capacity1,260 J·kg−1·K−1
Thermal conductivity27 W·m-1·K-1
Expansion coefficient6·10-6 K-1
Work function4.45 eV


Mohs hardness9.3
Vickers hardness4,900 NM·m-2

Elastic properties

Bulk modulus320 GPa

Electrical properties

Electrical conductivity1·10-4 S·m-1
Resistance10,000 Ωm


Magnetic susceptibility-6.7·10-6 cm3·mol−1

Chemical properties

Basicityslightly acidic
Oxidation state+3, +2, +1


Pauling scale2.04
Sanderson scale2.28
Allred-Rochow scale2.01
Mulliken scale2.04
Allen scale2.051
Ghosh-Gupta scale4.968 eV
Boyd-Edgecombe scale1.9
Nagle scale2.02
Pearson absolute negativity4.29 eV

Other properties

Natural occurrenceprimordial
Crystal structureRhombohedral
Goldschmidt Classificationlithophile
Superconductorwith transition tempperature under special conditions
Price/kg3.68 USD

Natural abundances

1 ppb ≈ 1·1010 M☉
2 ppb ≈ 3.97·109 Mt
1,600 ppb ≈ 1.59 g
Earth’s crust
8,700 ppb ≈ 240,000 Mt
4,440 ppb ≈ 6.08 Mt
Flowing water
10 ppb ≈ 160 kt
Human body
700 ppb ≈ 49 mg