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Boron,  5B
General properties
Name, symbol Boron, B
Allotropes α-, β-rhombohedral, α-, β-tetragonal, γ-orthorhombic, Amorphous
Appearance Black-brownish solid
Boron in the periodic table


Atomic number 5
Standard atomic weight (Ar) 10.81
Group, block (boron group); p-block
Period period 2
Electron configuration [He] 2s2 2p1
per shell
2, 3
Physical properties
Black or brown
Phase Solid
Melting point 2349 K ​(2076 °C, ​3769 °F)
Boiling point 4200 K ​(3927 °C, ​​7101 °F)
Density when liquid, at  2.08 g/cm3
Heat of fusion 50.2 kJ/mol
Heat of 508 kJ/mol
Molar heat capacity 11.087 J/(mol·K)
Atomic properties
Oxidation states 3, 2, 1, −1, −5​ ​(a mildly acidic oxide)
Electronegativity Pauling scale: 2.04
energies 1st: 800.6 kJ/mol
2nd: 2427.1 kJ/mol
3rd: 3659.7 kJ/mol
Atomic radius empirical: 90 pm
Covalent radius 84±3 pm
Van der Waals radius 192 pm
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)
Bulk modulus 185 GPa (α-rhombohedral)
224 GPa (β-rhombohedral)
227 GPa (γ-orthorhombic) GPa
CAS Registry Number 7440-42-8
Discovery Joseph Louis Gay-Lussac and Louis Jacques Thénard (30 June 1808)
First isolation Humphry Davy (9 July 1808)
· references

Boron is a metalloid with symbol B and atomic number 5. It is a black solid, resistant to the attack of most reagents at at room temperature, even aggressive haloacids, like hydrofluoric acid.



Boron's chemical properties are closer to silicon than to aluminium, the element next in group. Crystalline boron is chemically inert and resistant to attack by hot hydrohalic acids, such as hydrochloric acid and hydrofluoric acid. When finely divided, elemental boron is attacked slowly by hot oxidizing agents and mixtures, like hot concentrated hydrogen peroxide, hot concentrated nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids. Boron does not react with air at standard conditions, but it burns at high temperatures to form boron trioxide.

Boron halides are Lewis acids due to an unfilled valence shell.

Boron-containing compounds can be tested for in solution using curcumin. The normally yellow curcumin reacts with boron, turning deep red. This test will not work in basic conditions, however, as curcumin is normally red in strongly basic solutions.


Chemically, boron is a metalloid. It is a brown-black solid at standard conditions. It has a very high melting point, 2076 °C and boils at 3927 °C. Its density varies between 2.35 and 2.52 g/cm3.

Boron exists as two main allotrope forms:

  • Amorphous boron: brown powder, similar in aspect with cocoa powder. This one in turn consists of two forms: powder (brown-black) and glassy (opaque black).
  • Crystalline boron: black, extremely hard (about 9.5 on the Mohs scale). It is a poor electrical conductor at room temperature. Crystalline boron also consists of several varieties: α-rhombohedral, α-tetragonal, β-rhombohedral, β-tetragonal, γ-orthorhombic, cubic, high-pressure superconducting and borospherene.[1] Among these, β-rhombohedral is the most thermodynamically stable allotrope and the most commonly encountered.


Both amorphous and crystalline boron can be bought online. Metallium sells both types, sealed in glass ampoules.


Elemental boron can be extracted from boric acid, by dehydrating it to boron trioxide and reducing the oxide with magnesium. The resulting mass, more of a slag actually is treated with hydrochloric acid to remove the impurities. This part should be done slowly, as the residual magnesium diboride reacts with HCl to release borane and diborane that are very toxic and can be pyrophoric in moist air. Once the reaction has stopped, filter the black boron powder and dry it.[2]


  • Make boranes and organoboron compounds
  • Boron halides



Elemental boron, boron oxide, boric acid, borates, and certain organoboron compounds are non-toxic to humans and animals, but are toxic for insects. Boron halides are corrosive.


No special storage is required.


Boron poses little toxicity to the environment and can be dumped in the trash.



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