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Element Fact
There are around 120 known elements. Of these, 91 elements occur naturally on Earth. The rest no longer exist or are found only in traces as intermediate decay products alongside with other radioisotopes.

Boron, B

amorphous boron sample

Atomic number: 5
Atomic mass: 10.811(7)
Natural abundance: 9 ppm
10B (10.012937) - 19.9%
11B (11.009306) - 80.1%

History and Production

From Arabic buraq, meaning borax or Persian burah. Boron compounds have been known for thousands of years, but the element itself was only discovered in 1808 by L.J. Thenard and L.J. Lussac from Paris and Sir H. Davy from London. Pure boron can be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on heated filament. The less pure, or amorphous boron can be obtained by heating the trioxide with magnesium powder.

The amorphous element is used in pyrotechnic flares to provide a distinctive green color and as a rocket igniter. The isotope 10B is used as a nuclear reactor controller, as a shield for nuclear radiation.

Physical Data

Amorphous boron is a brownish-black powder (picture above) while the pure crystalline forms are dark red in transmitted light. It is extremely hard with high melting point but with low electrical conductivity. The elemental form does not occur in nature but exists as orthoboric acid and as borates in borax and colemanite minerals. The element shows complex allotropic modification and due to its unique bonding structure the chemistry of boron and its compounds are very complex, both in terms of structural and diversity.

Interatomic distance: 159.0 pm
Melting point: 2075C
Boiling point: 4000C
Thermal conductivity/Wm-1K-1: 27 (27C)
Density/kgm-3: 2340 (beta-rhombohedral, 20C)

Standard Thermodynamic Data (rhombic, atomic gas)

Enthalpy of formation: 565 kJ/mol
Gibbs free energy of formation: 521 kJ/mol
Entropy: 153.4 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [He] 2s2 2p1
Term symbol: 2P1/2
Electron affinity: 26.9892 kJ/mol   Electronegativity (Pauline): 2.04
Ionization energy (first, second, third): 800.6, 2427, 3660 kJ/mol

Chemical property

Boron forms a wide variety of inorganic compounds than all other elements. It has a very rich and extensive bonding chemistry due to its small size and high ionization energy. Important compounds are such as hydrides, borides, and borates. Each has their own unique bonding environment and chemistry behavior. Many form complexes and polyhedral structures. Boron is stable in air at ordinary temperature and does not react with water. But it readily reacts with steam at red heat, giving boric acid, H3BO3.

Test for boron:
(1) Volatile boron compounds burn with a green flame. If a solid borate is mixed with methanol and concentrated sulfuric acid, the volatile compound boron trimethoxide, B(OCH3)3, is formed and ignition of the alcohol therefore produces a green flame.
(2) Boric acid, or a borate to which acid has been added will turn yellow turmeric paper brown. Addition of an alkali then produces a bluish color. This test is more positive if a fiber of rayon, soaked and dried in turmeric, is used instead of a paper.

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