Boron

Boron 10.811(7)   B 5 1s22s22p1 13,2,p [ ? ] Metalloid: Properties: hard, semiconductor Compounds: Oxides, halides and hydrides Uses: Reducing agents, Lewis acid catalysis, nuclear industry

Boron, with atomic number Z = 5 and chemical symbol B, is a rare element present in only 0.0003% of the earth's crust, mostly found in dry lake beds in the western United States in the form of borax (Na₂B₄O₅(OH)₅• 8H₂O), and kernite (Na₂B₄O₅(OH)₅•2H₂O), which are hydrated sodium salts of tetraboric acid. Borax is mildly alkaline and is used as a cleansing agent. In addition, due its low melting temperature, boron is used in flux for soldering and welding. Some boron compounds are Lewis acids and elemental boron forms three-center two-electron bonds. Borohydrides are widely used as chemical reducing agents.

Boric acids

Boric acid, H₃BO₃, is a mildly acidic antiseptic compound formed from the reaction of borax with sulfuric acid. Boric acids include orthoboric acid (H₃BO₃), metaboric acid (HBO₂) and tetraboric acid, (H₂B₄O₇), which is also called pyroboric acid.

Boron halides

All four boron trihalides are planar, non-polar, covalent compounds. Boron trifluoride is a gas, boron triiodide is a solid and boron trichloride and tribromide are liquids. Boron trihalides are Lewis acids and react readily with water to produce boric acid and the corresponding hydrogen halide.

Boron hydrides

Boron hydrides are excellent sources of the hydride ion H^- and thus are good reducing agents. Sodium borohydride, NaBH₄, is a reducing agent used in many inorganic and organic reactions. Lithium borohydride (LiB₄) and aluminum borohydride (Al(BH₄)₃ are also common borohydrides used in chemistry. The reduction of ketones to secondary alcohols is a typical use for these reagents. Boron hydrides can fall into one of two categories, those with formula B_nH_n+4 or the less stable formula B_nH_n+6.

Allotropes

Elemental boron exists in a number of allotropes. $alfa$-rhombohedral boron is the simplest structure. All forms are polyhedral clusters of boron atoms, are semi-conductors and are very hard materials. All forms of elemental boron contain both three-center two-electron bonds as well as the typical two-center two-electron bonds found in most molecules.

Stable isotopes

The standard atomic mass—the average over different isotopes weighted by abundance—of boron is 10.811 u, where u is the unified atomic mass unit. Boron has two stable isotopes:

Isotope    atomic mass        natural abundance (%)

¹⁰B        10.012 937 0(4)        19.9(7)

¹¹B        11.009 305 5(5)        80.1(7)

Nuclear industry

Both ¹⁰B and ¹¹B are used in the nuclear industry. ¹¹B is used as a neutron reflector and ¹⁰B is used in boron neutron capture therapy. Both isotopes can be used to synthesize the elements ¹¹C and ¹³N. Unstable forms of boron include the following isotopes

Radio-isotope    atomic mass    half-life (%)

  ⁸B                      8.024607          0.770 s

  ⁹B                      9.013379          8 x 10^-19 s

¹²B                    12.014352          0.0202 s

¹³B                    13.017780          0.0174 s

It is used for hardening metals in unspecified nuclear weapons.