Boron

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 BnHn+4 or the less stable formula BnHn+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&mdash;the average over different isotopes weighted by abundance&mdash;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.