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 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.
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 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 aluminium 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.
Elemental boron exists in a number of allotropes. The simplest structure is <math>alfa</math>-rhombohedral boron. 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.
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)
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.