Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. A low-abundance element in both the solar system and the Earth's crust, boron is concentrated on Earth by the water-solubility of its more common naturally-occurring compounds, the borate minerals. These are mined industrially as evaporate ores, such as borax and kernite.
Elemental boron is not found naturally. Industrially, very pure isolated boron is produced with difficulty, as boron tends to form refractory materials containing small amounts of carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder and crystalline boron is black, extremely hard (about 9.5 on Mohs' scale), and a poor conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry.
The major uses of boron compounds are in sodium perborate bleaches, and the borax component of fiberglass insulation. Boron compounds play specialized roles as high-strength lightweight structural and refractory materials. They are used in glasses and ceramics to give them resistance to thermal shock. Boron-containing reagents are used for the synthesis of organic compounds, and as an intermediate in the synthesis of pharmaceuticals that do not contain boron.
In biology, borates have low toxicity in mammals (similar to table salt), but much more so to many arthropods. A boron-containing natural antibiotic is known. Small amounts of boron compounds play a strengthening role in the cell walls of all plants, making boron a necessary element in soils. Experiments indicate a role for boron as an ultratrace element in animals, but the nature of its role in animal physiology is unknown.
