Compounds of boron, notably borax, have been known since early times, but the pure element was first prepared in 1808 by the French chemists Joseph Gay-Lussac and Baron Louis Thénard, and independently by the British chemist Sir Humphry Davy. It is a trace element needed for plant growth, but toxic in excess. Research suggests that it is also nutritionally important for bone health in humans and other vertebrates.
Pure boron, as usually prepared, is a powder, though a crystalline form can be prepared by dissolving boron in molten aluminium and cooling slowly. The atomic weight of boron is 10.81; the element melts at about 2180° C (3956° F), boils at about 3650° C (6602° F), and has a relative density of 2.35.
Boron does not react with water or hydrochloric acid and is unaffected by air at ordinary temperatures. At red heat it combines directly with nitrogen to form boron nitride (BN), and with oxygen to form boron oxide (B2O 3). With metals it forms borides, such as magnesium boride (Mg3 B2). The original sources of boron compounds were the minerals borax and boric acid. More recently, important boron ores have been, in the United States, ulexite (NaCaB5O9· 8H2O), colemanite (Ca2B6O11· 5H 2O), and kernite (Na2B4O7· 4H2 O); and in Germany, boracite (Mg7Cl2B16O 30). Boron ranks about 38th in natural abundance among the elements in the Earth's crust.
Although boron has a valence of 3 and its position in the periodic table would indicate a close relationship to aluminium, it is actually much more like carbon and silicon in chemical properties. In its compounds, boron acts like a non-metal, but unlike most non-metals, pure boron is an electrical conductor, like the metals and like carbon (graphite). Crystalline boron is similar to diamond in appearance and optical properties, and is almost as hard as diamond. Most... [continues]
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