High-purity crystalline boron. Image source: www.smart-elements.com.
Boson's opsition in the periodic table.
Also described as a metalloid element, the discovery of boron was first announced in 1808 by Humphrey Davy in London and, independently, by the French chemists Joseph Louis Gay-Lussac and Louis-Jaques Thénard in Paris. It later turned out that in both cases the "element" claimed was actually a compound containing not more than 60–70% boron. It was identified as an element by Jons Jacob Berzelius in 1824; although reasonably pure boron – 99% pure was not obtained until 1909. This was not the end of the story, however, because even small amounts of impurities (1% of less) alter the structure and properties of boron profoundly. In fact, boron is affected by impurities to an extent unprecedented among elements and makes the study of its chemistry extremely difficult.
The difficulty in isolating and studying boron in its pure state has resulted in it being the only element for which the ground state is not experimentally known even at ambient conditions. Only in 2009 was work published which established the phase diagram of boron and the existence of a new most dense, hard phase of the element. This phase represents the first known case of an ionic crystal consisting of a single element.
|ionization energy||first: 899 kJ/mol
second: 2,422 kJ/mol
third: 3,657 kJ/mol
|melting point||2,300°C (4,172°F)|
|boiling point||2,550°C (4,622°F)|
|density (at 20°C)||2.34 g/cm3|
Uses of boron
Boron-10, one of two naturally occurring isotopes of boron (the other is boron-11), is a good absorber of neutrons and is therefore used in the control rods of nuclear reactors, as a radiation shield, and as a neutron detector (see boron counter). Boron filaments have found application in the aerospace industry because of their high strength and low weight. Boron is added to steels to make them hard. Pyrex glass is tough and heat resistant because of the boric oxide used to make it.
Chemistry of boron
Boron forms compounds, directly with fluorine, chlorine, bromine, oxygen, sulfur, and carbon. Boron carbide, which is almost as hard as diamond, is formed when boron and carbon are heated together in an electric furnace. With metals, boron forms borides, made by combining the elements at high temperature; all borides are extremely hard and are used as abrasives and refractories. Although the properties of boron and most of its compounds are those of a non-metal, the element is unusual in that in boron phosphate it behaves like a metal.
A great many uses have been found for the salts of boric acid – the borates. Borax – sodium borate – is a white soluble salt used in glass-making, soldering, the pottery industry, and in treating fabrics to make them fireproof. Boric acid, a white solid which can be made from borax, is used as a mild antiseptic, sometimes under the name of boracic acid.
If borax is heated sufficiently it melts to form a glassy substance in which metallic oxides will dissolve. The compounds that are formed by the reaction between borax and the metal oxides are sometimes highly colored, the color being characteristic of the metal. Thus it is possible to identify the metal present by the color it gives to a borax bead. On account of the ability of metallic oxides to dissolve in molten borax, borax is used as a flux component in removing oxides from metal surfaces in preparation for soldering or welding.
Borax is used in the manufacture of boro-silicate (Pyrex) glass which has good resistance to sudden temperature changes. The principal constituents of this type of glass (from which ovenware is made) are silica (about 80%) and borax (about 12%). Borax is also used in glazing pottery and other ceramic articles, in vitreous enamel finishes for stoves, cookers, and baths, and for coating paper.
Boron and life
Boron is an essential trace element for the growth of plants, but is not required by animals – in fact, it can be toxic in excess. We take in about 2 milligrams of boron each day in our food. The possibility of boron-based life has been considered.