Boron is the first element in Group 13 (IIIA) of the periodic table. The elements in this group are usually referred to as the aluminum family elements. Boron is quite different from other members of the family. One difference is that boron is not a metal. All other members of the family (aluminum, gallium, indium, and thallium) are metals. Compounds of boron have been used for centuries. For example, borax, a boron compound, has long been used to make glass and glazes. The element itself was not identified until 1808. The most important use of boron is still in glass manufacture. Agriculture, fire retardants, and soaps and detergents rely on boron compounds.
The first mention of boron compounds is found in a book by Persian alchemist Rhazes. Alchemists studied the nature of matter before modern chemistry was born. Rhazes classified minerals into six classes, one of which was the boraces, which included borax. Borax was widely used by crafts people. It reduces the melting point of materials used to make glass. It was also used to melt the ores of metals and to isolate the metals from those ores. In 1808, English chemist Humphry Davy had just learned how to isolate the most active metals, such as sodium and potassium. He was also working on a method to remove boron from its compounds.
News of Davy's success had traveled to France, where emperor Napoleon Bonaparte grew concerned about the scientific reputation of his country. He ordered larger and better equipment built for his scientists. He wanted them to surpass Davy in his work on metals. This equipment was designed especially for two French chemists, Louis Jacques Thênard and Joseph Louis Gay-Lussac. Thênard and Gay-Lussac found a new way to separate boron from its compounds. They heated boracic acid (also known as boric acid, H3 BO3) with potassium metal to produce impure boron. Thênard and Gay-Lussac were given credit for discovering the new element. In 1892, French chemist Henri Moissan (1852-1907) produced boron that was 98 percent pure. The names borax and boracic acid probably originated as far back as the time of Rhazes as Buraq (in Arabic) or Burah (in Persian).
One of the unusual properties of boron is its many physical forms, called allotropes, in which it occurs. While one form of boron consists of clear red crystals with a density of 2.46 grams per cubic centimeter, a second form consists of black crystals with a metallic appearance and a density of 2.31 grams per cubic centimeter. Boron can also occur as a brown powder with no crystalline structure. The density of this powder is 2.350 grams per cubic centimeter. All forms of boron have very high melting points, from 2,200 to 2,300°C (4,000 to 4,200°F). One property of special importance is boron's ability to absorb neutrons. Boron atoms are able to absorb a large number of neutrons. This makes boron useful in the control rods of nuclear reactors, devices for generating energy from nuclear fission reactions. In a nuclear reactor, it is essential that just the right number of neutrons is present. While too many neutrons can cause a fission reaction to get out of control, too few neutrons will make a fission reaction stop. Control rods are long tubes packed with boron (or some other element). The rods can be raised and lowered in the reactor. As the rods are lowered into the core, the boron absorbs neutrons, slowing the reaction.
Boron combines with oxygen in the air to form boron trioxide (B2O3) that forms a thin film on the surface which prevents further reaction with oxygen. Boron is not soluble in water. It normally does not react with acids. In powder form, it reacts with hot nitric acid (HNO3) and hot sulfuric acid (H2 SO4). It also dissolves in molten (melted) metals. The abundance of boron in the Earth's crust is estimated to be about 10 parts per million. That places it in about the middle among the elements in terms of their abundance in the earth.
Boron never occurs as a free element. The most common minerals of boron are borax, or sodium borate (Na2B4O7); kernite (another form of sodium borate); colemanite, or calcium borate (Ca2B6O11); and ulexite, or sodium calcium borate (NaCaB5O9). These minerals usually occur as white crystalline deposits in desert areas. The two largest world producers of boron compounds are Turkey and the United States. Smaller amounts come from Argentina and China. Nearly all of the boron in the United States comes from three California counties: Kern, San Bernadino, and Inyo.
Radiation can kill living cells. Light, X- rays, radio waves, and microwaves are all forms of radiant energy. These forms of radiation differ from each other in the amount of energy they carry with them. X rays carry a great deal of energy; light waves, less energy; and radio waves, very little energy. The bad news about high-energy radiation is that it can kill healthy cells. A person exposed to high levels of X-rays will become ill and may die. Because the X-rays kill so many cells, the person's body cannot survive. Essential body functions stop, and death occurs. The good news is that high-energy radiation can be used to kill cancer cells. Cancer cells are abnormal cells that reproduce faster than normal tissue. The rapidly dividing cells form tumors, crowd organs, and shut down some organ function. Radiation is one way to kill cancer cells. The problem lies in killing only the cancer cells. The radiation has to be "targeted" at the cancer (bad) cells, and not the healthy (good) cells. Scientists think that using boron may be one way of achieving this goal. A new procedure called boron neutron capture therapy (BNCT) is one method for targeting cancer cells.
With BNCT, a person with cancer receives an injection of boron. The boron tends to go directly to cancer cells. The patient receives an injection of boron that deposits only in the cancer cells. Scientists currently do not know why boron favors cancer cells. But it does. The patient's body is then bombarded with neutrons that pass through without harming health cells. They then collide with boron atoms. This causes boron to be converted into lithium atoms, alpha particles, and gamma rays. An alpha particle is a helium atom without electrons. Gamma radiation is very high-energy radiation that can kill cells. The lithium atoms and alpha particles travel only a short distance. They do not leave the cancer cell but have enough energy to kill the cell. Since they do not leave the cell, they pose no threat to healthy cells nearby. BNCT is not fully developed. But it holds great promise as a cancer treatment.
Two naturally occurring isotopes of boron: boron-10 and boron-11exist. Boron-10 is the isotope with high neutron-absorbing tendencies. Boron alloys are used in magnets in microphones and headphones. Three radioactive isotopes of boron are known also but none of the radioactive isotopes of boron have any important commercial uses. Boron is still produced by a method similar to that used by Thênard and Gay-Lussac. Boric oxide is heated with powdered magnesium or aluminum. The element can also be obtained by passing an electric current through molten (melted) boron trichloride.
Boron is used to make certain types of alloys. The most important of these alloys commercially are used to make some of the strongest magnets known. The rare earth magnets, for example, are made from boron, iron, and neodymium. These magnets are used for microphones, magnetic switches, loudspeakers, headphones, particle accelerators, and many technical applications. The use of boron in nuclear power plants was described above under "Physical properties."
The most important of boron compounds is sodium borate (Na2 B4 O7 ), used in the manufacture of borosilicate glass, glass fiber insulation, and textile glass fiber. The addition of sodium borate to glass makes it easier to work while it is molten. The final glass is not attacked by acids or water, is very strong, and resists thermal shock. Resistance to thermal shock means the glass can be heated and cooled very quickly without breaking. The Pyrex glass used in kitchenware and chemistry laboratories is a form of borosilicate glass. High quality optical glass, like that used in telescopes, is also made from borosilicate glass.
Glass fiber insulation is also made from borosilicate glass by forcing it through narrow openings. The glass comes out as a thin fiber and is then spun into insulation. These fibers trap air. Since neither the borosilicate fibers nor air is a good conductor of heat, it makes excellent insulation. Much of the insulation used in private homes, office buildings, and other structures is made of borosilicate fibers. Fibers made from borosilicate glass are also used in making cloth. Borosilicate fibers are blended with other synthetic fibers to make durable fabric for automobile seat covers and other long-wear applications.
Boron also forms important compounds with two other elements, carbon and nitrogen. Boron carbide and boron nitride are important compounds because of their hardness. In fact, boron nitride may be the hardest substance known. Both compounds have very high melting points: 2,350°C (4,300°F) for boron carbide and more than 3,000°C (5,400°F) for boron nitride. These properties make boron carbide and boron nitride useful as abrasives and refractories. An abrasive is a powdery material used to grind or polish other materials and a refractory material is one that can withstand very high temperatures by reflecting heat. Refractory materials line ovens to maintain high temperatures.
Studies suggest that a lack of boron may lead to arthritis and other disorders of the skeleton. Boron carbide and boron nitride are used in high-speed tools, military aircraft and spacecraft, heat shields, and specialized heat-resistant fibers. They also are found in face powders, cream make-ups, and lipsticks. Small amounts of boron compounds are also used to control the growth of weeds in agriculture, and as insecticides, fertilizers, and flame retardants. A flame retardant is a material that prevents another material from catching fire and burning with an open flame.
Though the role of boron in human health is not well understood, there is growing evidence that very small amounts of boron may be required to maintain healthy bones. Studies suggest that a lack of boron may lead to arthritis and other disorders of the skeleton. Boron may also be necessary for healthy brain functions, such as memory and hand-eye coordination. But no specific recommendations have been made by health authorities. However, some experts believe 1.5 to 3.0 milligrams of boron should be included in the daily diet. Most people get this much boron from fruits, green vegetables, nuts, and beans in their normal diet. A normal daily intake of fruits, green vegetables, nuts, and beans give most people the 1.5 to 3.0 milligrams of boron that health experts suggest should be included in the daily diet. Boron is also an essential trace mineral in plants. A trace mineral is an element needed in minute amounts for the good health of an organism. Boron is critical to production of certain essential plant proteins and to help plants extract water from the soil. Low levels of boron show up as yellowing, blackening, twisting, or crumpling of leaves.