Boron group elements

Type of physical science: Chemistry

Field of study: Chemistry of the elements

The nonmetal boron is the first element of group IIIA, which also contains the metals aluminum, gallium, indium, and thallium. While aluminum, boron, and their compounds are commercially important in a number of different applications, the chief market potentials for gallium, indium, and thallium are in the electronics industry.

Overview

Members of the boron group are representative elements forming group IIIA of the periodic table of the elements. Representative, or main-group, elements include those elements found in the A columns of the periodic table and group 0, the noble gases. In addition to the metalloid boron, group IIIA includes the aluminum family of metals: aluminum, gallium, indium, and thallium.

The boron group is characterized by a broad range of chemical and physical properties.

This is not surprising, since groups IIIA and IVA are found between the metals, which include 80 percent of all the elements, and the nonmetals of groups VA through VIIIA. The table summarizes several of these properties. In the table, atomic masses are given in atomic mass units, and temperatures in degrees Celsius. Density is given in grams per cubic centimeter. The Pauling scale is used for electronegativity. Radii are given in angstroms. One angstrom is 10^-10 meters.

This group contains the most abundant metal, aluminum, and some of the least abundant metals. None of the elements of group IIIA is found free in nature. Usually, they occur as oxides in numerous places. Aluminum is the third most abundant element in the earth's crust.

Its relative abundance is 75,000 parts per million. Aluminum is predominantly found in aluminosilicate minerals in rocks in the earth's outer crust. These are the same type of rocks that form the clay found in most soils when sufficiently weathered. Deposits of bauxite, an aluminum-containing ore, result from additional weathering of such clays. The hard mineral corundum is composed of aluminum oxide (Al₂O₃).

Iron and titanium impurities cause some of this hard mineral to be sought after as sapphire and ruby gems. Thallium and indium are among the least abundant, with only 1.8 parts per million each. The relative abundances of gallium and boron are 15 and 3 parts per million, respectively.

Gallium, indium, and thallium are obtained in the recovery of the by-products of the production of other metals such as aluminum. Gallium, for example, is recovered from the caustic, aqueous sodium hydroxide solution used to extract aluminum from bauxite in the Bayer process.

Although boron is not abundant, it occurs in concentrated amounts in accessible minerals found mainly in Southern California. Twenty-mule teams hauled borate ores out of Death Valley in the late 1800's. In the Mojave Desert, large deposits of borax (Na₂B₄O₇•10H₂O) are mined near Boron, California. Borax is a hydrate, a substance that has undergone hydration--a reaction with water (H₂O) in which the hydrogen-oxygen bonds of water are not broken. The number of water molecules added to the substance follow a centered dot in the formula of a hydrate.

Since the elements of group IIIA all have three valence electrons--that is, three electrons that may be used in bonding--each may lose these electrons to achieve the +3 oxidation state. The heavier elements may also form the +1 oxidation state. This is especially true for thallium. In some compounds, gallium and indium also achieve the +1 state. Reports of the +2 oxidation state may be the result of metal-to-metal bonding.

Elements tend to increase in metallic character as one moves down a column in the periodic table. This is particularly obvious in group IIIA. Boron, the first element, is a semiconducting metalloid. At higher temperatures, semiconductors conduct electricity, like metals, and at lower temperatures, they are insulators, like nonmetals. Compounds of boron, such as the oxide of boron (B₂O₃) and boric acid (H₃BO₃), which are both weak acids, clearly exhibit nonmetallic characteristics. As noted earlier, the remaining elements in group IIIA are metals. In contrast to analogous boron compounds, the oxides and hydroxides of aluminum and gallium are amphoteric, that is, each can behave as either an acid or a base. Formation of basic oxides is characteristic of metals. As anticipated, because of the greater metallic character of the heavier elements near the bottom of a group, the oxides and hydroxides of indium and thallium are basic.

As is characteristic of nonmetals, bonding to boron is covalent. Its valence electrons are shared to achieve the +3 oxidation state. Aluminum has many covalent compounds and many that are definitely ionic, such as the aluminum halides. Aqueous solutions of aluminum salts contain the aluminum cation, Al³⁺(aq). The remaining members of group IIIA, gallium, indium, and thallium, also form ionic compounds yielding cations in aqueous solution.

Elemental boron has a number of allotropes, diverse solid-state structures. Three have been studied well, and found to be semiconductors containing repeating units of twelve boron atoms arranged in regular icosahedrons, three-dimensional figures having twenty sides. The icosahedrons are bonded to one another in extensive networks of covalent bonds. Substances of this type are appropriately called covalent network solids. The precise arrangement of icosahedrons varies in the different allotropes. Elemental boron is hard and resistant to heat, in part because of this extended, covalent bonding. Rigidity is given to the solid structure by the network of strong covalent bonds. The most stable allotrope of boron is a hard, black solid having a very high melting point ,degrees Celsius), characteristic of a covalent network solid. Because chemical bonds must be broken when this type of solid melts, melting points of covalent network solids are comparatively high. For the same reason, crystalline boron resists chemical attack except at extremely high temperatures.

Applications

Aluminum is the most important group IIIA element commercially. Huge quantities of the soft metal are used in making alloys with many purposes. By adding metals such as copper, magnesium, and manganese, hard, corrosion-resistant alloys are obtained. These are important in building and construction, for structural and ornamental purposes. They are used in the manufacture of windows, awnings, blinds, siding, outdoor furniture, and storm doors. Alloys have been developed that can be used as armor plate for tanks and other military vehicles.

Aluminum has less than one-third the weight of the same volume of steel. The high strength-to-weight ratio of aluminum makes it an ideal component in the construction of aircraft, trains, buses, trucks, automobiles, motorcycles, and bicycles, in which the use of heavier materials would reduce mobility and energy conservation. Aluminum is a good conductor, but not as efficient as copper. Even so, the lesser weight of aluminum wires makes their use over long distances desirable. Since aluminum remains tough and becomes stronger when it gets colder, it is useful at very low temperatures. Because it absorbs very few neutrons, aluminum is useful in low-temperature nuclear reactors. Resistance to corrosion in salt water has made aluminum boat hulls popular. Aluminum foil, aluminum cans, and other types of recyclable aluminum packaging are used to protect perishables. Because of its high reflectivity and resistance to corrosion and tarnishing, aluminum is used on the surfaces of mirrors found in optical instruments including telescopes employed by astronomers. Because of its high thermal conductivity, aluminum is utilized in cookware and in the pistons of internal-combustion engines. Aluminum is also used to produce other metals. For example, the Goldschmidt process, in which powdered aluminum is used to prepare a metal by reducing its oxide, is used to produce chromium. Also, iron for welding is prepared using a mixture of powdered aluminum and iron oxide, called thermite. Because its oxidation at high temperatures is extremely rapid, finely divided aluminum is employed in explosives and solid fuels for rockets. Powdered aluminum is also used in photographic flashbulbs and aluminum paints.

The most important of the many compounds of aluminum is its only oxide, alumina (Al₂O₃) This porous, white solid is employed primarily in the production of aluminum metal. Aluminum oxide is utilized as a carrier for catalysts. In the production of borosilicate glass, a thermally stable glass, aluminum oxide is substituted for some of the limestone employed in making ordinary glass. Fused at the very high temperature of 2,045 degrees Celsius, alumina forms the hard mineral corundum, which is used as an abrasive or refractory, a heat-resistant material. Corundum may be found in sandpaper, grindstones, and sanding and shaping powders. Fireproof bricks and furnace linings also contain corundum.

A common salt of aluminum is aluminum sulfate octadecahydrate, which has the chemical formula Al₂(SO₄)3·18H₂O. Huge amounts of this compound are used by the paper industry and in treating wastewater. Dissolving alumina in hydrochloric acid yields aluminum chloride hexahydrate (AlCl₃·6H₂O), which is commonly found in antiperspirants and disinfectants. Reacting aluminum metal with chlorine gas under the proper conditions produces anhydrous aluminum chloride (AlCl₃), an important catalyst, used, for example, in the synthesis of ethylbenzene, which in turn is used to prepare styrene, which is the starting material in the manufacture of polystyrene plastic and foam, or Styrofoam.

Double salts containing aluminum, such as the sodium, potassium, and ammonium alums, are used as fixatives in dyeing fabrics, in sizing paper, in water purification, in medicine, as food additives, in tanning leather, as clarifying agents, and in sewage treatment. Sodium and potassium alums are also employed as waterproofing agents and in matches. Potassium alum is utilized in paints, and sodium alum is used in ceramics, inks, sugar refining, engraving, and in baking powder.

Boron is an important component in many special-purpose alloys. For example, in the production of boron steel, it is added to increase the hardness of the steel. Boron is used as a neutron absorber in nuclear reactor controls. It is also found in high-temperature resistors. Fibers and filaments of boron are used to reinforce composites of plastic, metals, or ceramics for special purposes. Significant amounts of boron are used in the manufacture of ignitors and in rocket fuels. In the treatment of copper and other metals, boron is the best known oxygen scavenger. It renders copper gas-free and thus very highly conductive. Copper-boron alloys are used to braze metals that otherwise could not be joined.

Borax is a major source of boron and is used to prepare other compounds of boron. The use of borax in detergents is a familiar application of the compound. By adding sulfuric acid to solutions of borax, a large portion of the borax produced is converted to boric acid. Much of the boric acid is heated to form boric oxide. Boric acid is also used as a mild antiseptic, and as a preservative for leather and wood. In addition, it is employed as a fire retardant for paper and cloth. For example, the recycled newspaper that is used for blown-in insulation of houses and other buildings is treated with boric acid to make it fire-resistant. Great quantities of boric oxide are utilized in the manufacture of fiber glass and borosilicate glass, in which it replaces much of the soda ash used in ordinary glass. Pyrex is the most familiar brand name for borosilicate glass.

Boric oxide is also used to produce ceramic glazes and porcelain enamels, such as those found on stoves and other kitchen appliances.

Uses for other group IIIA elements are becoming more numerous, especially in solid-state electronics. The relatively high cost of gallium limits commercial exploitation of its unique physical properties. The first important use of this rare metal, made in conjunction with the Atomic Energy Commission, involved the utilization of one of its compounds, gallium oxide (Ga₂O₃), to increase sensitivity in the spectroscopic analysis of uranium oxide. The most promising application of gallium is in the production of semiconductor compounds. Alloying very pure gallium with another very pure element such as arsenic, antimony, or phosphorus, facilitates the manufacture of intermetallic semiconductors with near-theoretical properties. Gallium antimonide and gallium arsenide are semiconductors used in a number of electronic devices, including voltage amplifiers and rectifiers, light-emitting diodes, and temperature and magnetic field sensors. They may be found in transistors, solar batteries, and infrared optical instruments. Gallium arsenide may also be used in lasers, in magnetoresistance devices, and in generating microwaves.

The first commercial use of indium, its utilization in the manufacture of bearings, remains a significant application of this rare metal. To make a typical bearing, indium is electroplated over layers of lead and silver on a steel shell. Heat treatment produces a lead-indium alloy rich in indium at the surface. This produces a harder, stronger bearing, with increased corrosion resistance. In addition, the surface is made more wetable; thus, the bearing is less likely to seize. Such higher-performance bearings are used in airplane, truck, and automobile engines. Some alloys of indium are useful in producing glass-to-glass, glass-to-metal, and nonmetal-to-metal seals at relatively low temperatures, for example, for soldering in transistors.

Indium itself is an essential component of the germanium transistor. It also is used in producing intermetallic semiconductors, such as indium antimonide, indium arsenide, and indium phosphide, which are used in various devices including lasers.

The chief commercial application of thallium utilizes its toxicity. The compound thallium sulfate (Tl₂SO₄) is used as a pesticide and rodenticide. Certain thallium compounds find applications in γ radiation detectors, infrared communications systems, semiconductors, and special types of optical glass. Low-melting-point glasses are formed by combinations of thallium, arsenic, and sulfur. Thallium-mercury alloys form low-freezing-point fluids useful in low-temperature thermometers, relays, and switches.

Context

Although compounds of boron have been used since ancient times, free boron was not obtained until 1808, when Sir Humphry Davy, in England, and Joseph Gay-Lussac and Baron Louis-Jacques Thenard, in France, independently isolated the element. Davy concluded that it was more like carbon than a metal, and in 1812 gave it the name boron.

Also in 1812, Davy formed a white solid from boron trifluoride and ammonia. This was the first recognized coordination compound. When Alfred E. Stock synthesized compounds containing boron and hydrogen in 1912, a new sector of covalent chemistry comparable to organic chemistry was generated. Understanding of these compounds grew later, in the 1940's, when the icosahedral arrangement of boron in boron carbide (B₁₂C₃) and decaborane was discovered. Ideas concerning three-centered boron-hydrogen-boron bonding and polycentered boron-boron bonding developed in the late 1940's through the 1950's. Such work has shown that boron forms a large number of coordination compounds, oxocompounds (compounds containing oxygen), and polyhedral borane derivatives.

Although the ancient Greeks and Romans used alum as a fixative and as an astringent, they may not have distinguished between it and other substances used for similar purposes. In 1760, Theodore Baron de Henouville concluded that alum contained an alkali metal and another metal.

In 1825, Hans Christian 0rsted isolated impure aluminum from alumina. Pure aluminum was first obtained by Henri Sainte-Claire Deville, who developed a commercial procedure for the production of aluminum from alumina. Working independently, Robert Wilhelm Bunsen, in Heidelberg, and Deville, in Paris, nearly simultaneously obtained pure metallic aluminum by electrolysis of fused sodium aluminum chloride. On February 23, 1886, Charles Martin Hall, a student at Oberlin College, produced aluminum by a less expensive, electrolytic method he developed in a woodshed using homemade batteries. Simultaneously, a young French scientist, PaulLouis-Toussaint Heroult, made the same discovery, now referred to as the Hall-Heroult process.

Prior to the invention of the spectroscope by Bunsen and Gustav Robert Kirchhoff in 1859, many elements remained undiscovered, because they existed in such limited quantities that they were undetectable by other methods. In 1861, while examining residues from a sulfuric acid plant, Sir William Crookes, another founder of spectroscopic science, observed a beautiful, bright green line with his spectroscope. Concluding it was a new element, he named it thallium, from the Latin word thallus, meaning "green sprout."

Ferdinand Reich and his assistant, Hieronymus Theodor Richter, discovered indium in 1863. While examining zinc ores for thallium, they noticed a brilliant indigo-blue line when crude zinc chloride liquor was heated in a flame. In the same year, they isolated from impure zinc a pure sample of the newly discovered element. Two blue lines were found in the spectrum of the new metal, which was appropriately named indium.

The periodic table developed by Dmitri Ivanovich Mendeleyev in 1869 contained blank spaces for several undiscovered elements. One such element was discovered in 1875 by Francois Lecoq de Boisbaudran, another spectroscopy pioneer, in sphalerite, a zinc ore. From several hundred kilograms of ore, Boisbaudran isolated and purified more than 1 gram of gallium, the last of the group IIIA elements to be discovered.

Principal terms

ALLOY: an intimate solid or liquid mixture of two or more metals, or of a metal or metals and one or more nonmetals, such as carbon

ATOMIC MASS UNIT: the unit employed in the scale of relative atomic masses of the elements; defined as one-twelfth the mass of an atom of carbon 12, equal to 1.6606 x 10^-24 grams

ATOMIC NUMBER: the number of protons in the nucleus of an atom of an element, which equals the number of electrons in the atom; represented by the symbol Z

CATION: a positively charged species, composed of an atom minus one or more electrons

COMPOUND: matter composed of atoms of two or more different elements chemically combined in fixed proportions

COORDINATION COMPLEX: a compound containing coordinate covalent bonds between metal ions and other ions or molecules called ligands; in such bonds, the shared electron pairs are donated by the ligands to the metal

COVALENT BOND: a bond, or interatomic attraction, consisting of electron pairs shared between the atoms

ELECTRONEGATIVITY: a measure of the ability of an atom in a compound to attract bonding electrons to itself

METALLOID: an element having characteristics of both metals and nonmetals

OXIDATION STATE: the charge an atom in a compound would have if all the bonding electron pairs belonged to the more electronegative atom involved in the bond

Bibliography

Bailar, John Christian, Jr., H. J. Emeleus, R. Nyholm, and A. F. Trotman-Dickenson. COMPREHENSIVE INORGANIC CHEMISTRY. Vol. 1. Elmsford, N.Y.: Pergamon Press, 1973. This is one of five volumes of a thorough but readable survey of inorganic chemistry. Two entries, one on boron and another on the aluminum family of metals, present background information and discuss the occurrence, isolation, and purification of each of these elements. They also contain a discussion of the structure and chemical and physical properties of the elements and their most important compounds.

Ebbing, Darrell D. GENERAL CHEMISTRY. Boston: Houghton Mifflin, 1990. This is a well-organized freshman chemistry text characterized by clear explanations and colorful illustrations. It contains two chapters on the main group elements, one of which includes a concise discussion of the properties, preparation, and uses of the boron group elements and their compounds.

Hampel, Clifford. RARE METALS HANDBOOK. London: Reinhold, 1961. Offers a convenient compendium of information about less-abundant elements including the non-metal boron and the rare metals gallium, indium, and thallium. Each chapter contains a brief history of the element and a discussion of its occurrence and industrial production, covering metallurgy, physical and chemical properties, analysis, and applications.

Muetterties, Earl L. THE CHEMISTRY OF BORON AND ITS COMPOUNDS. New York: John Wiley & Sons, 1967. Offers an excellent introduction to the chemistry of boron and helps the reader recognize the significance of boron chemistry in the development of theories of chemical bonding. Contains a list of numerous specific and general references for further reading at the end of each chapter.

Weeks, Mary Elvira, and H. M. Leicester. "Discovery of the Elements." JOURNAL OF CHEMICAL EDUCATION 44 (1968). This is a very readable history of the discovery of the elements. Contains a number of interesting photographs and drawings, as well as facts and historical background. Material is presented in a manner that illustrates how each discovery was a part of the development of a better understanding of chemistry.

Boron group elements

Acids and Bases