Group Iv Elements

Type of physical science: Chemistry

Field of study: Chemistry of the elements

Carbon is the central element in the chemistry of life processes, while much of the earth's surface is made of silicon compounds. Germanium, though rarer than carbon or silicon, has been important in solid-state electronics and in the historical development of the periodic table.

Overview

The two lighter group IV elements, carbon and silicon, are widely and abundantly distributed, while germanium is much rarer. Silicon makes up about 25 percent of the earth's crust, and is exceeded in crustal abundance only by oxygen. For every gram of germanium, the crust contains 114 of carbon and 36,700 of silicon. Although all three are solids, the elements differ considerably in physical properties. Carbon and silicon have low densities, comparable to those of metals such as aluminum, but germanium is more than twice as dense as silicon, comparable to the metal vanadium. The melting points of germanium ,Kelvins) and silicon ,Kelvins) are similar to those of many metallic elements, such as silver ,Kelvins) and iron ,Kelvins). Carbon, however, never melts at all at atmospheric pressure, but begins to sublime at 3,925 Kelvins, a temperature at which all other elements would melt or vaporize.

The group IV elements have four valence electrons and thus show a strong tendency for the formation of four bonds, and a preference for a formal oxidation state of +4 in most of their compounds, although the +2 oxidation state is known also for germanium in such compounds as germanium(II) iodide. The bonds formed by carbon are essentially covalent, except in some metal carbides, in which discrete negative ions of carbon exist. Only in the form of graphite does carbon show the metallic property of electrical conductivity; otherwise, it is nonmetallic in its behavior. Silicon and germanium are difficult to classify as either metals or nonmetals, and probably belong in a third class of elements: the metalloids, or semimetallic elements. They possess the valuable electrical property of semiconductivity, which places them between the nonmetals such as sulfur or phosphorus, which are insulators, and true metals such as tin or lead, which are electrical conductors.

Formation of positive ions (cations), which is characteristic behavior for true metallic elements, is avoided by silicon and germanium, which bond mainly by sharing pairs of electrons.

Germanium shows the beginnings of a trend toward metallic character by being somewhat soluble in acids (unlike silicon), and in forming a few compounds in the +2 oxidation state, in which it resembles metals such as tin or lead. Even here, however, the existence of discrete positive ions of germanium is doubtful.

Carbon chemistry is dominated by the strong ability of the element to form chains and rings of atoms joined by strong carbon-carbon bonds (catenation). Different patterns of catenation are evident in the two major crystalline forms of carbon, diamond and graphite. At the submicroscopic level, a diamond crystal is composed of carbon atoms arranged so that each atom lies at the center of a tetrahedron with other carbon atoms at its four corners. Strong covalent bonding results from the four pairs of electrons shared by each pair of carbon atoms. An intricate web of atoms results, forming a cubic structure that is simultaneously a crystal and a molecule.

Diamond crystals exhibit remarkable hardness and heat conductivity, which may be attributed to the interlocked lattice structure.

Graphite, softer and less dense than diamond, crystallizes in a structure in which layers are built up by carbon atoms strongly bonded together in a planar hexagonal "chicken-wire" pattern. The weakness of the bonding between layers is responsible for the lubricity of graphite, and for the property that allows one to write on paper with pencils (so-called pencil lead is actually graphite with a clay binder). Graphite occurs naturally and may be mined like other minerals, or it can be made in a furnace from coke. So-called intercalation compounds of graphite are known, in which atoms of other elements penetrate between the layers of carbon atoms, expanding the structure. Their formation can sometimes lead to the degradation of the graphite electrodes used in electrochemical processes.

Diamond was shown, in 1791, to be a form of carbon by Smithson Tennant (17611815), who oxidized a diamond and weighed the resulting carbon dioxide. The synthesis of diamond from less expensive forms of carbon was often attempted during the nineteenth and early twentieth centuries, and may have been achieved, although certainly less often than was claimed. By 1955, scientists at General Electric had achieved reproducible diamond synthesis by a high-temperature and very-high-pressure process. Eventually, about 40 percent of the world's industrial diamonds were made synthetically. Gem-quality synthetic diamonds can also be made, but are more expensive than the natural variety.

In the 1980's, a third form of carbon, called buckminsterfullerene (C₆₀) was discovered in carbon vapor using laser vaporization and mass-spectrometric detection techniques. This allotrope, which consists of sixty carbon atoms arranged in a polyhedral framework structure, was named by its discoverers for the architect and designer Buckminster Fuller, who popularized an architectural form called the geodesic dome (C₆₀ and related compounds are also called fullerites or fullerenes, and even, irreverently, buckeyballs). Although C₆₀ was initially detected in submicroscopic amounts under unusual conditions, chemists soon discovered how to obtain it in quantity by controlled syntheses. C₆₀ is a semiconductor, and dissolves in organic solvents.

Much remains to be discovered in the area of carbon-atom clusters. It seems likely that C₆₀ is only the first of many carbon clusters to be discovered. One can anticipate the development of substituted clusters and the incorporation of clusters into new solid structures. An exciting possibility is the encapsulation of atoms or molecules within the hollow cavity in C₆₀.

The elemental form of silicon is structurally similar to diamond. The element is dark blue-gray in color, and brittle. The hardness of silicon does not approach that of diamond, but exceeds that of glass. Unlike carbon, which was readily available in antiquity from natural sources, silicon does not occur uncombined, and was not recognized as an element until 1823, when Jons Jakob Berzelius (1779-1848) investigated the action of potassium metal upon potassium fluorosilicate, obtaining impure silicon.

In silicon chemistry, a striking type of catenation is found in the occurrence of structures with the repeating atomic pattern silicon-oxygen-silicon... , joined by covalent bonds.

These structures are found in the silicate minerals that compose most of the earth's crust, and in some synthetic materials, such as silicone polymers. The characteristic building block of the silicates is the silicon-oxygen tetrahedron: four oxygens evenly arranged around the silicon atom in space. Oxygen atoms may be shared between two silicons to produce a bewildering variety of extended structures: single or double chains, corrugated sheets, helical chains with clockwise or counterclockwise spirals, hollow cages, and many other types. Chemical resemblance between carbon and silicon is slight. Silicon-silicon covalent bonding does not occur to the extent that carbon-carbon bonding does, and the conditions required for stable silicon-silicon multiple bonds are much more restrictive than those for carbon. On the other hand, carbon-oxygen bonding is not known to produce any such rich structural variety as is found in the silicates. There is a preference for carbon to form double bonds to oxygen rather than single bonds; thus, extended polymeric structures are rarer for carbon-oxygen compounds.

Germanium is a brittle but shiny and metallic-appearing element, recovered as a by-product from zinc or copper mining. The structure is similar to that of silicon or of diamond, but the cohesive energy of the solid is less. Germanium is a semiconductor, and was used in the earliest transistors, but has now been replaced by silicon in most applications.

Catenation of germanium atoms has been found for the germanium hydrides (also called germanes). Volatile germanes with up to nine germanium atoms have been separated and characterized. Germanium can also form catenated networks with oxygen atoms and can share oxygen atoms with silicon in mixed silicon-germanium glasses. The germanium-oxygen bond is weaker than the silicon-oxygen bond, which allows hydrolysis of germanium-oxygen compounds by solutions of hydrochloric acid.

Germanium tetrachloride may be distilled from such solutions, whereas the analogous reaction with silicon is unknown.

Controversy exists over the electronegativities of the group IV elements, particularly germanium. In 1960, Linus Pauling assigned an electronegativity value of 1.8 to silicon, germanium, tin, and lead (as well as to iron and nickel). On the "electrostatic" electronegativity scale (developed by L. Allred and Eugene G. Rochow), however, germanium is assigned a value of 2.02, with silicon and tin nearly equal at 1.74 and 1.72, respectively. Certain chemical evidence supports the idea that germanium may be nearly as electronegative as arsenic: Both elements can be converted to hydrides in solution by reduction with zinc and hydrochloric acid.

The spectroscopic electronegativity values of L. C. Allen (1989) are 1.915 for silicon, 1.977 for germanium, and 1.758 for tin, with germanium only slightly higher than silicon.

Electronegativities alone are an insufficient basis for the interpretation of chemical reactions, but it is interesting to note the alternation in group IV values. More interesting than the controversy itself may be the possibility of new properties or reactions discovered while studying electronegativity effects.

Applications

The group IV elements find many applications because of their unique physical and chemical properties. Carbon, in its diamond form, is prized above all other precious stones for its luster and durability. Large stones are especially valued, particularly when clear, water-white, and free of flaws. The largest diamond ever found in nature was the Cullinan diamond from South Africa, which weighed 3,106 carats (a carat is 0.2 gram). Because its hardness exceeds that of all other solids, diamond finds uses in drills, cutting tools, and dies. The uniquely high heat conductivity of diamond helps to keep the tools cool in use. In the 1980's, it was discovered that diamond coatings can be applied to surfaces by a process involving decomposition of a hydrocarbon vapor at high temperature, under carefully controlled conditions. Such diamond coatings, it is hoped, may someday confer the durability of diamond to objects such as razor blades, bearings, or automobile parts. Research applications of diamond include diamond crystals used to detect radiation, diamond anvils for high-pressure research, and diamond sample cells for infrared spectroscopic measurements.

Graphite is used as a lubricant for gears, and as an additive in automotive lubricating oil. The electrical conductivity of graphite is exploited in the electrodes used in electrochemical processes such as the manufacture of aluminum and other metals. Graphite is a construction material for high-temperature apparatuses such as crucibles and ingot molds for melting and casting metals. Graphite was also used as a neutron moderator in the first nuclear reactor at the University of Chicago, and in the ill-fated Soviet reactor at Chernobyl, where the graphite actually burned in the aftermath of a nuclear accident.

Carbon and carbides are essential in metallurgy, especially the metallurgy of iron and steel. Pure iron (wrought iron) is relatively soft and useless for tools or blades. Addition of carbon to iron produces carbon steels, which are stiffer and stronger than pure iron, and which can be used in making springs, plates, pipe, and rails. Tungsten and boron carbides are materials of great hardness used in some cutting applications. Because of their brittleness, these materials are usually embedded in a metal matrix before use; an example is carballoy, which consists of tungsten carbide in a cobalt matrix.

Amorphous carbon, in its various forms, such as carbon black and charcoal, is manufactured and used in large quantities. Carbon black, very finely divided carbon, is made by burning hydrocarbons in limited air. Its uses include strengthening the rubber in automobile tires (several kilograms per tire) and conferring color to printing ink. Charcoal, which has been made from time immemorial by calcining wood sticks, is now made from many other carbonaceous materials by modern methods that allow control of the surface properties and particle size.

Charcoal particles exhibit the property of adsorption, the ability to trap and hold molecules on their surface. Charcoal adsorbents are widely used for purifying gases, liquids, and solids. For example, solutions of crude brown sugar can be made colorless by charcoal adsorption of the brown colorants, and stale air in buildings can be freshened by passing through charcoal filters.

Medical applications of charcoal include its use in poisoning cases to adsorb offending substances from the alimentary tract. Charcoal was also used in the gas masks that were rushed into service during World War I, and many patriotic families saved peach pits, which were collected and used to make a particularly active form of charcoal.

Silicon is used as an alloying element in special steels, and also in magnesium and aluminum. In these applications, metal silicides are formed, producing interlocking crystals that harden and strengthen the metal. Silicon carbide, which structurally resembles diamond, is a valuable abrasive (Carborundum) for grinding wheels, tool sharpeners, and abrasive paper.

Silicates in various form are used in glass, porcelain, cement, and vitreous enamel. The relative cheapness and versatility of these materials make them unbeatable for a variety of structural uses. A possibility, now in the experimental stages, is the all-ceramic automobile engine.

Silicone polymers, which have a silicon-oxygen backbone in their molecular structure, like the silicates, are used in caulking compounds, specialty rubber items, brake fluids, and many other applications. Since silicone rubber is physiologically inert, it can be used in implants to make "spare parts" for the human body.

Elemental silicon is now the material of choice for electronic applications in transistors, rectifiers, solar cells, and integrated circuits. For these uses, ultrapure silicon (99.9999 percent pure) is needed, and can be prepared by converting the crude material to volatile silicon hydrides, which are purified and reconverted to the element by heating. Single crystals can be "pulled" from molten silicon and, after cooling, sawed into disks, which serve as substrates for circuit chips. On these disks are deposited various layers of conducting or insulating materials. Laser beams are used to produce chemical changes that permit creation of complex circuits in miniature form directly on the silicon disk. These operations must be carried out in an absolutely clean and dust-free environment to prevent flaws. The microelectronics revolution based on silicon chips has made possible many devices, particularly microcomputers, which would have been impossible with vacuum tube technology.

Optical uses of germanium are now more important than electronic uses. Germanium is transparent to thermal (infrared) radiation, and is used in lenses, prisms, and coatings for infrared imaging systems. The gun sights used to help soldiers target their weapons at night ("sniperscopes") contain germanium components.

Radiation detectors for γ rays are made from germanium that has been doped with small amounts of lithium. These detectors permit accurate determination of γ-ray energies and intensities, and excel all other detectors in this application. Such detectors are used in procedures for trace analysis of metals by activation analysis.

Germanium oxide is used as an additive in the glass used in fiber optics to modify the optical properties of the fibers, which are used in telephone lines, computer cables, and cable television. The substitution of germanium oxide in the catalyst allows formation of polyester fibers free of unwanted colors caused by other metal oxide catalysts.

Minor amounts of germanium are used as components of phosphors, superconducting alloys such as niobium-germanium, and dental alloys such as gold-germanium. Some experimental work is under way with germanium compounds in medicine.

Context

Historically, group IV figured in the early development of the periodic table. Dmitry Ivanovich Mendeleyev, the Soviet chemist, in his periodic table of 1871, laid out the elements carbon, silicon, tin, and lead in a single column, which are now called group IV, but left a space for a new element, "eka-silicon," between silicon and tin. Predictions were made as to the physical and chemical properties to be expected for the new element "beyond silicon," but it was not until the discovery of germanium by Clemens Winkler in 1886 that Mendeleyev's predictions were tested against experimental fact and found to be surprisingly accurate. This prediction tended to draw attention to Mendeleyev's periodic table, which eventually became the basis for the modern periodic table.

Germanium and later silicon opened the way to the transistor and to the revolution in microelectronics. The electronics industries that evolved in California around Palo Alto in the 1970's added the name "Silicon Valley" to the English language. Microcomputers and modern communications equipment would be impossible without silicon, which will remain the basis for chips and circuits for the foreseeable future, although developments in "organic" metals and semiconductors will undoubtedly occur, and such materials may replace silicon in selected applications.

Organic compounds, composed of carbon, hydrogen, oxygen, and a few other elements, provide the material of construction for the bodies of animals (including humans) and plants.

Attempts to analyze these structures have made possible the identification of many of their components, and, in some cases, their synthesis, with unceasing benefits to medicine, agriculture, and daily life.

The synthetic polymer industry provides substitutes for many materials formerly obtained from natural sources; cotton, wool, silk, and wood now suffer competition from rayon, polyester, polyamides (such as nylon), and engineering plastics. The main importance of group IV elements will always be in the realm of biological carbon compounds and synthetic compounds inspired by them.

Carbon clusters, of which knowledge is still in its infancy, will be important to pure science and to technology. Graphite and diamond are merely the largest carbon clusters.

Controlled formation of clusters is likely to lead to new solids with valuable electrical and mechanical properties, and both superconductors and semiconductors with carbon cluster materials can be expected. The unique properties of diamond will be exploited in new ways as it becomes possible to produce diamond coatings and layers on a variety of substrates.

Principal terms

ALLOTROPY: the occurrence of multiple forms (allotropic forms) of a chemical element, usually as solids with different crystal structures

COVALENT BOND: a force of attraction that exists between two atoms as a result of sharing a pair of electrons

ELECTRONEGATIVITY: the tendency of an atom, in a compound, to attract electrons from the other atoms to which it is bonded

IONIC BOND: a force of attraction between electrically charged atoms (ions)

METALLOIDS: chemical elements that lie between the metals and the nonmetallic elements in the periodic table

PERIODIC TABLE: a matrix arrangement of the chemical elements in rows and columns by increasing atomic number; similar elements lie in the same column (group)

POLYMER: a large molecule made up of many small, identical units bonded together; plastics, rubber, cellulose, and starch are examples of polymers

SEMICONDUCTOR: a material of electrical conductivity less than that of a metal, but greater than that of an insulator; its resistance is less at higher temperatures (unlike a metal)

VALENCE ELECTRONS: the less firmly bound, outer electrons of atoms; may be lost to other atoms or shared with them, resulting in a force of attraction to the other atoms (chemical bond)

Bibliography

Asimov, Isaac. THE WORLD OF CARBON. New York: Collier Books, 1962. An introduction to carbon chemistry, with the main emphasis on organic chemistry and biochemistry. Written in an entertaining and undemanding style, but with solid scientific content.

Bruton, Eric. DIAMONDS. 2d ed. Radnor, Pa.: Chilton, 1978. A fascinating account of the history of diamonds and the mining, sorting, grading, and cutting of diamonds. Emphasis is on gem diamonds, with many photographs. Contains an interesting discussion of diamond synthesis.

Glockling, Frank. THE CHEMISTRY OF GERMANIUM. New York: Academic Press, 1969. A useful summary stressing organogermanium compounds and hydrides. Electronegativity and π-bonding are discussed, often with evidence from spectroscopy or reaction kinetics. Short discussion on applications of germanium.

Kroto, Harold. "Space, Stars, C₆₀, and Soot." SCIENCE 242 (1988): 1139-1145. A review article on C₆₀ and other carbon clusters. There is a summary of the events leading to the discovery and naming of C₆₀, and a discussion of how it may form during the vaporization of carbon. Many drawings of polyhedral structures are given, as well as a discussion of the possible importance of carbon clusters in astrophysics and environmental chemistry.

Queisser, Hans. THE CONQUEST OF THE MICROCHIP. Cambridge, Mass.: Harvard University Press, 1988. A history of the development of the transistor, the microprocessor, and the integrated circuit. Contains photographs of many of the scientists and engineers who created the field of microelectronics.

Reynolds, Warren L. PHYSICAL PROPERTIES OF GRAPHITE. Amsterdam: Elsevier, 1968. A monograph covering the structure of graphite, as well as the mechanical, electrical, optical, and nuclear properties of this form of carbon.

Rochow, Eugene G. THE METALLOIDS. Lexington, Mass.: D. C. Heath, 1966. Physical and chemical properties of the metalloids are discussed, and applications are stressed. The final chapter contains many suggestions for further reading.

Rochow, Eugene G. SILICON AND SILICONES. New York: Springer-Verlag, 1987. A highly readable and informative introduction to the chemistry of silicon and the silicones, containing much history and many personal vignettes. Includes a section on biologically active silicon compounds.

Weeks, Mary Elvira. DISCOVERY OF THE ELEMENTS. 5th ed. Easton, Pa.: Journal of Chemical Education, 1945. A scholarly and complete account of the discoveries of silicon and germanium is given. Contains photographs of the scientists involved in the discoveries and many references to the original literature.

Carbon and Carbon Group Compounds

Conductors and Resistors

The Periodic Table and the Atomic Shell Model

Electrical Properties of Solids