Silicon

Allotropic forms

Discovery and Naming

Silicon is an abundant element

Silicates

Silicones

Other uses of silicon

Resources

Silicon is the chemical element of atomic number 14, symbol Si and atomic weight 28.085. The seventh most abundant element in the universe, silicon is the second element in Group 14 of the periodic table. In its crystalline form of dark gray crystals, it has a specific gravity of 2.42 at 68°F (20°C), a melting point of 2,588°F (1,420°C) and a boiling point 5,936°F (3,280°C). It exists also in two amorphous (shapeless) forms, a brown powder and a black crystal. Silicon consists of three stable isotopes of mass numbers 28, 29 and 30.

Silicon, is a key component of microchips and microprocessors that allow the construction of inexpensive digital wristwatch to worldwide networks of computers. The conductive properties of silicon allow micro-devices to perform millions of calculations per second.

In terms of weight, silicon is the second most abundant element in the crust of Earth at 27.7%— second only to oxygen (46.6%). In rough terms, the Earth is essentially a spheroid of iron (the core) surrounded by layers (the mantle and the crust) of silicon and oxygen dominated compounds that include the other elements.

Earth was originally a molten ball of mostly iron, oxygen, silicon and aluminum that cooled. While still molten lighter atoms—including silicon and oxygen (atomic weights 28 and 16), moved outward from the core region, while the heavier iron atoms (atomic weight 56) dominated the central core. By about 3.5 billion years ago, the outermost layer had cooled to a crustal surface. The crustal composition is three-quarters oxygen and silicon.

Silicon exists in two allotropic forms, one of which consists of shiny, grayish black needle-like or crystal plates. The other allotrope is an amorphous brown powder. The melting point of the crystalline allotrope is about 2,570°F (1,410°C), its boiling point is about 4,270°F (2,355°C), and its density is 1.35 ounces per cubic inch (2.33 grams per cubic centimeter). It is a solid at room temperature. Silicon is a relatively hard element with a hardness of 7 on the Mohs scale. Classified as a semi-metal, silicon is a semiconductor, a property that determines some of its most important uses.

As its two allotropic forms might suggest, silicon is a metalloid. It is relatively inactive at room temperature, and resists attack by water and most acids. At higher temperatures, it reacts with many metals, oxygen, nitrogen, sulfur, phosphorus, and the halogens. It also forms a number of alloys in the molten state.

The discovery of silicon as an element evaded chemists for many years because of the stability of most silicon compounds. Most chemists had little reason to believe that a new element existed in sand, silicates, and other earthy materials. Even if they did, scientists did not have the technology to extract the element from its compounds. One researcher with perhaps the greatest reason to hope for success in producing silicon was English chemist and physicist Sir Humphry Davy (1778–1829). Davy had developed a technique by which unusually stable compounds could be decomposed into their constituent elements. He used this method to prepare sodium, potassium, calcium, and other elements for the first time. Davy was unsuccessful, however, in producing silicon by the same method.

The first successful effort in the search for silicon was achieved by Swedish chemist Jons Jakob Berzelius (1779–1848). In 1823, Berzelius electrolyzed a molten mixture of potassium metal and potassium silicon fluoride (K 2 SiF 6 ) and obtained a small sample of pure silicon: 4K + K 2 SiF 6 —heat and electricity → 6KF + Si. The new element was named by Scottish chemist Thomas Thomson (1773–1852) because of the element’s presence in the mineral flint (silex or silicis in Latin ). He added the ending -on because of the element’s similarity to carbon.

Silicon exists in the sun, stars, and in meteorites. It is found in plants and in animal bones. In Earth’s crust, there are at least 500 minerals—substances with definite chemical compositions and crystal forms. More than a third of these compounds contain silicon and oxygen.

Silicon and oxygen form silicon dioxide, SiO 2 , (as known as silica). Sand is mostly silica with some contributions from shells and corals. When mixed with lime (calcium oxide, CaO), soda (sodium carbonate, Na 2 CO 3 ) and trace substances, then melted in a furnace, silica become the key component of glass.

The purest form of silica, SiO 2 , is quartz, a common mineral that is found as nearly colorless crystals. Slightly impure quartz makes crystals of amethyst (purple or violet), opal (translucent, milky) and agate (striped), all of which are prized for their aesthetic value.

Practically all the rocks and clays contain silicon and oxygen combined chemically with metallic elements in compounds called silicates. A common exception is limestone, which is calcium carbonate.

The atoms of carbon can bond to each other to make long chains that include branches, and rings of carbon atoms onto which atoms of hydrogen and several other elements (including oxygen) can bond. The entire field of organic chemistry, with its millions of different organic compounds, is based on this ability of the carbon atom.

Silicon also has increased bonding abilities. On the periodic table, silicon is directly beneath carbon in group 14, which means that it, like carbon, has four electrons in its outermost shell that are available to share in chemical bonds with other elements. Like carbon, it can share those electrons with other silicon atoms. Because silicon atoms are about one and a half times larger in diameter that carbon atoms, however, the atoms can not pack as tightly and therefore can not to bond into long -Si-Si-Si-Si- chains that allow as much access as do carbon chains. Oxygen atoms can act as separators, or bridges, between the Si atoms to make -Si-O-Si-O-Si-O-Si- chains. Oxygen has a valence of two, and it can bond to two silicon atoms to bridge a chain. Such bridged structures open up the possibility of vast networks of silicon and oxygen based silicates.

The network in a quartz crystal consists of silicon and oxygen atoms. Each silicon atom is bonded to four oxygen atoms. Each silicon atom has only half possession of the four oxygen atoms surrounding it, so the overall formula is SiO2, not SiO4. Half of four oxygen atoms per silicon atom equal two oxygen atoms per silicon atom. In other silicate minerals, this network incorporates the presence of other atoms such as aluminum, iron, sodium, and potassium, that allow crystals to take on different shapes and properties.

Talc is a silicate mineral whose silicon and oxygen atoms are bonded together in sheets rather than in quartz-like three-dimensional solid crystals. These thin sheets can slide over one another. The low friction of talcum powder (ground-up talc) results from this sheet like configuration. Asbestos is a silicate mineral with silicon and oxygen atoms are bonded in long strings. Asbestos is therefore a mineral rock that can be shredded into fibers.

A silicate material widely used in industry is cement. Recent estimates place use of this cement at more than 100 million tons in the United States each year. Cement is manufactured from two minerals: clay or shale (both aluminum silicates) plus limestone (calcium carbonate, CaCO 3 ). These minerals are mixed, then heated together at a temperature of 2,732°F (1,500°C). At this temperature, the limestone converts to lime, CaO. The mixture is then cooled, and it is ground to a very fine, gray powder. When this cement powder is mixed with sand, gravel, and water, it sets into concrete. Accordingly, although the terms are sometimes inappropriately used synonymously, concrete is actually an aggregate material containing cement. Concrete is a very hard and strong material, largely because strong Si-O-Si bridges in the clay.

Like silicates, silicones are a family of compounds held together by strong Si-O-Si bridges. But where silicates have two additional, non-bridging oxygen atoms attached to each silicon atom, the silicones have organic groups for example, two methyl groups, CH 3 . The resulting (CH 3 ) 2 SiO- groups can build up into long chains, just as the silicates. In contrast, however, are organic groups in the chains, that allow the compounds to resemble organic materials such as oils, greases, and rubbers.

As with organic compounds, a variety of silicone compounds can be composed of various-length silicon-oxygen chains with organic groups attached. The smaller molecules are the basis of silicone oils that, as with the all-organic petroleum oils, are used as lubricants, which resist decomposition at higher temperatures. Very large silicone molecules make silicone rubbers with high compression elasticity. These compounds are incorporated into ranging from super-bouncing balls to high impact bumpers. The first human footprint on the moon was made with a silicone-rubber-soled boot.

Between the oils and rubbers are hundreds of kinds of silicones that are used in electrical insulators, rust preventives, soaps, fabric softeners, hair sprays, hand creams, furniture and auto polishes, paints, adhesives, and chewing gum. Silicones are also used in surgical implants because they less prone that organic material to rejection by the immune system.

On the periodic table, silicon lies on the borderline between the metals and nonmetals. Silicon is essentially a semi-metal (i.e., has some metallic properties such as metallic conductivity) that allows it to be used in semiconductor devices (i.e., silicon is a semiconductor). Thin slices of ultra-pure silicon crystals, generally known as chips, can have as many as half a million microscopic, interconnected electronic circuits etched into them. These circuits can act as electron gates and perform incredibly complex manipulations of voltages, that can be treated as binary numbers (e.g., voltage on = 1, voltage off = 0).

Silica gel is a porous form of silica, SiO 2 , that absorbs water vapor from the air. In its most common form, silica gel is manufactured for use as a drying agent and small packages of silica gel are often packed with shipped products such as electronics that may be sensitive to moisture. Absorption by silica acts to maintain the humidity levels in a package as the package undergoes temperature changes.

Silicon carbide (SiC), is an extremely hard crystalline material, manufactured by fusing sand (SiO2) with coke (C) in an electric furnace at a temperature above 3,992°F (2,200°C). Silicon carbide, also known by its trade name, Carborundum, is often used as an abrasive, By attaching an ultrasonic impact grinder to a magnetostrictive transducer and using an abrasive liquid containing silicon carbide, holes of practically any shape can be drilled in hard, brittle materials such as tungsten carbide or precious stones.

Silicon based semiconductors are also used in the search for weapons of mass destruction, especially nuclear materials. The interactions of radiation with semiconducting crystals such as silicon can also be measured and semiconducting radiation detectors have the advantages of small size, high sensitivity, and high accuracy. Silicon chips also are key components of hand-held advanced nucleic acid analyzers (HANAA) that allow real-time polymerase chain reaction (PCR) based tests for pathogens (disease-causing organisms) that can be used by potential bioterrorists.

Silicon is also used in chips to which DNA (deoxyribonucleic acid) binds during hybridization procedures.

BOOKS

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford, UK: Oxford University Press, 2003.

Oxtoby, David W., et al. The Principles of Modern Chemistry. 5th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

PERIODICALS

Bennewitz, R., et al. “Atomic Scale Memory at a Silicon Surface.” Nanotechnology, 13 (2000): 499-502.

Cao, Y.W.C, R. Jin, C.A. Mirkin. “Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection.” Science, no. 5586 (2002): 1536-1540

Robert L. Wolke

K. Lee Lerner