The other elements of group 4A, in addition to carbon, are silicon, germanium, tin, and lead. The general trend from nonmetallic to metallic as we go down a family is strikingly evident in group 4A. Carbon is a nonmetal; silicon and germanium are metalloids; tin and lead are metals. In this section we will consider a few general characteristics of group 4A and then look more thoroughly at silicon.
Some properties of the group 4A elements are given in Table 22.8. The elements possess the outer-shell electron configuration ns2np2. The electronegativities of the elements are generally low; carbides that formally contain C4- ions are observed only in the case of a few compounds of carbon with very active metals. Formation of 4+ ions by electron loss is not observed for any of these elements; the ionization energies are too high. However, the +2 oxidation state is found in the chemistry of germanium, tin, and lead; it is the principal oxidation state for lead. The vast majority of the compounds of the group 4A elements are covalently bonded. Carbon forms a maximum of four bonds. The other members of the family are able to form higher coordination numbers through valence-shell expansion.
Carbon differs from the other group 4A elements in its pronounced ability to form multiple bonds both with itself and with other nonmetals, especially N, O, and S. The origin of this behavior was considered earlier. (For more information, see Section 22.1)
Table 22.8 shows that the strength of a bond between two atoms of a given element decreases as we go down group 4A. Carbon-carbon bonds are quite strong. As a consequence, carbon has a striking ability to form compounds in which carbon atoms are bonded to one another. This property permits the formation of extended chains and rings of carbon atoms and accounts for the large number of organic compounds that exist. Other elements, especially those in the vicinity of carbon in the periodic table, can also form chains and rings. However, such self-linkage is far less important in the chemistries of these other elements. For example, the SiSi bond strength (226 kJ/mol) is far smaller than the SiO bond strength (386 kJ/mol). As a result, the chemistry of silicon is dominated by the formation of SiO bonds, and SiSi bonds play a rather minor role.
Silicon is the second most abundant element, after oxygen, in Earth's crust. It occurs in SiO2 and in an enormous variety of silicate minerals. The element is obtained by the reduction of molten silicon dioxide with carbon at high temperature:
Elemental silicon has a diamond-type structure [see Figure 11.41(a)]. Crystalline silicon is a gray metallic-looking solid that melts at 1410°C (Figure 22.49). The element is a semiconductor (Section 23.5) and is thus used in making transistors and solar cells. To be used as a semiconductor, it must be extremely pure. One method of purification is to treat the element with Cl2 to form SiCl4. The SiCl4 is a volatile liquid that is purified by fractional distillation and then converted back to elemental silicon by reduction with H2:
The element can be further purified by the process of zone refining. In the zone-refining process, a heated coil is passed slowly along a silicon rod, as shown in Figure 22.50. A narrow band of the element is thereby melted. As the molten area is swept slowly along the length of the rod, the impurities concentrate in the molten region, following it to the end of the rod. The purified top portion of the rod is retained for manufacture of electronic devices.
FIGURE 22.50 Zone-refining apparatus.
Silicon dioxide and other compounds that contain silicon and oxygen comprise over 90 percent of Earth's crust. Silicates are compounds in which a silicon atom is surrounded in a tetrahedral fashion by four oxygens, as shown in Figure 22.51. In silicates, silicon is found in its most common oxidation state, +4. The simple SiO44- ion, which is known as the orthosilicate ion, is found in very few silicate minerals. However, we can view the silicate tetrahedra as "building blocks" that are used to build mineral structures. The individual tetrahedra are linked together by a common oxygen atom that serves as a vertex of both tetrahedra.
FIGURE 22.51 Structure of the SiO4 tetrahedron of the SiO44- ion. This ion is found in several minerals, such as zircon, ZrSiO4.
For example, we can link two silicate tetrahedra together by sharing one oxygen atom, as shown in Figure 22.52. The resultant structure, called the disilicate ion, has two Si atoms and seven O atoms. We can easily determine the charge on this ion by realizing that in all silicates, Si and O are in the +4 and -2 oxidation states, respectively. The overall charge of the ion must be consistent with these oxidation states. Thus, the charge on Si2O7 is 2 × (+4) + 7 × (-2) = -6; it is the Si2O76- ion. The mineral thortveitite, which has the formula Sc2Si2O7, contains Si2O76- ions. The disilicate ion is also represented in Figure 22.53(b); each silicate tetrahedron is represented as in Figure 22.53(a).
Figure 22.52 Geometrical structure of the Si2O76- ion, which is formed by the sharing of an oxygen atom by two silicon atoms. This ion occurs in several minerals, such as hardystonite, Ca2Zn(Si2O7).
In most silicate minerals a large number of silicate tetrahedra are linked together to form chains, sheets, or three-dimensional structures. For example, we can connect two vertices of each tetrahedron to two other tetrahedra, leading to an infinite chain with a OSiOSi backbone. This structure, called a single-strand silicate chain, is represented in Figure 22.53(c). As shown, this chain can be viewed as repeating units of the Si2O64- ion, or, in terms of its simplest formula, SiO32-. The mineral enstatite, MgSiO3, consists of rows of single-strand silicate chains with Mg2+ ions between the strands to balance charge.
In Figure 22.53(d) each silicate tetrahedron is linked to three others, forming an infinite two-dimensional sheet structure. The simplest formula of this infinite sheet is Si2O52 -. The mineral talc, also known as talcum powder, has the formula Mg3(Si2O5)2(OH)2 and is based on this sheet structure. The Mg2+ and OH- ions lie between the silicate sheets. The slippery feel of talcum powder is due to the sliding of the silicate sheets relative to one another, much like the sliding of the sheets of carbon atoms in graphite, which gives that substance its lubricating properties. (For more information, see Section 11.8)
FIGURE 22.53 Silicate structures are represented as tetrahedra linked together through their vertices. Each tetrahedron represents an SiO4 unit, as shown in (a). Tetrahedra are linked through a shared oxygen atom. (b) Representation of the disilicate ion, Si2O76-, which was shown in Figure 22.52. (c) Representation of an infinite single-strand silicate chain. Each tetrahedron is linked to two others. The box shows the repeating unit of the chain, which is similar to the unit cell of solids (Section 11.7); the chain can be viewed as an infinite number of repeating units, laid side by side. The repeating unit has a formula of Si2O64-, or as a simplest formula, SiO32-. (d) Representation of a two-dimensional sheet structure. Each tetrahedron is linked to three others. The repeating unit of the sheet has the formula Si2O52-.
Asbestos is a general term applied to a group of fibrous silicate minerals. These minerals possess chainlike arrangements of the silicate tetrahedra, or sheet structures in which the sheets are formed into rolls. The result is that the minerals have a fibrous character, as shown in Figure 22.54. Asbestos minerals have been widely used as thermal insulation, especially in high-temperature applications, because of the great chemical stability of the silicate structure. In addition, the fibers can be woven into asbestos cloth, which can be used for fireproof curtains and other applications. However, the fibrous structure of asbestos minerals poses a health risk. Tiny asbestos fibers readily penetrate soft tissues, such as the lungs, where they can cause diseases, including cancer. The use of asbestos as a common building material has therefore been discontinued.
When all four vertices of each SiO4 tetrahedron are linked to other tetrahedra, the structure extends in three dimensions. This linking of the tetrahedra forms quartz, SiO2, which was depicted in two-dimensional fashion in Figure 11.27(a). Because the structure is locked together in a three-dimensional array much like diamond [Figure 11.41(a)], quartz is harder than strand- or sheet-type silicates.
The mineral chrysotile is a noncarcinogenic asbestos mineral that is based on the sheet structure shown in Figure 22.53(d). In addition to silicate tetrahedra, the mineral contains Mg2+ and OH - ions. Analysis of the mineral shows that there are 1.5 Mg atoms per Si atom. What is the simplest formula for chrysotile?
SOLUTION As shown in Figure 22.53(d), the silicate sheet structure is based on the Si2O52- ion. (Note that the charge on this ion is consistent with the +4 and -2 oxidation states of Si and O, respectively.) The observation that the Mg:Si ratio equals 1.5 is consistent with three Mg2+ ions per Si2O52- ion. The addition of three Mg2+ ions would make Mg3(Si2O5)4+. In order to achieve charge balance in the mineral, there must be four OH - ions per Si2O52- ion. Thus, the simplest formula of chrysotile is Mg3(Si2O5)(OH)4.
The cyclosilicate ion consists of three silicate tetrahedra linked together in a ring. The ion contains three Si atoms and nine O atoms. What is the overall charge on the ion? Answer: 6-
Quartz melts at approximately 1600°C, forming a tacky liquid. In the course of melting, many silicon-oxygen bonds are broken. When the liquid is rapidly cooled, silicon-oxygen bonds are re-formed before the atoms are able to arrange themselves in a regular fashion. An amorphous solid, known as quartz glass or silica glass, results (see Figure 11.27). Many different substances can be added to SiO2 to cause it to melt at a lower temperature. The common glass used in windows and bottles is known as soda-lime glass. It contains CaO and Na2O in addition to SiO2 from sand. The CaO and Na2O are produced by heating two inexpensive chemicals: limestone, CaCO3, and soda ash, Na2CO3. These carbonates decompose at elevated temperatures:
Other substances can be added to soda-lime glass to produce color or to change the properties of the glass in various ways. For example, addition of CoO produces the deep blue color of "cobalt glass." Replacement of Na2O by K2O results in a harder glass that has a higher melting point. Replacement of CaO by PbO results in a denser "lead crystal" glass with a higher refractive index. Lead crystal is used for decorative glassware; the higher refractive index gives this glass a particularly sparkling appearance. Addition of nonmetal oxides, such a B2O3 and P4O10, which form network structures related to the silicates, also causes a change in properties of the glass. Addition of B2O3 creates a glass with a higher melting point and a greater ability to withstand temperature changes. Such glasses, sold commercially under trade names such as Pyrex® and Kimax®, are used where resistance to thermal shock is important, for example, in laboratory glassware or coffeemakers.
Silicones consist of OSiO chains in which the remaining bonding positions on each silicon are occupied by organic groups such as CH3:
Depending on the length of the chain and the degree of cross-linking between chains, silicones can be either oils or rubberlike materials. Silicones are nontoxic and have good stability toward heat, light, oxygen, and water. They are used commercially in a wide variety of products, including lubricants, car polishes, sealants, and gaskets. They are also used for waterproofing fabrics. When applied to a fabric, the oxygen atoms form hydrogen bonds with the molecules on the surface of the fabric. The hydrophobic (water-repelling) organic groups of the silicone are then left pointing away from the surface as a barrier.