The Group 4A Elements

An old ad from The Saturday Evening Post for Boraxo, a hand-cleaning product containing sodium tetraborate (NaB40y). Extensive natural deposits of borax (Na2B40y IOH2O) found in saline lakes near Death Valley, California, were hauled to a factory in wagons pulled by teams of 20 mules—hence the name 20 Mule Team Borax. 

2M t 3X2 2MX3 X2 = any halogen molecule Tl gives TlX as well, but no TII3 

4M -r 3O2 - — 2M2O3 High temperatures Ti gives Ti20 as weii 

2M -r 3S — — M2S3 High temperatures Ti gives Ti2S as weii 

The chemistry of indium is similar to that of aluminum and gallium except that compounds containing the 1 -1- ion are known, such as InCI and hjO, in addition to those with the more common 3+ ion. The chemistry 0 thallium is completely metallic. 

A new form of elemental carbon, the fullerenes, was discussed in Chapter 2. 

All the Group 4A elements can form four covalent bonds to nonmetals— for example, CH4, SiF4, GeBr4, SnCl4, and PbCU. In each of these tetrahedral molecules the central atom is described as sp hybridized by the localized electron model. 

We have seen that carbon also differs markedly from the other members of Group 4A in its ability to form tt bonds. This accounts for the completely different structures and properties of CO2 and SiOi. Note from Table 18.11 that C—C bonds and Si—O bonds are stronger than Si—Si bonds. This partly explains why the chemistry of carbon is dominated by C—C bonds, whereas that of silicon is dominated by Si—O bonds. 

Carbon occurs in the allotropic forms graphite, diamond, and fullerenes, whose structures were given in Section 16.5. The most important chemistry of carbon is organic chemistry, which is described in detail in Chapter 21. 

Silicon, the second most abundant element in the earth s crust, is a semimetal found widely distributed in silica and silicates (see Section 16.5). About 85% of the earth s crust is composed of these substances. Although silicon is found in some steel and aluminum alloys, its major use is in semiconductors for electronic devices (see Chapter 16). 

TABLE 18.11 Selected Physical Properties, Sources, and Methods of Preparation for the Group 4A Elements  

Element Electronegativity Point (°C) Point (°C) Sources Preparation 

Silicon 1.9 1410 2355 Silicate minerals, silica Reduction of K2SiF6 with Al, or reduction of Si02 with Mg 

Germanium 2.0 937 2830 Germinate (mixture of copper, iron, and germanium sulfides) Reduction of Ge02 with H2 or C 

Element Electronegativity Melting Point (°C) Boiling Point (°C) Source Method of Preparation 

Carbon 2.6 3727 (sublimes) - Graphite, diamond, petroleum, coal - 

Lead 2.3 327 1740 Galena (PbS) Roasting of PbS with O2 to form Pb02 and then reduction with C 

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Look at their positions in the periodic table. The group 4A element germanium has four valence-shell electrons and thus has four relatively low ionization energies, whereas the group 5A element arsenic has five valence-shell electrons and has five low ionization energies. 

The group 4A elements—carbon, silicon, germanium, tin, and lead—are especially important, both in industry and in living organisms. Carbon is present in all plants and animals, accounts for 23% of the mass of the human body, and is an essential constituent of the molecules on which life is based. Silicon is equally important in the mineral world It is present in numerous silicate minerals and is the second most abundant element in the earth s crust. Both silicon and germanium are used in making modern solid-state electronic devices. Tin and lead have been known and used since ancient times. 

The group 4A elements exemplify the increase in metallic character down a group in the periodic table Carbon is a nonmetal silicon and germanium are semimetals and tin and lead are metals. The usual periodic trends in atomic size, ionization energy, and electronegativity are evident in the data of Table 19.4. 

The group 4A elements—C, Si, Ge, Sn, and Pb—exhibit the usual increase in metallic character down the group. Their most common oxidation state is +4, but the +2 state becomes increasingly more stable from Ge to Sn to Pb. In elemental form, carbon exists as diamond, graphite, fullerene, coke, charcoal, and carbon black. 

Identify the group 4A element that best fits each of the following descriptions ... 

Which of the group 4A elements have allotropes with the diamond structure Which have metallic allotropes How does the variation in the structure of the group 4A elements illustrate how metallic character varies down a periodic group ... 

The MOs of a semiconductor are similar to those of an insulator, but the band gap in a semiconductor is smaller (Figure 21.10). As a result, a few electrons have enough thermal energy to jump the gap and occupy the higher-energy conduction band. The conduction band is thus partially filled, and the valence band is partially empty because it now contains a few unoccupied MOs. When an electrical potential is applied to a semiconductor, it conducts a small amount of current because the potential can accelerate the electrons in the partially filled bands. Table 21.3 shows how the electrical properties of the group 4A elements vary with the size of the band gap. 

Sketch the electron populations of the bands of MO energy levels for elemental carbon (diamond), silicon, germanium, gray tin, and white tin. (Band gap data are given in Table 21.3.) Your sketches should show how the populations of the different bands vary with a change in the group 4A element. 

TABLE 18.14 Selected Reactions of the Group 4A Elements Reaction Comment... 

Table 18.14 summarizes some important reactions of the Group 4A elements. 

The Group 4A elements form compounds in both the +2 and +4 oxidation states. For carbon and silicon, the +4 oxidation state is the more stable one. For example, CO2 is more stable than CO, and Si02 is a stable compound, but SiO does not exist under normal conditions. As we move down the group, however, the trend in stability is reversed. In tin compounds the -1-4 oxidation state is only slightly more stable than the + 2 oxidation state. In lead compounds the -1-2 oxidation state is unquestionably the more stable one. The outer electron configuration of lead is 6s 6p, and lead tends to... 

A number of elements are semiconductors, that is, they normally are not conductors, but will conduct electricity at elevated temperatures or when combined with a small amount of certain other elements. The Group 4A elements silicon and germanium are especially suited for this purpose. The use of semiconductors in transistors and solar cells, to name two applications, has revolutionized the electronic industry in recent decades, leading to increased miniaturization of electronic equipment. 

The Group 2A Elements The Group 3A Elements The Group 4A Elements The Group 5A Elements The Chemistry of Nitrogen... 

The group 4A elements possess the outer-shell electron configuration ns np. The electronegativities of the elements are generally low (V TABLE 22.8) carbides that formally contain C ions are observed only in the case of a few compounds of carbon with ver)y active metals. Formation of 4-1- ions by electron loss is not observed for any of these elements the ionization energies are too high. The +2 oxidation state is found in the... 

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