Shared oxygens, between silicon atoms

Figure 3.43. Relation between the degree of polymerization and the ratio of shared oxygen to silicon atoms Circles. Hoebbel and Wieker (84) squares. Bcchtold. Vest, and Plambeck (149) solid line, theoretical relation for silica particles.

The layered structures of clays (Figure 17.4) consist of sheets of silicon oxide alternating with sheets of aluminum oxide. The silicon oxide sheets are made up of tetrahedra in which each silicon atom is surrounded by fom o g gen atoms. Of the four oxygen atoms in each tetrahedron, three are shared with other silicon atoms that are components of other tetrahedra. This sheet is called the tetrahedral sheet The alumimun oxide is contained in an octahedral sheet, so named because each alum-imun atom is surroimded by 6 oxygen atoms in an octahedral configuration. The structure is such that some of the oxygen atoms are shared between aluminum atoms and some are shared with the tetrahedral sheet. 

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

The topological analysis of the density and of the ELF funetion provides new information to understand the nature of the Si-0 bond. On the one hand, the atomic population and the bond ellipticities tell us that the SiO bond is partly ionic and also that there is no evidence for a partial double bond character. This latter point is confirmed by the analysis of the ELF function since there is only one attractor between the oxygen and silicon cores and that its basin population is always less than or equal to 2 electrons. Moreover, it appears more important to consider the oxygen than the silicon to discuss the bonding in silica. The Si-0 bond is found to belong to the electron shared interaction by the ELF analysis. 

For many years, the limited similarity between silicon and carbon excited the scientific community. Carbon and silicon share the same outer shell electronic structure, s, which permits sp hybridization and dominant tetrahedral coordination, as well as dominance of the tetravalent oxidation state. Nevertheless, silicon chemistry is markedly poorer compared to that of carbon. Double silicon bonds and silicon catenation are scarce, and crystalline silicon, which is so widely used in the electronics industry, is never encountered in nature. Instead, sUicon-oxygen bonds dominate natural silicon chanistry, and solid silica and silicates have no common physicochemical features with carbon dioxide and carbonates. The silicon atom is larger than carbon, it is less electronegative, has lower nuclear electric charge shielding and, perhaps most importantly, it has vacant d-orbitals in its outer shell all these dictate the reactivity of silicon. Several consequences of these differences are especially significant, and they are also relevant to sol-gel electrochemistry. 

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