Bond orbitals silicon dioxide

Because carbon stands at the head of its group, we expect it to differ from the other members of the group. In fact, the differences between the element at the head of the group and the other elements are more pronounced in Group 14/IV than anywhere else in the periodic table. Some of the differences between carbon and silicon stem from the smaller atomic radius of carbon, which explains the wide occurrence of C=C and G=Q double bonds relative to the rarity of Si=Si and Si=0 double bonds. Silicon atoms are too large for the side-by-side overlap of p-orbitals necessary for -it-bonds to form between them. Carbon dioxide, which consists of discrete 0=C=0 molecules, is a gas that we exhale. Silicon dioxide (silica), which consists of networks of —O—Si- O - groups, is a mineral that we stand on. 

As mentioned before, carbon and silicon mostly differ in their ability to from multiple p,-p, bonds E=Y with suitable partners (E = C, Si Y = element of group 14 to 16 ). While the p orbital overlap in compounds >C=Y is sufficient to yield stable multiple bonded species, this overlap is strongly reduced in the case of silicon (classical double bond rule of Pitzer and Mulliken). Consequently, under comparable conditions the equivalents of many unsaturated monomeric compounds of carbon, such as H2C=CH2, R2C=0 or CO2 are silicon single bonded polymeric products, e g. polysilanes (-H2Si-SiH2-)n, silicones (-R2Si-0-) and silicon dioxide (Si02)n... 

Silicon, also in period 3, has p orbitals significantly larger than those of oxygen. Their differences in size and energy mean that extensive overlap does not occur. Consequently silicon dioxide, Si02, has a giant covalent structure (see Chapter 4) where the bonds are -O-Si-O-bonds. No stable Si02 (0=Si=0) molecules are possible in the solid state. 

Silicon dioxide is quite different from carbon dioxide. Instead, in accordance with the diagonal relationship, it resembles boron oxide. Only one kind of direct bond is known for each natural compound of these two elements, namely B—O and Si—O. The linkage of the oxygen free electron pair with the free d-orbital of silicon or the free p-orbital of boron increases the strength of the bond, which then takes the form... 

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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