Silicon Lewis symbols

Write the Lewis symbols for potassium, oxygen, silicon, and chlorine. 

Because elements in the same group have the same number of valence electrons, the Lewis symbols for subsequent periods are the same, except for the identity of the elements. For example, the Lewis dot symbols for carbon, silicon, and germanium are as follows ... 

The direct conversion of light energy to electrical energy can be accomplished in photovoltaic cells, also called solar cells. These consist basically of two layers of silicon, Si, one doped with about 1 atom per million of arsenic. As, and the other doped with about 1 atom per million of boron, B. To understand what happens, consider the Lewis symbols of the three elements involved ... 

As shown by its Lewis symbol above and discussed in Chapter 3, each silicon atom has 4 valence electrons. In a crystal of Si, each of the silicon atoms is covalently bonded to four other Si atoms, as shown in Figure 8.13. If a B atom replaces one of the Si atoms, the 3 valence electrons in B result in a shortage of 1 electron, leaving what is called a positive hole. Each As atom has 5 valence electrons, so that replacement of one of the Si atoms by As leaves a surplus of 1 electron, which can move about in the Si crystal. A layer of Si atoms doped with As makes up a donor layer because of the extra valence electron introduced by each As atom, and Si doped... 

Lewis developed a special set of symbols for his theory. A Lewis symbol consists of a chemical symbol to represent the nucleus and core (inner-shell) electrons of an atom, together with dots placed around the symbol to represent the valence (outer-shell) electrons. Thus, the Lewis symbol for silicon, which has the electron configuration [Ne]3s 3p, is... 

In Lewis theory, we ignore the inner or core electrons and represent the four valence electrons with four dots surrounding the chemical symbol for silicon. Because carbon also has four valence electrons, its Lewis structure also has four... 

Using only a periodic table, determine the correct Lewis dot symbol for a silicon (Si) atom. 

Elements in the same column as one of the above elements have analogous electron dot structures. For example, silicon s structure is Si. An important feature of Lewis s symbols for atoms is that the number of unpaired electrons in the symbol is usually the number of chemical bonds that element will form. Thus, a hydrogen atom can form only one bond. 

In the catalytic cycle, a simplified version of which is shown in Scheme 5.72 for the acetate aldol addition of 246, the highly electrophilic silyl cation 251 plays a key role, as assumed by the authors. It forms from the reaction of tetrachlorosilane with the corresponding phosphoramide ((Me2N)3PO symbolizing the catalyst 235). When loaded with benzaldehyde, silicon enlarges its coordination sphere and adopts an octahedral geometry in 252. After the carbon-carbon bond has been established, cation 253 forms. It then decomposes to liberate phosphoramide 235, chlorotrialkylsilane, and the aldolate 254. By NMR studies, it was shown that the intermediate of this procedure is the tric/i/orosilyl-protected aldolate 254. This makes a substantial mechanistic difference to conventional Lewis acid-catalyzed Mukaiyama aldol protocols that deliver tri /Ay/silyl-protected aldolates. In accordance with the catalytic cycle shown in Scheme 5.72, tetrachlorosilane is consumed and therefore required to be used in stoichiometric amounts. Thus, the reaction is catalyzed by phosphoramides and mediated by tetrachlorosilane or, more generally, by Lewis base-activated Lewis acids [126]. 

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