Silicon and Its Simpler Compounds

Silicon is a brittle steel-gray metalloid. Some of its physical properties are given in Table 18-3. It can be made by reduction of silicon tetrachloride by sodium  

The element has the same crystal structure as diamond, each silicon atom forming single covalent bonds with four adjacent silicon atoms, which surround it tetrahedrally. It is used in transistors, especially for service at elevated temperatures (Section 18-13). 

Silicon contaminated with carbon can be obtained by reduction of silica, Si02, with carbon in an electric furnace. An alloy of iron and silicon. 

Ferrosilicon, which has composition approximately FeSi, is used in the manufacture of acid-resisting alloys, such as dnriron, which contains about 15% silicon. Duriron is used in chemical laboratories and manufacturing plants. A mild steel containing a few percent of silicon may be made which has a high magnetic permeability, and is used for the cores of electric tranformers. 

Silicon carbide, SiC, is made by heating a mixture of carbon and sand in a special electric furnace  

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Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

Matter can be broadly classified into three types—elements, compounds, and mixtures. An element is the simplest type of matter with unique physical and chemical properties. An element consists of only one kind of atom. Therefore, it cannot be broken down into a simpler type of matter by any physical or chemical methods. An element is one kind of pure substance (or just substance), matter whose composition is fixed. Each element has a name, such as silicon, oxygen, or copper. A sample of silicon contains only silicon atoms. A key point to remember is that the macroscopic properties of a piece of silicon, such as color, density, and combustibility, are different from those of a piece of copper because silicon atoms are different from copper atoms in other words, each element is unique because the properties of its atoms are unique. 

In principle, the same basic methods of heat treatment, ion bombardment and cleavage which are used to produce clean silicon surfaces can be used to generate clean GaAs surfaces, and the same general reservations apply. However, the fact that GaAs is a compound whose surface stoichiometry is potentially variable introduces additional problems for those techniques which depend on removal of material. Cleavage is not subject to these effects, however, and the cleavage plane is 110, which contains equal numbers of Ga and Aa atoms. Furthermore, the cleaved surface structure does not appear to be metastable, at least in terms of the LEED patterns produced [106], so in some ways it represents a simpler case than silicon. 

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