Diamond covalent bonds

Covalent Atoms Covalent bonds Diamond (carbon) Paper 0.3... 

Owing to its single composition and pure covalent bonding, diamond is a standard solid material, and takes the role of the most appropriate sample in explaining the effects upon structure-sensitive properties when the structure of solid material deviates from the ideal state. 

Diamond has the highest atom-number density of any known material at terrestrial pressures. Because of its high atom-number density and strong covalent bonding, diamond has the highest hardness and elastic modulus of any material and is the least compressible substance known. 

Figure 2.12. Three-dimensional representation of sp covalent bonding (diamond structure). Shaded regions are regions of high electron probabilities where covalent bonding occurs.

A covalent network solid is a solid that consists of atoms held together in large networks or chains by covalent bonds. Diamond is an example of a three-dimensional network solid. Every carbon atom in diamond is covalently bonded to four others, so an entire crystal might be considered an immense molecule. It is possible to have similar two-dimensional sheet and one-dimensional chain molecules, with atoms held together by covalent bonds. Examples are diamond (three-dimensional network), graphite (sheets), and asbestos (chains). Figure 11.27 shows the structures of diamond and graphite. 

Covalent network Atoms Covalent bonding Diamond, graphite, asbestos... 

The structures of covalent network solids are determined primarily by the directions of covalent bonds. Diamond is a simple example. It is one allotropic form of the element carbon, in which each carbon atom is covalently bonded to four other carbon atoms in tetrahedral directions to give a three-dimensional covalent network... 

Pure carbon occurs naturally in two modifications, diamond and graphite. In both these forms the carbon atoms are linked by covalent bonds to give giant molecules (Figure S.2). 

Covalent bonding appears in its pure form in diamond, silicon and germanium - all materials with large moduli (that of diamond is the highest known). It is the dominant... 

If the polymer is completely cross-linked (/= 1) then the modulus (Ej) is known it is that of diamond, 10 GPa. If it has no covalent bonds at all, then the modulus (E2) is that of a simple hydrocarbon like paraffin wax, and that, too, is known it is 1 GPa. 

In diamond, each carbon atom forms single bonds with four other carbon atoms arranged tetrahedrally around it The hybridization in diamond is sp3. The three-dimensional covalent bonding contributes to diamond s unusual hardness. Diamond is one of the hardest substances known it is used in cutting tools and quality grindstones (Figure 9.12). 

Even though silicon is metallic in appearance, it is not generally classified as a metal. The electrical conductivity of silicon is so much less than that of ordinary metals it is called a semiconductor. Silicon is an example of a network solid (see Figure 20-1)—it has the same atomic arrangement that occurs in diamond. Each silicon atom is surrounded by, and covalently bonded to, four other silicon atoms. Thus, the silicon crystal can be regarded as one giant molecule. 

FIGURE 5.21 The structure of diamond, Each sphere represents the location of the center of a carbon atom. Each atom is at the center of a tetrahedron formed hy the sp1 hybrid covalent bonds to each of its four neighbors. 

Questions such as, for example, whether sphalerite contains Zn++ and S= ions or has a covalent structure similar to that of diamond, and whether ionic or covalent bonds are present in complexes such as [FeF% —, [Fe(CN)e]=, etc., have been extensively discussed it has, indeed, until recently not been at all clear whether or not they could be definitely... 

Ionic bonds may be fully as strong as covalent bonds, so that properties such as hardness, solubility, melting point, ionization in solution, and chemical character are not especially valuable criteria as a rule. Sometimes comparison of properties with those of compounds of known bond type permits reasonably certain conclusions to be drawn. Thus the similarity in physical properties as well as in atomic arrangement of SiC, AIN, and diamond suggests that all three substances contain covalent bonds. PbS is like FeS2, MoS2, etc. in properties rather than like CaS, so that it is improbable that PbS is an ionic substance. 

Replacement of each water molecule by an MnAli2 icosahedron with shared faces leads to an infinite framework with 136 Mn and 816 Al atoms in the unit cube. This framework is similar to the framework of covalently bonded carbon atoms in a diamond crystal, with one body diagonal of each pentagonal dodecahedron in place of each C—C covalent bond. 

Diamondoids, when in the solid state, melt at much higher temperatures than other hydrocarbon molecules with the same number of carbon atoms in their structures. Since they also possess low strain energy, they are more stable and stiff, resembling diamond in a broad sense. They contain dense, three-dimensional networks of covalent bonds, formed chiefly from first and second row atoms with a valence of three or more. Many of the diamondoids possess structures rich in tetrahedrally coordinated carbon. They are materials with superior strength-to-weight ratio. 

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