Giant covalent lattices

When A and B are both electronegative they form covalent compounds. These may consist of individual molecules (02, H20, etc.) or of giant covalent lattices (polymeric solids) with a... 

Giant covalent lattices usually consist of a three-dimensional lattice of covalently bonded atoms. These atoms can be either all of the same type, as in silicon and carbon (diamond and graphite), or of two different elements, as in silicon dioxide. 

If the solid is a giant atomic lattice the covalent bonds throughout the lattice are strong and so the solid has a very high melting temperature. 

Bonding and structure Giant metallic lattice Giant metallic lattice Giant metallic lattice Giant covalent (three- dimensional) Simple molecular (P4) Simple molecular (Sg) Simple molecular (CI2) Monatomic (Ar)... 

In covalent network solids, covalent bonds join atoms together in the crystal lattice, which is quite large. Graphite, diamond, and silicon dioxide (Si02) are examples of network solids. The crystal is one giant molecule. 

CoSii, and VaSi, and some have been known for more than 100 years (59). Other more complex types containing three or more components, such as MnsSiaC, are also known. They all form three-dimensional giant lattices, often of an unusual and complicated kind, and can best be considered as intermediate in nature between alloy systems and macromolecular covalent compounds. The study of silicides has considerably intensified in the past decade, since it has been found that some show promise as electronic materials, while at the same time they are strong and highly resistant to chemical attack. A number of books and general reviews may be noted (18, 257, 318, 340, 360, 423). 

Only those atoms that form four covalent bonds produce a repetitive three-dimensional structure using only covalent bonds. The diamond structure. Fig. 27.11, is one of several related structures in which only covalent bonds are used to build the solid. The diamond structure is based on a face-centered cubic lattice wherein four out of the eight tetrahedral holes are occupied by carbon atoms. Every atom in this structure is surrounded tetrahedrally by four others. No discrete molecule can be discerned in diamond. The entire crystal is a giant molecule. 

Univalent atoms united by covalencies in molecules such as Cla have no bonds left, but polyvalent atoms can form lattices in which each atom is joined to one or more neighbours by a covalency, making the whole array into a giant molecule. The molecules or ions in other lattices are held by van der Waals or Coulomb forces. The physical characteristics of the varied types of solid show very wide variations which refiect these different modes of assemblage. 

So far we have considered solids in which atoms occupy the lattice positions. In some of these substances (network solids), the solid can be considered to be one giant molecule. In addition, there are many types of solids that contain discrete molecular units at each lattice position. A conunon example is ice, where the lattice positions are occupied by water molecules [see Fig. 10.12(c)], Other examples are dry ice (solid carbon dioxide), some forms of sulfur that contain Sg molecules [Fig. 10.32(a)], and certain forms of phosphorus that contain P4 molecules [Fig. 10.32(b)]. These substances are characterized by strong covalent bonding within the molecules but relatively weak forces between the molecules. For example, it takes only 6 kJ of energy to melt 1 mole of solid water (ice) because only intermolecular (H2O—H2O) interactions must be overcome. However, 470 kJ of energy is required to break 1 mole of covalent O—H bonds. The differences between the covalent bonds within the molecules and the forces between the molecules are apparent from the comparison of the interatomic and intermolecular distances in solids shown in Table 10.6. 

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