Solids network

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

However, in some covalent substances, known as network solids, atoms are bonded together in a way that forms a network structure. 

The most typical example of a network solid is diamond. In diamond each carbon atom is covalently bonded to four other carbon atoms forming a tetrahedral shape. (The type of hybridization that corresponds to this tetrahedral structure is sp3) This structure is extremely strong and this makes diamond the hardest natural substance. 

Silicon carbide SiC is another network solid. Silicon carbide is used as an abrasive because of its hard structure. 

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Molecular formula metal metal metal network solid Pi S6 Cls Ar... 

Fig. 17-2. Carbon forms network solids diamond and graphite.

The elements that form network solids lie on the right side of the periodic table, bordering the elements that form molecular crystals on one side and those that form metals on the other. Thus they are intermediate between the metals and the nonmetals. In this borderline region classifications are sometimes difficult. Whereas one property may suggest one classification, another property may lead to a different conclusion. Figure 17-3 shows some elements that form solids that are neither wholly metallic nor wholly molecular crystals. 

We have seen that the pure elements may solidify in the form of molecular solids, network solids, or metals. Compounds also may condense to molecular solids, network solids, or metallic solids. In addition, there is a new effect that does not occur with the pure elements. In a pure element the ionization energies of all atoms are identical and electrons are shared equally. In compounds, where the most stable electron distribution need not involve equal sharing, electric dipoles may result. Since two bonded atoms may have different ionization energies, the electrons may spend more time near one of the positive nuclei than near the other. This charge separation may give rise to strong intermolecular forces of a type not found in the pure elements. 

Contrast the bonds between atoms in metals, in van der Waals solids, and in network solids in regard to ... 

Discuss the conduction of heat by copper (a metal) and by glass (a network solid) in terms of the valence orbital occupancy and electron mobility. 

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. 

Network solids consist of atoms covalently bonded to their neighbors throughout the extent of the solid. 

In this part of the chapter, we begin with molecular solids and distinguish them from network solids. Then we examine metallic solids, which, if consisting of a single element, are built from identical atoms stacked together in orderly arrays. The structures of ionic solids are based on the same kinds of arrays but are complicated by the need to take into account the presence of ions of opposite charges and different sizes. 

Network solids are typically hard and rigid they have high melting and boiling points. Ceramic materials are commonly network solids. 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

Distinguish metallic solids, ionic solids, network solids, and molecular solids by their structures and by their properties (Sections 5.8-5.11 and 5.14). 

Explain why ionic solids such as NaCl have high melting points yet dissolve readily in water, whereas network solids such as diamond have very high melting points and do not dissolve in solvents. 

Silica, Si02, is a hard, rigid network solid that is insoluble in water. It occurs naturally as quartz and as sand, which consists of small fragments of quartz, usually colored golden brown by iron oxide impurities. Some precious and semiprecious stones are impure silica (Fig. 14.36). Flint is silica colored black by carbon impurities. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

In sharp contrast to molecular solids, network solids have very high melting points. Compare the behavior of phosphorus and silicon, third-row neighbors in the periodic table. As listed in Table 11-2. phosphorus melts at 317 K, but silicon melts at 1683 K. Phosphorus is a molecular solid that contains individual P4 molecules, but silicon is a network solid in which covalent bonds among Si atoms connect all the atoms. The vast array of covalent bonds In a network solid makes the entire stmcture behave as one giant molecule. ... 

Several oxides and sulfides display the characteristics of network solids. The bond network of silica appears in Section 9-. Other examples are titania (Ti02) and alumina (AI2 O3). These two substances have extremely high melting points because their atoms are held together by networks of strong a covalent bonds. Like graphite, M0S2 is a two-dimensional network solid that serves as a solid lubricant. 

Much of the Earth s silicon is found in durable rock formations based on silica. Granite is a common example. This is consistent with a network solid. Phosphorus, in contrast. 

We have touched briefly on three simple ionic lattices, but there are many others. Moreover, the stmctures of many crystalline network solids can also be described by the methods we have introduced here for NaCl, CsCl, and CaF2. 

Network solids such as diamond, graphite, or silica cannot dissolve without breaking covalent chemical bonds. Because intermolecular forces of attraction are always much weaker than covalent bonds, solvent-solute interactions are never strong enough to offset the energy cost of breaking bonds. Covalent solids are insoluble in all solvents. Although they may react with specific liquids or vapors, covalent solids will not dissolve in solvents. 

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