Network Covalent, Ionic, and Metallic Solids

Virtually all substances that are gases or liquids at 25°C and 1 atm are molecular. In contrast, 

Several nonmetallic elements and metalloids have a network covalent structure. The most important of these is carbon, which has two different crystalline forms of the network covalent type. Both graphite and diamond have high melting points, above 3500°C. However, the bonding patterns in the two solids are quite different 

The structure of a diamond. Diamond has a three-dimensional structure in which each carbon atom is surrounded tetrahedrally by four other carbon atoms. 

Diagrams of four types of substances (see text discussion). X represents a nonmetal atom, — represents a covalent bond, M+ a cation, X- an anion, and e an electron. 

The structure of graphite. Graphite has a two-dimensional layer structure with weak dispersion forces between the layers. 

At 25°C and 1 atm, graphite is the stable form of carbon. Diamond, in principle, should slowly transform to graphite under ordinary conditions. Fortimately for the owners of diamond rings, this transition occurs at zero rate unless the dieunond is heated to about 1500°C, at which temperature the conversion occurs rapidly. For imderstandable reasons, no one has ever become very excited over the commercial possibihties of this 

At high pressures, diamond is the stable form of carbon, since it has a higher density than graphite (3.51 vs 2.26 g/cm ). The industrial synthesis of diamond from graphite or other forms of carbon is carried out at about 100,000 atm and 2000°C. 

Among the three-dimensional silicates are the zeolites, which contain cavities or tunnels in which Na+ or Ca + ions may be trapped. Synthetic zeolites with made-to-order holes are used in home water softeners. When hard water containing Ca + ions 

Sodium ions migrate out of the cavities Ca + ions from the hard water move in to replace them. 

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Section 9.5 extends the discussion to nonmolecular solids (network covalent, ionic, and metallic). 

Solids are usually described according to the forces that hold the particles together. The four types of solids are molecular solids, ionic solids, covalent network solids, and metallic solids. 

Crystalline solids can be classified into five categories based on the types of particles they contain atomic solids, molecular solids, covalent network solids, ionic solids, and metallic solids. Table 13-4 summarizes the general characteristics of each category and provides examples. The only atomic solids are noble gases. Their properties reflect the weak dispersion forces between the atoms. 

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 this investigation, you will study the properties of five different types of solids non-polar covalent, polar covalent, ionic, network, and metallic. You will be asked to identify each substance as one of the five types. In some cases, this will involve making inferences and drawing on past knowledge and experience. In others, this may involve process-of-elimination. The emphasis is on the skills and understandings you use to make your decisions. Later, you will be able to assess the validity of your decisions. 

From weakest to strongest, the four solid bonding types are molecular, metallic, ionic, and covalent network. 

The nature of the bonds between the structural units of crystalline solids impart other physical properties to these solids. Metals are good conductors of electricity because metallic bonds allow a free flow of electrons. Covalent network, molecular, and ionic solids do not conduct electricity because their bonds do not provide for mobile electrons. Remember, however, that ionic solids in a water solution have free electrons and are good conductors of electricity. Metallic solids are malleable and ductile covalent network solids are brittle and hard. These differences in physical properties are caused by the chemical bonds between the units It is all in the bonds ... 

The particles in a solid are held together with sufficient force to maintain a rigid structure. In some cases, these forces consist of intermolecular forces, while in others, chemical bonds. Solids are typically classified according to the types of forces that hold the particles together. When classified this way, the four types of solid are molecular, ionic, covalent network, and metallic. 

Solution KCl has a metal and a nonmetal ion attracted to one another and it will be ionic. H301+ has polar covalent bonds and one coordinate covalent bond. The bond between C and Cl will be polar covalent because of the difference in electronegativities. Si02 is sand and is a network solid. A sample of iron will have metallic bonds because only metal atoms are present. Fluorine is diatomic and will have nonpolar covalent bonds. HBr will have a polar covalent bond because of the great difference in electronegativity between these two nonmetals. 

Crystalline solids may be classified as (1) ionic solids, in which the repeating units are ions (2) network solids (or macromolecular solids), in which covalently bonded atoms are the repeating units (3) molecular solids, in which individual molecules are the repeating units and (4) metallic solids, in which individual metal atoms are held together by their loosely held valence electrons. 

The crystal structure of graphene illustrates two important characteristics of crystals. First, we see that no atoms lie on the lattice points. While most of the structures we discuss in this chapter do have atoms on the lattice points, there are many examples, like graphene, where this is not the case. Thus, to build up a structure you must know the location and orientation of the atoms in the motif with respect to the lattice points. Second, we see that bonds can be formed between atoms in neighboring unit cells. This happens in many crystals, particularly metallic, ionic, and network-covalent solids. 

Most metals are malleable, which means that they can be hammered into thin sheets, and ductile, which means that they can be drawn into wires ( FIGURE 12.10). These properties indicate that the atoms are capable of slipping past one another. Ionic and covalent-network solids do not exhibit such behavior. They are typically brittle and fracture easily. Consider, for example, the difference between dropping a ceramic plate and an aluminum cooking pan onto a concrete floor. 

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