Crystalline solids covalent network

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. 

Carbon exists in more than 40 known structural forms, or allotropes, several of which are crystalline but most of which are amorphous. Graphite, the most common allotrope of carbon and the most stable under normal conditions, is a crystalline covalent network solid that consists of two-dimensional sheets of fused six-membered rings (Figure 10.26a). Each carbon atom is sp2-hybridized and is connected to three other carbons. The diamond form of elemental carbon is a covalent network solid in which each carbon atom is sp3-hybridized and is bonded with tetrahedral geometry to four other carbons (Figure 10.26b). 

Solids can be characterized as amorphous if their particles are randomly arranged or crystalline if their particles are ordered. Crystalline solids can be further characterized as ionic solids if their particles are ions, molecular solids if their particles are molecules, covalent network solids if they consist of a covalently bonded array of atoms without discrete molecules, or metallic solids if their particles are metal atoms. 

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 ... 

Answer The correct answer is Si02(quart —> Si02(g). Si02(< t/ariz) is a covalent network solid. Therefore, in its crystalline form, the rigid solid structure is maintained by covalent bonds. The other examples only involve the breaking of various intermolecular forces. 

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. 

Covalent solids (or network solids ) can be considered giant molecules that consist of covalently bonded atoms in an extended, rigid crystalline network. Diamond (one crystalline form of carbon) and quartz are examples of covalent solids (Figure 13-32). Because of their rigid, strongly bonded structures, mst covalent solids are very hard and melt at high temperatures. Because electrons are localized in covalent bonds, they are not freely... 

Depending on their coordination in the backbone network, group IV elements can form extended, covalently bonded structures of different dimensionality. As the most commonly known example, the fourfold coordination of the sp hybridized atoms leads to the three-dimensional (3D) crystalline solids diamond, c-Si, c-Ge, and a-Sn with their well-known semiconducting properties. On the other hand, linear (ID) polymer chains (XR2) with X = C, Si, Ge, Sn are based on a twofold coordination of the backbone atoms and are of great importance in organic and inorganic polymer chemistry. ... 

Of the solids given, ionically bonded sodium chloride is expected to be crystalline, a poor electrical conductor in the solid form, and a good conductor when fused. Diamond, formed of covalently bonded carbon atoms, is a network substance that does not form cubic crystalline patterns, and does not conduct electricity either when solid or fused. None of the allotropic forms of sulfur is expected to conduct electricity. Choice (D), the metal chromium, could possibly form a cubic solid crystalline form, but can be eliminated because it is expected to conduct electricity both when a solid and when fused. The correct choice is (A), because sodium chloride is a crystalline solid that is a poor conductor in the solid state and a good conductor when fused. ... 

Two Co(ll) coordination polymers with formulas [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n (H2oda=oxydiacetic acid) were characterized by single crystal XRD and TG. [Co(oda)(H20)2 H20]n has a covalently linked 1-D chain structure, while [Co(oda)(H20) H20]ii has a covalently linked 3-D chiral network with channels. The structures showed an unusual example of topological isomerism, and the structural interconversion between [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n revealed that self-assembly in the synthesis and interconversion of crystalline solids is a thermodynamically controlled process [135]. 

Ceramics are inorganic, nonmetallic, solid materials. They can be crystalline or noncr5 talline. Noncrystalline ceramics include glass and a few other materials with amorphous structures. Ceramics can possess a covalent-network structure, ionic bonding, or some combination of the two. (Section 11.8, Table 11.6) They are normally hard and brittle and are stable to very high temperatures. Ceramic materials include familiar objects such as pottery, china, cement, roof tiles, refractory bricks used in furnaces, and the insulators in spark plugs. 

Covalent solids (or network solids ) can be considered giant molecules that consist of covalendy bonded atoms in an extended, rigid crystalline network. Diamond (one crystalline... 

We have described the structure of crystals in a general way. Now we want to look in detail at the structure of several crystalline solids that represent the different types molecular, metallic, ionic, and covalent network. 

Distinguish among the following types of crystalline solids ionic, molecular, covalent network, and metallic. 

Amorphous solid Crystalline solid Materials science Polycrystalline sohd Solid state Section 15.8 Covalent network sohd Ionic crystal Metallic crystal Molecular crystal... 

In this chapter, we will focus on crystalline solids, although many of the concepts are applicable to molecular, covalent network, and metallic solids as well. There are techniques for studying the structures of amorphous solids however, they will not be considered here. 

---