Covalent network crystals

What kinds of attractive forces exist between particles in (a) molecular crystals, (b) covalent-network crystals, (c) ionic crystals, (d) metaUic crystals ... 

Covalent network crystals include three-dimensional network solids, such as this quartz, (Si02) f, shown here with its three-dimensional atomic structure. 

A covalent network crystal is composed of atoms or groups of atoms arranged into a crystal lattice that is held together by an interlocking network of covalent bonds. 

Covalent bonds (the result of the sharing of one or more pairs of electrons in a region of orbital overlap between two or perhaps more atoms) are directional interactions as opposed to ionic and metallic bonds, which are nondirectional. A good example of a covalent network crystal is diamond, shown in Figure 7.3. Note that each carbon atom is best thought of as being r -hybridized and that to maximize the overlap of these hybrid orbitals, a C-C-C bond angle of 109.5° is necessary. 

These interactions are therefore directional in nature. Other compounds that form covalent network crystals are silicon dioxide (quartz or cristobalite, Si02), graphite, elemental silicon (Si), boron nitride (BN), and black phosphorus. 

Network crystals (Ex C, SiC, Si02) Covalent Atoms - very high melting point - very hard - do not conduct electricity (except graphite)... 

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. 

Covalent Network atoms covalent bonds very high low hard crystals that are insoluble in most liquids formed usually from elements belonging to Group 14 (IV A) graphite, diamond, Si02... 

The bonding features in the charge density are pronounced in crystals with extended covalent networks. The availability of perfect silicon crystals has allowed the measurement of uncommonly accurate structure factors, of millielectron accuracy. The data have served as a test of experimental formalisms for charge density analysis, and at the same time have provided a stringent criterion for quantum-mechanical methods. 

Homonuclear aggregates of phosphorus atoms exist in many forms discrete molecules, covalent networks in crystals, polyphosphide anions, and phosphorus fragments in molecular compounds. 

Covalent network solids contain atoms held together by a network of covalent bonds that link every atom in the solid to every other atom. The molecules are gigantic each particle of the crystal is essentially one molecule. This type of solid is hard, nonvolatile, with a very high melting point and insoluble in both water and inorganic solvents. They do not conduct electricity. 

Motherwell, W. D. S., Shields, G. P. and Allen, F. H. (1999). Visualization andcharac-terization of non-covalent networks in molecular crystals automated assignment of graph-set descriptors for asymmetric molecules. Acffl Crystallogr. B, 55, 1044-56. [57]... 

A number of solids are composed only of atoms interconnected by a network of covalent bonds. These solids are often called covalent network solids. Quartz is a network solid, as is diamond. See Figure 9-20. In contrast to molecular solids, network solids are typically brittle, nonconductors of heat or electricity, and extremely hard. In a diamond, four other carbon atoms surround each carbon atom. This tetrahedral arrangement forms a strongly bonded crystal system that is extremely hard and has a very high melting point. 

Boric oxide B203 is very hard to crystallize the glass has a linked covalent network in which both bridging B—O—B and terminal B=0 bonds may be present. The hydroxide boric acid B(OH)3 is... 

Trends in properties, such as decreasing hardness and melting point, are due to changes in types of bonding within the solid covalent network in C, Si, and Ge metallic in Sn and Pb [see text). Down the group, density increases because of several factors, including differences in crystal packing. 

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