Crystal types covalent network

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. 

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. 

Indicate the type of crystal (molecular, metalhc, ionic, or covalent-network) each of the foUowing would form upon... 

Indicate the type of crystal (molecular, metalhc, ionic, or covalent-network) each of the following would form upon sohdification (a) InAs, (b) MgO, (c) H, (d) In, (e) HBr. 

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. 

Crystalline solids A solid whose atoms, ions, or molecules are arranged in an orderly, geometric, three-dimensional structure (lattice) is called a crystalline solid. The individual pieces of a crystalline solid are called crystals. Crystalline solids are divided into five categories based on the types of particles they contain atomic solids, molecular solids, covalent network solids, ionic solids, and metallic solids. Noble gases are atomic solids whose properties reflect the weak dispersion forces between the atoms. 

It is useful as a point of departure, to briefly describe the basic crystal lattice common to phyllosilicates. The elementary character is the SiO tetrahedral linkage of an essentially two-dimensional, hexagonally symmetric, network. One side of this "sheet network is coordinated with other cation-oxygen complexes joined by an important component of covalent bonding while the other is coordinated by essentially ionic bonding or van der Waals type bonds. The key to phyllosilicate structures is the oxygen network which determines the shape and extent of the structure. 

The big difference in melting points suggests a difference in type of crystal binding. The intermolecular forces in solid CO2 must be very low to be overcome by a low-temperature sublimation. CO2 is actually a molecular lattice held together only by the weak van der Waals forces between discrete CO2 molecules. Si02 is a covalent lattice with a three-dimensional network of bonds each silicon atom is bonded tetrahedrally to four oxygen atoms and each oxygen is bonded to two silicon atoms. 

Covalently bonded solids such as quartz, diamond, and graphite form another class of crystals. Quartz is a continuous network of silicon dioxide bonded in a uniform, crystalline arrangement. Sand is a mixture of quartz and other rocks. Glass is solid quartz that has melted and resolidified without the same crystalline uniformity, in the way that melted butter does not re-form the same type of solid when it cools. Glass has been known to form naturally in lightning strikes on sand. 

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