Solids Held Together by Covalent Bonding

Consider Ihe battery in Fig. 7.18. The sodium bela alumina barrier allows sodium ions formed at Ihe anode lo flow across to the sulfur compartment, where, together with Ihe reduction products of Ihe sulfur, il forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 °C lo keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At Ihe present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times Ihe power on a weight basis. 

Because some of the properties of solids that contain no ionic bonds may be conveniently compared with those of ionic solids, it is useful lo include them here despite the fact that this chapter deals primarily with ionic compounds. 

We may classify solids broadly into three types based on their electrical conductivity. Metals conduct electricity very well. In contrast, insulators do not. Insulators may consist of discrete small molecules, such as phosphorus triiodide, in which the energy necessary to ionize an electron from one molecule and transfer it to a second is too great to be effected under ordinary potentials.20 We have seen that most ionic solids are nonconductors. Finally, solids that contain infinite covalent bonding such as diamond and quartz are usually good insulators (but see Problem 7.3). 

The third type of solid comprises the group known as semiconductors. These are either elements on the borderline between metals and nonmetals, such as silicon and 

20 Given sufficient energy, of course, any insulator can be made lo break down. 

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I2 is a molecular solid, held together by covalent bonds. 

This relationship is illustrated in Figure 1. The science of solids is the science of supramolecular systems in which the three-dimensional solid structure is held together by covalent bonds... 

Molecular Orbital Theory a model that uses wave functions to describe the position of electrons in a molecule, assuming electrons are delocalized within the molecule Molecular Solid a solid that contains molecules at the lattice points Molecule a group of atoms that exist as a unit and are held together by covalent bonds... 

Comparing polarity between components is often a good way to predict solubility, regardless of whether those components are liquid, solid, or gas. Why is polarity such a good predictor Because polarity is central to the tournament of forces that underlies solubility. So solids held together by ionic bonds (the most polar type of bond) or polar covalent bonds tend to dissolve well in polar solvents, like water. 

Another property of great interest is the ionicity of the bonding. To what extent do the atoms of the solid resemble neutral atoms, held together by covalent bonds, and to what extent are they like ions held together by electrostatic forces This is a difficult question. Even if we have an accurate X-ray picture of the electron density of a compound, it is very hard to say whether atoms or ions are being shown. The same is true for an electron density calculated by accurate quantum mechanical methods. 

Covalent solids. The units that comprise a covalent solid are atoms held together by covalent bonds. They have very high melting points (1200°C to 2000°C or more is not unusual) and are extremely hard. They are insoluble in most solvents. Diamond is a covalent solid composed of covalently bonded carbon atoms. Diamonds are used for industrial cutting because they are so hard and as gemstones because of their crystalline beauty. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

First of all, comparatively few solids are held together exclusively by covalent bonds. The majority of solids incorporating covalent bonds are bound also by either ionic or van der Waals bonds. The common occurrence is to find distinct molecules held together by covalent bonds and the molecules bound in the crystal by van der Waals bonds. The covalent bonds may hold a complex anion or cation together the cations and anions are bound in the crystal by ionic bonds. 

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