TYPES OF BONDING IN SOLIDS

Types of Bonding in Solids Table 2.1. Glass Transition Temperatures 

Other amorphous solids such as polymers, being rigid and brittle below. Tg, and elastic above it, also exhibit this behavior. Table 2.1 lists the glass transition temperatures of common solid materials. Although most solid-state textbooks deal almost exclusively with crystalline materials, this text will attempt to address both the crystalline and amorphous states, describing the structure/property relationships of major amorphous classes such as polymers and glasses. 

---

Type of bonding in solid Structure of solid Expected Conductivity of solid Expected Conductivity if solid is fused... 

In oixr discussion on the formation of solids from atoms we encountered hve general types of bonding in solids ... 

The electrodynamic forces proposed for stabilizing jellium provide the principal type of bonding in molecular crystals such as solid methane, rare gas crystals, solid anthracene, and the like. These forces also form the inter-chain bonding of long-chain molecules in polymeric materials (the intra-molecular bonding within the chains is usually covalent). 

Another technique that has been employed for studying certain types of changes in solids is infrared spectroscopy, in which the sample is contained in a cell that can be heated. By monitoring the infrared spectrum at several temperatures, it is possible to follow changes in bonding modes as the sample is heated. This technique is useful for observing phase transitions and isomerizations. When used in combination, techniques such as TGA, DSC, and variable-temperature spectroscopy make it possible to learn a great deal about dynamic processes in solids. 

There are several methods you can use to predict the type of bond in an unknown substance. For example, you can consider the substance s physical properties. In contrast to ionic solids, covalent (molecular) compounds typically have the following properties ... 

The type of bonding in a substance depends upon the kinds of atoms it contains and the forces of attraction between those atoms. If you know the physical properties of a substance, you can often predict the type of bonding in the substance. Table 4.5 summarizes the types of forces of attraction and the physical properties of solids with different types of bonding discussed in this section. In Investigation 4-B, you will design an experiment in which you predict the type of bonding in various substances, then test your prediction. 

Based on your observations, classify the solids. Identify the type of bonding in each substance. Explain your decisions about classification in detail. 

The qualitative properties of a defect such as a sulfur vacancy in ZnS are fortunately independent of the type of bonding in the compound. If we consider first that ZnS is an ionic compound composed of Zn+2 and S-2 ions, the removal of a neutral S atom to the gas phase to form S2 molecules leaves behind a neutral sulfur vacancy, Vs°, since charge neutrality must be preserved in the crystal. The two electrons left behind can be considered as being trapped in the vicinity of the vacancy and can be removed one at a time into the conduction band of the solid by thermal ionization. These processes can be written as ordinary chemical equations ... 

We have developed a model to study the basic structural properties of solid C<,q. The model consists of two distinct types of intermolecular interactions. The dominant one is the van der Waals-type interactions between carbon atoms on different Cm molecules. A secondary short-range Coulomb interaction is modeled by a small charge transfer between the two types of bonds in the C60 molecule. In contrast to early calculations [6] which include the van der Waals interactions only, our model predicts correctly the observed cubic ground-state structure Pa3. Many structural properties calculated, such as the compressibility, cohesive energy, and specific heat, are in good agreement with experiments l7l. 

These misconceptions regarding two different types of bonding in an ionic lattice are not only to be found by Hilbing [13] and Taber [15] the Australian scientists Butts and Smith [20] also identified them in their studies. In addition, they found misconceptions, which, in analogy to scientific historical development, assume that molecules first exist in solid salt crystals, and then the ions develop because of the solution of salts in water. 

Table salt, an ionic solid, and table sugar, a covalent solid, are similar in appearance. However, these compounds behave differently when heated. Salt does not melt, but sugar melts at a relatively low temperature. Does the type of bonding in a compound affect its properties ... 

Year 12 Australian students asked about sodium chloride were found to often volunteer a description of how ions might be formed through an electron transfer event (i.e. from sodium atom to chlorine atom). It was also common for these students to refer to molecules of NaCl, and some believed that there were two types of bond in sodium chloride either that the NaCl molecules had internal covalent bonds, but were ionically bonded to other molecules, or vice versa (Butts Smith, 1987). Some of the students thought that this assumed molecular nature of sodium chloride explained why the solid did not conduct electricity, believing that ions were only formed from the molecules on dissolving. One student thought that each ion would have one ionic bond, and five physical bonds (p. 196). 

Ions are commonly found in a class of compounds called salts, or ionic solids. Salts, when melted or dissolved in water, yield solutions that conduct electricity. A substance that conducts electricity when melted or dissolved in water is called an electrolyte. Table salt — sodium chloride — is a good example. On the other hand, when table sugar (sucrose) is dissolved in water, it becomes a solution that doesn t conduct electricity. So sucrose is a nonelectrolyte. Whether a substance is an electrolyte or a nonelectrolyte gives clues to the type of bonding in the compound. If the substance is an electrolyte, the compound is probably ionically bonded (see Chapter 6). If it s a nonelectrolyte, it s probably covalently bonded (see Chapter 7). 

Conductivity measurements show triphenyltin sodium and trimethyltin sodium to be strong electrolytes in liquid ammonia232,2S3, but no solid structural information is available. It is possible that these materials have an electron deficient type of bonding in the crystals, similar to the alkali metal alkyls. 

It is useful to ask what types of bonds characterize solids in general and elemental sohds in particular. In answering this question, it is common to decompose the physical continuum of interactions that hold nature s many and varied soHds together and conclude that there are five ideahzed types of bonding interactions, namely van der Waals, ionic, covalent, metallic, and hydrogen bonding. Let us now consider each briefly in turn. 

---