Structure of Ionic Compounds

In Chapter 1 (Section 1.4.2), we made a distinction between the fundamental unit of an ionic compound and the fundamental unit of a covalent compound, saying that for an ionic compound the fundamental unit is the formula unit but for a covalent compound it is the molecule. Molecules are distinct individual units that exist independent of other molecules. Formula units, on the other hand, are collections of ions in the ratio indicated in the formula (and in their Lewis electron-dot structures). They do not exist as independent distinct units, however. The ions that make up ionic compounds, both anions and cations, exist in three-dimensional (3-D) space like a stack of marbles, with one layer placed upon another. The cations and anions interact equally with all ions in their immediate vicinity, which is more than what is shown in the formula. It 

FIGURE 6.21 The structure of NaCI. Note that no single sodium ion is associated with any single chloride ion—no molecule. (From Kenkel, J., Kelter, P., and Hage, D., Chemistry An Industry-Based Introduction with CD-ROM, CRC Press, Boca Raton, FL, 2001. With Permission.) 

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On the other hand, the crystal structures of ionic compounds with small molecular ions depend mainly on how space can be filled most efficiently by the ions, following the principle of cations around anions and anions around cations. Geometric factors such as the relative size of the ions and the shape of molecular ions are of prime importance. More details are given in Chapter 7. 

The structures of ionic compounds comprising complex ions can in many cases be derived from the structures of simple ionic compounds. A spherical ion is substituted by the complex ion and the crystal lattice is distorted in a manner adequate to account for the shape of this ion. 

In practice, this hard-core model is too simple to predict reliably the ground-state structure of ionic compounds such as the alkali halides that are located in the upper left-hand corner of the AB structure map in Fig. 1.9. Nevertheless, it provides a simple introduction to the importance of the radius ratio in determining structural stability. 

Substances may be related rather simply to the crystal structure of ionic compounds, namely,... 

This exercise will help you relate the structures of ionic compounds to some of their key properties. 

This theory is applicable to the molecules formed by covalent bonds because these are directional. The force that holds ions together in the ionic compounds is the coulombic force. This force is non-directional and depends on the distance between the ions only. The crystal structure of ionic compound is, therefore, determined by relative sizes and charge of the ions. 

What is the structure of ionic compounds in the liquid, solid, and dissolved states ... 

Several properties distinguish ionic compounds from covalent compounds. These may be related rather simply to the crystal structure of ionic compounds, namely, a lattice composed of positive and negative ions in such a way that the attractive forces between oppositely charged ions are maximized and the repulsive forces between ions of the same charge are minimized. Before discussing some of the possible geometries, a few simple properties of ionic compounds may be mentioned ... 

The structure of diamond, a covalent crystal, is shown in Fig. 7.1. How is it related to some of the structures of ionic compounds discussed in this chapter ... 

We have commented on the absence of structures of ionic compounds A X with coordination numbers of A greater than eight or nine. If we derive 2-dimensional nets in which A has some number (p) of X atoms and X has some number (q) of A atoms as nearest neighbours we find that the only possible (p, <7)-connected nets (p and <7 > 3) in which p- and <7-connected points alternate are the (3,4), (3, 5), and (3,6) nets. Since there are upper limits to the values of p and q in plane nets, it is reasonable to assume that the same is true of 3D nets. We are not aware that this problem has been studied. However, the existence or otherwise of crystal structures... 

Recall from Chapter 4 that the submicroscopic structure of ionic compounds helps explain why they share certain macroscopic properties such as high melting points, brittleness, and the ability to conduct electricity when molten or when dissolved in water. What is it about the structure of these compounds that gives them properties such as the one shown in Figure 5.1 The answer involves the ions of which they are made. 

Structures of Ionic Compounds We write the formula of an ionic compound such as lithium fluoride simply as LiF, but this is really the empirical, or simplest, formula. The actual solid contains huge and equal numbers of... 

What if ions stayed the same sizes as their parent atoms How would that change the structures of ionic compounds ... 

I Describe the formation of ionic bonds and the structure of ionic compounds. 

The chemical bonds in a compound determine many of its properties. For ionic compounds, the ionic bonds produce unique physical structures, unlike those of other compounds. The physical structure of ionic compounds also contribute to their physical properties. These properties have been used in many applications, discussed in Figure 7.6. 

In Chapter 7, you learned about the structure of ionic compounds— substances formed from ionic bonds. The covalent molecules you have read about in this chapter have structures that are different from those of ionic compounds. In studying the molecular structures of covalent compounds, models are used as representations of the molecule. 

Chapter 2 The Structure of Ionic Compounds Closest Packing... 

In order to analyze the structure of ionic compounds more carefully we need to know the number of tetrahedral and octahedral voids in the closest-packed structures. Figure 18 shows all of the tetrahedral voids surrounding one sphere between two layers of closest-packed spheres. Of course, each of these voids is shared by four spheres. 

SECTION 12.5 Ionic solids consist of cations and anions held together by electrostatic attractions. Because these interactions are quite strong, ionic compounds tend to have high melting points. The attractions become stronger as the charges of the ions increase and/or the sizes of the ions decrease. The presence of both attractive (cation nion) and repulsive (cation-cation and anion anion) interactions helps to explain why ionic compounds are brittle. Like metals the structures of ionic compounds tend to be symmetric, but to avoid direct contact between ions of like charge the coordination numbers (typically 4 to 8) are necessarily smaller than those seen in close-packed metals. The exact structure depends on the relative sizes of the ions and the cation-to-anion ratio in the empirical formula. 

Table 2.1. The Observed Structures of Ionic Compounds, the Ratio of the Ion Radii, and the Prediction According to the Pauling Rules ...

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