NaCI, ionic crystal

Anions and cations pack together into a crystal lattice as shown to the right for NaCI. Ionic compounds are also known as salts. 

Ionic crystal of NaCI Fig. 2.3. Dissolution of an ionic crystal by the action of a solvent. 

Because the electric force weakens with increasing distance between the oppo.sitc charges, the KCl ionic bond is weaker than the NaCI ionic bond. Weaker ionic bonds mean that KCl crystals are less resilient to stress and impact than are NaCl crystals, accounting for the rounder edges you observed in the KCl crystals and for the fact that it was easier to grind the KCl to a powder. 

Sodium chloride, NaCI, is an ionic crystal. The purple spheres represent sodium ions, Na, and the green spheres are chloride ions, Cl . Each ion is surrounded by six oppositely charged ions, except for those ions that are at the surface of the crystal. 

Figure 9.19 Three types of lattices in ionic crystals. In LiCI, the Ch ions are in contact with each other, forming a face-centered cubic lattice, In NaCI, the Cl ions are forced slightly apart by the larger Na+ ions. In CsCI, the large Cs+ ion at the center touches the Cl ions at each corner of the cube. 

Portion of an ionic crystal and a formula unit of NaCI... 

FIGURE 1 4 An ionic bond IS the force of attrac tion between oppositely charged ions Each Na ion (yellow) in the crystal lattice of solid NaCI IS involved in ionic bonding to each of six surrounding Cl ions (green) and vice versa... 

Thus, there is no discrete "molecule" of NaCI. Instead, the entire crystal is an ionic solid. 

Ionic compounds are high-melting solids because of their ionic bonds. As discussed previously in Section 6.6, a visible sample of sodium chloride consists, not of NaCI molecules, but of a vast three-dimensional network of ions in which each Na+ cation is attracted to many surrounding Cl- anions and each Cl- ion is attracted to many surrounding Na+ ions. For sodium chloride to melt or boil so that the ions break free of one another, every ionic attraction in the entire crystal must be overcome, a process that requires a large amount of energy. 

The above considerations do not provide hard-and-fast criteria for distinguishing between ionic and polymeric solids. There is little doubt that covalency makes some contribution to the bonding even in crystals such as NaCI, while there is likely to be some ionic component in solids... 

The ammonium ion is about the same size ( ,. = 151 pm) as the potassium ion (r = 152 pm) and this is a useful fact to remember when explaining the resemblance in properties between these two ions. For example, the solubilities of ammonium salts are similar to those of potassium salts. Explain the relation between ionic radius and solubility. On the other hand, all of the potassium halides crystallize in the NaCI structure with C.N. = 6 (see Chapter 4). but none of the ammonium halides does so. The coordination numbers of the ammonium halides are either four or eight. Suggest an explanation. 

Lithium iodide crystallizes in the NaCI lattice despite the fact that r+/r is less than 0.414. Its density is 3.49g/cm Calculate from these data the ionic radius of the iodide ion. 

FIGURE 2.21 Formation of an ionic compound, (a) The transfer of an electron from a Na atom to a Cl atom leads to the formatbn of a Na ion and a Cl ion. (b) Arrangement of these ions in solid sodium chloride, NaCI. (c) A sample of sodium chloride crystals. 

Several different types of crystal defects can be present often, more than one type is present at the same time. A Schottky defect occurs when one or more atoms in the crystalline lattice are missing. For example, NaCI contains approximately I missing ion per 430,000 ions at 1000 K and the crystalline lattice of CrO consists of 8% vacancies. For an ionic solid, the charges need to be balanced. Thus, a number... 

Figure 13-29 Crystal structures of some ionic compounds of the MXtype.The gray circles represent cations. One unit cell of each structure is shown, (a) The structure of cesium chloride, CsCI, is simple cubic. It is not body-centered, because the point at the center of the cell (Cs+, gray) is not the same as the point at a corner of the cell (cr, green), (b) Sodium chloride, NaCI, is face-centered cubic.

FIGURE 6.1 The elements sodium and chlorine react to form the ionic compound sodium chloride, the compound that makes up table salt. The magnification of NaCI crystals shows the arrangement of Na+ ions and Cl ions in a crystal of NaCI. 

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