Giant ionic crystal structures

Figure 3.l5d shows only a tiny part of a small crystal of sodium chloride. Many millions of sodium ions and chloride ions would be arranged in this way in a crystal of sodium chloride to make up the giant ionic structure. Each sodium ion in the lattice is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. 

The diagram shows a model of the structure of sodium chloride and similar ionic crystals. The ions are arranged in a regular lattice structure - a giant ionic lattice. 

Perhaps the most common example of an ionically bonded substance is NaCl, or table salt. In this, the sodium (Na) atom gives up an electron to the much more electronegative chlorine (Cl) atom, and the two atoms become ions, Na and Cl. The electrostatic bonding force between the two oppositely charged ions extends outside the local area attracting other ions to form giant crystal structures. For this reason most ionically bonded materials are solid at room temperature. 

In ionic crystals, ions of oppositely charged ions are held together by strong electrostatic forces of attraction. These crystals are giant ionic structures and no single molecule exist. Some of the common properties are -... 

Unlike the truly ionic CdFg (fluorite structure), the iodide forms electrically neutral layers of large extent (Fig. 95). The force between the layers is small (Van dcr Waals) and the crystal easily cleaves into parallel sheets. Layer lattices are commonly formed by the iodides and bromides of bipositive metals and even by the chlorides of certain metals with very small cations. This results from polarisation of the anions. The. same effect is exhibited by most hydroxides of the type M(OH)2, but in these the two-dimensional, giant molecules are held together by the rather stronger forces between OH ions of adjacent layers—the so-called liydrox d bonds. 

Ionic impurities can be effectively and cheaply removed from water by an interesting process that involves the use of giant molecules — n o QC-ular structures that are so big as to constitute visible particles. A crystal of diamond is an example of such a giant molecule (Chapter 7). Some complex inorganic crystals, such as the minerals called zeolites, are of this nature. These minerals are used to soften hard water, to remove the heavy metal ions from the water. Such a process is termed ion exchange. 

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