Crystalline solid closest-packed

The right hand side of Fig. A.4.6 is contained in Fig. 3.3. Capacity measurements can readily be made at solid electrodes to study adsorption behavior. For a review see Parsons (1987). As Fig. A.4.7 illustrates, capacity potential curves of three low-index phases of silver, in contact with a dilute aqueous solution of NaF, show different minimum capacities (corresponding to the condition o = 0) and therefore remarkably different potentials of pzc. The closest packed surface (111) has the highest pzc and the least close-packed (110) has the lowest pcz these values differ by 300 mV. Such complications observed with single crystal electrodes, seem likely to have their parallel at other solid surfaces. For example, it is to be expected that a crystalline oxide will have different pzc values at its various types of exposed faces. 

In Activity 4.1, students described and modeled crystalline solids according to the way their atoms packed into a regular structure. We found that their atoms could pack into face-centered, body-centered, or hexagonal-closest arrangements. Another way to understand the structure of a crystalline solid is to consider the bonding forces between its structural units. 

Define the following terms crystalline solid, lattice point, unit cell, coordination number, closest packing. 

Unlike metallic solids, however, ionic solids are unable to closest pack in the crystalline state for two different reasons. First, the sizes of the anions and the cations in an ionic solid are rarely the same. Second, and more importantly, because the ions have opposite charges, the lowest energy configuration is one that minimizes the repulsion of like charges and maximizes the attraction between ions of opposite charge. 

A similar trend exists for the halogens. The structure of crystalline iodine is shown in Figure 13.9. The ratios r2/r for crystalline CI2 (s), Br2 (s), and I2 (s) are 1.68, 1.46, and 1.29, respectively. Although each of the halogens is classified as a nonmetal, the percent metallic character increases down the series. Iodine is the most metallic of the group, a fact that is supported by its grayish color as a solid and its metallic reflectance. In fact, iodine can even adopt a distorted cubic closest-packed structure similar to many metallic solids when it is crystallized at high pressure. 

By now, you should have some appreciation for the different types of crystalline lattices and have some idea of the difficulty in predicting the exact structure that a compound will form. There are a variety of factors involved in the process of crystallization, including (but not limited to) closest packing of spheres, closest packing of polyhedra, electrostatic interactions, hydrogen bonding, hybridization, crystal field effects, and the degree of polarization. When it comes to the solid state, simple stoichiometries do not necessarily imply simple structures. 

The structures adopted by crystalline solids are those that bring particles in closest contact to maximize the attractive forces between them. In many cases the particles that make up the solids are spherical or approximately so. Such is the case for atoms in metallic solids. It is therefore instructive to consider how equal-sized spheres can pack most efficiently (that is, with the minimum amount of empty space). 

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