The Close Packing Concept

When we add a third layer C of atoms on top of the two layers, we find there are two close packed possibilities this can be tested with small disks on Fig. 4.6. Each atom of layer C must rest on three of layer B. One possibility is to place atoms of layer C directly above atoms of layer A. Thus, we create a new layer just like A, and further layers are added to 

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The concept of close packing is particularly useful in describing the crystal structures of metals, most of which fall into one of three classes hexagonal close packed, cubic close packed (i.e., fee), and body-centered cubic (bcc). The bcc unit cell is shown in Fig. 4.8 its structure is not close packed. The stablest structures of metals under ambient conditions are summarized in Table 4.1. Notable omissions from Table 4.1, such as aluminum, tin, and manganese, reflect structures that are not so conveniently classified. The artificially produced radioactive element americium is interesting in that the close-packed sequence is ABAC..., while one form of polonium has... 

The concept of the close-packing of ions in ionic solids had already been widely accepted, so it was not too much of a stretch to apply the same basic idea to the ligands in covalendy bonded species. Bartell assigned a nonbonded radius to each of... 

Let us now look at the c.p.h. unit cell as shown in Fig. 5.4. A view looking down the vertical axis reveals the ABA stacking of close-packed planes. We build up our c.p.h. crystal by adding hexagonal building blocks to one another hexagonal blocks also stack so that they fill space. Here, again, we can use the unit cell concept to open up views of the various types of planes. 

In summary, for Leukipp and Demokrit, the empty space between the atoms was a key assumption in their model, because, if particles were closely packed, they could not move and substances could not be mixed. When asking students to philosophise about the nature of matter, we indeed find parallels to the ancient Greek thinking, both to the so-called atomists and to the continuous ideas of Aristotle and others. For example, Leukipp s and Demokrit s explanation for the specific weight of substances corresponds to one student conception younger students especially tend to explain differences in the specific weight (but also hardness of substances) with differences in the closeness of particles (Fig. 10.6). They seldom take into account that the particles could have a different weight themselves. 

Accordingly, Neurock and co-workers have developed models for the electrochemical interface that retain this concept of hexagonal stmcture over close-packed metal surfaces [FiUiol and Neurock, 2006 Taylor et al., 2006c]. With the use of a screening charge as described in Section 4.3, the sensitivity of the stmctural parameters of water with respect to the electrochemical environment were explored [Taylor et al., 2006a]. The predominant effect stems from the polar nature of the water molecule, in which the water molecules are observed to rotate as a function of the applied potential. 

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