Close-packing of atoms

Figure 1.4 (a) Close packing of atoms in a cubic structure, showing six... 

Figure 1.4 (a) Close packing of atoms in a cubic structure, showing six in-plane neighbours for each atom (b) An expanded diagram of the packing of atoms above and below the plane. A above and A below represents the location of atoms in the hexagonal structure, and A above with B below, the face-centred cubic structure... 

Many systems of notation and classification have been proposed. The well-known books by R. W. G. Wyckoff, A. F. Wells, F. C. Phillips, L. Bragg, M. J. Buerger, L. V. Azakoff, D. M Adams, and W. B. Pearson (Appendix A, Further Reading) have discussed these proposals. These proposals include close packing of atoms, nets, or prism connections, stacking of coordination polyhedra and even a crystal-algebra method. Application of most of these proposals requires familiarity with the features of many structures. Only specialists can be expected to have... 

There are, of course, more complex forms of closest packing than those considered so far. Besides, the species to be packed need not be identical. Thus, close packing of atoms of two kinds could be considered. Close-packed structures with atoms in the interstices are also important. The interstice arrays may have very different... 

Figure 2.2. Close Packing of Atoms Crystal Lattice Types...

The elements have not the close-packing of atoms typical of true metals. Zinc and cadmium have distorted h.c.p. structure with axial ratios 1.87 instead of the ideal 1.63 (Fig. 275). These are contributory factors to their low tensile strengths the ultimate tensile stress of zinc is 7.5 tons per sq. in., of copper 15 tons. Mercury gives rhombohedral crystals with 6 6 co-ordination... 

The structures obtained by closed packing of atoms in metals given in the literature are equihbrium structures. The arrangement of layers is a long-range order and is, especially... 

The first question to be answered is that as to the nature of the core of the three-chain -hehcal molecule—the part of the molecule closest to the axis. It is important for stability of the molecule that atoms be well packed together, and the problem of packing atoms together is a more difiicult one to solve in the neighborhood of the axis than at a distance away from the axis, where there is a larger distance between an atom and the equivalent atom in the next unit. (An example of a helical structure which seems to satisfy all of the structural requirements except that of close packing of atoms in the region near the helical axis is the 5.2-residue helix of polypeptide chains. This structure seems not to be represented in proteins, whereas... 

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