The ABABAB. . . and ABCABC. . . packing arrangements
are called hexagonal close-packing (hcp) and cubic
close-packing (ccp), respectively. In each structure, any
given sphere is surrounded by (and touches) 12 other spheres
and is said to have 12 nearest neighbours, to have a coordination
number of 12, or to be 12-coordinate. Figure 5.3 shows
representations of the ABABAB. . . and ABCABC. . .
arrangements which illustrate how this coordination
number arises; in these diagrams, ‘ball-and-stick’ representations
of the lattice are used to allow the connectivities to be
seen. This type of representation is commonly used but does