What arrangement results in hexagonally close packed structures?

Hexagonally close packed (hcp) structures are characterized by a specific arrangement of spheres that maximizes packing efficiency. The ABAB pattern signifies that layers of spheres alternate between two types: one layer has spheres positioned in a manner that fits snugly into the gaps of the layer below it. This arrangement allows for the densest possible packing of spheres, achieving a coordination number of 12, where each sphere is in contact with 12 others – six in the same layer and three in each adjacent layer above and below.

This method of stacking results in a hexagonal symmetry when viewed from above, giving rise to the term "hexagonally close packed." The alternating layers (A and B) ensure optimal use of space and minimize voids, resulting in a highly efficient packing configuration.

Other potential arrangements, such as random packing, do not achieve the same efficiency and regularity. In random packing, spheres are not systematically arranged; hence, this type of packing typically leads to a lower packing efficiency due to increased voids. The ABCABC layering refers to a face-centered cubic (FCC) arrangement, which though also highly efficient, does not describe a hexagonally close packed structure specifically. Thus, the ABAB pattern is uniquely identifiable as the arrangement