Final answer:
The simple cubic structure (SC) is not close-packed, with only polonium (Po) crystallizing in this inefficient arrangement, unlike the CCP and HCP structures, which have higher coordination numbers and more efficient packing.
Step-by-step explanation:
Of the three common crystal structures encountered in structural metals, the one that is not close-packed is the simple cubic structure (SC). In a simple cubic structure, the spheres are not packed as closely as they could be, and the atoms fill approximately 52% of the volume of the container, making it a relatively inefficient arrangement. This structure consists of planes in which each atom contacts only the four nearest neighbors in its layer; one atom directly above it in the layer above; and one atom directly below it in the layer below, with a coordination number of six. In contrast, more efficiently packed structures like face-centered cubic (FCC or CCP) and hexagonal close-packed (HCP) consist of layers of hexagonally arranged atoms, where each atom has a coordination number of 12.
Only one metal, polonium (Po), is known to crystallize in a simple cubic structure due to this inefficiency. The CCP and HCP structures are close-packed, where each atom in the second layer rests above the spaces of the first layer, and atoms are closely packed together to minimize empty space and maximize atomic interactions.