Final answer:
The atomic packing factor for hexagonal closest packing (HCP) is indeed 0.74, which means 74 percent of space is filled in such structures, making the statement true.
Step-by-step explanation:
The question concerns the atomic packing factor (APF) for hexagonal closest packing (HCP) structures in metallic crystals. The atomic packing factor is the fraction of volume in a crystal structure that is occupied by atoms. In the case of HCP, as well as cubic closest packing (CCP), the APF is 0.74. This means that 74 percent of the available space is filled when the atoms are the same size.
The HCP structure has layers of atoms, where each atom in one layer sits in the indentations of the adjacent layer, creating a very dense structure. The arrangement of the atoms in HCP consists of alternating type A and type B close-packed layers (ABABAB...). In contrast, CCP has a stacking sequence of type A, type B, and type C close-packed layers (ABCABCABC...). In both HCP and CCP structures, each atom is surrounded by 12 equidistant nearest neighbors, leading to a high coordination number and maximizing the packing efficiency.
Since both HCP and CCP structures fill 74 percent of the space, the statement that the atomic packing factor for HCP is 0.74 is A) True.