Final answer:
The atomic packing factor for a FCC crystal structure is 0.74, and the density would generally decrease if the crystal structure is changed from FCC to BCC.
Step-by-step explanation:
The atomic packing factor (APF) for a face-centered cubic (FCC) crystal structure is 0.74. In an FCC structure, atoms are arranged in a cubic closest packing (CCP) arrangement, which consists of three repeating layers of hexagonally arranged atoms. Each atom in an FCC structure contacts 12 near neighbors, resulting in a coordination number of 12.
The difference in density (%) if the crystal structure is changed from FCC to body-centered cubic (BCC) depends on the specific elements or compounds involved. However, in general, atoms in a BCC arrangement are more efficiently packed than in a simple cubic structure, occupying about 68% of the total volume. Therefore, there would typically be a decrease in density when changing from FCC to BCC, although the exact percentage decrease would vary depending on the specific elements or compounds involved.