Which crystalline structure has more tightly packed atoms compared to BCC?

The face-centered cubic (FCC) structure is known for its efficient packing of atoms, leading to a higher atomic packing factor compared to body-centered cubic (BCC) structures. In the FCC arrangement, atoms are located at each corner of the cube and at the centers of each face, resulting in a total of four atoms per unit cell. The close proximity of atoms in this configuration allows for more efficient packing and maximizes the number of atoms in a given volume.

In contrast, the BCC structure, while also efficient, only has two atoms per unit cell, with one atom located at each corner and one in the center. This results in less dense packing of atoms compared to FCC. As such, FCC structures display a packing efficiency of 74%, while BCC has a packing efficiency of about 68%. This distinction is crucial in material science and engineering, as it affects the mechanical properties of metals and alloys, influencing strength and ductility.

Thus, the face-centered cubic structure stands out for its more tightly packed configuration, making it the correct answer.