Which structure is more densely packed: FCC or BCC?

The face-centered cubic (FCC) structure is indeed more densely packed than the body-centered cubic (BCC) structure. In FCC, the atoms are positioned at each of the corners and the centers of all the faces of the cube. This arrangement allows for a higher coordination number of 12, meaning each atom is in contact with 12 neighboring atoms.

The packing efficiency of FCC is about 74%, which is a measure of how closely packed the atoms are within the structure. This high percentage indicates that a significant amount of the volume of the structure is occupied by atoms, contributing to its density.

In contrast, the BCC structure has a coordination number of 8, with atoms located at the corners of the cube and a single atom in the center. The packing efficiency of BCC is lower, at around 68%, which indicates that it has more empty space compared to FCC.

These structural characteristics highlight why FCC is more densely packed. The arrangement of atoms in FCC maximizes the space utilization within the crystalline lattice, resulting in a higher density compared to BCC.