Which unit cell type is associated with the stacking sequence ABCABCABCABC in its close-packed layers?

• A. Body Centered Cubic
• B. Face Centered Cubic
• C. Hexagonal Close-Packed
• D. Simple Cubic

Answer: (B)

Stacking of close-packed layers determines the type of close-packed crystal you have. When the second layer sits in the holes of the first, the third layer can occupy a different hole, producing a three-layer repeating sequence: ABCABCABC. This three-step repeating pattern is the hallmark of cubic close packing, where the close-packed planes are the {111} planes in a face-centered cubic lattice. The unit cell that hosts this arrangement is the face-centered cubic structure, with atoms at the corners and at the centers of the faces, giving the characteristic dense packing along the [111] direction.

In contrast, hexagonal close packing shows an ABAB stacking, not ABC, and the simple cubic and body-centered cubic structures do not exhibit this close-packed layering pattern.