How many layers are in the hexagonal close-packed unit cell?

How many layers are in the hexagonal close-packed unit cell?

• A. Two layers
• B. Three layers
• C. Four layers
• D. Five layers

Answer: (B)

In hexagonal close-packed materials, layers stack in an ABAB sequence along the vertical direction. If you look at one hexagonal unit cell and follow that stacking from bottom to top, you encounter a bottom A layer, a middle B layer, and then another A layer at the top. So there are three close-packed planes within the unit cell, even though the two A layers are arranged the same way. The unit cell spans across these three planes to capture the full stacking pattern. This is why three layers is the correct count.

In hexagonal close-packed materials, layers stack in an ABAB sequence along the vertical direction. If you look at one hexagonal unit cell and follow that stacking from bottom to top, you encounter a bottom A layer, a middle B layer, and then another A layer at the top. So there are three close-packed planes within the unit cell, even though the two A layers are arranged the same way. The unit cell spans across these three planes to capture the full stacking pattern. This is why three layers is the correct count.