Question

The number of atoms in a hcp unit cells is:

• A. $4$
• B. $8$
• C. $12$
• D. $6$

Answer 1

Answer: (D)

$6$

The number of atoms in a hep unit cells is 6.
3 atoms are at the middle of the unit cell and each atom contributes 1 atom to the unit cell.
2 atoms are at the two face centres of the unit cell and each atom contributes 1/2 atom to the unit cell.
12 atoms are at the 12 corners of the unit cell and each atom contributes 1/6 atom to the unit cell.
The number of atoms in one unit cell
$(3\times 1)+(2\times \frac{\text{1}}{\text{2}})+(12\times \frac{\text{1}}{\text{6}})=3+1+2=6$