The effective number of atoms in a face-centered cubic (FCC) unit cell is equal to 4.

Detailed Explanation

In a Face-centred cubic lattice (FCC) this structure considers one corner atom, there are four face-centered atoms at equal distances in the same plane. Similarly, the four face-centered atoms below this plane and the four face-centered atoms above this plane are also at equal distances.

a) 8 corners × 1/8 per corner atom = 8 × 1/8 = 1 atom
b) 6 face-centered atoms × 1/2 atom per unit cell = 3 atoms

Hence, the total number of atoms in a unit cell = 4 atoms

Thus, in a face-centered cubic unit cell, we have:

• 8 corners × 1/8 per corner atom = 8 × 1/8 = 1 atom
• 6 face-centered atoms × 1/2 atom per unit cell = 3 atoms

Therefore, the total number of atoms in a unit cell = 4 atoms.