SOLUTION

One of the close-packed structures is the face-centered cubic structure. The face-centered cubic structure has atoms located at the eight corners and the centers of all the cubic faces as shown in Fig.

• Each of the corner atoms is the corner of eight cubes so the corner atoms are shared among eight unit cells. Additionally, each of its six face-centered atoms is shared with an adjacent unit cell as shown in Fig.
• It has a coordination number of 12.
• The FCC unit cell consists of a net total of four atoms; one eight of eight corner atoms and six halves of the face-centered atoms.

Number of atoms per unit cell

8 corners × 18 per corner atom = 8 × 18 = 1 atom

6 face-centered atoms × 12 atoms per unit cell = 3 atoms

Hence, the total number of atoms in a unit cell =  3+ 1 = 4 atoms