Question

In an fcc unit cell, atoms are numbered as shown below.

The atoms not touching each other are (Atom numbered 3 is face center of front face)

• A. 3 and 4
• B. 1 and 3
• C. 1 and 2
• D. 2 and 4

Answer 1

Answer: (C)

In fcc structure corner atoms do no touch each other (atoms 1 and 2), but every face center atom touches corners. Moreover, every face center atom touches every other face center atom provided it is not the opposite face center atom in an fcc unit cell.
(i) Atoms 3 and 4 are touching each other where centerto-center distance $=\frac{a}{\sqrt{2}}$.
(ii) Atoms 1 and 2 are not touching each other.
(iii) Atoms 2 and 4 are touching each other where centerto-center distance $=\frac{a}{\sqrt{2}}$