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

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

Options

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

Explanation

In fcc structure , corner atoms do not 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 oppostie face center atom in an fcc unit cell. i. Atoms 3 and 4 are touching each other where center-to-center distance `=(a)/sqrt(2)`
ii. Atom 1 and 2 are not touching each other.
iii. Atoms 2 and 4 are touching each otehr where center-to-center distance `=(a)/sqrt(2)`.