Question

Na and Mg crystallize in bcc and fcc type crystals respectively, then the number of atoms of Na and Mg present in the unit cell of their respective crystal is ............

(A) 4 and 2

(B) 9 and 14

(C) 14 and 9

(D) 2 and 4

Answer 1

Correct answer is: (D) 2 and 4

The bcc cell consists of 8 atoms at the corners and one atom at centre.

\(\therefore\) \(n=\left(8\times\cfrac{1}{8}\right)+1=2\)

The fcc cell consists of 8 atoms at the eight corners and one atom at each of the six faces. This atom at the face is shared by two unit cells.

\(\therefore\) \(n=8\times\cfrac{1}{8}+\left(6\times\cfrac{1}{2}\right)=4\)