Question

Na and Mg crystallise in Bee and Fcc type crystals respectively. "The no. of atoms in the unit cells of their respective crystals is:

Options

• 4 and 2

• 9 and 14

• 14 and 9

• 2 and 4

Answer 1

2 and 4

Explanation:

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

∴ Number of atoms (n) = `(8 xx 1/8) + 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.

∴ Number of atoms (n) = `8 xx 1/8 + (6 xx 1/2)` = 4.