(a) Why does presence of excess of lithium makes Li Cl crystals pink?
(b) A solid with cubic crystal is made of two elements P and Q. Atoms of Q are at the comers of the cube and Pat the body-centre. What is the formula of the compound?

Explanation

(a) When a crystal of LiCl is heated in an atmosphere of Li vapour, the Li atoms lose electron to for `Li^(+)` ions. The released electrons diffuse into the crystal and occupy anionic sites. These electrons impact pink colour to the LiCl crystal.
(b) It is given that the atoms of Q are present at the corners of the cube.
Therefore, number of atoms of Q in one unit cell `=8xx(1)/(8)=1`.
It is also given that the atoms of P are present at the body-centre. Therefore, numbers of atoms of P in one unit cell = 1. This means that the ratio of the number of Q atoms P: Q = 1 : 1.
Hence, the formula of the compound is PQ.