(a) Why does the presence of an excess of lithium make LiCl crystals pink?
(b) A solid with cubic crystal is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body-center. What is the formula of the compound?
(a) When crystals of LiCl is heated in presence of excess of lithium, Cl- ions from crystal diffuse on the surface and combine with ionized Li to form LiCl. The released unpaired electrons occupy the anionic sites known as F-centers. The pink colour results by excitation of these electrons when they absorb energy from visible light falling on them.
(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 =
Also, it is also given that the atoms of P are present at the body-center.
Therefore, number of atoms of P in one unit cell 
= 1
This means that the ratio of the number of P atoms to the number of Q atoms, P: Q = 1:1
Hence, the formula of the compound is PQ.
