Question

(i) Why does the presence of excess lithium make $LiCl$ crystals pink$?$
(ii) A solid with cubic crystal is made up of two elements $P$ and $Q$. Atoms of $Q$ are at the corners of the cube and $P$ at the body centre. What is the formula of the compound$?$

Answer 1

Solid crystals show various kinds of lattice defects. Some units of the crystals may have one or more atoms less in their crystal, this gives rise to imperfections in the crystal also known as defects of crystals. When a colour is exhibited by a crystal it thus means that the crystal has some kind of defect in it.

Complete step-by-step answer:
(i) The $LiCl$ crystal develops a pink colour when lithium is present in excess due to metal excess defect, where some of the chloride ions are missing from the lattice site which thus form holes. And to maintain the electrical neutrality of the compound, the vacancies are occupied by the electrons. If the site of an anion is replaced by electrons then it is known as F- Centre. So, when light falls on the crystal, due to the absorption of energy these electrons get excited and we see the complementary pink colour.
(ii) We know that Atoms of $Q$ are at the corners of the cube and $P$ at the body Centre so,
The numbers of atoms of $Q$ in a one-unit cell $= \,\dfrac{1}{8} \times 8\, = \,1$.
The number of atoms of $P$ in a one-unit cell $= \,1$.
Which means that the ratio of the number of $P$ atoms to the number of $Q$ atoms $= \,\,1:1$
Therefore, the formula of the compound will be $PQ$.

Note: For doing this kind of questions you must have an idea about different types of crystals and the different lattice points present in a crystal and also their contribution in one unit cell. You must also know about various lattice defects that can be observed in a crystal in order to understand the colour of the crystal.