Question

On heating crystals of KCl in potassium vapours, the crystals start exhibiting violet colour. Why?

Answer 1

Heating of the crystals of the compound in the vapours of the corresponding element leads to the formation of anionic gap in the crystal lattice. Thus, those gaps show the different colour of the resulting component.

Complete step by step solution:
Let us see the phenomenon of the crystal lattice structures and its properties. Space lattice is a regular pattern of atoms forming a crystal. It is a definite array of points in three-dimensional structure where the points are at the intersection of the three sets of parallel planes. On heating the crystals of KCl in potassium vapours, there results in the vacancy of chloride anion. The hole or a gap created by the anion is filled with the electron of K. The anionic sites occupied by the unpaired electrons (one or more) are called F-centres. It is a type of crystallographic defect. Therefore, F-centres are the lattice sites containing electrons. Such defects impart colour to the lattice as electrons (when in excited state) absorb the energy coming from the visible light falling on the crystal. This strategy is also used to identify the compound as the compounds will absorb different colours of light when visible light is absorbed by the anionic sites now containing unpaired electrons. Particularly metal oxides show this phenomenon naturally as heating them to high temperature, ions become excited and hence, are displaced from their crystallographic positions. In our case, the hole will appear violet in colour.

Note: Every other compound shows different colour when F-centres are created in their crystal lattice. This is the type of defect in the crystallographic studies along with other defects as line defects, point defects, etc.