Ionic solid which have an ionic vacancy due to metal excess defect develop colour explain with the help of suitable example
Answers
The colouration in ionic compounds with metal excess defects can be explained by the following -
-In metal excess defects, the anions leave the crystal lattice which leads to a deficiency in the negative charge .
-The free electrons rush to take their place.
-The sites occupied by these electrons impart colour and are called F centres.
For example in NaCl crystal, Cl ions leave their place this leads to a difference in positive and negative charges in the atoms which leads to instability thus the free electrons occupy these vacancies.
Answer:
The colour develops because of the presence of electrons in the anionic sites.
These electrons absorb energy from the visible part of radiation and get excited.
For example, when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the chloride ions from the crystal diffuse to the surface to form NaCl with the deposited Na atoms.
During this process, the Na atoms on the surface lose electrons to form Na+ ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites.
These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.