Explain the following terms with suitable examples :
F-centres (P.B.E.B. 2003)

Explanation

F-centres. The anionic occupied by unpaired electrons and responsible for the colour of the crystal ae called F-centres.
For example, When crsytals of a NaCl are heated in an atmosphere of sodium vap[our, the Na atom are deposited on the surface of the crystal. The `Cl^(-)`ions diffuse to the surface of the crystal and combine with Na atoms to form NaCl. This happens with the loss of electron by Na atoms to form `Na^(+)` ions. The released electron
`NararrNa^(+)+e^(-)`
diffuse into the crystal and occupy the anionic sites. As a result the crystal now has excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres. They impact yellow colour to the crystals of NaCl. The colour is due to the excitation of these electrons when they absorb energy from the visible light falling on the crystal. Similarly, excess of Li and LiCl crystals makes it pink and excess of K in KCl crystals makes it violet.