Question

What does F- centre mean in Chemistry?

Answer 1

Hint : F- centre also known as Farbe centre is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or unpaired electrons.

Complete Step By Step Answer:
When non – stoichiometric compounds have excess metal ions, the crystal lattice has vacant anion site. The anion sites that are occupied by electrons are called F – centres. Such defects also impart colour to the crystal. This is a type of defect and is called a metal excess defect.
The electrons which are known to be present in such vacancy tend to absorb light in the visible spectrum such that a transparent material becomes coloured.
A metal excess defect is a defect seen in the crystal structures. Metal Excess defects are of two types:
-Metal excess defect due to anionic vacancies
-Metal excess defect due to the presence of extra cations at interstitial sites
Here is an example, when crystals of $NaCl$ are heated in an atmosphere of sodium vapour , the sodium atoms are deposited on the surface of the crystal. The $Cl$ ions diffuse from the crystal to its surface and combine with $Na$ atoms, forming $NaCl$ . During this process the $Na$ atoms on the surface of the crystal lose electrons. The electrons that are released diffuse into the crystal and occupy the vacant anionic sites creating F-centres. This gives $NaCl$ yellow colour.
F-centres give different colours like in $KCl$ , it gives a violet colour and in $HCl$ , it gives pink colour.

Note :
In metal excess defect due to anionic vacancies, the negative ion is missing from the lattice site leaving behind a hole which is then occupied by an electron to maintain balance. These electrons are trapped in the anionic vacancies.