Question

Define F- centers.

Answer 1

Hint: F- centers, the name comes from the German word i.e. Farbzentrum in which Farbe means colour and Zentrum means center. These are responsible for imparting colours to crystals. These are the type of defects in which anionic vacancy is occupied by an unpaired electron.

Complete answer:
An F-center, also called Farbe centers or color centers. These words derived from the original German Far Zentrum where Farbe means colour and Zentrum means center. It is defined as the type of crystallographic defects in which the anionic sites are occupied by one or more unpaired electrons in an ionic compound. These anionic sites are referred to as F- centers. The unpaired electrons trapped in the F-centers get excited when they absorb energy from the visible light range and impart colour to the crystal. Hence, these types of defects are responsible for imparting colour to the crystal.
The F- centers are present in metal excess defects in a crystal structure. Metal excess defects occur when the anionic vacancies and extra cations are present in the interstitial sites of the metal. And when these anionic sites are occupied by unpaired electrons are called F- centers. Anionic vacancy are present in alkyl halides like $$HCl$$,$$KCl$$ and $$NaCl$$ etc. Some examples of F- centers are:
$$HCl$$ gives pink colour
$$KCl$$ gives violet colour
$$NaCl$$ gives yellow colour

Note:
Metal excess defects are present in the non-stoichiometric inorganic solids that contain constituent elements in non-stoichiometric ratio. Also, the anionic vacancies are created when alkyl halides are heated in the atmosphere of vapours of alkali metals. The unpaired electron in the anionic sites impart colour to the alkali metal halide crystal.