Question

I. $NaCl$ and $KCl$ show metal excess defect due to anionic vacancies.
II. $F$-centres are the anionic sites occupied by unpaired electrons.
III. $F$-centres is derived from the German word 'Farbenzenter' for colour centre.
IV. $F$-centres impart yellow colour to the $NaCl$ crystals.
Which of the above statement(s) is/are correct?

• A. $II, III$ and $IV$
• B. $I, III$ and $IV$
• C. $I, II$ and $III$
• D. All of these

Answer 1

Answer: (D)

Alkali metals solid like $KCl$ and $NaCl$ show non-stoichiometric defect. 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 get diffused to the surface of the crystal and combine with $Na$ atoms to give $NaCl$. This happens by loss of electron by $Na$ atoms to form $Na ^{+}$ions. The released electron diffused into the crystal and occupy anionic sites. Hence, the crystal now has anionic sites. Therefore, the crystal now has an excess of sodium. These anionic sites occupied by unpaired electrons are called $F$-centres (derived from the German word Farbenzenter for colour centre). These $F$-centres impart yellow colour to the $NaCl$ crystal.