Question

Paragraph :In metal excess defect, anions all missed from their lattice sites and they are occupied by electrons to maintain electrical neutrality. These sites occupied by electrons are called F-centers.

Question: The colour of NaCl changes when heated in sodium vapour due to

• A. Excess of sodium in the crystal
• B. Electrons in the voids
• C. Excess of chlorine in the crystal
• D. Deficiency of sodium in the crystal

Answer 1

The correct option is B

Electrons in the voids

As anions are missing from the lattice points, $e^{-}$ occupy these voids. There $e^{-}$ impart colour to the crystal. So there voids are called F-centers(Farbe means color in german) and colour centers.