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
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.