When pure crystals are heated in an atmosphere of sodium vapor, the crystals acquire a distinct yell — Solid State Chemistry Question

💡 Solution & Explanation

Heating $NaCl$ in sodium vapor leads to a metal excess defect. The excess $Na$ atoms ionize on the surface, and the released electrons diffuse into the crystal and occupy the vacant anionic ($Cl^-$) lattice sites. These trapped electrons are called F-centers (Farbe centers) and are responsible for the yellow color upon excitation.