The presence of excess sodium in sodium chloride makes the crystal appearance yellow. This is due to — Solid State Chemistry Question

💡 Solution & Explanation

When NaCl is heated in an atmosphere of sodium vapor, excess sodium atoms deposit on the surface. $Cl^-$ ions diffuse to the surface, and the lost electrons from Na diffuse into the crystal and occupy the vacant anionic sites. These trapped electrons in anionic vacancies are called F-centres and are responsible for the yellow colour. Therefore, correct answer is C.