 Solution

One standard description of sodium-rich NaCl is removal of lattice chloride into a NaCl reservoir, leaving a positively charged chloride vacancy and an electron:This Kröger–Vink notation equation conserves atoms, chloride sites and effective charge. The electron can be trapped in the vacancy, forming an electrically neutral F center:Thus sodium excess introduces electronic defects and can give coloration. It does not imply that every added electron is a mobile conduction electron: electronic transport depends on trapping and thermal release. Chloride vacancies can also assist anion transport. An alternative microscopic incorporation route is a sodium interstitial plus an electron, but the vacancy–electron model describes the usual alkali-metal-colored halide.