= 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>:
$$
\boxed{\mathrm{Na(s)+Cl_{Cl}^{\times}\longrightarrow
NaCl(s)+V_{Cl}^{\bullet}+e'.}}
$$
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>:
$$
\mathrm{V_{Cl}^{\bullet}+e'\rightleftharpoons
(V_{Cl}^{\bullet}e')^{\times}}.
$$
Thus sodium excess introduces electronic defects and can give coloration. \b[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.
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