Q.Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.

When an ionic solid has metal excess due to anion vacancies, the missing anion's place is taken by an electron (to keep neutrality). Such electron-occupied anion sites are F-centres. The trapped electrons absorb energy from visible light and get excited, imparting colour to the crystal. e.g. $NaCl$ heated in sodium vapour bec …

Step 1 – Origin of the anion vacancy (metal excess).

Heat $NaCl$ crystals in an atmosphere of sodium vapour. The excess $Na$ atoms deposit on the crystal surface; $Cl^-$ ions from inside diffuse to the surface to combine with $Na^+$, leaving anion ($Cl^-$) vacancies behind. The electron released when each $Na$ ionises:

migrates into these anion vacancies.

Step 2 – Formation of F-centres.

Each anion vacancy occupied by an unpaired electron is called an F-centre (colour centre). The crystal now has slightly more metal than the ideal formula — a metal-excess defect.

Step 3 – How colour arises. …

Explain the metal-excess/anion-vacancy mechanism (electron released fills the anion vacancy = F-centre), then link the trapped electron's visible-l …

• Saying the vacancy is filled by an ion; it is filled by an electron.
• Not naming the phenomenon as F-centres / metal-excess defect. …