Step 1: Understand F-centres.
F-centres are crystal defects where electrons are trapped in anion vacancies. They give colour to the crystal by absorbing visible light.
Step 2: Describe NaCl heating in Na vapour.
When NaCl crystal is heated in sodium vapour, sodium atoms from the vapour enter the crystal surface and ionise: \[ Na \rightarrow Na^+ + e^- \]
Step 3: Trace the extra ions and electrons.
The extra \( Na^+ \) ions fill interstitial or lattice positions. To maintain charge neutrality, the released electrons migrate and get trapped in chloride ion vacancies (empty anion sites). These are the F-centres.
Step 4: Classify the defect type.
The crystal now has more metal ions \( Na^+ \) than anions \( Cl^- \). This is a metal excess defect.
Step 5: Eliminate other defect types.
Schottky defect: equal removal of cations and anions. Dislocation defect: atoms displaced from normal positions. Metal deficiency defect: fewer cations than anions (opposite case).
Step 6: State the answer.
F-centres in NaCl formed by heating with Na vapour are due to metal excess defect. \[ \boxed{\text{Metal excess defect}} \]