Assertion: Metal excess defect is a non-stoichiometric defect.

Reason: F-centers are occupied by paired electrons.

Assertion and Reason both are correct statements and reason is not the correct explanation for assertion.

Non stoichiometric defects: Ratio of number of cations and anions is not maintained as shown by chemical formula.

Metal excess defect is a non-stoichiometric defect which is created by anion vacancy or extra cation in the interstitial sites.

It develops a hole trapping electron e-.

F-centers are occupied by paired electrons which imparts colour to the crystals like NaCl, KCl and LiCl.