Which among the following compounds show metal excess defect due to anionic vacancy?

$NaCl$

The correct answer is Option (2) → $NaCl$.

Let us clarify the concept of metal excess defects in the context of the compounds you listed, specifically focusing on sodium chloride (NaCl).

Understanding Metal Excess Defect

1. Metal Excess Defect:

A metal excess defect occurs when there is a higher concentration of metal ions compared to what would be expected based on the stoichiometric ratio of the compound. This typically happens in ionic solids when anionic vacancies exist, leading to the need for additional cations to maintain charge neutrality.

2. Role of Anionic Vacancies:

Anionic vacancies occur when some of the anions are missing from the crystal lattice. This creates a scenario where there are excess cations present to balance the charge.

Analysis of the Compounds

1. ZnO (Zinc Oxide):

ZnO can exhibit both metal excess and deficiency defects, but it is primarily known for showing metal deficiency defects under certain conditions.

2. NaCl (Sodium Chloride):

NaCl can exhibit metal excess defects when there are anionic vacancies. If some chloride ions (Cl⁻) are missing from the lattice, extra sodium ions (Na⁺) can exist in the structure to maintain charge neutrality. Therefore, it can indeed show metal excess defects due to anionic vacancies.

3. FeO (Iron(II) Oxide):

FeO can have varying oxidation states of iron and tends to show metal deficiency rather than excess due to its stoichiometry.

4. CdO (Cadmium Oxide):

CdO may also exhibit metal excess defects, similar to ZnO, depending on the presence of anionic vacancies.

So, the correct answer is: NaCl.