Table salt sometimes appear yellow in colour because:

It has metal excess defect

The correct answer is option 1. It has metal excess defect.

In a perfect crystal lattice, every atom or ion is in its precise, regular position. However, real crystals often contain imperfections or defects, which can influence the physical properties of the material, including color.

There are several types of defects:

Schottky defects: Equal numbers of cations and anions are missing from the lattice, maintaining electrical neutrality.

Frenkel defects: A cation or anion leaves its regular position and occupies an interstitial site within the crystal.

Metal excess defects: Occur when there are more metal ions than expected in the stoichiometric formula of the crystal, often leading to the formation of color centers.

Metal Excess Defect in Sodium Chloride

Structure of Sodium Chloride: Sodium chloride (\(NaCl\)) has a face-centered cubic (FCC) structure where sodium ions (\(Na^+\)) and chloride ions (\(Cl^-\)) are arranged in a regular, repeating pattern. Each sodium ion is surrounded by six chloride ions, and vice versa, forming a stable ionic lattice.

Formation of Metal Excess Defect:

Vacancy and Electron Trapping:  If a chloride ion (\(Cl^-\)) is missing from its regular position in the lattice, it creates an anion vacancy. This vacancy can trap an electron to maintain electrical neutrality.  The trapped electron occupies the vacancy, and this site is known as an F-center (from the German "Farbe," meaning color).

Charge Compensation: The removal of a \(Cl^-\) ion leaves the crystal with an excess positive charge from the \(Na^+\) ions. The electron trapped at the F-center compensates for this positive charge.

Optical Properties and Yellow Color:

The electron trapped in the F-center is in an excited state. This electron can absorb light in the visible spectrum. Specifically, the electron absorbs light in the blue region of the spectrum (~450 nm). Since blue light is absorbed, the complementary color, yellow, is transmitted, giving the salt a yellow appearance. This absorption of light due to the presence of F-centers is what gives the \(NaCl\) crystal its yellow hue when the metal excess defect is present.

Why the Other Options Are Not Correct

Metal Deficiency Defect: Metal deficiency defect occurs when there are fewer metal ions than the stoichiometric amount in the crystal. This defect usually involves the creation of vacancies or the introduction of extra non-metal ions. It typically leads to changes in electrical properties rather than color changes associated with F-centers. \(NaCl\) appearing yellow is not due to a deficiency of sodium ions but rather due to an excess of them (or a deficiency of chloride ions)

Oxidation: Oxidation involves the loss of electrons from atoms or ions, potentially changing the color of a material if the oxidation state of a component changes. However, in the case of \(NaCl\), the yellow color is due to electron trapping at F-centers, not due to oxidation.

Reduction: Reduction involves the gain of electrons, but in this context, the yellow color in \(NaCl\) is specifically related to the formation of F-centers via metal excess defects, rather than a broad reduction process.

Summary and Conclusion

Metal Excess Defect in \(NaCl\): The yellow color in sodium chloride arises due to a metal excess defect where chloride ions are missing from their regular positions in the crystal lattice, creating anion vacancies.

F-centers: The vacancies created by the missing chloride ions trap electrons, forming F-centers. These F-centers absorb blue light, resulting in the yellow color observed in the crystal.

Impact: This defect explains why sometimes table salt, which is normally colorless, may appear yellow.

Thus the correct answer is option 1: It has a metal excess defect.