Yellow colour in \(NaCl\) is due to

Metal excess defect due to anionic vacancies

The correct answer is option 2. Metal excess defect due to anionic vacancies.

The yellow color in sodium chloride (\(NaCl\)) due to the metal excess defect with anionic vacancies can be explained by the presence of certain impurities or defects in the crystal lattice.

1. Metal Excess Defect:  In a metal excess defect, also known as a Schottky defect, there is an excess of metal ions (sodium ions, \(Na^+\)) relative to the stoichiometric composition of the compound. This excess of metal ions can occur due to various reasons, such as the presence of impurities or the conditions under which the crystal forms.

2. Anionic Vacancies:  Within the crystal lattice of sodium chloride, each sodium ion (\(Na^+\)) is surrounded by six chloride ions (\(Cl^-\)), and each chloride ion is surrounded by six sodium ions. However, in the presence of anionic vacancies, some of the chloride ions may be missing from their lattice sites. These vacancies result in an imbalance in the ionic composition of the crystal lattice, leading to an excess of sodium ions relative to chloride ions.

3. Coloration Mechanism:  The presence of anionic vacancies in the sodium chloride lattice can affect its optical properties. Specifically, it can lead to the absorption of certain wavelengths of light. In the case of the yellow color observed in \(NaCl\), the crystal lattice may absorb light in the blue or violet region of the spectrum, while allowing other wavelengths, such as yellow, to be transmitted or reflected. This selective absorption and transmission of light result in the perceived yellow coloration of the crystal.

4. Impurities and Defects: The formation of anionic vacancies and the resulting coloration can be influenced by impurities present in the crystal lattice. For example, the presence of metal ions such as iron(III) (\(Fe^{3+}\)) or manganese(II) (\(Mn^{2+}\)) can introduce defects and contribute to coloration in sodium chloride crystals. These impurities may occupy lattice sites or interact with the crystal structure in a way that affects its optical properties.

In summary, the yellow color observed in sodium chloride crystals can be attributed to the presence of anionic vacancies, resulting in a metal excess defect. This defect leads to an imbalance in the ionic composition of the crystal lattice, affecting its optical properties and causing selective absorption of certain wavelengths of light, ultimately resulting in the perceived yellow coloration. Additionally, impurities present in the crystal lattice can further influence the coloration process.