The correct answer is (2) Metal excess defect due to anionic vacancies.
The yellow color in sodium chloride (\(NaCl\)) is associated with the presence of metal excess defect, specifically a type of metal excess defect known as F-center or color center. Let's break down the details:
1. F-Center (Color Center):
F-center stands for Farbe, the German word for color. It refers to a type of crystallographic defect in ionic crystals that results in the absorption of visible light, leading to the appearance of color.
In the case of sodium chloride, the F-center is formed when an electron is missing from a chloride ion (\(Cl^-\)) site in the crystal lattice.
2. Formation of F-Center:
The formation of an F-center involves the loss of an electron from a chloride ion, leaving behind a chloride ion vacancy or anionic vacancy (\(Cl^-\) site without an electron).
The electron that has been removed becomes a free electron (\(e^-\)) and is free to move through the crystal lattice.
3. Absorption of Light:
The presence of a free electron in the crystal lattice creates an electronic transition in the visible light spectrum.
This free electron can absorb energy from visible light, particularly in the blue or ultraviolet region, leading to the appearance of a yellow color in the transmitted light.
4. Color Appearance:
The yellow color associated with F-centers in sodium chloride crystals is often visible when the crystal is exposed to light. This coloration is commonly observed in certain types of rock salt or salt crystals.
5. Related to Metal Excess Defect:
The formation of F-centers is related to a metal excess defect, as it involves the presence of excess electrons (metal excess) due to the creation of anionic vacancies.
In summary, the yellow color in sodium chloride is attributed to the presence of F-centers, which are formed by the loss of an electron from a chloride ion site, resulting in an anionic vacancy. This type of coloration is associated with metal excess defects in the crystal lattice. |
