Sodium Chloride Yellow Color Explained

When sodium chloride (NaCl) crystals are heated in the presence of excess sodium vapor, they absorb sodium atoms. This process leads to the formation of point defects within the crystal lattice, causing the characteristic yellow coloration.

Formation of F-Centres

The yellow color observed is specifically due to the creation of F-centres. An F-centre is a type of lattice defect where an anion (in this case, a chloride ion, Cl⁻) is missing from its normal position, and an electron occupies the resulting vacant site (anion vacancy).

• Excess sodium vapor leads to sodium atoms diffusing into the NaCl crystal.
• Some chlorine atoms leave the lattice sites.
• Electrons released during this process become trapped in the anion vacancies.
• These electron-filled vacancies are known as F-centres.

Color Origin

The trapped electrons within the F-centres absorb energy from specific wavelengths of visible light. This absorption causes the crystal lattice to reflect or transmit light in the complementary color, which in this case is yellow.

Other Defects

While Schottky defects (pairs of cation and anion vacancies) and Frenkel defects (displaced ions) are also types of crystal imperfections, they do not typically cause the distinct yellow coloration observed under these specific conditions. H-centres involve interstitial halogen atoms and are not the primary cause of color in this scenario.

Therefore, the yellow color arises from the formation of F-centres.