F Centres and the Yellow Colour of NaCl - Colour Hint

The video explains the process of heating sodium in sodium vapor, leading to the deposition of sodium atoms. This causes chloride ions to escape, creating anion vacancies. Electrons then occupy these vacancies, forming F centers, which result in the yellow coloration of sodium chloride crystals. Additionally, the video discusses how excess lithium and potassium can cause pink and violet coloration in crystals, respectively, as examples of metal excess defects.

Describe the role of the unpaid electron in the F center.

How do excess lithium and potassium affect the coloration of crystals?