Question

An excess of potassium ions makes $KCl$ crystals appear violet or lilac in colour since______.

• A. some of the anionic sites are occupied by a pair of electrons.
• B. some of the anionic sites are occupied by an unpaired electron.
• C. $F$-centers are created which impart colour to the crystals.
• D. there are vacancies at some anionic sites.

Answer 1

Answer: (B), (C)

$F$-centers are created which impart colour to the crystals.

When $KCl$ is heated in an atmosphere of potassium $\left(K\right)$ vapours, the excess of $K$ atoms deposits on the surface of $KCl$ crystals. Chloride ions $C{l}^{-}$ then diffuse to the surface and creates anionic vacancy.

These $C{l}^{-}$ ions then combine with $K^{+}$ ions which are formed by losing electrons. The released electrons diffuse into the crystal and occupy anionic sites.

As a result, the crystal now has an excess of potassium.
The anionic sites occupied by unpaired electrons are called $F$-centers.
They impart violet(or lilac) color to the crystals of $KCl$.

Hence, the options $\left(A\right)$ and $\left(D\right)$ are correct..