An excess of potassium ions makes \(KCl\) crystal appear violet or liliac in colour since:

(A) Some of the anionic sites are occupied by an unpaired electron

(B) Some of the anionic sites are occupied by a pair of electrons

(C) There are vacancies at some anionic sites

(D) F-centres are created which impart colour to the crystals

Choose the correct answer from the options from the options given below:

A and D only

The correct answer is option 2. A and D only.

Let us break down the reasoning behind why the correct answer is Option 2: A and D only.

Why \( KCl \) Crystals Appear Violet or Lilac

When potassium chloride (\( KCl \)) crystals contain an excess of potassium ions, the crystals exhibit a violet or lilac color. This color is due to a specific type of defect in the crystal structure known as F-centres.

F-centres (from the German word "Farbe," meaning color) are electron-rich defects in the crystal lattice. These occur when an anion (in this case, a chloride ion, \( Cl^- \)) is missing from its position in the lattice. In the absence of the chloride ion, the site (anionic site) where the ion was supposed to be is now vacant. To maintain electrical neutrality, an unpaired electron from the potassium atom occupies this vacancy. This unpaired electron in the anionic vacancy (now an F-centre) can absorb light. The absorbed light energy corresponds to certain wavelengths, and the remaining light that is reflected or transmitted by the crystal is perceived by our eyes as violet or lilac.

Evaluating the Statement:

(A) Some of the anionic sites are occupied by an unpaired electron:

The statement is correct. This describes the formation of an F-centre. When a \( Cl^- \) ion is missing, an unpaired electron takes its place, creating the F-centre responsible for the color.

(D) F-centres are created which impart color to the crystals:

The statement is correct. The F-centres are the reason why the \( KCl \) crystal appears violet or lilac. The unpaired electron at the F-centre absorbs light, leading to the observed coloration.

Why the Other Options Are Incorrect

(B) Some of the anionic sites are occupied by a pair of electrons:

The statement is incorrect. The color is specifically due to unpaired electrons in the F-centres, not pairs of electrons. Pairs of electrons would not produce the same color effect.

(C) There are vacancies at some anionic sites:

The statement is incorrect. While anionic vacancies exist, the key reason for the violet/lilac color is the presence of unpaired electrons at these sites (i.e., the F-centres), not just the vacancies alone.

Conclusion

The correct answer is Option 2: A and D only because the violet or lilac color of \( KCl \) crystals with excess potassium ions is specifically due to the presence of unpaired electrons at anionic sites (statement A) and the formation of F-centres (statement D), which are responsible for the observed color.