COMEDK · Chemistry · 16. Solid State

If the distance between \(\mathrm{Na}^{+}\)and \(\mathrm{Cl}^{-}\)ions in sodium chloride crystal is \(x \mathrm{pm}\), the length of the edge of the unit cell is

A \(\frac{x}{2} \mathrm{pm}\)
B \(2 x \mathrm{pm}\)
C \(4 x \mathrm{pm}\)
D \(\frac{x}{4} \mathrm{pm}\)

Verified Solution

Answer & Solution

Correct Answer

(B) \(2 x \mathrm{pm}\)

Step-by-step Solution

Detailed explanation

In fcc unit cell of \(\mathrm{NaCl}, \mathrm{Na}^{+}\)ion is present at edge centre and \(\mathrm{Cl}^{-}\)ion is present at corner. \(\therefore\) Edge length, \(a=2 r^{+}+2 r^{-}\) \(\therefore \quad a=2\left(r^{+}+r^{-}\right)\) \(=2 x\) \(\left[\because x=r^{+}+r^{-}\right]\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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