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If the observed density of a crystal is less than its ideal (theoretical) density, then the crystal defect responsible may be:

1. Schottky defect
2. Frenkel defect
3. substitutional impurity
4. metal deficiency defect

Correct answer: Schottky defect

Solution

In a Schottky defect, equal numbers of cations and anions are missing from their lattice positions (to maintain electrical neutrality). The unit cell has fewer particles but essentially the same volume, so the observed density is less than the ideal density. Frenkel defect does not change density because the ion merely shifts to an interstitial site within the same crystal.

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