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Which of the following statements about the voids formed in a three-dimensional hexagonal close-packed (HCP) arrangement of identical spheres is/are INCORRECT?

1. A tetrahedral void forms when a sphere of the second layer sits directly above a triangular void of the first layer.
2. Not all triangular voids of the first layer are covered by spheres of the second layer.
3. Tetrahedral voids are formed when a triangular void in the second layer aligns with a triangular void in the first layer such that their triangular outlines do not overlap.
4. Octahedral voids are formed when a triangular void in the second layer exactly overlaps with a triangular void in the first layer.

Correct answer: Tetrahedral voids are formed when a triangular void in the second layer aligns with a triangular void in the first layer such that their triangular outlines do not overlap.

Solution

Option C incorrectly describes the formation of octahedral voids (aligned but non-overlapping triangular voids from two layers) as tetrahedral voids. Tetrahedral voids are formed when a sphere, not a void, of the second layer sits above a triangular void of the first layer.

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