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Compare the HCP (hexagonal close-packed) and CCP (cubic close-packed) structures of the same element, assuming the atomic radius is identical in both. Which of the following properties are the same for both structures?

1. Density of the crystal
2. Distance between two consecutive layers (A and B)
3. Coordination number of each atom
4. Fraction of unoccupied (void) space

Correct answer: Coordination number of each atom

Solution

HCP and CCP are both close-packed structures with identical packing efficiency (74%) meaning the same fraction of unoccupied space (26%), the same coordination number (12), and since the same element is used with the same radius the density is also equal. The interlayer distance (A-B spacing) can differ as the geometry of stacking differs (ABAB vs ABCABC). Among the options, coordination number and void fraction are definitely the same; density is also the same.

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