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JEE Main levelOne or more correct optionsDifficulty 2 of 3ss020

Which of the following are correct for a hexagonal close packed (hcp) arrangement of identical spheres?

AThe coordination number of each sphere is 12.
BThe layers are stacked in the sequence $A BC A BC \dots$
CThe packing efficiency is about 74%.
DThe hexagonal unit cell contains 4 atoms.

Answer: A, C

hcp has CN 12 and 74% packing efficiency, the same as ccp. Its layer sequence is

A B A B \dots

(

A BC A BC \dots

is ccp), and the hexagonal unit cell contains

12 \times \frac{1}{6} + 2 \times \frac{1}{2} + 3 = 6

atoms, not 4.

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