Question

ASSERTION
Packing fraction of FCC and HCP unit cells are the same.
REASON
Both FCC and HCP unit cells have same packaging pattern i.e. ABCABCABC………….. a.) Both assertion and reason are correct and reason is correct explanation of assertion
b.) assertion and reason are correct and reason is not correct explanation of assertion
c.) assertion is correct but reason is incorrect
d.) assertion is incorrect and reason is correct

Answer 1

Hint: FCC stands for face centered unit cell and HCP stands for hexagonal closed packing. They both are of 3D type packing.

Complete answer:
Cubic close packing is called face centered cell packing. It has atoms at the corner and at the center of the face of the unit cell. The packing efficiency of FCC type structure is 74%. Coordination number of the face centered cubic unit cell is 12. FCC structure stacking is done in ABCABCABC….. pattern. Stacking of the third layer is done in voids of the second layer.
HCP stands for hexagonal closed packing and has the packing efficiency of 74%. Coordination number of the hexagonal close packing unit cell is 12. In HCP structure stacking of layers is done in ABABABA….. pattern. Stacking the third layer is exactly like the first layer.
Therefore, assertion is true that is packing fraction or packing efficiency of FCC and HCP unit cells are same but the reason is not true that is Both FCC and HCP unit cells does not have same packaging pattern i.e. ABCABCABC………….. . They both have different packaging patterns. FCC has ABCABCABC….. type packing and HCP has ABABABA…. Pattern.

Hence, option C is correct that assertion is true but reason is incorrect.


Note: There are many similarities between FCC and HCP unit cells like they have the same coordination number 12 and they have the same packing efficiencies but they have dissimilarities too between them like they don’t have the same pattern of stacking of layers. FCC has ABCABCABC….. type packing and HCP has ABABABA…. Pattern.