Question

The arrangement of the first two layers, one above the other, in hcp and ccp arrangement is:
A. Exactly the same in both cases
B. Partly same and partly different
C. Different from each other
D. Nothing definite

Answer 1

Hint: Inside a unit cell, the most powerful conformation atomic spheres can take is known as the closest packing configuration. There are two modes of tightly packed atomic spheres: hexagonal closest packing (HCP) and cubic closest packing: (CCP). Tight packing can only be achieved in a single manner of arrangement.

Complete step by step solution:
The structure of a crystalline material is usually visualised as being made up of unit cells. These cells are organised regularly to give rise to the lattice structure of a crystal.
Through the periodic patterning of the atoms that make up the crystal, the three-dimensional structure of a solid crystalline material is formed. The closest packing formation is regarded as the most successful conformation of atomic spheres within a unit cell. There are two methods to tightly pack the cell.
In the first form, gaps in the previous layer cover each successive layer of spheres. In the first layer, three neighbouring spheres can form a hollow space where they intersect. In the hollows formed in the previous layer, spheres in one layer align to fit. Directly above the first layer, the third layer aligns. This arrangement is referred to as "ABA" since the third layer is aligned the same way as the first layer and results in the closest hexagonal packing (HCP).
Alternatively, the gaps in the first layer are covered by the second layer. The third layer of spheres does not align with the first layer. This configuration is known as "ABC" and results in the closest cubic packing (CCP).
In the arrangement of hcp and ccp, the arrangement of the first two layers, one above the other, is exactly the same in both cases. It is an arrangement type of AB.
 So, from the above discussion it is clear that the first two layers in both types of packing have the same arrangement.
So the correct answer is option A.

Note:
Atomic coordination numbers, interatomic distances, and the forms and strengths of bonding that exist within the solid are influenced by the arrangement of the atoms in a crystalline solid. An understanding of atomic packing in a unit cell and crystal lattice can provide insight into a given crystalline material's physical, chemical, electrical, and mechanical properties.