Question

Choose the correct statement for 3-D hexagonal closed packing :

Answer 1

3-D hexagonal close packing is formed from 2-D hexagonal closed packing. Here, the second layer of spheres is placed in the interstices or depressions of the first layer spheres of 2-D hexagonal close packing.

Spheres of third layer are placed over second layer covering the
tetrahedral voids so that, spheres of third layer are exactly aligned with the spheres of first layer.
This leads to arrangement of ABABAB... pattern. This structure is called hexagonal close packing (hcp) structure. Packing in Mg, Zn, Cd etc. follows this type of arrangement.
This can be diagrammatically represented as


Here, we can see easily that $1^{st}$ layer and $4^{th}$ layer are not exactly aligned.
Only option (b) is correct