Question

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

• A. Third layer is covering the octahedral voids of second layer
• B. First layer is identical to the third layer
• C. Spheres of the fourth layer are exactly aligned with those of the first layer.
• D. All of the above.

Answer 1

The correct option is B First layer is identical to the third layer

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