When third layer of 2 D hexagonal close packed layer of spheres is placed over 2nd layer, the second possibility of arrangement is: Covering the Octahedral voids:

Place 2 layers of spheres such that the second layer fits in the depressions of first layer. The two layers can be named as A and B.

Then, place a third layer above the second layer in a manner that its spheres cover the octahedral voids.
When placed in this manner, the spheres of third layer are not aligned with those of either the first or second layer
So, the third layer can be named as ‘C’.

Only when fourth layer is placed, the spheres of 4th layer align with the spheres of first layer. So, the fourth layer can also be named as A.
Similarly 5th layer aligns with 2nd layer and can be named as B and so on..
Hence, this type of arrangement is ABCABC… type of arrangement and is called as cubic close packed(ccp) or face-centred cubic (fcp) structure.
Example: Silver, Copper, Iron, Nickle, Platinum have this type of lattice.
This type of packing is very efficient because of very less space between the spheres, and the packing efficiency of this lattice is 74%.
Each sphere is in contact with 12 nearest atoms. So, the coordination number is 12.
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