Question

Third layer of sphere is added to second layer so as to form hcp or ccp structure .what is the difference between the addition of the third layer to form these hexagonal close packed structure?​

Answer 1

When the third layer of sphere is added to the second layer, then the first dimensional row exhibits the depressions between neighbouring atoms.

• By arranging the second row in such a way that spheres fit into such depressions then a staggered arrangement is obtained. If A is the first row then row B is the second.  
• When the third row is in contacted with the second row in a staggered manner then A style structure is obtained.
• Likewise, the fourth row spheres can be organized as layers of type B. This leads to ABAB, thereby providing a form for tighter hexagonal packing.