Question

Distinguish between hexagonal close packing an cubic close packing.

Answer 1

• When the tetrahedral voids of the second layer is covered by the spheres of the third layer. So, that the spheres of the third layer are exactly aligned with those of the first layer, we get a pattern of sphere which is repeated in alternate layers. This pattern can be written in the form of $ABAB.....$ pattern. This structure is called hexagonal closed packing $\left(HCP\right)$.
  
• When the third layer is placed above the second layer in a manner such that its sphere cover the octahedral voids and the sphere of the third layer are not aligned with first or the second layer, the arrangement is called ${}^{\prime }{C}^{\prime }$ type. This pattern is written as $ABCABC.......$. This structure is called cubic closed packing(CCP).