Question

How will you distinguish between the following pairs of terms:
(i) Hexagonal close-packing and cubic close-packing?
(ii) Crystal lattice and unit cell?
(iii) Tetrahedral void and octahedral void?

Answer 1

$\left(i\right)$ Hexagonal close-packing and cubic close-packing
HCP arrangement is $ABAB$ .... type arrangement. The spheres of third layer are exactly aligned with those of the first layer.
CCP arrangement is $ABCABC$ .... type arrangement. The spheres of fourth layer are exactly aligned with those of the first layer.
In hcp, the tetrahedral voids of the second layer may be covered by the spheres of the third layer. In ccp, the octahedral voids of the second layer may be covered by the spheres of the third layer
(ii) Crystal lattice and unit cell
Crystal lattice is the three dimensional arrangement of constituent particles (atoms, ions or molecules) of a substance, whereas unit cell is the smallest repeating pattern in a crystal lattice. When unit cell is repeated in three dimensions, entire crystal is obtained.
(iii) Tetrahedral void and octahedral void
Tetrahedral void is surrounded by four spheres while an octahedral void is surrounded by six spheres.