How will you distinguish between the following pairs of terms?
a. Hexagonal close-packing and cubic close-packing
b. Crystal lattice and unit cell
c. Tetrahedral void and octahedral void

Explanation

(i) Cubic close packing when the thrid layer is placed over the second layer in such a way that the spheres cover the octahedral voids, a layer diffeent from first (A) and second(B) is produced if we contnue packing in this manner, then a packing is obtained where the spheres in every fourth layer will vertically aligned this pattern of packing spheres is called ABCABC pattern or cubic close packing. Hexagonal close packing when a third layer is placed over the second layer in such a manner that the spheres cover the tetrahedral void. a three diemensional close packing is obtained whre the spheres in every third or alternate layers are vertically aligned if we continue packing in this manner, then the packing obtained would be called ABAB pattern or hexagonal close packing.
(ii) Crystal lattice, it is a regular arrangement of the consituent particles (i.e., ion, atoms or molecuels) of a crystal in three dimensional space unitcell. The smallest three dimensional portion of a complete space lattice which when repeated over and voer agian in different directions produces the complete crystal lattice is called the unit cell.
(iii) Tetrahedral void. A simple) triangular void is a crystal is surrounded by four spheres and is called a tetrahedral void.
Octahedral void. A double triangular void is surrounded by six spheres and is called a octahedral void.