question_answer 31)

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?  

Answer:

(a) In a hexagonal close packing (hcp), the spheres of the third layer are vertically above the spheres of the first layer (ABABAB …… type). On the other hand, in cubic close packing (ccp), the spheres of the fourth layer are present above the spheres of the first layer (ABCAB …….. type). (b) The three dimensional arrangement of atoms/ions/molecules is called a crystal lattice. On the other hand, the smallest repeating pattern in this which when repeated in three dimensions generates the crystal is called a unit cell. (c) Tetrahedral void is a simple triangular void in a crystal and is surrounded by four spheres arranged tetrahedrally around it. On the other hand, an octahedral void is a double triangular void with one triangle vertex upwards and the other triangle tertex downwards and is surrounded by six spheres.