Question

In an arrangement of type ABABA ....., identical atoms of first layer A and third layer A are joined by a line passing through their centres.
Choose the correct statement from the following.

• A. No void is found on the line
• B. Only tetrahedral voids are found on the line
• C. Only octahedral voids are found on the line
• D. Equal number of tetrahedral and octahedral voids are found on the line

Answer 1

The correct option is B Only tetrahedral voids are found on the line

3-D hexagonal close packing is formed from 2-D hexagonal closed packing. Here, the second row of spheres is placed in the interstices or depressions of the first-row spheres of 2-D hexagonal close packing. Only 50% void of one layer can be occupied by sphere of other layer (II layer). Then, sphere of third layer occupy those voids of II layer under which there are sphere of first layer. This leads to hexagonal closed packing.
This follows the pattern of ABABAB.....
The ABABAB.. arrangement results in hexagonal lattice.
In such an arrangment, spheres of third layer A occupy the tetrahedral void of the B layer and is identical to the first layer A. So the line joining AA layers passing through the centre of spheres will go through tetrahedral void.