Question

Three spheres of the first layer and three spheres of the second layer enclose a site at the centre in a close packing arrangement, this site is called?
A. Interstitial void
B. tetrahedral void
C. octahedral void
D. cubic void

Answer 1

Hint: The type of void is decided on the basis of the number of spheres forming that void. The tetrahedral void is formed by four spheres. The octahedral void is formed by six spheres. The cubic void is formed by eight spheres.

Complete answer:
Interstitial void simply means the space between the spheres. It does not specify any regular arrangement of spheres so, option (A) is incorrect.
When two similar two-dimensional close-packed layers are placed as such, the spheres of the second layer get positioned above the void of the first layer, two types of voids form.
One type of void, in which a sphere of second layer is placed just above the void of the first layer and the second type of the void, in which three spheres of the second layer are placed just above the three spheres of the first layer,
The arrangement of spheres in all type of voids is shown follows:

So, when three spheres of the first layer and one sphere of the second layer enclose a site at the centre in a close packing arrangement, this site is called tetrahedral void so option (B) is incorrect.
So, when three spheres of the first layer and three-sphere of the second layer enclose a site at the centre in a close packing arrangement, this site is called an octahedral void so option (C) is correct.
When four spheres of the first layer and four spheres of the second layer enclose a site at the centre in a close packing arrangement, this site is called a cubic void. So, option (D) is incorrect.

Therefore the correct option is (C).

Note: On joining the centres of the four spheres a tetrahedral forms so the void is known as tetrahedral void. On joining the centres of these six spheres octahedron forms so, the void is known as octahedral void and on joining the centres of the eight spheres a cube forms so, the void is known as cubic void.