The Solid State

Question 93

What is meant by octahedral void in a close-packed structure?

If a triangular void pointing up in one close-packed layer is covered by a triangular void pointing down in the adjacent layer, then a void surrounded by six spheres results Such a void is called an octahedral void since the six spheres surrounding it lie at the corners of a regular octahedron.

                    octahedral void
Question 94

How many vertex, edges, faces and centres are there in a cubic unit cell?

 In a cubic unit cell,there are 8 vertex, 12 edges, 6 faces and one body centre.
Question 95

Agl crystallises in cubic close packed ZnS structure. What fraction of tetrahedral sites are occupied by Ag+ ions?

In AgI, if there are nI- ions, there will be nAg+ ions. As I- ions form the lattice, number of tetrahedral voids = 2n.
As there are nAg+
 ions to occupy these voids, therefore fraction of tetrahedral voids occupied by Ag+ ions = n/2n = ½ = 50%.
Question 96

Write the formula of a compound which is made of two elements A and B. Atom A forms close-packed lattice while atom B occupies all tetrahedral voids.

For every atom in a close packed structure, there are two tetrahedral voids.

Let the number of close packed spheres be N, then:
The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N

Therefore formula of the compound is AB2

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