Placing third layer over the second layer : Covering Tetrahedral Voids
- Tetrahedral voids of the second layer can get covered by the spheres of the third layer.
- Here the spheres of the third layer get exactly aligned with the spheres of the first layer.
- Therefore it can be observed that the pattern of spheres is repeated in alternate layers.
- This pattern is often written as ABAB ....... pattern and the structure is called hexagonal close packed (hcp) structure found in many metals like magnesium and zinc.
Question : A compound forms hexagonal close-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids?
Answer: Number of particles in 1 mol of compound = 6.022 X 1023
Number of particles in 0.5 mol = 0.5 X6.022 X 1023 = 3.011 X 1023
Number of octahedral voids = number of atoms or particles.
Number of tetrahedral voids = 2 X number of particles
Therefore number of octahedral in the given compound = 3.011 X 1023
Number of tetrahedral voids = 2 X 3.011 X 1023 = 6.022 X 1023
The total number of voids = 3.011 X 1023 + 6.022 X 1023
= 9.033 X 1023
Number of tetrahedral voids = 6.022 X 1023
Question. Atoms of element B form hcp lattice and those of the element A occupy 2/3rd of tetrahedral voids. What is the formula of the compound formed by the elements A and B?
Answer: Number of tetrahedral voids = 2 X number of particles of atom B
Number of atoms of A is equal to of the number of atoms of B.
Hence the ratio A:B = (4/3):1 or 4:3
Therefore, the formula of the compound is A4B3.
