Class 12 Chemistry The Solid State | Placing third layer over the second layer , Covering Tetrahe |

** Placing third layer over the second layer** :

- 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 10^{23}

Number of particles in 0.5 mol = 0.5 X6.022 X 10^{23} = 3.011 X 10^{23}

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 10^{23}

Number of tetrahedral voids = 2 X 3.011 X 10^{23} = 6.022 X 10^{23}

The total number of voids = 3.011 X 10^{23} + 6.022 X 10^{23}

= 9.033 X 10^{23}

Number of tetrahedral voids = 6.022 X 10^{23}

**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 A_{4}B_{3}.

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