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 A4B3.
Placing third layer over the second layer: Covering Octahedral Voids
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The third layer may be placed above the second layer in a manner such that its spheres cover the octahedral voids.
When placed in this manner, the spheres of the third layer are not aligned with those of either the first or the second layer. This arrangement is called“ C’
Only when fourth layer is placed, its spheres are aligned with those of the first layer.
This pattern of layers is often written as ABCABC........... This structure is called cubic close packed( ccp) or face-centered cubic( fcc) structure.