Question : A compound forms hexagonal close packed structure. What is the number of voids in 0.5 mole of it ? how many of these are tetrahedral voids ?
key points to answer this question : When asked to calculate the number of voids in hexagonal close packed structure , we have to calculate octahedral and tetrahedral voids , and these voids depends on the number of atoms present in 0.5 mole , that is represent by N , tetrahedral voids are equal to 2N and total voids.
Solution : The total numbers of atoms (N) in a closed packed structure in 0.5 mole
One mole contain 6.022 × 1023 particles
so the total numbers of atoms (N) in 0.5 moles = 0.5 × 6.022 × 1023 = 3.011 × 1023
as we know or we have discussed here that the octahedral voids = numbers of atoms (N)
hence number of octahedral voids = 3.011 × 1023
we also know that the tetrahedral voids are equal to 2N
means tetrahedral voids = 2 x numbers of atoms (N)
hence number of tetrahedral voids = 2 × 3.011 × 1023 = 6.022 ×1023
Total voids = octahedral voids + tetrahedral voids
so The total number of voids are = 3.011 × 1023 + 6.022 × 1023 = 9.033 × 1023