Write a short note on: Formula of a compound and number of voids filled.

(N/A) In close-packed structures such as $ccp$ or $hcp$,two types of voids are formed: tetrahedral voids and octahedral voids.
The number of octahedral voids present in a lattice is equal to the number of close-packed particles $(N)$,while the number of tetrahedral voids is twice the number of close-packed particles $(2N)$.
In ionic solids,the larger ions (usually anions) form the close-packed structure,while the smaller ions (usually cations) occupy the voids.
If the cation is small,it occupies tetrahedral voids; if it is larger,it occupies octahedral voids.
The fraction of octahedral or tetrahedral voids occupied in a given compound depends on the chemical formula of the compound. It is not necessary that all tetrahedral or octahedral voids are filled.