Q: In which of the following crystals alternate tetrahedral voids are occupied?

Explanation:

In crystal structures, voids or interstices are spaces between the atoms where smaller ions or molecules can fit. These voids come in different shapes and sizes, with tetrahedral voids being one type. A tetrahedral void is a cavity surrounded by four atoms positioned at the corners of a tetrahedron.

Different crystal structures occupy these voids in various ways:

1. NaCl (Sodium Chloride): This structure adopts a face-centered cubic (FCC) lattice where each sodium ion is surrounded by six chloride ions and vice versa. The structure primarily involves octahedral coordination rather than tetrahedral.

2. ZnS (Zinc Sulfide): ZnS commonly exists in the zinc blende structure, which is similar to the diamond structure. In this arrangement, each zinc ion is tetrahedrally coordinated by four sulfide ions, and each sulfide ion is similarly coordinated by four zinc ions. This means that the tetrahedral voids in the lattice are alternately occupied by zinc and sulfide ions.

3. CaF2 (Calcium Fluoride): This crystal structure features calcium ions in a cubic arrangement with fluoride ions occupying all the tetrahedral voids. Here, all tetrahedral voids are filled rather than just alternate ones.

4. Na2O (Sodium Oxide): This structure does not typically involve the occupation of alternate tetrahedral voids in the same manner as ZnS.

Therefore, among the given options, ZnS is the crystal where alternate tetrahedral voids are occupied.