Final answer:
Zinc sulfide adopts a face-centered cubic packing, with four S²- ions and four Zn²+ ions per unit cell, which corresponds to the formula ZnS. Each ion type is surrounded tetrahedrally by the other, similar to the diamond structure when all atoms are carbon.
Step-by-step explanation:
The student asked about the packing arrangement and the number of ions in the unit cell for zinc sulfide, or sphalerite, when it crystallizes.
The sulfide ions in zinc sulfide adopt a face-centered cubic (FCC) packing. Every FCC lattice has four atoms per unit cell, and since the zinc ions occupy half of the available tetrahedral holes, there are also four zinc ions present.
This results in the unit cell containing four S²- ions and four Zn²+ ions, giving the empirical formula ZnS.
The FCC structure and the occupation of tetrahedral holes by Zn²+ ions lead to a stoichiometry of 1:1, matching the formula.
It is also noted that in the zinc-blende structure, each sulfur atom is tetrahedrally surrounded by four zinc atoms, and vice-versa.
An interesting fact is that if all the atoms in this structure were replaced with carbon, it would correspond to the diamond structure.