It is stated that ZnS does not crystallise in the NaCl structure it is due to:

Options

A. The `r^(+)//r^(-)` ratio is `0.402` too low to avoid anion anion contact as in the NaCl stucture
B. ZnS is water insoluble, NaCl is water soluble
C. ZnS is water soluble, NaCl is water insoluble
D. Zn belongs to d-block, Na belongs to s-block

Answer

To determine why ZnS does not crystallize in the NaCl structure, we need to analyze the factors that influence crystal structures, particularly the radius ratio of the cations and anions involved.

### Step-by-Step Solution:

1. **Understanding the NaCl Structure**:
- The NaCl structure is characterized by a face-centered cubic (FCC) arrangement of ions. In this structure, sodium ions (Na⁺) and chloride ions (Cl⁻) are arranged in such a way that each Na⁺ is surrounded by six Cl⁻ ions and vice versa.

2. **Radius Ratio Concept**:
- The radius ratio (r⁺/r⁻) is crucial in determining the type of crystal structure that a compound can adopt. It is defined as the ratio of the radius of the cation (r⁺) to the radius of the anion (r⁻).
- For the NaCl structure, the radius ratio is approximately 0.414. This ratio allows for the stable coordination of cations and anions in the FCC lattice.

3. **Radius Ratio of ZnS**:
- In the case of ZnS, zinc (Zn²⁺) has a smaller ionic radius compared to sodium (Na⁺), and sulfide (S²⁻) has a larger ionic radius compared to chloride (Cl⁻).
- The radius ratio for ZnS is approximately 0.38 (Zn²⁺: 0.074 nm, S²⁻: 0.184 nm), which is less than the required ratio for the NaCl structure.

4. **Coordination and Structure**:
- Due to the smaller radius ratio of ZnS, it prefers a tetrahedral coordination rather than octahedral coordination found in the NaCl structure. This means that in ZnS, each Zn²⁺ ion is surrounded by four S²⁻ ions, leading to a different crystal structure.

5. **Conclusion**:
- Therefore, ZnS does not crystallize in the NaCl structure primarily because the radius ratio of Zn²⁺ to S²⁻ is not suitable for the FCC arrangement that characterizes the NaCl structure. Instead, ZnS adopts a different structure, typically the zinc blende structure, which is tetrahedral.

### Final Answer:
ZnS does not crystallize in the NaCl structure because the radius ratio (r⁺/r⁻) of Zn²⁺ to S²⁻ is approximately 0.38, which is less than the required 0.414 for the NaCl structure, leading to tetrahedral coordination instead of octahedral.