In which of the following structures, the anion has maximum coordination number?

Options

A. (a) NaCl
B. (b) ZnS
C. (c) `CaF_(2)`
D. (d) `Na_(2)O`

Answer

To determine which structure has the anion with the maximum coordination number, we will analyze the coordination numbers of the anions in each of the given structures: NaCl, ZnS, CaF2, and Na2O.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of an ion in a crystal structure refers to the number of nearest neighboring ions surrounding it.

2. **Analyzing NaCl**:
- NaCl has a rock salt structure.
- In this structure, Na⁺ ions occupy octahedral voids in a face-centered cubic (FCC) arrangement of Cl⁻ ions.
- The coordination number of Cl⁻ (the anion) in NaCl is **6**.

3. **Analyzing ZnS**:
- ZnS can exist in two forms: zinc blende and wurtzite.
- In both forms, Zn²⁺ ions occupy tetrahedral voids, while S²⁻ ions form a face-centered cubic structure.
- The coordination number of S²⁻ in both structures is **4**.

4. **Analyzing CaF2**:
- In the CaF2 structure, Ca²⁺ ions are arranged in a cubic close-packed (CCP) structure.
- The F⁻ ions occupy the tetrahedral voids.
- The coordination number of F⁻ in CaF2 is **4**.

5. **Analyzing Na2O**:
- Na2O has an anti-fluoride structure.
- In this structure, Na⁺ ions occupy tetrahedral voids, while O²⁻ ions occupy the cubic close-packed structure.
- The coordination number of O²⁻ in Na2O is **8**.

6. **Comparing Coordination Numbers**:
- NaCl: Cl⁻ has a coordination number of **6**.
- ZnS: S²⁻ has a coordination number of **4**.
- CaF2: F⁻ has a coordination number of **4**.
- Na2O: O²⁻ has a coordination number of **8**.

7. **Conclusion**:
- The maximum coordination number among the anions in the given structures is **8**, which occurs in Na2O for the O²⁻ ion.

### Final Answer:
The structure in which the anion has the maximum coordination number is **Na2O**.

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