Which of the following pair contains iso-structural species ?

Options

A. `CH_(3)^(-)and CH_(3)^(+)`
B. `NH_(4)^(+)and NH_(3)`
C. `SO_(4)^(2-)and BF_(4)^(-)`
D. `NH_(2)^(-)and BeF_(2)`

Explanation

To determine which pair contains iso-structural species, we need to analyze the given species based on their shapes and hybridizations. Iso-structural species have the same shape and hybridization.

### Step-by-Step Solution:

1. **Identify the Species in Each Pair:**
- Pair 1: \( \text{CS}_3^- \) and \( \text{CH}_3^+ \)
- Pair 2: \( \text{NH}_4^+ \) and \( \text{NH}_3 \)
- Pair 3: \( \text{SO}_4^{2-} \) and \( \text{BF}_4^- \)
- Pair 4: \( \text{NH}_2^- \) and \( \text{BeF}_2 \)

2. **Analyze Pair 1: \( \text{CS}_3^- \) and \( \text{CH}_3^+ \)**
- **Shape of \( \text{CS}_3^- \):** Pyramidal (due to one lone pair)
- **Hybridization of \( \text{CS}_3^- \):** \( sp^3 \)
- **Shape of \( \text{CH}_3^+ \):** Trigonal planar
- **Hybridization of \( \text{CH}_3^+ \):** \( sp^2 \)
- **Conclusion:** Different shapes and hybridizations, so not iso-structural.

3. **Analyze Pair 2: \( \text{NH}_4^+ \) and \( \text{NH}_3 \)**
- **Shape of \( \text{NH}_4^+ \):** Tetrahedral
- **Hybridization of \( \text{NH}_4^+ \):** \( sp^3 \)
- **Shape of \( \text{NH}_3 \):** Pyramidal
- **Hybridization of \( \text{NH}_3 \):** \( sp^3 \)
- **Conclusion:** Different shapes, so not iso-structural.

4. **Analyze Pair 3: \( \text{SO}_4^{2-} \) and \( \text{BF}_4^- \)**
- **Shape of \( \text{SO}_4^{2-} \):** Tetrahedral
- **Hybridization of \( \text{SO}_4^{2-} \):** \( sp^3 \)
- **Shape of \( \text{BF}_4^- \):** Tetrahedral
- **Hybridization of \( \text{BF}_4^- \):** \( sp^3 \)
- **Conclusion:** Same shape and hybridization, so they are iso-structural.

5. **Analyze Pair 4: \( \text{NH}_2^- \) and \( \text{BeF}_2 \)**
- **Shape of \( \text{NH}_2^- \):** Bent (due to one lone pair)
- **Hybridization of \( \text{NH}_2^- \):** \( sp^3 \)
- **Shape of \( \text{BeF}_2 \):** Linear
- **Hybridization of \( \text{BeF}_2 \):** \( sp \)
- **Conclusion:** Different shapes and hybridizations, so not iso-structural.

### Final Answer:
The pair that contains iso-structural species is **Pair 3: \( \text{SO}_4^{2-} \) and \( \text{BF}_4^- \)**.

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