In which of the following pair, both the species are isoelectronic but the first one is large in size than the second?

Options

A. `S^(2-) , O^(2-)`
B. `Cr, S^(2-)`
C. `F^(-), Na^(+)`
D. `N^(3-) , p^(3-)`

Explanation

To solve the question of which pair contains isoelectronic species where the first one is larger in size than the second, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Isoelectronic Species**:
Isoelectronic species are atoms or ions that have the same number of electrons. This means they will have similar electronic configurations.

2. **Analyze Each Option**:
- **Option A**: Sulfur (S²⁻) and Oxygen (O²⁻)
- Sulfur has an atomic number of 16, so in S²⁻, it has 16 + 2 = 18 electrons.
- Oxygen has an atomic number of 8, so in O²⁻, it has 8 + 2 = 10 electrons.
- **Conclusion**: Not isoelectronic.

- **Option B**: Chromium (Cr) and Sulfur (S²⁻)
- Chromium has an atomic number of 24, so it has 24 electrons.
- Sulfur (S²⁻) has 18 electrons (as calculated above).
- **Conclusion**: Not isoelectronic.

- **Option C**: Fluorine (F⁻) and Sodium (Na⁺)
- Fluorine has an atomic number of 9, so F⁻ has 9 + 1 = 10 electrons.
- Sodium has an atomic number of 11, so Na⁺ has 11 - 1 = 10 electrons.
- **Conclusion**: Both F⁻ and Na⁺ are isoelectronic with 10 electrons.
- **Size Comparison**: Anions (F⁻) are larger than cations (Na⁺) due to electron-electron repulsion.

- **Option D**: Nitrogen (N³⁻) and Titanium (Ti³⁺)
- Nitrogen has an atomic number of 7, so N³⁻ has 7 + 3 = 10 electrons.
- Titanium has an atomic number of 22, so Ti³⁺ has 22 - 3 = 19 electrons.
- **Conclusion**: Not isoelectronic.

3. **Final Conclusion**:
The only pair that is isoelectronic and where the first species is larger than the second is **Option C: F⁻ and Na⁺**.

### Final Answer:
**Option C: Fluorine (F⁻) and Sodium (Na⁺)**

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