Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion.According to VSEPR model, molecules adopt geometries in which their valence electron pairs position themselves as far from each other as possible.The VSEPR model consider double and triple bonds to have slightly greater repulsive effects than single bonds because of the repulsive effect of `pi` electrons.how ever, the lone pair creates the maximum repulsive effect.
Which of the following does not represent the isostructural pair ?

Options

A. `SF_5^-` and `IF_5`
B. `CIO_2F_3` and `SOF_4`
C. `SeF_3^-` and `XeO_3`
D. None

Explanation

To determine which of the given pairs does not represent isostructural molecules, we will analyze the molecular geometries of each option using the Valence Shell Electron Pair Repulsion (VSEPR) theory.

### Step-by-Step Solution:

1. **Identify the first pair: SF₅⁻ and IF₅**
- **For SF₅⁻:**
- Sulfur (S) is the central atom with 6 valence electrons. The negative charge adds one more electron, giving a total of 7 electrons.
- Attach 5 fluorine (F) atoms to sulfur.
- This results in 5 bond pairs and 1 lone pair (7 total).
- According to VSEPR, the geometry for 6 electron pairs (5 bond pairs + 1 lone pair) is octahedral, which leads to a square pyramidal shape.

- **For IF₅:**
- Iodine (I) is the central atom with 7 valence electrons.
- Attach 5 fluorine atoms.
- This results in 5 bond pairs and 1 lone pair (6 total).
- The geometry is also octahedral, leading to a square pyramidal shape.

- **Conclusion:** SF₅⁻ and IF₅ are isostructural.

2. **Identify the second pair: ClO₂F₃ and SOF₄**
- **For ClO₂F₃:**
- Chlorine (Cl) is the central atom with 7 valence electrons.
- Attach 2 double-bonded oxygen (O) and 3 fluorine (F) atoms.
- This results in 5 bond pairs (2 from O and 3 from F) and 0 lone pairs.
- The geometry is trigonal bipyramidal.

- **For SOF₄:**
- Sulfur (S) is the central atom with 6 valence electrons.
- Attach 1 double-bonded oxygen and 4 fluorine atoms.
- This results in 5 bond pairs and 0 lone pairs.
- The geometry is also trigonal bipyramidal.

- **Conclusion:** ClO₂F₃ and SOF₄ are isostructural.

3. **Identify the third pair: SeF₃⁻ and XeO₂**
- **For SeF₃⁻:**
- Selenium (Se) is the central atom with 6 valence electrons. The negative charge adds one more electron, giving a total of 7 electrons.
- Attach 3 fluorine atoms.
- This results in 3 bond pairs and 2 lone pairs (5 total).
- The geometry is trigonal bipyramidal, leading to a T-shaped structure.

- **For XeO₂:**
- Xenon (Xe) is the central atom with 8 valence electrons.
- Attach 2 double-bonded oxygen atoms.
- This results in 2 bond pairs and 2 lone pairs (4 total).
- The geometry is tetrahedral, leading to a pyramidal shape.

- **Conclusion:** SeF₃⁻ and XeO₂ are not isostructural.

### Final Answer:
The pair that does not represent isostructural molecules is **C. SeF₃⁻ and XeO₂**.