Which among the following species has unequal bond lengths ?

Options

A. `BF_4^-`
B. `XeF_4`
C. `SF_4`
D. `SiF_4`

Answer

To determine which species has unequal bond lengths, we need to analyze the molecular geometry and hybridization of the given compounds. Let's go through the analysis step by step.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question mentions several species, but we will analyze the common ones that typically exhibit unequal bond lengths, such as BF3, XeF4, SF4, SiCl4, and others.

2. **Determine the Valence Electrons**:
- **BF3**: Boron (B) has 3 valence electrons, and each fluorine (F) has 7. Thus, BF3 has a total of 3 + (3 × 7) = 24 valence electrons.
- **XeF4**: Xenon (Xe) has 8 valence electrons, and each fluorine contributes 7. Thus, XeF4 has 8 + (4 × 7) = 36 valence electrons.
- **SF4**: Sulfur (S) has 6 valence electrons, and each fluorine contributes 7. Thus, SF4 has 6 + (4 × 7) = 34 valence electrons.
- **SiCl4**: Silicon (Si) has 4 valence electrons, and each chlorine (Cl) has 7. Thus, SiCl4 has 4 + (4 × 7) = 32 valence electrons.

3. **Calculate Steric Numbers**:
- **BF3**: Steric number = 3 (3 bonds, no lone pairs) → sp2 hybridization → trigonal planar geometry (all bond lengths are equal).
- **XeF4**: Steric number = 6 (4 bonds, 2 lone pairs) → sp3d2 hybridization → octahedral geometry (all bond lengths are equal).
- **SF4**: Steric number = 5 (4 bonds, 1 lone pair) → sp3d hybridization → seesaw geometry (unequal bond lengths due to lone pair repulsion).
- **SiCl4**: Steric number = 4 (4 bonds, no lone pairs) → sp3 hybridization → tetrahedral geometry (all bond lengths are equal).

4. **Analyze Bond Lengths**:
- **BF3**: All bond lengths are equal due to trigonal planar geometry.
- **XeF4**: All bond lengths are equal due to octahedral geometry.
- **SF4**: The seesaw geometry results in unequal bond lengths because the lone pair affects the bond angles and lengths.
- **SiCl4**: All bond lengths are equal due to tetrahedral geometry.

5. **Conclusion**: The species with unequal bond lengths is **SF4**.

### Final Answer:
**SF4** has unequal bond lengths.