In which of the following molecules /ions , are all the bonds not equal ?

Options

A. `XeF_(4)`
B. `BF_(4)^(-)`
C. `SF_(4)`
D. `SiF_(4)`

Explanation

To determine which of the given molecules or ions have unequal bond lengths, we will analyze each one step by step.

### Step 1: Analyze Xenon Tetrafluoride (XeF4)
- **Valence Electrons**: Xenon (Xe) has 8 valence electrons (group 18), and each fluorine (F) contributes 1 electron. Thus, total = 8 + 4 = 12 electrons.
- **Structure**: The molecular geometry is square planar due to the presence of two lone pairs on xenon.
- **Bond Lengths**: All four Xe-F bonds are equal because of the symmetrical arrangement in a square planar geometry.

### Step 2: Analyze Boron Tetrafluoride (BF4-)
- **Valence Electrons**: Boron (B) has 3 valence electrons, and with the addition of one electron due to the negative charge, we have 4 total. Each fluorine contributes 1 electron, so total = 4 + 4 = 8 electrons.
- **Structure**: The molecular geometry is tetrahedral.
- **Bond Lengths**: All four B-F bonds are equal due to the tetrahedral symmetry.

### Step 3: Analyze Sulfur Tetrafluoride (SF4)
- **Valence Electrons**: Sulfur (S) has 6 valence electrons, and each fluorine contributes 1 electron. Thus, total = 6 + 4 = 10 electrons.
- **Structure**: The molecular geometry is trigonal bipyramidal with one lone pair, leading to a seesaw shape.
- **Bond Lengths**: The axial bonds (SF) are longer than the equatorial bonds due to the different angles (90° for axial and 120° for equatorial). Therefore, the bond lengths are not equal.

### Step 4: Analyze Silicon Tetrafluoride (SiF4)
- **Valence Electrons**: Silicon (Si) has 4 valence electrons, and each fluorine contributes 1 electron. Thus, total = 4 + 4 = 8 electrons.
- **Structure**: The molecular geometry is tetrahedral.
- **Bond Lengths**: All four Si-F bonds are equal due to the tetrahedral symmetry.

### Conclusion
Among the analyzed molecules, only **Sulfur Tetrafluoride (SF4)** has unequal bond lengths.

### Final Answer
**The molecule with unequal bond lengths is SF4.**

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