The species that cannot exist is

Options

A. `SiF_(6)^(2-)`
B. `BF_(6)^(3-)`
C. `SF_(6)`
D. `AlF_(6)^(3-)`

Answer

To solve the question regarding which species cannot exist, we will analyze the given species based on their central atoms and their electronic configurations.

### Step-by-Step Solution:

1. **Identify the Species**: The species given are SiF6^2-, BF6^3-, SF6, and AlF6^3-.

2. **Analyze SiF6^2-**:
- Central atom: Silicon (Si)
- Atomic number (Z): 14
- Electronic configuration: [Ne] 3s² 3p²
- Silicon has a vacant 3d orbital, allowing it to accommodate six fluorine atoms.
- **Conclusion**: SiF6^2- can exist.

3. **Analyze BF6^3-**:
- Central atom: Boron (B)
- Atomic number (Z): 5
- Electronic configuration: [He] 2s² 2p¹
- Boron has no vacant d orbitals, which means it cannot expand its octet to accommodate six fluorine atoms.
- **Conclusion**: BF6^3- cannot exist.

4. **Analyze SF6**:
- Central atom: Sulfur (S)
- Atomic number (Z): 16
- Electronic configuration: [Ne] 3s² 3p⁴
- Sulfur has a vacant 3d orbital, allowing it to accommodate six fluorine atoms.
- **Conclusion**: SF6 can exist.

5. **Analyze AlF6^3-**:
- Central atom: Aluminium (Al)
- Atomic number (Z): 13
- Electronic configuration: [Ne] 3s² 3p¹
- Aluminium has a vacant 3d orbital, which allows it to accommodate six fluorine atoms.
- **Conclusion**: AlF6^3- can exist.

6. **Final Conclusion**: Among the species analyzed, BF6^3- is the only species that cannot exist due to the absence of a vacant d orbital in boron.

### Answer:
The species that cannot exist is **BF6^3-**.