Which of the following pentahalides is not formed ?

Options

A. `NF_5`
B. `PF_5`
C. `AsF_5`
D. `BiF_5`

Answer

To determine which of the following pentahalides is not formed, we need to analyze the ability of each central atom to expand its valence shell to accommodate five halide atoms. The options given are:

1. NF5
2. PF5
3. AsF5
4. BiF5

**Step 1: Understand the concept of pentahalides formation**
- Pentahalides are formed when a central atom can accommodate five halide atoms. This requires the central atom to have available d-orbitals to expand its octet.

**Step 2: Analyze the electronic configuration of each central atom**
- **Nitrogen (N)**: Atomic number 7
- Electronic configuration: 1s² 2s² 2p³
- Valence electrons: 5 (2s² 2p³)
- Nitrogen does not have any d-orbitals available for expansion of its octet.

- **Phosphorus (P)**: Atomic number 15
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p³
- Valence electrons: 5 (3s² 3p³)
- Phosphorus has available d-orbitals in its excited state, allowing it to form PF5.

- **Arsenic (As)**: Atomic number 33
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p³
- Valence electrons: 5 (4s² 4p³)
- Arsenic can also utilize its d-orbitals to form AsF5.

- **Bismuth (Bi)**: Atomic number 83
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p⁶ 5s² 5p³ 6s² 6p³
- Valence electrons: 5 (6s² 6p³)
- Bismuth can form BiF5 as it has d-orbitals available.

**Step 3: Conclusion**
- Among the given options, only nitrogen (N) does not have vacant d-orbitals to accommodate additional electrons and thus cannot form NF5. Therefore, the pentahalide that is not formed is **NF5**.

**Final Answer: NF5**