Which of the following pentahalides is not formed ?

Options

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

Answer

To solve the question "Which of the following pentahalides is not formed?" we will analyze the given options and the electronic configurations of the elements involved.

### Step-by-Step Solution:

1. **Identify the Elements and Their Pentahalides:**
- The options provided are:
1. NF5
2. PF5
3. AsF5
4. BiF5

2. **Understand the Concept of Pentahalides:**
- Pentahalides are compounds formed when an element bonds with five halogen atoms. For an element to form a pentahalide, it must have the ability to expand its valence shell to accommodate five bonding pairs.

3. **Analyze Nitrogen (N) for NF5:**
- Nitrogen has an atomic number of 7 and an electronic configuration of 1s² 2s² 2p³.
- It has three electrons in its outer shell (2s and 2p orbitals) and can form a maximum of three covalent bonds (as seen in compounds like NH3 and NF3).
- Importantly, nitrogen does not have vacant d-orbitals to accommodate additional bonds beyond three.

4. **Analyze Phosphorus (P) for PF5:**
- Phosphorus has an atomic number of 15 and an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p³.
- It can expand its valence shell and has vacant d-orbitals, allowing it to form PF5.

5. **Analyze Arsenic (As) for AsF5:**
- Arsenic has an atomic number of 33 and an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³.
- Similar to phosphorus, arsenic can also expand its valence shell and has vacant d-orbitals, allowing it to form AsF5.

6. **Analyze Bismuth (Bi) for BiF5:**
- Bismuth has an atomic number of 83 and an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p³.
- Bismuth can also expand its valence shell and has vacant d-orbitals, allowing it to form BiF5.

7. **Conclusion:**
- Among the options, NF5 cannot be formed because nitrogen lacks the necessary vacant d-orbitals to accommodate five bonds. Therefore, the correct answer is **NF5**.

### Final Answer:
**NF5 is the pentahalide that is not formed.**