Nitrogen does not form pentahalides. Give reason.

Nitrogen does not form pentahalides due to its inability to expand its valence shell beyond an octet. Here's a detailed explanation:

### **1. Valence Electrons and Octet Rule**

- **Nitrogen's Valence Electrons:** Nitrogen has five valence electrons. In bonding, it typically forms three covalent bonds to achieve a stable octet (eight electrons in its valence shell).

\[
\text{Nitrogen (N)}: 5 \text{ valence electrons}
\]

- **Formation of Common Compounds:** For example, in ammonia (\(\text{NH}_3\)), nitrogen forms three single bonds with hydrogen atoms, sharing three pairs of electrons and achieving an octet:

\[
\text{H} - \text{N} - \text{H}
|
\text{H}
\]

### **2. Inability to Expand the Octet**

- **No Available d-Orbitals:** Elements in the second period of the periodic table (like nitrogen) do not have accessible d-orbitals. This limits their ability to accommodate more than eight electrons in their valence shell.

- **Pentahalide Requirement:** To form a pentahalide (e.g., \(\text{NF}_5\)), nitrogen would need to form five bonds, which would require ten electrons around the nitrogen atom:

\[
\text{F} - \text{N} - \text{F}
| \quad |
\text{F} \quad \text{F}
\]

- **Electron Count:** This arrangement would give nitrogen ten electrons in its valence shell, exceeding the octet rule. Since nitrogen cannot utilize d-orbitals to accommodate these extra electrons, such a compound is not feasible.

### **3. Comparison with Other Elements**

- **Phosphorus Example:** Unlike nitrogen, phosphorus (which is in the same group but in the third period) can form pentahalides like phosphorus pentachloride (\(\text{PCl}_5\)) because it has available d-orbitals to expand its octet.

### **Conclusion**

Nitrogen's position in the periodic table restricts it to a maximum of three covalent bonds, preventing the formation of pentahalides. Its inability to exceed the octet rule due to the lack of available d-orbitals makes the formation of such compounds chemically unfavorable.

**Summary:** Nitrogen cannot form pentahalides because, lacking d-orbitals, it cannot expand its valence shell beyond eight electrons and thus cannot accommodate five bonds.