`N_2` forms `NCl_3` whereas `P` can form both `PCl_3` and `PCl_5`. Why ?

Options

A. `P` has d-orbitals which can be used for bonding but `N` does not have.
B. `N` atom is larger than `P` in size
C. `P` is more reactive towards `Cl` than `N`
D. None of the above

Answer

To understand why nitrogen (`N2`) forms `NCl3` while phosphorus (`P`) can form both `PCl3` and `PCl5`, we need to analyze the electronic configurations and bonding capabilities of both elements.

### Step 1: Analyze the Electronic Configuration of Nitrogen
- Nitrogen has an atomic number of 7, which means its electronic configuration is `1s² 2s² 2p³`.
- In the outermost shell (n=2), nitrogen has 5 electrons (2 in `2s` and 3 in `2p`).
- It has 3 unpaired electrons in the `2p` subshell, which allows it to form a maximum of 3 covalent bonds.

### Step 2: Bond Formation in Nitrogen
- Since nitrogen does not have any vacant d-orbitals (as it is in the second period of the periodic table), it cannot expand its octet.
- Therefore, nitrogen can only form `NCl3`, where it shares its 3 unpaired electrons with 3 chlorine atoms.

### Step 3: Analyze the Electronic Configuration of Phosphorus
- Phosphorus has an atomic number of 15, giving it the electronic configuration of `1s² 2s² 2p⁶ 3s² 3p³`.
- In the outermost shell (n=3), phosphorus has 5 electrons (2 in `3s` and 3 in `3p`), similar to nitrogen.

### Step 4: Bond Formation in Phosphorus
- In its ground state, phosphorus also has 3 unpaired electrons, allowing it to form `PCl3` by bonding with 3 chlorine atoms.
- However, phosphorus has vacant d-orbitals in the third shell (3d), which allows it to promote one of its `3s` electrons to the `3d` orbital when it is excited.

### Step 5: Excited State of Phosphorus
- When phosphorus is in an excited state, it can have 5 unpaired electrons (2 in `3s`, 3 in `3p`, and 2 in `3d`).
- This allows phosphorus to form 5 bonds, leading to the formation of `PCl5`.

### Conclusion
- Thus, nitrogen can only form `NCl3` due to its inability to expand its octet, while phosphorus can form both `PCl3` and `PCl5` because it has vacant d-orbitals that allow it to expand its covalency.

### Final Answer
- Nitrogen forms `NCl3` because it has a maximum covalency of 3 and lacks d-orbitals for expansion. Phosphorus can form both `PCl3` and `PCl5` because it can utilize its vacant d-orbitals to expand its covalency to 5.