`PCl_(5)` on heating dissociates to `PCl _(3) and Cl _(2)` because :

Options

A. P can expand its octet
B. It is polar covelent compound
C. It has unsymmetrical geometry
D. None of these

Answer

To answer the question of why \( PCl_5 \) dissociates into \( PCl_3 \) and \( Cl_2 \) upon heating, we can follow these steps:

### Step-by-Step Solution:

1. **Determine the Structure of \( PCl_5 \)**:
- \( PCl_5 \) has phosphorus (P) as the central atom and five chlorine (Cl) atoms surrounding it.
- Phosphorus has 5 valence electrons, and each chlorine has 1 valence electron.

2. **Calculate the Steric Number**:
- The steric number can be calculated using the formula:
\[ \text{Steric Number} = \frac{1}{2} \left( V + M + \text{(charge of anion)} - \text{(charge of cation)} \right) \]
- Here, \( V = 5 \) (valence electrons of P) and \( M = 5 \) (monovalent Cl atoms).
- Plugging in the values:
\[ \text{Steric Number} = \frac{1}{2} (5 + 5) = 5 \]

3. **Identify Hybridization and Geometry**:
- A steric number of 5 indicates \( sp^3d \) hybridization.
- The geometry of \( PCl_5 \) is trigonal bipyramidal.

4. **Understand Bonding in \( PCl_5 \)**:
- In \( PCl_5 \), there are three equatorial bonds and two axial bonds.
- The axial bonds are longer than the equatorial bonds due to steric hindrance.

5. **Effect of Heating**:
- Upon heating, the axial bonds, which are longer and thus weaker, break more easily than the equatorial bonds.
- This leads to the dissociation of \( PCl_5 \) into \( PCl_3 \) and \( Cl_2 \).

6. **Conclusion**:
- The dissociation occurs because the axial bonds are weaker and break upon heating, resulting in the formation of \( PCl_3 \) and \( Cl_2 \).

### Final Answer:
The reason \( PCl_5 \) dissociates to \( PCl_3 \) and \( Cl_2 \) upon heating is that the axial bonds are longer and weaker, making them easier to break.