`PCl_(5)` on heating dissociates to `PCl _(3) and Cl _(3)` 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 understand why \( PCl_5 \) dissociates into \( PCl_3 \) and \( Cl_2 \) upon heating, we can analyze the molecular structure and bonding involved.

### Step-by-Step Solution:

1. **Identify the Structure of \( PCl_5 \)**:
- \( PCl_5 \) has a phosphorus atom at the center surrounded by five chlorine atoms.
- The phosphorus atom has five valence electrons and forms five bonds with chlorine atoms.

2. **Determine the Hybridization**:
- The hybridization of phosphorus in \( PCl_5 \) can be determined by the number of bonding pairs.
- Phosphorus uses \( sp^3d \) hybridization to accommodate five bonding pairs (one for each Cl atom).

3. **Molecular Geometry**:
- The molecular geometry of \( PCl_5 \) is trigonal bipyramidal.
- This means there are three chlorine atoms in a plane (equatorial positions) and two chlorine atoms above and below this plane (axial positions).

4. **Bonding Characteristics**:
- The bonds in \( PCl_5 \) are polar covalent due to the difference in electronegativity between phosphorus and chlorine.
- However, the overall molecule is non-polar due to its symmetrical geometry.

5. **Thermal Stability**:
- Upon heating, \( PCl_5 \) can dissociate into \( PCl_3 \) and \( Cl_2 \).
- This is because the bond energies of \( P-Cl \) in \( PCl_5 \) are weaker compared to the energy released when forming \( Cl_2 \) (a diatomic molecule).

6. **Dissociation Reaction**:
- The dissociation can be represented as:
\[ PCl_5 \rightarrow PCl_3 + Cl_2 \]
- This reaction is favored at higher temperatures as it leads to a more stable configuration (lower energy state).

### Conclusion:
The dissociation of \( PCl_5 \) into \( PCl_3 \) and \( Cl_2 \) upon heating is due to the thermal instability of \( PCl_5 \) and the formation of more stable products.