To understand why `PCl_3` and `PCl_5` both exist while only `PH_3` exists and `PH_5` does not, we need to analyze the hybridization and the factors influencing the stability of these compounds.
### Step-by-Step Solution:
1. **Understanding Hybridization**:
- Hybridization is the mixing of atomic orbitals to form new hybrid orbitals that can form bonds. The type of hybridization depends on the number of bonds and the types of atoms involved.
2. **Hybridization of `PCl_3` and `PH_3`**:
- For `PCl_3`, phosphorus has a valency of 5 and forms three bonds with chlorine. The hybridization is `sp³`, which is suitable for forming three equivalent bonds.
- For `PH_3`, phosphorus also forms three bonds with hydrogen, and the hybridization is similarly `sp³`. The comparable sizes and energies of the orbitals involved (s and p orbitals) allow for effective overlap, making both `PCl_3` and `PH_3` stable.
3. **Hybridization of `PCl_5` and `PH_5`**:
- For `PCl_5`, phosphorus forms five bonds with chlorine, leading to `sp³d` hybridization. The presence of chlorine, which is more electronegative than phosphorus, causes a significant d-orbital contraction due to the increased effective nuclear charge on phosphorus. This contraction allows for effective overlap and stabilizes `PCl_5`.
- In contrast, for `PH_5`, if it were to exist, it would also require `sp³d` hybridization. However, hydrogen is less electronegative than phosphorus, meaning that the effective nuclear charge on phosphorus does not increase significantly. Therefore, there is no d-orbital contraction, and the d-orbitals remain larger than the s and p orbitals, making it impossible for `sp³d` hybridization to occur.
4. **Conclusion**:
- The existence of `PCl_5` is facilitated by the electronegativity of chlorine, which allows for d-orbital contraction and effective hybridization. In contrast, `PH_5` cannot exist because the lack of electronegativity of hydrogen prevents d-orbital contraction, making `sp³d` hybridization impossible.
### Final Answer:
`PCl_3` and `PCl_5` both exist due to effective hybridization facilitated by electronegative chlorine, while only `PH_3` exists because `PH_5` cannot stabilize its hybridization due to the lack of d-orbital contraction from hydrogen.
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