Assertion : Formation of `PH_(4)^(+)` ion is relatively difficult in comparison to `NH_(4)^(+)` ion.
Reason : Lone pair of phosphorus atom in `PH_(3)` resides in stereochemically inactive pure s-orbital.

Options

A. If assertion is true but the reason is false
B. If assertion is false but reason is true
C. If both assertion and reason are true and the reason is the correct explanation of assertion
D. If both assertion and reason are true and the reason is not the correct explanation of assertion

Answer

To solve the question regarding the assertion and reason about the formation of \( PH_4^+ \) and \( NH_4^+ \), we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the formation of \( PH_4^+ \) is relatively difficult compared to \( NH_4^+ \).
- To analyze this, we need to consider the basicity of \( PH_3 \) and \( NH_3 \). \( NH_3 \) is a stronger base than \( PH_3 \), which implies that \( PH_4^+ \) is harder to form because it is less likely to donate its lone pair to form a bond with \( H^+ \).

**Hint**: Compare the basicity of \( NH_3 \) and \( PH_3 \) to understand the assertion.

2. **Understanding the Reason**:
- The reason provided is that the lone pair of the phosphorus atom in \( PH_3 \) resides in a stereochemically inactive pure s-orbital.
- In \( PH_3 \), the lone pair is in the 3s orbital, which is not hybridized, leading to a higher percentage of s-character (100% s-character). This results in lower electronegativity and poorer ability to donate the lone pair compared to \( NH_3 \), where the lone pair is in an sp³ hybridized orbital (25% s-character).

**Hint**: Analyze the hybridization and s-character of the lone pairs in \( NH_3 \) and \( PH_3 \).

3. **Connecting Assertion and Reason**:
- Since \( PH_3 \) has a lone pair in a pure s-orbital, it is less effective at donating that lone pair to form \( PH_4^+ \). Thus, the assertion that \( PH_4^+ \) is harder to form is supported by the reason given.
- Both the assertion and reason are true, and the reason correctly explains the assertion.

**Hint**: Consider how the nature of the orbital affects the basicity and formation of the ions.

4. **Conclusion**:
- Therefore, both the assertion and the reason are true, and the reason is a correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is a correct explanation for the assertion.