Assertion : `PH_(4)^(+)` ion is having tetrahedron geometry.
Reason : P-atom is unhybridised in `PH_(4)^(+)` ion.

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 but reason is not the correct explanation of assertion

Answer

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

### Step 1: Understanding the Assertion
The assertion states that the \( PH_4^+ \) ion has a tetrahedral geometry.

- **Explanation**: The \( PH_4^+ \) ion consists of a phosphorus atom bonded to four hydrogen atoms. In this ion, the phosphorus atom has a positive charge, which indicates that it has lost one electron. Phosphorus is in group 15 of the periodic table and normally has 5 valence electrons. However, due to the positive charge, it has only 4 valence electrons available for bonding.

### Step 2: Determining the Hybridization
To determine the geometry, we need to find the hybridization of the phosphorus atom in \( PH_4^+ \).

- **Calculation of Steric Number**: The steric number is calculated as the sum of the number of sigma bonds and lone pairs around the central atom.
- In \( PH_4^+ \), there are 4 hydrogen atoms bonded to phosphorus, and there are no lone pairs on the phosphorus atom.
- Therefore, the steric number = 4 (4 bond pairs + 0 lone pairs).

- **Hybridization**: The steric number of 4 corresponds to \( sp^3 \) hybridization.

### Step 3: Geometry Based on Hybridization
- **Geometry**: The \( sp^3 \) hybridization leads to a tetrahedral geometry. Thus, the assertion is correct.

### Step 4: Analyzing the Reason
The reason states that the phosphorus atom is unhybridized in \( PH_4^+ \).

- **Explanation**: Since we have established that the phosphorus atom undergoes \( sp^3 \) hybridization to form four equivalent \( P-H \) bonds, it cannot be unhybridized. The presence of hybridization is essential for the formation of the tetrahedral shape.

### Conclusion
- **Assertion**: True (The \( PH_4^+ \) ion has tetrahedral geometry).
- **Reason**: False (The phosphorus atom is hybridized, not unhybridized).

Thus, the correct answer is that the assertion is true, but the reason is false.

### Final Answer
- **Assertion**: True
- **Reason**: False
- **Conclusion**: The correct option is A (Assertion is true, Reason is false).