A : `PH_(5)` is not possible .
[R] : `-5` oxidation state of phosphorus is not possible .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion , then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

To solve the assertion-reason question regarding the compound \( PH_5 \) and the oxidation state of phosphorus, we can follow these steps:

### Step 1: Understand the Assertion
The assertion states that \( PH_5 \) is not possible. We need to analyze why this is the case.

### Step 2: Determine the Oxidation State of Phosphorus in \( PH_5 \)
In \( PH_5 \):
- Let the oxidation state of phosphorus be \( x \).
- Each hydrogen atom has an oxidation state of \( +1 \).
- Since there are 5 hydrogen atoms, their total contribution to the charge is \( +5 \).
- The overall charge of the compound is neutral (0).

Using the equation:
\[ x + 5(+1) = 0 \]
This simplifies to:
\[ x + 5 = 0 \]
Thus, solving for \( x \):
\[ x = -5 \]

### Step 3: Analyze the Oxidation State
The calculated oxidation state of phosphorus in \( PH_5 \) is \( -5 \). We need to determine if this oxidation state is possible for phosphorus.

### Step 4: Consider the Properties of Phosphorus
Phosphorus is a non-metal and typically exhibits oxidation states ranging from \( -3 \) to \( +5 \). The oxidation state of \( -5 \) is not observed in phosphorus due to its inability to accommodate such a high negative charge. Therefore, phosphorus cannot exist in the \( -5 \) oxidation state.

### Step 5: Conclusion
Since the oxidation state of phosphorus in \( PH_5 \) is \( -5 \), which is not possible, the assertion that \( PH_5 \) is not possible is correct. Additionally, the reason provided, that the \( -5 \) oxidation state of phosphorus is not possible, is also correct and explains the assertion.

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

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