Assertion : Catenation tendency is weaker in nitrogen.
Reason : Nitrogen exists as diatomic gas.

Options

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

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Catenation tendency is weaker in nitrogen."

- **Catenation** refers to the ability of an element to form chains or rings by bonding with itself.
- In the context of p-block elements, catenation is observed in elements like carbon, which can form long chains due to its ability to bond with itself.
- Nitrogen, being a second-period element, has a weaker catenation tendency compared to carbon and even phosphorus. This is because nitrogen primarily forms diatomic molecules (N₂) rather than chains of nitrogen atoms.

**Conclusion for Step 1**: The assertion is true.

### Step 2: Analyze the Reason
The reason states that "Nitrogen exists as a diatomic gas."

- Nitrogen does exist as a diatomic molecule (N₂) in its most stable form, which is indeed a true statement.
- However, the existence of nitrogen as a diatomic gas does not directly explain why its catenation tendency is weaker. Catenation is more related to the ability of an element to bond with itself in multiple ways, which is not significantly influenced by its diatomic nature.

**Conclusion for Step 2**: The reason is true, but it does not correctly explain the assertion.

### Final Conclusion
- The assertion is true: Catenation tendency is weaker in nitrogen.
- The reason is true, but it does not provide a correct explanation for the assertion.

Thus, the correct answer is: **Option B** - Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

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