A : Dihedral angle of `H_2O_2` in gas phase is greater than in solid phase.
R : `H_2O_2` has planar structure.

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 explanantion of the assertion, then mark (2)
C. If assertion is true statements but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to analyze the two statements provided:

**Assertion (A):** The dihedral angle of H₂O₂ in the gas phase is greater than in the solid phase.
**Reason (R):** H₂O₂ has a planar structure.

### Step-by-Step Solution:

1. **Understanding the Assertion (A):**
- The dihedral angle of H₂O₂ in the gas phase is indeed greater than in the solid phase.
- In the gas phase, the dihedral angle is approximately 111.5 degrees.
- In the solid phase, the dihedral angle decreases to about 90.2 degrees.
- This decrease in angle in the solid phase is due to the interactions and packing of molecules in the solid state, which restricts the angle.

2. **Understanding the Reason (R):**
- The reason states that H₂O₂ has a planar structure.
- However, this statement is incorrect. H₂O₂ does not have a planar structure; it has a non-planar, open book-like structure due to the presence of lone pairs on the oxygen atoms.
- The non-planarity is a result of the repulsion between the lone pairs of electrons on the oxygen atoms, which prevents the molecule from adopting a planar configuration.

3. **Conclusion:**
- Since the assertion (A) is true (the dihedral angle in gas phase is greater than in solid phase), but the reason (R) is false (H₂O₂ does not have a planar structure), we conclude that the correct option is that the assertion is true, and the reason is false.

### Final Answer:
The correct option is that the assertion is true, but the reason is false.

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