Which of the following oxides exists as trigonal planar molecule in gaseous state and a cyclic trimer in the solid state ?

Options

A. `SO_2`
B. `SeCO_2`
C. `SeO_3`
D. `SO_3`

Answer

To solve the question, we need to identify which oxide exists as a trigonal planar molecule in the gaseous state and as a cyclic trimer in the solid state. Let's analyze each of the given options step by step.

### Step 1: Analyze SO2 (Sulfur Dioxide)
- **Gaseous State**: SO2 has a bent molecular geometry due to the presence of a lone pair on sulfur, resulting in an angle of approximately 120 degrees. Therefore, it does not have a trigonal planar structure.
- **Solid State**: In the solid state, SO2 does not form a cyclic trimer; it typically exists as a molecular solid.

**Conclusion**: SO2 does not fit the criteria.

### Step 2: Analyze SeCO2 (Selenium Dioxide)
- **Gaseous State**: Similar to SO2, SeCO2 also has a bent structure due to the presence of lone pairs. It does not exhibit trigonal planar geometry.
- **Solid State**: SeCO2 does not form a cyclic trimer in the solid state.

**Conclusion**: SeCO2 does not fit the criteria.

### Step 3: Analyze SeO3 (Selenium Trioxide)
- **Gaseous State**: SeO3 is a trigonal planar molecule in the gaseous state, with bond angles of 120 degrees.
- **Solid State**: In the solid state, SeO3 does not form a cyclic trimer; it typically exists in a different arrangement.

**Conclusion**: SeO3 does not fit the criteria.

### Step 4: Analyze SO3 (Sulfur Trioxide)
- **Gaseous State**: SO3 has a trigonal planar structure in the gaseous state, with bond angles of 120 degrees.
- **Solid State**: In the solid state, SO3 forms a cyclic trimer (S3O9), which consists of three SO3 units bonded together in a ring structure.

**Conclusion**: SO3 fits both criteria.

### Final Answer
The oxide that exists as a trigonal planar molecule in the gaseous state and as a cyclic trimer in the solid state is **SO3 (Sulfur Trioxide)**.

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