Assertion (A) : `CO_2` is a gas but `SiO_2` had a three -dimenstional network structurte .
Reason (R): `CO_2` bonds and `SiO_2` has a three-dimensional network structure .

Options

A. If both (A) and (R) are correct and (R) is correct explanation of (A)
B. If both (A) and (R) are correct and (R) is not correct explanation of (A)
C. If (A) is correct , but (R) is incorrect
D. If (A) is incorrect but (R) is correct .

Answer

To solve the assertion and reason question, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
The assertion states: "CO₂ is a gas but SiO₂ has a three-dimensional network structure."

- **CO₂ (Carbon Dioxide)**: At room temperature and pressure, CO₂ exists as a gas. It is a simple molecular compound consisting of one carbon atom double-bonded to two oxygen atoms. In solid form (dry ice), it has a different structure, but typically, it is found as a gas.

- **SiO₂ (Silicon Dioxide)**: SiO₂ has a three-dimensional network structure. It is commonly found in nature as quartz and has a tetrahedral arrangement of silicon and oxygen atoms. This structure gives it a high melting point and hardness.

**Conclusion for Step 1**: The assertion is true because CO₂ is indeed a gas, and SiO₂ does have a three-dimensional network structure.

### Step 2: Analyze the Reason
The reason states: "CO₂ bonds and SiO₂ has a three-dimensional network structure."

- **CO₂ Bonds**: CO₂ has a linear structure due to sp hybridization of carbon, where the carbon atom forms double bonds with two oxygen atoms. This means that the bonding in CO₂ is not indicative of a three-dimensional network.

- **SiO₂ Structure**: As mentioned, SiO₂ has a three-dimensional network structure, where each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement.

**Conclusion for Step 2**: The reason is incorrect because it does not accurately describe the bonding in CO₂. It fails to mention that CO₂ has a linear structure instead of a three-dimensional network.

### Final Conclusion
- **Assertion (A)**: True
- **Reason (R)**: False

Thus, the correct answer is that the assertion is correct, but the reason is incorrect.