For which pair of allotropes is one a molecular solid and the other a network covalent solid?

Options

A. Calcium fluoride
B. Silicon dioxide
C. Carbon dioxide
D. Sodium chloride

Answer

To determine which pair of allotropes consists of one molecular solid and the other a network covalent solid, we can analyze the given options step by step.

### Step 1: Understand the Definitions
- **Molecular Solid**: A solid composed of molecules held together by intermolecular forces (e.g., van der Waals forces, hydrogen bonds). Examples include ice (solid water) and carbon dioxide (dry ice).
- **Network Covalent Solid**: A solid where atoms are bonded covalently in a continuous network, resulting in very high melting points and hardness. Examples include diamond (a form of carbon) and silicon dioxide (quartz).

### Step 2: Analyze the Options
1. **Calcium Fluoride (CaF2)**:
- This is an ionic compound, not a molecular solid or a network covalent solid. Therefore, it does not fit the criteria.

2. **Silicon Dioxide (SiO2)**:
- Silicon dioxide is a network covalent solid. It forms a strong three-dimensional network of silicon and oxygen atoms. However, it does not pair with a molecular solid.

3. **Carbon Dioxide (CO2)**:
- Carbon dioxide is a molecular solid when it is in its solid state (dry ice). It consists of discrete CO2 molecules held together by intermolecular forces.

4. **Sodium Fluoride (NaF)**:
- Sodium fluoride is also an ionic compound, similar to calcium fluoride, and does not fit the criteria.

### Step 3: Identify the Correct Pair
From the analysis:
- **Carbon Dioxide (CO2)** is a molecular solid.
- **Silicon Dioxide (SiO2)** is a network covalent solid.

Thus, the correct answer is that the pair consisting of **Carbon Dioxide (molecular solid)** and **Silicon Dioxide (network covalent solid)** fits the criteria.

### Final Answer
The correct pair of allotropes is **Carbon Dioxide (molecular solid)** and **Silicon Dioxide (network covalent solid)**.

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