Which of the following is an example of covalent crystal solid?

Options

A. Si
B. NaF
C. Al
D. Ar

Explanation

To determine which of the given options is an example of a covalent crystal solid, we can analyze each option based on their bonding characteristics.

### Step-by-Step Solution:

1. **Understanding Covalent Crystal Solids**:
- Covalent crystal solids are characterized by a three-dimensional network of covalent bonds. In these solids, atoms are held together by strong covalent bonds, resulting in high melting and boiling points.

2. **Analyzing the Options**:
- **Option A: Silicon (Si)**:
- Silicon forms a covalent network solid where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement. This structure is characteristic of covalent crystals.
- **Conclusion**: Silicon is a covalent crystal solid.

- **Option B: Sodium Fluoride (NaF)**:
- Sodium fluoride is composed of sodium ions (Na⁺) and fluoride ions (F⁻) held together by ionic bonds. This structure is typical of ionic solids.
- **Conclusion**: NaF is not a covalent crystal solid; it is an ionic solid.

- **Option C: Aluminum (Al)**:
- Aluminum is a metallic solid. It consists of positively charged aluminum ions surrounded by a sea of delocalized electrons, which is characteristic of metallic bonding.
- **Conclusion**: Aluminum is not a covalent crystal solid; it is a metallic solid.

- **Option D: Argon (Ar)**:
- Argon is a noble gas and exists as individual atoms. The forces holding argon atoms together in a solid state are weak van der Waals (London dispersion) forces, not covalent bonds.
- **Conclusion**: Argon is not a covalent crystal solid; it is a noble gas solid.

3. **Final Answer**:
- Among the options provided, the only example of a covalent crystal solid is **Silicon (Si)**.

### Answer:
**Silicon (Si)** is an example of a covalent crystal solid.