Which of the following exists as covalent crystals in the solid state?

Options

A. Silicon
B. Sulphur
C. Phosphorous
D. Iodine

Answer

To determine which of the following substances exists as covalent crystals in the solid state, we first need to understand the characteristics of covalent crystals.

### Step-by-Step Solution:

1. **Understanding Solid State Classifications**:
- Solids can be classified into different categories based on their bonding types. The main categories include:
- Ionic solids
- Molecular solids
- Covalent solids
- Metallic solids

2. **Identifying Covalent Crystals**:
- Covalent crystals, also known as network solids, consist of atoms connected by covalent bonds in a continuous network. This results in a solid with high melting and boiling points, and they are typically hard and brittle.

3. **Examples of Covalent Crystals**:
- Common examples of covalent crystals include:
- Silicon (Si)
- Diamond (a form of carbon)
- Graphite (another form of carbon, but with different properties)
- These materials have a strong covalent bond structure throughout the crystal lattice.

4. **Analyzing the Options**:
- If given a list of substances, we need to identify which one is a covalent crystal. For instance, if the options include:
- Silicon (Si)
- Sodium chloride (NaCl)
- Iodine (I2)
- Phosphorus (P)
- Among these, Silicon is the only one that forms a covalent crystal due to its network structure.

5. **Conclusion**:
- Therefore, the substance that exists as a covalent crystal in the solid state is **Silicon (Si)**.