Covalent molecules are usually held in a crystal structure by

Options

A. dipole-dipole interactions
B. electrostatic attraction
C. hydrogen bonds
D. van der Waals' attraction.

Explanation

To determine how covalent molecules are held in a crystal structure, we can analyze the options provided:

1. **Understanding Covalent Molecules**:
- Covalent molecules are formed when non-metal atoms share electrons, resulting in covalent bonds. These bonds create a continuous network of atoms throughout the crystal, forming a three-dimensional structure.

2. **Evaluating the Options**:
- **Option 1: Dipole-Dipole Interaction**:
- Dipole-dipole interactions occur between polar molecules, where there is a permanent dipole due to the unequal sharing of electrons. Examples include HCl and SO2. However, covalent molecules can be non-polar, and thus dipole-dipole interactions are not the primary forces holding covalent crystals together.
- **Option 2: Electrostatic Interaction**:
- Electrostatic interactions are characteristic of ionic compounds, where positive and negative ions attract each other. This is not applicable to covalent molecules, which do not consist of ions.
- **Option 3: Hydrogen Bond**:
- Hydrogen bonds occur between a hydrogen atom covalently bonded to a highly electronegative atom (like O, N, or F) and another electronegative atom. While hydrogen bonding is significant in some molecular crystals (like water), it is not the primary interaction in all covalent crystals.
- **Option 4: Van der Waals Attraction**:
- Van der Waals forces are weak attractions that occur between all molecules, including covalent ones. These forces can play a role in holding covalent crystals together, especially in non-polar covalent substances.

3. **Conclusion**:
- The primary interactions holding covalent molecules in a crystal structure are Van der Waals attractions. Therefore, the correct answer to the question is **Option 4: Van der Waals attraction**.

### Step-by-Step Solution:
1. **Identify what covalent molecules are**: They are formed by non-metal atoms sharing electrons, resulting in a network of covalent bonds.
2. **Analyze the first option (Dipole-Dipole Interaction)**: This is not applicable to all covalent molecules, as it only occurs in polar molecules.
3. **Analyze the second option (Electrostatic Interaction)**: This is characteristic of ionic compounds, not covalent molecules.
4. **Analyze the third option (Hydrogen Bond)**: While important in some cases, it is not the primary interaction in all covalent crystals.
5. **Analyze the fourth option (Van der Waals Attraction)**: This is a general interaction present in all molecular substances, including covalent crystals.
6. **Conclude that the correct answer is Option 4: Van der Waals attraction**.