Van der Waals attraction

Van der Waals forces are weak attractive forces that exist between molecules. These forces arise due to temporary fluctuations in electron distribution within the molecules. They are named after the Dutch scientist Johannes Diderik van der Waals, who first described them.

Crystal Structure of Covalent Molecules

Covalent molecules are composed of atoms that are held together by covalent bonds, which involve the sharing of electron pairs between atoms. In a crystal structure, the covalent molecules are arranged in a regular, repeating pattern.

Explanation

The correct answer is option 'A', Van der Waals attraction. This is because covalent molecules are usually held in a crystal structure by weak Van der Waals forces.

- Van der Waals Forces: Van der Waals forces are attractive forces that arise due to temporary fluctuations in electron distribution within the molecules. There are three types of Van der Waals forces: dispersion forces, dipole-dipole forces, and hydrogen bonding.
- Dispersion Forces: Dispersion forces, also known as London forces, are the weakest type of Van der Waals forces. They arise due to temporary imbalances in electron distribution that create temporary dipoles in molecules. These temporary dipoles induce dipoles in neighboring molecules, resulting in an attractive force between the molecules.
- Dipole-Dipole Forces: Dipole-dipole forces occur between polar molecules. A polar molecule has a permanent dipole moment due to the presence of polar bonds or an asymmetrical distribution of electron density. The positive end of one polar molecule is attracted to the negative end of another polar molecule, resulting in a dipole-dipole attraction.
- Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to an electronegative atom (such as nitrogen, oxygen, or fluorine) and is attracted to another electronegative atom in a different molecule. Hydrogen bonding is stronger than regular dipole-dipole forces.
- Electrostatic Attraction: Electrostatic attraction refers to the attraction between positively and negatively charged particles. While covalent molecules do contain polar bonds, the overall charge of the molecule is usually neutral, so the electrostatic attraction between molecules is not a significant factor in holding them in a crystal structure.

Therefore, the most significant force holding covalent molecules in a crystal structure is the weak Van der Waals attraction, specifically the dispersion forces. These forces are relatively weak compared to ionic or covalent bonds, but they are sufficient to maintain the regular arrangement of covalent molecules in a crystal lattice.