The Van der waals' forces are dominant intermolecular forces in_______?

Analyze the question: We need the substance in which Van der Waals' forces (weak intermolecular forces) are the main forces holding the particles together.

Concept:

• Ionic compounds (like NH₄Cl and NaCl) are held by strong electrostatic attraction between ions. These are ionic bonds, not Van der Waals' forces.
• Water has O-H bonds that are very polar, so its molecules are held mainly by hydrogen bonding, which is much stronger than ordinary Van der Waals' forces.
• Chlorine (Cl₂) is a non-polar, simple covalent molecule. There is no permanent dipole and no hydrogen bonding, so the only forces between its molecules are weak Van der Waals' (London dispersion) forces.

Step-by-step check:

• A. Ammonium chloride: ionic bonding (NH₄⁺ and Cl⁻), with covalent bonds inside the ammonium ion. Not correct.
• B. Chlorine: non-polar diatomic molecule, so Van der Waals' forces dominate. Correct.
• C. Sodium chloride: ionic lattice. Not correct.
• D. Water: hydrogen bonding dominates. Not correct.

Exam tip: Match the substance to its dominant force:

• Metal + non-metal → ionic bonding
• Molecules containing H bonded to N, O or F → hydrogen bonding
• Non-polar molecules such as Cl₂, O₂, N₂, I₂ and the noble gases → Van der Waals' forces

This is why chlorine is a gas at room temperature and iodine easily sublimes: the forces between their molecules are weak.