Which is the weakest type of cohesive force?

• A. Dipole-dipole interactions
• B. Hydrogen bonding
• C. London dispersion forces
• D. Ionic bonds

Answer: (C)

London dispersion forces are the weakest among these because they arise from temporary fluctuations in electron density that create brief, induced dipoles. All molecules experience them, but they are especially weak in small or light molecules. Their strength grows with molecular size and polarizability, since larger electron clouds produce bigger transient dipoles. In contrast, dipole–dipole interactions involve permanent or more persistent partial charges and are stronger; hydrogen bonding is an especially strong dipole interaction when hydrogen is bonded to highly electronegative atoms and can strongly attract lone pairs on another molecule; ionic bonds are much stronger still, reflecting full electrostatic attraction between ions. So the weakest cohesive force here is London dispersion forces.