van der Waals Forces & Ion-Dipole Interactions

1. Weak chemical forces — why they matter

Atoms and molecules in liquids and solids are held together by weak intermolecular forces. These forces explain boiling points, solubility, the boiling of water, DNA base-pairing and protein folding. They are much weaker than covalent (300–600 kJ/mol) and ionic bonds, ranging from a few kJ/mol to about 40 kJ/mol.

2. London dispersion forces

A temporary fluctuation in electron distribution creates an instantaneous dipole in one atom, which induces a dipole in a neighbour. The attraction between the instantaneous and induced dipoles is the dispersion (London) force. It is the only force between non-polar molecules (e.g., noble gases, CH₄, I₂) and increases with molecular size and surface area.

3. Dipole–dipole interactions

Between polar molecules, the permanent δ+ end of one molecule attracts the δ− end of another. These forces are stronger than dispersion forces and explain why polar compounds have higher boiling points than non-polar compounds of similar molar mass.

4. Ion–dipole interactions

When an ion (e.g., Na⁺) approaches a polar molecule (e.g., H₂O), the ion attracts the δ− end of the dipole. These strong interactions explain the dissolving of salts in water (hydration of ions).

5. Induced dipole interaction

A permanent dipole can distort the electron cloud of a neighbouring non-polar molecule, inducing a temporary dipole. Example: O₂ and N₂ become slightly soluble in water through such induced interactions.

Weak forces = dispersion (instantaneous-induced dipoles), dipole-dipole, ion-dipole, induced-dipole. H-bonding is an especially strong dipole-dipole case.

• Dispersion: non-polar molecules, increases with size.
• Dipole-dipole: polar molecules, dominant between H₂S, HCl etc.
• Ion-dipole: solutions of electrolytes in water.
• Hydrogen bond: strongest common weak force in biological water systems.