Intermolecular Forces

About this topic

Intermolecular forces act between molecules and determine physical properties like boiling point. They include London dispersion forces (in all molecules, from temporary dipoles), dipole–dipole forces (between polar molecules) and hydrogen bonds (the strongest, involving H bonded to N, O or F).

These forces are far weaker than covalent bonds, which is why molecules stay intact when a liquid boils. Dispersion forces grow with the number of electrons, so larger molecules have higher boiling points: methane boils at minus 162 degrees Celsius and octane at 126. Polar molecules add dipole attractions. A substance with stronger intermolecular forces has higher melting and boiling points, higher viscosity and surface tension, and lower vapour pressure.

Compare propane (nonpolar, boils at minus 42) with ethanol (hydrogen bonding, boils at 78), though they have similar masses. The geckos that climb walls use dispersion forces from millions of tiny hairs. Water drops form beads because of surface tension from these attractions.

Students often confuse intermolecular forces with the bonds inside molecules. When water boils, the O–H bonds do not break; the molecules separate. Remember the order of strength: dispersion is weakest, then dipole–dipole, then hydrogen bonding, though dispersion can win in very large molecules.