3.6.3—Van der Waals’ forces as the intermolecular

Van der Waals’ forces are intermolecular attractions between molecular entities. The two assessed types are instantaneous dipole–induced dipole (London dispersion) forces and permanent dipole–permanent dipole forces; hydrogen bonding is a special permanent-dipole case and is treated separately for the detailed water consequences.

In every atom or molecule, electron motion can momentarily make the charge cloud uneven. The temporary dipole then induces an opposite dipole in a neighbour, so δ+ and δ− ends attract. These forces generally become stronger as electron number increases and as molecules have more close-contact positions.

A polar molecule has a lasting charge separation, so neighbouring molecules can align opposite partial charges and form permanent dipole–permanent dipole attractions. For small molecules with the same electron count, this interaction is stronger than London dispersion alone; for example, propanone has stronger intermolecular attraction and a higher boiling point than non-polar butane.

Keep the scale clear: covalent bonds are intramolecular, whereas van der Waals’ forces act between molecules. Do not call every intermolecular force a hydrogen bond, treat an instantaneous dipole as permanent, or compare force types without considering the stated electron count and molecular contact.