The stronger the intermolecular forces, the more energy it takes to overcome these interactions and to cause the substance to boil. Therefore, molecules with strong bonds have high boiling points, and molecules with weak bonds have lower boiling points.
The four types of intermolecular forces, listed from strongest to weakest, are ionic, hydrogen bonding, dipole-dipole interactions and Van Der Waals dispersion forces, or London forces.
Ionic forces are interactions between positively and negatively charged atoms or molecules, which attract and bond to each other like magnets. Hydrogen bonding occurs when molecules like oxygen, fluorine and nitrogen, which contain highly electronegative elements, bond to hydrogen. The electronegative atoms pull the hydrogen's electrons toward themselves, creating a partial positive charge on the hydrogen atom and a partial negative charge on the electronegative atom. These partial charges contribute to the strength of intermolecular forces. Dipole-dipole interactions occur when partial charges are created within a molecule that contains electronegative atoms, and the partial charges attract other molecules.
London forces are the weakest intermolecular force. These forces are temporary and occur when electrons are not evenly distributed around the nucleus of the atom. If there are more electrons on one side, a partial charge is created on the atom itself, allowing it to attract other charged atoms or molecules.