Intermolecular Forces

Molecular crystals, as mentioned above, and most liquids (except those that are molten forms of
ionic solids) are held together by intermolecular attractions that are generally weaker than ionic
and covalent interactions. These attractive forces are electrostatic in nature, and affect the atoms or
molecules even though they are neutral because of the asymmetric distribution of charge density.
They can be roughly categorized into three types: dipole-dipole interactions, London dispersion
forces, and hydrogen bonding. These intermolecular forces are sometimes referred to collectively as
van der Waals forces.

BASIC CONCEPT

From    strongest   to  weakest:

DIPOLE-DIPOLE INTERACTIONS

Polar molecules tend to orient themselves so that the positive region of one molecule is close to the
negative region of another molecule. This arrangement is energetically favorable because of the
electrostatic attraction between unlike charges. The magnitude of this kind of interaction increases
with increasing polarity of the molecules.

Dipole-dipole interactions are present in the solid and liquid phases when the constituent molecules
are polar, but often become negligible in the gas phase because the molecules are generally much
farther apart. Polar species tend to have higher boiling points than nonpolar species of comparable
molecular weight.

Hydrogen    bonding

Dipole-dipole   interactions

Dispersion  (London)    forces