Intermolecular Forces of Attraction

Intermolecular forces are attractive forces between molecules.  This forces are responsible  for the non-ideal behavior of gases at high pressures and low temperatures.  This forces are also important in establishing  the form and behavior of matter.

What is the difference between intermolecular forces and intramolecular forces?  Intermolecular forces are forces that holds molecules together while intramolecular forces are hold atoms together in a molecule.  Intramolecular forces are the chemical bond, the covalent, ionic, and metallic.  When intermolecular forces is broken only change in phase occur but when intramolecular forces are broken chemical change occur.  With regards to the strength of these forces, generally intermolecular forces are weaker than intramolecular forces.

The different types of intermolecular forces are hydrogen bond, dipole-dipole, and London  dispersion forces (Dipole-dipole and London dispersion forces are also known as Van der Waals Forces).  This is after Joannes Van der Waals, who developed the equation for predicting the deviation of gases from ideal behavior.  All intermolecular forces are electrostatic which involve the attraction of positive and negative species like ionic bond but weaker than the ionic bond.

Hydogen Bond

Hydrogen bond is a special kind of dipole-dipole interaction.  This exist when hydrogen is attracted to highly electronegative atoms like N, O, F.  Example of this are the polar molecules of NH₃, H₂O and HF.

Above photo shows the hydrogen bond exist in water molecules.  This bond is quite stronger than dipole-dipole interaction due to some reasons.  One is that hydrogen is covalently bonded to high electronegative atoms which lead to special polar X - H bond.  Second is due to small size of hydrogen atom, the dipoles come close together and produce strong dipole-dipole interactions.  

Hydrogen bond are possible only with hydrogen containing compounds because all atoms other than H have inner-shell electrons to shield their nuclei from attraction by lone-pair electrons of the nearby atoms.  Only F, N and O only meet the hydrogen bonding formation due to their high electronegativity value.

Dipole -dipole interaction

Dipole-dipole forces are attractive forces that exist between polar molecules.  Polar molecules are molecules with dipole moments.  The larger the dipole moment the greater the force.  Below shows what happens during dipole-dipole interaction between molecules.  Molecules are arranged in a way that their positive pole is attracted to the negative pole of the other molecule.  

Example of molecules with dipole-dipole interactions are CO, HCl, PCl₃, HBr.

Dispersion forces

Dispersion forces are forces that exist between nonpolar molecules.  Examples of nonpolar molecules are the diatomic molecules, CO₂, CH₄.  This was proposed by Fritz London, a German-American physicist in 1930.  London recognized that the motion of electrons in an atom or molecule can create an instantaneous, or momentary, dipole moment. The strength of dispersion forces tends to increase with increasing atomic or molecular size. Below is an example of dispersion forces between Cl₂ molecules.