Intermolecular force

Intermolecular forces are electromagnetic forces which act between molecules or between widely separated regions of a macromolecule. Listed in order of decreasing strength, these forces are:

Ionic interactions
Hydrogen bonds
Dipole-dipole interactions
London Dispersion Forces

Ionic Interactions

This is an interactions that occurs between charged species, like charges will have a repulsive force while opposite charges will have an attractive force.

Hydrogen Bonding

hydrogen bonding can be formed when a hydrogen atom is bound to a highly electronegative atom such as nitrogen, oxygen, or fluorine. The hydrogen atom has a partial positive charge and can interacts with another electronegative atoms found in a different molecule (again N, O, or F). This results in a stabilizing interaction that loosely binds the two molecules together. A common example is water:

H₂O---H-O-H

Hydrogen bonds are found throughout nature. Hydrogen bonds give water its unique properties, which allowed life to develop on earth; hydrogen bonds are also the intermolecular force that binds together the two strands in a molecule of DNA

Dipole-Dipole Interactions

Dipole-dipole interactions, also called Keesom interactions after Willem Hendrik Keesom who produced the first mathematical description in 1921, are the forces that occur between two molecules with permanent dipoles. These work in a similar manner to ionic interactions, but are weaker because only partial charges are involved. An example of this can be seen in hydrochloric acid:

(+)(-) (+) (-)
H-Cl-----H-Cl

London Dispersion Forces

Also called London forces or Van der Waals forces, these involve the attraction between temporaily induced dipoles in nonpolar molecules. This polarization can be induced either by a polar molecule or by the repulsion of negatively charged electron clouds in nonpolar molecules. An example of the former is chlorine dissolving in water:

                 (+)(-)(+)  (-) (+)
[Permanent Dipole] H-O-H-----Cl-Cl [Induced Dipole]

An example of the second scenario is found in molecular chlorine:

               (+) (-)    (+) (-)
[Induced Dipole]Cl-Cl------Cl-Cl [Induced Dipole]

HI

See also: polymer