Dipole-dipole attraction

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

These are interactions that occur between charged species. Like charges repel, while opposite charges attract.

Hydrogen Bonding

Hydrogen bonding occurs 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 interact with another electronegative atom 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. They give water its unique properties, which allowed life to develop on earth. Hydrogen bonds are also the intermolecular force that bind 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 temporarily 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:

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

See also: polymer