Intermolecular Forces

Intermolecular forces exist between independent particles, such as atoms, ions, or molecules. They can be forces of either attraction or repulsion. The amount of charge, how it is distributed, and the length of time that a charge distribution exists can affect the strength of intermolecular forces. And despite having variable force strengths, all intermolecular forces are considered weak compared to chemical bonds, or intramolecular forces. Chemical bonds are not only stronger; they are also more permanent. The energy costs involved in breaking chemical bonds are much higher than ones needed to overcome intermolecular forces.

There are five types of intermolecular forces: ion-ion forces, ion-dipole forces, ion-induced dipole/dipole-induced dipole forces, dipole-dipole forces, and London dispersion forces. Generally, we expect ion-ion forces to be the strongest, followed by ion-dipole, dipole-dipole, and then London dispersion forces. Of course, many exceptions to this hierarchy of strength exist.

In order to properly differentiate between these forces, it is important to know what must be present in order for each interaction to occur. Ion-ion forces only involve ions in mixtures of substances. Ion-ion forces can be either attractive (cation-anion) or repulsive (cation-cation/anion-anion) and the strength varies depending on charge and size. Ion-dipole forces occur in mixtures between ions and polar molecules. The anions gravitate toward positive regions of dipoles while the cations gravitate toward negative regions. With dipoles, the strength of the forces depends upon the polarity of the molecule (or charge magnitude) and how compact the molecule is. If a molecule is more compact, there is better access to the center of charge and stronger attraction to its neighbors.

Induced dipoles occur when nonpolar molecules come in the vicinity of polar or charged particles and become polar themselves. As an ion or dipole moves closer to the nonpolar molecule, a