### Intermolecular Forces

Molecules are made of positively charged nuclei and negatively
charges electrons, therefore, it is useful to think of many of the
interactions between molecules as electronic in nature. The
electric field intensity is given by:

$\vec E = \frac{\vec F}{Q} = \frac{-\vec \nabla
\Gamma}{Q}$

where $\vec \nabla$ denotes a spatial derivative, and
$\Gamma$ is the molecular potential energy.

The electrostatic intermolecular interactions that we will
consider are:

- Point charges: between ions, or in plasmas
- Permanent Dipoles
- Induced Dipoles
- Dispersion Forces

##### DEFINITION

Interactions between **point charges** are the
simplest kind of electrostatic interactions. Here we assume that
all of the charge of a molecule is concentrated at a point.

The interaction force/potential between point charges is given
as:

$F_{12} \propto \frac{Q_1Q_2}{r^2}$

$\Gamma_{12} \propto \frac{Q_1Q_2}{r}$

##### Note:

If the charges are the same, then the force is positive
(repulsive).