MODULE 2 - Intermolecular forces

An intramolecular force - Any force that holds together the atoms making up a molecule or compound.

examples of intramolecular forces are:

- Ionic bonds which hold ions together through the strong electrostatic attraction between oppositely charged ions

- Convalent bonds which hold atoms together through the sharing of electrons. 

An Intermolecular forces - The forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions). These are weak compared to the intramolecular forces, the forces which keep a molecule together.

There are three types of intermolecular forces (ordered by strength):

- Hydrogen bonding 

- Permanent dipole-dipole interactions

- Van der waals' forces (induced dipole forces)

Permanent dipole-dipole interactions - only between polar molecules

- A weak attractive force between permanent dipoles in neighbouring polar molecules.

These forces occur when the permanent dipole of one molecule attracts the permanent dipole of another, this is because the slightly negative part of the molecule becomes electrostatically attracted to the slightly positive part of a neighbouring molecule. 

Van der waals' forces (induced dipole-dipole interactions)

These are the weakest forces of attraction are present between all molecules wether they are polar or non-polar. 

They are formed when eletrons move around and create uneven distributions of electrons inside molecules or atoms.   

Electrons in an atom are always moving, therefore at any given time, the majority of them may be found at one pole of the atom, causing it to become slightly negative, whilst the other side becomes slightly positive (an instantaneous dipole is formed).

As this atom moves near another atom, the slightly negative side of the atom begins to repel the neighbouring atom's electron, therefore inducing a tempory dipole in the neighbouring atom. The slightly positive sides of the dipoles in the neighbouring atoms are then weakly electrostatically attracted to one another by what is known as a Van der waals' force.

Factors which affect the strength of van der waals' forces

- As the amount of electrons increase, the more uneven the distribution would be therefore there will be a larger induced dipole in neighbouring molecules creating a stronger van der waals force.

Hydrogen bonding 

A hydrogen bond is the electrostatic attraction between polar molecules that occurs when an electron deficient hydrogen atom of a molecule experiences attraction to a lone pair of electrons on a highly electronegative neighbouring atom of a different molecule.

Hydrogen bonds are usually formed between molecules where hydrogen is bonded with nitrogen, oxygen or flourine, this is due to their large difference in electronegativity.