Intermolecular forces (or intermolecular attractions) are the attractive forces between neighbouring molecules. These forces are much weaker than covalent or ionic bonds.
There are three types of intermolecular forces: dispersion forces, permanent dipole-dipole forces and hydrogen bonds.
This post focuses on dispersion forces and permanent dipole-dipole forces.
Key Definition
Dipole – a difference in charge across a bond or molecule.
Dispersion Forces
Dispersion forces are also sometimes called induced dipole-dipole interactions or London forces. They are experienced by both polar and non polar molecules. The forces of attraction are small and are caused by temporary dipoles in neighbouring molecules.
What causes the temporary dipoles?
The electrons in a molecule are constantly moving around. This movement of electrons means that at any time more electrons could be at one side of the molecule than the other.
As electrons are negative in charge more electrons on one side of the molecule will give that side a partially negative charge. The side with fewer electrons will have a partially positive charge. This creates an instantaneous dipole across the molecule.
Remember the electrons are constantly moving, hence why this is only instantaneous.
The diagram below illustrates this and the partial charges are represented with a lowercase delta (𝛿) and a negative/positive sign:
How does this cause forces of attraction?
The instantaneous dipole of one molecule can induce a temporary dipole in a neighbouring molecule.
In the molecule in which an instantaneous dipole has occurred, the partially positive side of the molecule will attract the electrons in the neighbouring molecule. This gives the neighbouring molecule a dipole:
As the electrons in the molecules are constantly moving, the electrons in one molecule could move to the other side, however this will just reverse the polarity:
This can happen between multiple molecules, each inducing dipoles in their neighbouring molecules:
The induced dipoles cause the dispersion forces between the molecules that attract them to each other.
Permanent Dipole-Dipole Forces
Polar molecules have a permanent dipole caused by differences in the electronegativity of the atoms they are composed of. The dipole of one molecule can therefore attract the permanent dipole of another polar molecule.
This attraction is very weak and can be called a permanent dipole-dipole force (or dipole-dipole interaction).
For example, hydrogen chloride molecules (HCl) have a permanent dipole. The hydrogen atom in each molecule has a slightly positive charge and the chlorine atom in each molecule has a slightly negative charge.
This means that the hydrogen atom in one HCl molecule is slightly attracted to the chlorine atom in another HCl molecule. This is illustrated in the diagram below:
Remember as mentioned above, in addition to the permanent dipole-dipole attractions between the molecules, polar molecules still experience dispersion forces. This means that there is a greater attraction overall between polar molecules than non-polar molecules and so polar molecules have higher boiling points.
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