Intermolecular forces: types and examples

In a nutshell

Intermolecular forces hold molecules together. There are several different types of intermolecular forces that can hold molecules together.

Intermolecular forces

Definition

Intermolecular forces occur between molecules to keep them together. They are weaker than the types of bonding.

There are three types of intermolecular forces:

London forces
Permanent dipole-permanent dipole bonds
Hydrogen bonding

London forces

Definition

London forces are between all atoms and molecules, causing them to be attracted to one another. London forces are also know as instantaneous dipole-induced dipole bonds.

How do London forces work?

1.	Electrons moving quickly in charged clouds move to one side more than the other. A temporary (instantaneous) dipole will form.
2.	The instantaneous dipole induces an instantaneous dipole in the opposite direction on another atom.
3.	The two dipoles are attracted to one another.
4.	The second instantaneous dipole induces a dipole in another atom. This happens as a continuous cycle for each atom.

Note: Dipoles are being created and destroyed constantly due to the fast moving electrons. The atoms will constantly be attracted to one another throughout.

London forces are able to hold molecules in a lattice structure.

Permanent dipole-permanent dipole bonds

Definition

Permanent dipole-permanent dipole bonds are result of weak electrostatic forces of attraction between the particle charges on atoms in a polar molecule.

These forces occur alongside London forces, therefore polar molecules with these interactions in addition to London forces will have a higher melting and boiling point.

Example

Hydrogen bromide is a polar molecule due to the polar hydrogen - bromine bonds. Therefore, electrostatic forces will form between each individual molecule forming permanent dipole-permanent dipole bonds.

Chemistry; Bonding and structure; KS5 Year 12; Intermolecular forces: types and examples

Note: London forces and permanent dipole-permanent dipole bonds are both types of van der Waals forces.

Melting and boiling points

Large molecules will have larger electron clouds, therefore stronger London forces will form. Molecules with a larger surface area will have a bigger exposed electron cloud, forming stronger London forces. More energy is needed to break these stronger forces, so the melting and boiling point will be higher.

Example

Alkanes contain London forces. The longer the carbon chain the stronger the London forces, due to more surface contact and more electron interaction. More energy is needed to break these forces with increased strength, therefore the melting and boiling point will increase as the chain length increases.

Branched alkanes have a smaller surface area compared to straight chain alkanes, therefore are left with less London forces and a lower melting and boiling point.