Intermolecular Forces

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London Forces

An intermolecular force is the electrostatic attraction between positive and negative charges.

An intermolecular force occurs between dipoles on DIFFERENT MOLECULES.

A London force is an induced dipole-dipole force. Non-polar molecules form instantaneous dipoles due to random electron movement. The attraction between the oppositely charged dipoles is called a London force.

Factors affecting:

Number of electrons - The more there are, the stronger the London force

The bigger the atom, the stronger the London force

London forces are the weakest intermolecular force.

Examples of molecules with London forces are

CO2

H2

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Permanent dipole-dipole force

When a molecule has an overall dipole, slightly positive and slightly negative adjacent dipoles will attract. This type of bonding occurs between polar bonds as they have permanent dipoles.

When do stronger permanent dipole-dipole forces occur?

There is an increase in the electronegativity difference between covalently bonded atoms.

There is an increase in the number of polar groups on the molecule.

Again this type of force occurs between molecules; they are stronger than London forces.

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Hydrogen Bonding

Hydrogen bonding only occurs between N/O/F and H. The slightly positive hydrogen is attracted to the negative lone pair of N/O/F. The H-F bond is the strongest as fluorine is the most electronegative element and it has the most lone pairs.

Atoms with lone pairs have high electron densities.