A Level - Intermolecular Forces

Intermolecular forces are weak bonds which form between molecules. They are different to the bonds between atoms within molecules - these are intramolecular forces (intra- meaning inside). All molecules and even the monoatomic noble gases will experience these forces, though in some cases, you'll need to lower the temperature in order to reduce the kinetic energy to such a level that the molecules can 'stick' together (i.e. the intermolecular forces can properly take hold).

There are different types of intermolecular forces, but all of them are based on electrostatic attraction; the force of attraction between positive and negative charges.

At A Level, there are three types of intermolecular forces that you need to be aware of:

• Hydrogen bonds
• Permanent dipole-dipole interactions
• London forces (sometimes called Van der Wall's, depending on your specification)

Of these, hydrogen bonds are the strongest and they only occur between molecules which have a O, N or F atom bonded to a hydrogen atom. Because of the difference in electronegativity of these atoms, there is a dipole present across the bond. As the oxygen is more electronegative, there is a partial positive charge on the hydrogen and a partial negative charge on the other atom.

When these two partial charges interact, a hydrogen bond is formed, represented here by a dashed straight line, as shown here in water:

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If the above was a student answer to the question 'draw a diagram to show how molecules of water undergo hydrogen bonding', have a think about what this student may have missed.

From an exam perspective, it is important to remember to draw the dipoles and any lone pairs, as well as the hydrogen bond itself as this is where students very often lose marks - and easy marks too! The lone pairs is what the student would have missed if the diagram above was an exam answer - they'd thrown away easy marks! The diagram below would be a superior answer:

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The comparative strength of hydrogen bonds, against to the other types of intermolecular forces experienced by other simple covalent molecules is what gives water an unusually high melting and boiling point as they require more energy to overcome. Hydrogen bonds also play a part in reducing the density of water as it transitions form a liquid to a solid; they hold the water molecules apart in an open lattice, as depicted below:

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After hydrogen bonding, the next strongest intermolecular force is permanent dipole-dipole interactions. These interactions are between dipoles which have irreversibly formed due to a difference in electronegativity between the two bonded atoms in a molecule. One single molecule may contain more than one permanent dipole. 

An example of such a molecule would be chloromethane, in which the chlorine atom is significantly more electronegative than the carbon atom, resulting a permanent dipole. The partial positive charge on the carbon can interact with a partial negative charge on the chlorine atom of another molecule, creating a weak intermolecular bond. Whilst they are weak, may of these bonds can require a good amount of energy to overcome, thus raising the boiling point of a substance. 

Finally, we have London Forces. These forces are the weakest of the three intermolecular forces studies at A Level. They arise due to a asymmetrical distribution of a electrons in a species, which gives rise to an instantaneous (sometimes termed 'transient') dipole. Once this dipole forms, it creates dipoles in other nearby species and these are aptly referred to as 'induced dipoles', as shown below:

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When these dipoles interact, they produce London forces, which, although incredibly weak, when in number they can raise the boiling point of a molecule by raising the energy required to overcome the intermolecular forces and bring about a change of state.

So, to summarise:

• Molecules with O - H, N - H and F - H bonds form hydrogen bonds, the strongest type of intermolecular force. 
• The permanent dipoles within molecules can interact and these forces are weaker than hydrogen bonding. 
• Instantaneous dipoles can induce dipoles in atoms of other molecules, forming London Forces - the weakest of the intermolecular forces. 
• The stronger or more numerous the intermolecular forces, the greater the melting/boiling point of a chemical.