Intermolecular forces

Intermolecular forces are the attractions between molecules. Compared with the covalent bonds that hold each molecule together they are weak, yet they decide the melting and boiling points of molecular substances. There are three types: induced dipole-dipole forces (also called van der Waals, London or dispersion forces), permanent dipole-dipole forces and hydrogen bonding.

Intermolecular forces vs covalent bonds

When water boils, the molecules separate from each other, but every H₂O molecule stays in one piece. Boiling overcomes the forces between molecules, not the covalent bonds within them. That is why simple molecular substances often have low boiling points even though their covalent bonds are strong.

AQA's A-level specification names the three forces between molecules and states that the melting and boiling points of molecular substances are influenced by their strength.

The three types of intermolecular force

1. Induced dipole-dipole forces

These act between all molecules, polar or not. Electrons are always moving, so at any instant they may be slightly more on one side of a molecule, making a temporary dipole. That dipole pulls or pushes the electrons in a neighbouring molecule, inducing a dipole in it, and the two attract. The dipoles constantly appear and disappear.

• More electrons means stronger forces, because bigger electron clouds are easier to distort. That is why boiling points rise down group 7 and along the alkanes.
• More surface contact means stronger forces. Long, straight molecules touch along more of their length than compact, branched ones.

2. Permanent dipole-dipole forces

These act between polar molecules, which have a permanent δ+ end and δ− end because their atoms have different electronegativities and the molecule isn't symmetrical. The δ+ end of one molecule attracts the δ− end of the next. They act as well as induced dipole forces, not instead of them.

3. Hydrogen bonding

The strongest of the three. It happens when hydrogen is bonded directly to nitrogen, oxygen or fluorine, the most electronegative elements. The hydrogen becomes strongly δ+ and is attracted to a lone pair on an N, O or F atom in another molecule. Water, ammonia, hydrogen fluoride and alcohols all form hydrogen bonds.

How intermolecular forces affect boiling points

Stronger intermolecular forces need more energy to overcome, so the substance boils at a higher temperature. Three sets of data from OpenStax Chemistry show each effect.

More electrons: the halogens

All are non-polar, so only induced dipole-dipole forces act. Going down the group, each molecule has more electrons, the forces get stronger and the boiling point rises.

Shape: the pentane isomers

Pentane, isopentane and neopentane all have the formula C₅H₁₂, so the same number of electrons. Their boiling points are 36 °C, 27 °C and 9.5 °C. The straight chain of pentane gives the most surface contact and the strongest induced dipole forces; the compact, ball-like neopentane gives the least.

Polarity: HCl and F₂

HCl and F₂ have similar numbers of electrons, but HCl boils at 188 K and F₂ at 85 K. HCl is polar, so it has permanent dipole-dipole forces as well as induced ones.

Hydrogen bonding and water's unusual properties

AQA highlights the importance of hydrogen bonding in the anomalous boiling points of compounds and the low density of ice.

In group 14, boiling points rise steadily from CH₄ to SnH₄ as the molecules get more electrons. Group 16 follows the same pattern from H₂S to H₂Te, but water breaks it. OpenStax notes that the trend would put water's boiling point near −80 °C; it is actually 100 °C. The difference is hydrogen bonding, which only the first member of the group can form. NH₃ and HF stand out in groups 15 and 17 for the same reason.

Ice is less dense than liquid water. In ice, each water molecule hydrogen bonds to others in a fixed, open arrangement that holds the molecules further apart than in the liquid. When ice melts, some hydrogen bonds break and the molecules pack closer together. That is why ice floats.

How to work out which forces a molecule has

1. Every molecule has induced dipole-dipole forces.
2. Is the molecule polar? Check for polar bonds and whether the shape lets their dipoles cancel. If it is polar, add permanent dipole-dipole forces. CO₂ has polar bonds but is linear, so the dipoles cancel and it is non-polar.
3. Is H bonded directly to N, O or F? If so, add hydrogen bonding.

So methane has only induced dipole forces; hydrogen chloride has induced and permanent dipole forces; water has all three. Molecular shape matters here, so the shapes of molecules guide is useful alongside this one.

Common mistakes

• Saying covalent bonds break when a molecular substance boils. Only the intermolecular forces are overcome.
• Saying non-polar molecules have no intermolecular forces. Every molecule has induced dipole-dipole forces.
• Calling any H in a molecule a hydrogen bond. H must be bonded to N, O or F, and there must be a lone pair on N, O or F in a neighbouring molecule.
• Calling a hydrogen bond a covalent bond. It is an intermolecular force, much weaker than a covalent bond.
• Comparing molecules of very different sizes by polarity alone. A large non-polar molecule can boil higher than a small polar one, because of its strong induced dipole forces.

Forces between molecules are part of the Chemical Bonding course. For practice in a short timed paper, try today's A-level Chemistry: Year 1 or GCSE Chemistry Higher paper.

Read next

Chemistry · Bonding and structureHydrogen bonding: what it is, examples and effectsWhat hydrogen bonding is, the conditions for it, how to draw it, examples such as water and ammonia, and why it raises boiling points and makes ice float.