London Dispersion Forces Explained for IB Chemistry

London dispersion forces are weak intermolecular attractions caused by temporary dipoles. They exist between all atoms and molecules, including nonpolar molecules, because electron clouds are constantly moving.

The exam skill is to explain the sequence clearly: an instantaneous dipole forms, it induces a dipole in a neighbouring particle, and the opposite partial charges attract. Larger electron clouds usually create stronger dispersion forces.

Quick Answer

What London Dispersion Forces Are

At any instant, electrons may be distributed slightly more on one side of a particle than the other. This creates a temporary dipole. That dipole can then induce a dipole in a nearby particle, causing a weak attraction.

These attractions are temporary, but there are many of them. In larger molecules, the combined effect can become significant.

Why Dispersion Forces Exist in Nonpolar Molecules

Nonpolar molecules do not have permanent dipoles, but their electrons are still moving. A momentary shift in the electron cloud can create a temporary negative side and a temporary positive side.

This is why noble gases and nonpolar molecules can condense at low temperatures. They do not have permanent dipole-dipole forces, but they still have London dispersion forces.

Why Larger Molecules Have Stronger Dispersion Forces

Larger molecules usually have more electrons. Their electron clouds are easier to distort, which means they are more polarisable. More polarisable particles create stronger temporary and induced dipoles.

This explains the trend in halogens:

F2 < Cl2 < Br2 < I2

The boiling point increases down the group because dispersion forces become stronger as molecular size and electron number increase.

Shape and Surface Contact

Molecular shape can also matter. Long, straight-chain molecules can have more surface contact than compact branched molecules of similar mass. More contact can mean stronger dispersion forces and a higher boiling point.

When comparing molecules, check both size and shape before deciding which has stronger London dispersion forces.

Common Mistakes

Mini Practice

1. Explain how a temporary dipole forms.
2. State why I2 has stronger dispersion forces than F2.
3. Explain why noble gases can liquefy at low temperatures.
4. Compare a straight-chain alkane and a branched alkane of similar formula.
5. State whether London dispersion forces are intra- or intermolecular.

Answers:

1. Electrons momentarily shift to one side, creating partial charges.
2. I2 has more electrons and a more polarisable electron cloud.
3. Temporary dipoles create weak attractions between atoms.
4. The straight-chain alkane may have stronger dispersion forces because of greater surface contact.
5. They are intermolecular forces.

Practice This Topic

Try this exam-style task:
Explain why iodine has a higher boiling point than fluorine, even though both are nonpolar diatomic molecules.

Answer guide:
Both molecules have London dispersion forces. Iodine has more electrons and a larger, more polarisable electron cloud than fluorine. This creates stronger temporary dipoles and stronger induced dipoles, so more energy is needed to separate the molecules.

Practice related Chemistry questions
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Related Study Links

• IB Chemistry Acids and Bases: pH, Neutralisation, and Common Mistakes
• How to Complete Ionization and Net Ionic Equations
• Spectator Ions in IB Chemistry: Net Ionic Equations
• Practice IB Chemistry SL covalent model questions

FAQ

What are London dispersion forces?

London dispersion forces are weak intermolecular attractions caused by temporary dipoles. A momentary uneven electron distribution creates an instantaneous dipole, which can induce a dipole in a nearby particle.

Are London dispersion forces present in all molecules?

Yes. London dispersion forces are present in all atoms and molecules because all electron clouds move and can become temporarily uneven. They are especially important in nonpolar molecules because other intermolecular forces may be absent.

Why do larger molecules have stronger dispersion forces?

Larger molecules usually have more electrons and more polarisable electron clouds. Their temporary dipoles can be stronger, so the induced dipole attractions between molecules are stronger.

Do London dispersion forces affect boiling point?

Yes. Stronger London dispersion forces mean more energy is needed to separate molecules, so boiling point usually increases when other factors are similar.

Final Takeaway

London dispersion forces come from moving electrons. For exam answers, use the chain: temporary dipole, induced dipole, attraction, stronger with larger and more polarisable electron clouds.