AP Chemistry London Dispersion Forces

What makes them stronger, and how to rank intermolecular forces, with three worked examples.

London dispersion forces are the one intermolecular force every substance has, which is why they decide so many boiling-point questions. They are also the force students most often get backwards, assuming they are always the weakest. The examples below show when they are, and when they are not.

How to Identify and Compare Intermolecular Forces

London dispersion forces (LDF) come from temporary, fluctuating dipoles: electrons are always moving, so a molecule's charge is momentarily uneven, and that induces a matching dipole in its neighbor. They grow with polarizability, which rises with the number of electrons, the size of the electron cloud, and the contact area between molecules. To compare two substances:

List every force each substance has. All molecules have LDF; add dipole-dipole if the molecule is polar (its shape matters, see the VSEPR chart), hydrogen bonding if H is bonded to N, O, or F, and ion-dipole if an ion meets a polar molecule.
If the force types match, compare LDF. More electrons, a larger electron cloud, or more contact area means stronger dispersion forces.
Predict the property. Stronger intermolecular forces mean a higher boiling point and melting point and a lower vapor pressure.

These forces act between molecules, unlike the ionic and covalent bonds inside them, which is why boiling a molecular substance does not break its covalent bonds.

Worked Example 1: Ranking the Boiling Points of Cl₂, Br₂, and I₂

Rank Cl₂, Br₂, and I₂ by boiling point, and explain why.

All three are nonpolar diatomic molecules, so London dispersion is the only intermolecular force each has. The force types match, so compare LDF strength: iodine has the most electrons and the largest, most polarizable electron cloud, then bromine, then chlorine.
Predicted order: I₂ > Br₂ > Cl₂.

This matches the measured boiling points of roughly 184°C, 59°C, and −34°C, which is why iodine is a solid, bromine a liquid, and chlorine a gas at room temperature.

Worked Example 2: When London Dispersion Forces Beat Dipole-Dipole Forces

Br₂ is nonpolar and HCl is polar. Which has the higher boiling point?

HCl has dipole-dipole forces plus a small amount of LDF, since it has only 18 electrons. Br₂ has only LDF, but it has 70 electrons and a far larger, more polarizable electron cloud. The dispersion forces in Br₂ are strong enough to outweigh HCl's dipole-dipole attraction, so Br₂ boils higher: about 59°C versus about −85°C for HCl.

The usual rule that dipole-dipole forces are stronger than LDF holds for molecules of comparable size. It fails once one molecule is much larger.

Worked Example 3: Why Two Isomers of C₅H₂₂ Boil at Different Temperatures

Pentane (a straight chain) and neopentane (2,2-dimethylpropane, a compact, nearly spherical molecule) both have the formula C₅H₁₂. Which boils higher?

Both are nonpolar hydrocarbons with the same number of electrons, so their LDF differ only by contact area. Straight-chain pentane can lie alongside its neighbors along its whole length, while compact neopentane touches neighbors at fewer points. Greater contact area gives stronger dispersion forces, so pentane boils higher: about 36°C versus about 10°C for neopentane.

Common London Dispersion Force Mistakes

Saying nonpolar molecules have no intermolecular forces. Every molecule has London dispersion forces; that is why nonpolar substances such as Cl₂ can still be liquids and solids.
Assuming LDF are always weaker than dipole-dipole forces. The comparison only holds for molecules of similar size.
Confusing intermolecular forces with the bonds inside molecules. Melting or boiling a molecular substance overcomes intermolecular forces, not covalent bonds.
Calling it "van der Waals" instead of naming the force. The AP framework asks for the specific force: LDF, dipole-dipole, hydrogen bonding, or ion-dipole.

This same compare-the-forces reasoning applies across the rest of AP Chemistry too, including how phase changes on a heating curve work -- for every other free tool and guide on this site, start from the AP Chem Score Calculator.

Frequently Asked Questions

What are London dispersion forces?

Attractions between molecules caused by temporary, fluctuating dipoles. The electrons in any atom or molecule are constantly moving, so at any instant the charge distribution can be slightly uneven, and that temporary dipole induces a matching dipole in a neighbor. They exist between all molecules, polar or nonpolar.

What makes London dispersion forces stronger?

Greater polarizability, meaning the electron cloud is easier to distort. Polarizability rises with more electrons and a larger electron cloud. Greater contact area between molecules also strengthens the attraction, so a long, extended molecule has stronger dispersion forces than a compact one with the same formula.

Are London dispersion forces always the weakest intermolecular force?

No. For small molecules they are weak, but they add up with size. Between large, highly polarizable molecules they can be the strongest net intermolecular force, strong enough to outweigh dipole-dipole forces in a much smaller polar molecule.

What is the difference between intermolecular forces and chemical bonds?

Chemical bonds (ionic, covalent, metallic) hold atoms or ions together within a substance. Intermolecular forces act between separate molecules and decide properties like boiling point. Boiling a covalent molecular substance overcomes intermolecular forces, it does not break the covalent bonds inside each molecule.

Is London dispersion the same thing as van der Waals forces?

No. The AP Chemistry framework specifically says the terms should not be used synonymously. Van der Waals forces is a broader, older umbrella term, so name the specific force: London dispersion, dipole-dipole, hydrogen bonding, or ion-dipole.

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