London dispersion forces

London dispersion forces are weak Coulombic attractions between temporary, fluctuating dipoles that form when electron clouds shift unevenly; they exist between all molecules and become the strongest net intermolecular force in large, highly polarizable molecules (AP Chem EK 3.1.A.1).

What are London dispersion forces?

London dispersion forces (LDFs) come from the fact that electrons are always moving. At any instant, the electrons in a molecule can pile up on one side, creating a momentary lopsided charge called an instantaneous dipole. That tiny dipole nudges the electron cloud of a neighboring molecule, creating an induced dipole, and the two briefly attract each other. The attraction is Coulombic, just like every other intermolecular force, but it flickers on and off as electrons shuffle around.

Here's the part the CED really cares about. LDF strength scales with polarizability, which is how easily a molecule's electron cloud gets distorted. More electrons, a bigger electron cloud, and the presence of pi bonding all increase polarizability. LDFs also get stronger with more contact area between molecules, which is why a long straight-chain molecule has stronger dispersion forces than its compact branched isomer. And don't let the word "weak" fool you. Per interaction LDFs are the weakest IMF, but they add up. For large molecules like I₂ or long hydrocarbons, dispersion forces are often the strongest net intermolecular force, which is exactly why iodine is a solid at room temperature with zero polarity.

Why London dispersion forces matter in AP Chemistry

LDFs live in Topic 3.1 (Intermolecular Forces) in Unit 3, under learning objective 3.1.A. That LO asks you to connect chemical structure to the relative strength of intermolecular forces, both within one substance and when comparing two different substances. Dispersion forces are your baseline in every one of those comparisons because every molecule has them. The whole of Unit 3 builds on this idea, since boiling points, vapor pressure, solubility, and chromatography all trace back to which IMFs are present and how strong they are. If you can't reason about LDFs, you can't explain why nonpolar substances condense at all, and that question shows up on the exam constantly.

How London dispersion forces connect across the course

Polarizability (Unit 3)

Polarizability is the dial that controls LDF strength. More electrons, a larger electron cloud, and pi bonds all make the cloud squishier and easier to distort, so temporary dipoles get bigger and the attractions get stronger. This is the single fact behind "why does Br₂ boil higher than Cl₂" questions.

Instantaneous Dipole and Induced Dipole (Unit 3)

These two terms are the mechanism of an LDF. A random electron fluctuation creates an instantaneous dipole in one molecule, which then induces a matching dipole in its neighbor. If an FRQ asks you to explain dispersion forces at the particle level, this is the language the rubric wants.

Dipole-Dipole Forces (Unit 3)

Polar molecules have permanent dipoles, so they get dipole-dipole attractions on top of their LDFs, not instead of them. The 2018 FRQ on CS₂ vs. COS is the classic setup. Both molecules have similar LDFs, so the permanent dipole in COS is what tips the boiling point comparison.

Heating Curves and Phase Changes (Unit 6)

Vaporization means breaking intermolecular attractions, so substances with stronger net LDFs have higher enthalpies of vaporization. When a question asks you to rank ΔH_vap values, you're really ranking IMF strength, and for nonpolar compounds that means ranking dispersion forces.

Are London dispersion forces on the AP Chemistry exam?

LDFs are a workhorse of both multiple choice and FRQs. The classic MCQ stem gives you two or more substances and asks you to rank boiling points, vapor pressures, or enthalpies of vaporization. Your job is to identify every IMF present in each substance and compare net strength, remembering that big nonpolar molecules can beat small polar ones. On FRQs, the College Board loves structure-based explanations. The 2018 short FRQ gave the structures and boiling points of CS₂ and COS and asked for particle-level reasoning. The 2021 long FRQ on silicon compounds and the 2022 FRQ on methyl salicylate both leaned on IMF reasoning too. A full-credit answer never just names the force. It explains WHY one substance has stronger LDFs (more electrons, larger electron cloud, greater polarizability, more contact area) and connects that to the macroscopic property. Writing "CCl₄ is bigger so it has stronger forces" without the polarizability logic will cost you points.

London dispersion forces vs Van der Waals forces

The CED explicitly warns that "London dispersion forces" should not be used as a synonym for "van der Waals forces." Van der Waals is the umbrella category that includes both dispersion forces and dipole-dipole interactions. LDFs are one specific type underneath it. On the AP exam, name the specific force. Saying "van der Waals forces" when the question wants the temporary-dipole mechanism is too vague for the rubric.

Key things to remember about London dispersion forces

• London dispersion forces are Coulombic attractions between temporary, fluctuating dipoles, and they exist between ALL molecules, polar or not.

• LDF strength increases with polarizability, which grows with more electrons, a larger electron cloud, and the presence of pi bonding.

• Greater contact area between molecules strengthens LDFs, which is why straight-chain isomers boil higher than branched ones.

• For large molecules, dispersion forces are often the strongest NET intermolecular force, so a big nonpolar molecule can out-boil a small polar one.

• On FRQs, explain the why (more electrons, more polarizable cloud, stronger temporary dipoles), because just naming the force doesn't earn the point.

• Don't say "van der Waals forces" when you mean London dispersion forces; van der Waals is the broader category and the AP exam wants the specific name.

Frequently asked questions about London dispersion forces

What are London dispersion forces in AP Chem?

They're temporary attractions created when shifting electrons form an instantaneous dipole in one molecule, which induces a dipole in a neighbor. Per EK 3.1.A.1, they exist between all molecules and are often the strongest net IMF between large molecules.

Are London dispersion forces always the weakest intermolecular force?

No. Each individual LDF interaction is weak, but they scale with molecular size, so in large or highly polarizable molecules the total dispersion attraction can exceed dipole-dipole forces and even hydrogen bonding. That's why nonpolar I₂ is a solid at room temperature.

How are London dispersion forces different from dipole-dipole forces?

Dipole-dipole forces require permanent dipoles from polar bonds and asymmetric geometry, while LDFs come from temporary, fluctuating dipoles and exist in every molecule. Polar molecules have both; nonpolar molecules have only LDFs.

What makes London dispersion forces stronger?

Two things from the CED: polarizability and contact area. Polarizability increases with more electrons, a bigger electron cloud, and pi bonding, while greater molecular contact area (think long chains vs. branched isomers) gives more surface for temporary dipoles to interact.

Are London dispersion forces the same as van der Waals forces?

No, and the CED specifically says not to use the terms interchangeably. Van der Waals is the umbrella term covering dispersion forces and dipole-dipole interactions, so always name London dispersion forces specifically on the exam.