Intermolecular-force vocabulary is often used loosely, but AP Chemistry expects precision. This page clarifies what London dispersion forces (LDF) are, what van der Waals forces means, and how to avoid common wording mistakes.

Core Terminology: What Each Phrase Means

London dispersion forces (a specific force)

LDF are a single, specific type of intermolecular attraction. They occur between all particles (because all particles have electrons), but the phrase “London dispersion forces” should be reserved for this temporary-dipole/induced-dipole mechanism.

A key language point: saying “the molecule has van der Waals forces” is vague; saying “the dominant attraction is LDF” is specific.

London dispersion forces arise when an instantaneous (temporary) dipole forms in one particle and induces a complementary dipole in a neighboring particle. The diagram explicitly labels the unequal electron distribution, the resulting temporary dipoles ($\delta^+$ and $\delta^-$), and the net attractive force between them. Source

van der Waals forces (a broader umbrella term)

Because van der Waals is broader, it is not interchangeable with LDF. The syllabus requirement is essentially a terminology rule: do not use “London dispersion forces” as a synonym for “van der Waals forces.”

How to Use the Terms Correctly (AP-Appropriate Precision)

Acceptable statements

• “LDF are present between all atoms and molecules.”

• “The intermolecular attractions can be described as van der Waals forces, including LDF.”

• “In this nonpolar substance, the relevant intermolecular attraction is London dispersion forces.”

Statements to avoid (and better replacements)

• Avoid: “This substance has van der Waals forces, meaning London dispersion forces.”

  • Better: “This substance has van der Waals attractions, primarily London dispersion forces.”

• Avoid: “Van der Waals forces are the same as LDF.”

  • Better: “LDF are one type of van der Waals attraction.”

Why the Distinction Matters in Explanations

Preventing overgeneralisation

If you call all weak intermolecular attractions “LDF,” you lose the ability to communicate which attraction is being discussed. AP Chemistry often rewards identifying the correct category of interaction rather than using a generic label.

Communicating mechanism vs category

• LDF communicates a mechanism (temporary dipole induces another dipole).

This sequence diagram shows how a momentary electron-density fluctuation produces an instantaneous dipole, which then induces a dipole in a neighboring molecule. The final panel highlights that the attraction is between opposite partial charges, reinforcing that LDF is a specific mechanism rather than a catch-all term. Source

• van der Waals communicates a category (a collection of weak attractions between neutral particles).

When writing explanations, choose the term that matches what the question asks:

• If asked for the type/mechanism, state London dispersion forces.

• If asked for a general description of weak attractions, van der Waals may be acceptable, but specifying LDF is usually clearer.

Common Exam Pitfalls and How to Avoid Them

Pitfall: treating “van der Waals” as a single force

Students sometimes write as if “van der Waals” is one distinct interaction with one cause. On AP-style items, it is safer to treat van der Waals as an umbrella and then name LDF when you mean temporary-dipole attractions.

Pitfall: using LDF to mean “any weak intermolecular force”

In AP Chemistry, LDF is not a synonym for “weak forces” or “intermolecular forces in general.” Use intermolecular forces (IMFs) as the general term, and use LDF for the specific temporary-dipole attraction.