---
title: "Van Der Waals Forces | Physical Chemistry II"
description: "Van der Waals forces are weak intermolecular attractions that shape polymer conformation, radius of gyration, and bulk behavior in Physical Chemistry II."
canonical: "https://fiveable.me/physical-chemistry-ii/key-terms/van-der-waals-forces"
type: "key-term"
subject: "Physical Chemistry II"
unit: "Unit 7"
---

# Van Der Waals Forces | Physical Chemistry II

## Definition

Van der Waals forces are weak attractions between molecules caused by temporary or permanent dipoles. In Physical Chemistry II, they help explain polymer shape, packing, and radius of gyration.

## What It Is

Van der Waals forces are the weak attractions that act between atoms, groups, or whole molecules in Physical Chemistry II when electron density is not perfectly even. They include London dispersion forces, dipole-dipole interactions, and dipole-induced dipole interactions, and they show up any time molecules get close enough for their electron clouds to influence one another.

The most common piece is London dispersion. Even a nonpolar molecule has electrons moving around, so for a brief moment one side can become slightly electron-rich and the other slightly electron-poor. That temporary dipole can induce a dipole in a nearby molecule, creating a weak attraction. This effect is present in every molecule, and it becomes stronger as molecules get larger and more polarizable.

Dipole-dipole interactions happen when molecules already have permanent dipoles. The positive end of one molecule aligns with the negative end of another, so the molecules attract each other. Dipole-induced dipole interactions happen when a polar molecule distorts the electron cloud of a nearby nonpolar molecule and makes it attractive too.

In polymer chemistry, these interactions matter because a long chain is constantly sampling many conformations in solution or in the solid state. If segment-segment attractions are stronger, the chain tends to fold or pack more tightly, which can lower the radius of gyration and favor a more compact conformation. If the attractions are weaker, thermal motion can keep the chain more extended.

That is why van der Waals forces are not just a tiny detail in this course. They help determine whether a polymer chain behaves like a loose coil, a collapsed structure, or something in between, and they feed directly into properties like melting point, solubility, and mechanical stiffness.

## Why It Matters

Van der Waals forces are one of the main reasons polymer chains do not behave like ideal, featureless strings. In Physical Chemistry II, you use them to connect molecular-scale interactions to measurable properties like chain size, packing, and phase behavior.

For topic 7.3 on polymer conformations and radius of gyration, this term shows up in the balance between segment-segment attraction and segment-solvent attraction. If the polymer segments attract each other more strongly than they interact with the solvent, the chain can contract into a collapsed conformation. That changes the average shape you would infer from a scattering experiment or from a chain-size calculation.

These forces also explain why two polymers with similar backbones can behave differently in solution or in the solid state. A chain with stronger dispersion interactions may pack more tightly, which can raise its melting point or make it less soluble in a given solvent. A chain with weaker intermolecular attraction may stay more expanded and mix more easily with the surrounding solvent.

If you are reading a graph, solving a polymer problem, or comparing materials, van der Waals forces give you the molecular reason behind the trend. They are the bridge between electronic structure and bulk behavior.

## Connections

### Polymer Chain Conformation

Van der Waals forces help decide which conformations a polymer visits most often. If attractions between nearby segments are stronger, the chain can fold inward or pack more tightly. If those attractions are weaker, thermal motion has more freedom to keep the chain in a looser, more extended shape.

### Radius of Gyration

Radius of gyration measures how spread out a polymer chain is around its center of mass. Stronger van der Waals attractions usually pull segments closer together, which can reduce the average size and lower the radius of gyration. Weaker attractions tend to produce a larger, more expanded coil.

### [Flory-Huggins Interaction Parameter](/physical-chemistry-ii/key-terms/flory-huggins-interaction-parameter)

The Flory-Huggins interaction parameter helps describe whether polymer and solvent interactions favor mixing or separation. Van der Waals forces are part of the molecular interactions that feed into that picture, especially when comparing segment-segment attraction with segment-solvent attraction. They help explain why one solvent swells a polymer while another collapses it.

### [good solvent](/physical-chemistry-ii/key-terms/good-solvent)

In a good solvent, polymer-solvent attractions compete successfully with polymer-polymer attractions. That can overcome some van der Waals-driven self-attraction inside the chain and keep the polymer more expanded. When the solvent is poor, the chain is more likely to shrink because segment-segment interactions win out.

## On the AP Exam

A problem set might ask you to predict whether a polymer coil expands or collapses when the solvent changes, and van der Waals forces are part of the reasoning. You would compare segment-segment attractions with segment-solvent attractions, then connect that balance to radius of gyration, chain packing, or solubility.

In a lab or data-analysis question, you might interpret scattering data, viscosity trends, or melting behavior using the idea that stronger intermolecular attractions pull chains closer together. If a chain looks more compact than expected, van der Waals forces are one of the first molecular explanations to check. If a chain is unusually extended, you would look for weaker attractions or better solvent compatibility.

Short-answer questions often ask you to distinguish van der Waals forces from stronger specific interactions, or to explain why large nonpolar polymers still attract each other. The move is to name the type of interaction, describe the electron-cloud origin, and connect it to the observed polymer behavior.

## van der Waals forces vs Hydrogen Bonding

Hydrogen bonding is a more specific and usually stronger intermolecular attraction that requires H bonded to N, O, or F interacting with a lone pair. Van der Waals forces are broader and include dispersion and dipole-based attractions, so they occur in all molecules, not just hydrogen-bonding ones.

## Key Takeaways

- Van der Waals forces are weak intermolecular attractions caused by temporary or permanent dipoles.
- In Physical Chemistry II, they matter because they connect molecular interactions to polymer shape, packing, and size.
- Stronger van der Waals attractions usually make polymer chains more compact and can lower the radius of gyration.
- These forces also help explain differences in solubility, melting point, and how a polymer behaves in different solvents.
- Every molecule has some dispersion interaction, but the total effect gets larger when molecules are bigger, more polarizable, or packed close together.

## FAQs

### What is van der Waals forces in Physical Chemistry II?

Van der Waals forces are weak attractions between molecules or parts of molecules caused by dipoles and electron-cloud fluctuations. In Physical Chemistry II, you use them to explain why polymer chains attract, fold, pack, or stay more spread out. They are one of the molecular forces behind conformational behavior and bulk material properties.

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

Not exactly. London dispersion forces are one type of van der Waals force, and they come from temporary dipoles created by electron motion. The larger van der Waals category also includes dipole-dipole and dipole-induced dipole interactions. So dispersion is part of the family, not the whole story.

### How do van der Waals forces affect polymer conformation?

They influence whether segments on the same chain attract each other enough to make the polymer contract. Stronger attractions usually favor more compact conformations and a smaller radius of gyration. Weaker attractions, or a better solvent, let the chain stay more extended.

### Why do bigger molecules often have stronger van der Waals forces?

Bigger molecules usually have more electrons and a more easily distorted electron cloud, which makes them more polarizable. That increases the size of dispersion interactions, especially when molecules can pack closely. In polymer problems, that can show up as tighter packing or stronger cohesion between chains.

## Related Study Guides

- [7.3 Polymer Conformations and Radius of Gyration](/physical-chemistry-ii/unit-7/polymer-conformations-radius-gyration/study-guide/ZZkPanXFxDIZiVtt)

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