Topic 3.1 Notes – Intermolecular and Interparticle Forces

1. What Intermolecular and Interparticle Forces Are

Intermolecular forces (IMFs) are electrostatic attractions between separate particles. Those particles can be:

• Neutral molecules
• Atoms (like noble gases)
• Ions

They are weaker than intramolecular forces (covalent, ionic, metallic bonds) because:

• They act over larger distances.
• They often involve partial charges instead of full charges.

All IMFs come from Coulombic attraction between:

• Full charges (ions)
• Permanent partial charges (dipoles)
• Temporary dipoles (fluctuating electron clouds)

Everything in this unit follows one chain:

Structure → electron distribution → charge separation → force strength

If you can explain how structure affects electron distribution, you can explain the IMF strength. That’s what FRQs usually want.

2. Types of Intermolecular and Interparticle Forces

London Dispersion Forces (LDFs)

These exist in all atoms and molecules.

They come from:

• Constant motion of electrons
• Temporary uneven distribution
• A temporary dipole that induces another dipole nearby

The sequence below shows how an instantaneous dipole forms in one atom, induces a dipole in a neighbor, and creates an attractive intermolecular force.

Formation of a London dispersion force

What increases LDF strength?

• More electrons → larger electron cloud → more polarizable
• Larger size
• Greater surface contact (long chains > compact shapes)
• Presence of π bonds (π electrons are more easily distorted)

Huge exam idea:
A large nonpolar molecule can have stronger overall IMFs than a small polar molecule because dispersion can dominate.

Also important:
London dispersion forces are a type of van der Waals force, but those terms are not interchangeable.

Dipole-Dipole Forces

These occur between polar molecules.

A polar molecule has a permanent dipole moment because of unequal electronegativity and molecular shape.

The diagrams below show that opposite partial charges attract and like partial charges repel. On the exam, you usually focus on the attractive orientation where δ+ is near δ−.

Attractive and repulsive dipole orientations

Strength depends on:

• Magnitude of dipole
• Distance between molecules
• Orientation (δ+ to δ− is attractive)

Because polar molecules also have dispersion forces, dipole-dipole interactions add to the total attraction. That’s why polar substances of similar size usually have higher boiling points than nonpolar ones.

Hydrogen Bonding

This is a strong special case of dipole-dipole.

It only happens when:

• H is bonded to N, O, or F
• That H is attracted to a lone pair on N, O, or F on another molecule (or another part of the same molecule)

Why it’s strong:

• Very polar bond
• Small atoms allow close approach
• Large partial charges

It can be:

• Intermolecular (between molecules)
• Intramolecular (within one large molecule)

Hydrogen bonding dramatically increases boiling points.

Dipole-Induced Dipole Forces

These occur between:

• A polar molecule
• A nonpolar molecule

The permanent dipole distorts the electron cloud of the nonpolar molecule.

Always attractive.
Stronger when:

• The polar molecule has a large dipole
• The nonpolar molecule is highly polarizable

Ion-Dipole Forces

Between:

• An ion
• A polar molecule

Stronger than dipole-dipole because ions have full charges.

Orientation matters:

• Cations align with δ− ends
• Anions align with δ+ ends

This explains why ionic compounds dissolve in polar solvents like water.

Ion-Ion Forces

These are attractions between fully charged ions in ionic solids.

Strongest interaction here.
Responsible for high melting points of ionic compounds.

3. Relative Strength and Structure Effects

General trend from weakest to strongest:

London dispersion < dipole-induced dipole < dipole-dipole < hydrogen bonding < ion-dipole < ion-ion

When comparing same substance:

• More electrons → stronger LDF
• More surface area → stronger LDF
• Greater polarity → stronger dipole forces

When comparing different substances, check:

• Is it polar?
• How large is it?
• Does it contain N-H, O-H, or F-H?
• Are ions present?

In many AP questions, you’re given boiling points. Your job is to justify the ranking using structure and the types of forces present. Always mention dispersion forces because they are always present.

4. Noncovalent Interactions in Large Biomolecules

Large biomolecules rely on many weak forces working together.

Examples:

• Hydrogen bonds stabilize protein secondary structure
• Ion-ion interactions between charged side chains
• Dipole-dipole between polar groups
• London dispersion between nonpolar regions

Individually weak. Collectively powerful.
That cumulative effect determines 3D structure and biological function.