London dispersion forces (LDFs) are the weakest but most universal type of intermolecular force in chemistry. They appear in IB Chemistry Topic 4 (Bonding) and play a major role in explaining why nonpolar substances can still condense into liquids and solids. Even though LDFs are weak, they are essential for understanding trends in boiling points, molecular size, and the behavior of noble gases and hydrocarbons.
What Are London Dispersion Forces?
London dispersion forces are temporary intermolecular attractions caused by momentary fluctuations in electron distribution that create temporary dipoles.
In simple terms:
- Electrons move constantly
- At any moment, they may be unevenly distributed
- This creates a temporary partial positive and partial negative side
- Nearby molecules react to this temporary dipole
- A weak attraction forms
LDFs are present in all molecules, even if they are completely nonpolar.
How London Dispersion Forces Form
Electrons are always moving around the nucleus.
At any instant, they may cluster on one side, producing a temporary dipole. This dipole:
- Creates a matching dipole in a neighboring molecule
- Causes a brief attractive force
- Breaks and reforms rapidly
Because electrons move constantly, these temporary dipoles form continuously.
Why London Dispersion Forces Matter
Even though LDFs are weak, they are crucial because:
- They are the only intermolecular force in nonpolar molecules
- They explain why noble gases can liquefy
- They help determine boiling points and melting points
- They affect solubility
- They influence viscosity and volatility
Without LDFs, substances like methane and iodine would never condense into liquids or solids.
Factors That Affect the Strength of LDFs
London dispersion forces vary widely. Their strength depends on:
1. Number of electrons (molar mass)
More electrons = stronger LDFs.
This is why:
- Helium is a gas
- Argon liquefies at very low temperatures
- Iodine (I₂) is a solid at room temperature
2. Size of electron cloud
Larger electron clouds polarize more easily, increasing LDF strength.
3. Molecular shape
Longer, more linear molecules have stronger LDFs because they have larger surface areas that can interact.
Example:
- n-pentane (straight chain) has a higher boiling point
- 2,2-dimethylpropane (branched) has weaker LDFs and a lower boiling point
Shape matters greatly in boiling point trends.
Examples of London Dispersion Forces in Real Substances
Noble gases
He, Ne, Ar, Kr, Xe
They condense at low temperatures due only to LDFs.
Hydrocarbons
Alkanes, alkenes, alkynes
As chain length increases:
- Electron count increases
- LDFs increase
- Boiling point increases
Nonpolar molecules
O₂, N₂, CO₂
All rely on LDFs for condensation.
London Dispersion Forces vs Other Intermolecular Forces
Compared to dipole–dipole forces:
LDFs are weaker because they rely on temporary dipoles rather than permanent ones.
Compared to hydrogen bonding:
Hydrogen bonding is much stronger and requires specific bonds with N, O, or F.
Compared to ionic or covalent bonds:
Intramolecular bonds are drastically stronger than any intermolecular force.
LDFs help explain physical properties, not chemical reactivity.
London Dispersion Forces in IB Exam Questions
You must be able to:
- Explain why boiling points increase down Group 17
- Predict which molecule has stronger IMFs based on size and shape
- Compare linear vs branched hydrocarbons
- Identify LDFs as the only force in nonpolar substances
- Explain why larger molecules have lower volatility
These concepts appear frequently in Paper 1 and Paper 2.
Common IB Misunderstandings
“Nonpolar molecules have no intermolecular forces.”
Incorrect—LDFs are always present.
“LDFs are too weak to matter.”
False—LDFs are responsible for physical states of many substances.
“Hydrogen bonding is always required for condensation.”
Not true—noble gases condense solely due to LDFs.
“More mass always means stronger IMFs.”
Mostly true, but molecular shape also matters significantly.
FAQs
Why do larger molecules have stronger dispersion forces?
Their electrons are more spread out and more easily distorted, leading to stronger temporary dipoles.
Can LDFs exist alongside other IMFs?
Yes—polar molecules have LDFs in addition to dipole–dipole forces or hydrogen bonding.
Why do branched molecules have lower boiling points?
Their compact shape reduces surface contact and weakens LDFs.
Conclusion
London dispersion forces are temporary intermolecular attractions caused by electron movement. Although they are the weakest IMFs, they are universally present and essential for explaining key physical properties like boiling point trends, solubility, and molecular condensation. Mastering LDFs helps IB Chemistry students understand the behavior of nonpolar substances and trends across the periodic table.
