Worked example 1
Rank the boiling points of F₂, Cl₂, Br₂ and I₂ and justify the order.
Try it first: Ask what the only intermolecular force in these molecules can be.
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Types of Solids and IMFSIntro to IMFSContent level: Core 18 min
London Dispersion Forces
What you'll be able to do: Explain why every substance has dispersion forces and predict their strength from size and shape.
Best after: Bond Type and Electronegativity Difference
Introduction
Even helium liquefies if you cool it enough, so something must attract two neutral atoms. That something is the dispersion force, and it is present in every substance you will ever meet.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Describe how an instantaneous dipole induces a dipole in a neighbour
- Relate polarisability to number of electrons and molecular size
- Predict boiling point order within a family from dispersion strength
- Explain the effect of molecular shape on dispersion
Lesson
Instantaneous dipoles are always forming
Electrons move constantly, so at any instant one side of an atom is slightly electron rich. That momentary dipole polarises a neighbouring cloud, creating an induced dipole, and the two attract. The attraction is weak but universal.
Dispersion is present in every substance, including polar ones.
Polarisability controls the strength
A cloud that distorts easily makes bigger instantaneous dipoles. More electrons and a larger, more diffuse cloud means higher polarisability, so dispersion strength rises down a group and with molar mass within a family.
Shape matters as much as size
n-pentane and neopentane share the formula C₅H₁₂, but the straight chain packs alongside its neighbours over a long surface while the compact sphere touches less. n-pentane boils at 36 °C, neopentane at 10.
Compare surface contact, not just molar mass, when two isomers are involved.
When dispersion wins outright
For large molecules dispersion can beat hydrogen bonding. I₂ is a solid at room temperature with no polar bonds at all, while HF with strong hydrogen bonds is a gas, because iodine has far more electrons.
Control the comparison before ranking dispersion
Dispersion arises from fluctuating electron clouds in every atom and molecule. With similar shapes, a larger, more polarizable electron cloud usually gives stronger dispersion. For molecules with the same formula, greater surface contact can strengthen dispersion. Keep one factor as constant as possible: comparing both size and shape at once makes a one-cause explanation weak.
Same shape: compare polarizability. Same formula: compare contact area.
Key ideas
Definition
London dispersion force
Attraction between an instantaneous dipole and the dipole it induces in a neighbour.
Definition
Polarisability
How easily an electron cloud is distorted by a nearby charge.
Rule
Size rule
More electrons means stronger dispersion and a higher boiling point within a family.
Rule
Shape rule
Elongated molecules have more contact area and stronger dispersion than compact isomers.
Worked examples
Worked example 2
Explain why n-pentane boils higher than neopentane despite identical formulas.
Try it first: Sketch one long chain and one compact ball.
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Worked example 3
Why is xenon easier to liquefy than krypton under comparable conditions?
Try it first: Both are monatomic noble gases; compare electron clouds.
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Common mistakes
Saying polar molecules do not have dispersion forces.
Why it's wrong: Every substance has dispersion; polar molecules simply add dipole-dipole on top.
Check instead: List dispersion first, then add any extra forces.
Assuming dispersion is always the weakest force present.
Why it's wrong: In large molecules such as I₂ dispersion dominates over stronger force types in small molecules.
Check instead: Compare total attraction, weighing size against force type.
Confusing intermolecular forces with covalent bonds.
Why it's wrong: Boiling breaks intermolecular attractions, not the bonds inside molecules.
Check instead: Say which attraction is being broken before predicting the energy needed.
After the lesson · Question bank
Independent practice
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What you should now know
Dispersion forces arise from instantaneous fluctuations in electron density that induce dipoles in neighbours. They strengthen with more electrons and larger, more polarisable electron clouds, and with elongated shapes that allow more contact.
- Dispersion arises from instantaneous and induced dipoles
- More electrons means greater polarisability and stronger dispersion
- Elongated shapes beat compact isomers
- Dispersion is present in every substance
Sources and further reading
- Chemistry 2e, Section 10.1: Intermolecular ForcesOpenStax · Rice University · 10.1View source
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