Van der Waals Forces
Van der Waals forces are a collective term for weak, short-range intermolecular attractions that arise from transient or permanent electric dipoles, including London dispersion forces, dipole-dipole interactions, and dipole-induced dipole interactions. Named after Dutch physicist Johannes Diderik van der Waals, these forces explain deviations of real gases from ideal behaviour and govern properties such as boiling points, surface tension, and the adhesion of geckos to surfaces. Their strength scales with molecular size and polarisability, making them significant for large molecules and noble gases.
Key Formula
U(r) = -C / r^6
LaTeX: U(r) = -\frac{C}{r^6}
| Symbol | Meaning | Unit |
|---|---|---|
| U(r) | Potential energy of interaction | J |
| C | Interaction-specific constant | J·m⁶ |
| r | Distance between interacting molecules | m |
Worked Example
Problem
Arrange the following noble gases in order of increasing boiling point and explain using van der Waals forces: He, Ne, Ar, Kr, Xe.
Solution
Step 1: Identify that all noble gases are nonpolar and monoatomic, so only London dispersion forces act between them. Step 2: London dispersion force strength increases with polarisability, which increases with the number of electrons and atomic size. Step 3: Electron counts — He: 2, Ne: 10, Ar: 18, Kr: 36, Xe: 54. Step 4: Greater electron count → larger electron cloud → greater instantaneous dipole → stronger London forces → higher boiling point.
Answer
He (–269 °C) < Ne (–246 °C) < Ar (–186 °C) < Kr (–153 °C) < Xe (–108 °C). Boiling point increases with atomic number due to stronger London dispersion forces.
Types of Van der Waals Forces
| Type | Origin | Approximate Strength (kJ/mol) | Example |
|---|---|---|---|
| London dispersion | Instantaneous dipole–induced dipole | 0.1–40 | He···He, I₂···I₂ |
| Dipole–dipole | Permanent dipole–permanent dipole | 1–20 | HCl···HCl |
| Dipole–induced dipole | Permanent dipole–induced dipole | 0.5–5 | HCl···Ar |
| Ion–induced dipole | Ion–induced dipole | 2–10 | Na⁺···Ar |
Interactive Tools
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Related Terms
London Dispersion Forces
London dispersion forces (LDFs) are the weakest form of van der Waals forces, arising from temporary, instantaneous dipoles caused by the random fluctuation of electron density in any atom or molecule. Although individually very weak (0.1–40 kJ/mol), they are the only intermolecular force present in nonpolar molecules and noble gases, and become significant in large, polarisable molecules. LDFs increase with molecular size, surface area, and number of electrons, explaining why larger alkanes have higher boiling points than smaller ones.
Dipole-Dipole Interaction
Dipole-dipole interactions are intermolecular forces that occur between polar molecules, where the partially positive end (δ+) of one molecule is attracted to the partially negative end (δ–) of a neighbouring molecule. These forces are stronger than London dispersion forces but weaker than hydrogen bonds, typically ranging from 1–20 kJ/mol. They are responsible for the elevated boiling points of polar molecules such as HCl, SO₂, and acetone compared to nonpolar molecules of similar molecular weight.
Hydrogen Bond
A hydrogen bond is an attractive intermolecular or intramolecular force between a hydrogen atom covalently bonded to a highly electronegative atom (N, O, or F) and a lone pair on another electronegative atom. With strengths typically ranging from 5 to 40 kJ/mol, hydrogen bonds are much weaker than covalent bonds but far stronger than other van der Waals forces. They are responsible for water's unusually high boiling point, the structure of DNA base pairs, and the secondary and tertiary structures of proteins.
Named after Johannes Diderik van der Waals (1837–1923), the Dutch physicist who introduced these forces in his 1873 doctoral thesis to account for deviations of real gases from the ideal gas law. He was awarded the Nobel Prize in Physics in 1910.