Intermolecular and Interparticle Forces
Draw the available interactions for a pair of particles, including mixed-pair hydrogen bonding and dispersion.
AP Chemistry Topic 3.1 · CED printed p. 58, 59 / PDF p. 65, 66. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Molecular geometry and polarity
- Coulombic charge orientation
Every molecular pair has dispersion
Even a nonpolar molecule has moving electron density. A temporary uneven distribution can induce an uneven distribution in a neighboring particle, creating attractive London dispersion interactions. Dispersion exists between all atoms and molecules, including polar ones. Its strength generally grows with polarizability: larger, more easily distorted electron clouds with more electrons respond more strongly. Delocalized π electron density can enhance polarizability. Greater molecular contact area also strengthens the net attraction, so branching may reduce attractions relative to a similar extended isomer.
Large nonpolar molecules can have stronger net intermolecular attractions than much smaller polar molecules. Never rank “hydrogen bonding > dipole > dispersion” as an absolute ranking for arbitrary substances. London dispersion is a specific interaction, whereas van der Waals is a broader terminology grouping in many conventions; the terms are not interchangeable.
Permanent charge patterns make orientation matter
Polar molecules also have dipole–dipole interactions. At comparable separation and a fixed favorable orientation, larger permanent dipole magnitudes give stronger dipole–dipole contributions; compare orientation as well as magnitude. Favorable arrangements bring δ+ regions near δ− regions; orientation changes the interaction, and thermal motion continually changes orientations. A permanent dipole can induce a dipole in a nonpolar molecule. These dipole-induced-dipole attractions grow with the polar molecule’s dipole magnitude and the other molecule’s polarizability.
An ion attracts the oppositely signed region of a polar molecule through ion–dipole interaction. Around a cation in water, the oxygen δ− side points inward; around an anion, hydrogen δ+ sides point inward. These forces generally exceed comparable dipole–dipole interactions, but their strengths still depend on charge, size, distance and surroundings. The polarity of the whole molecule, not merely one polar bond, determines its permanent-dipole interaction.
Hydrogen bonding needs the right partners
In the AP model, a hydrogen covalently attached to N, O or F can be strongly attracted to a lone-pair-bearing N, O or F site in another molecule, or another part of the same molecule. The X–H covalent bond and the hydrogen-bond contact are different interactions. Methanol can both donate an O–H hydrogen and accept at O. An ether can accept from water, but lacks an O–H, N–H or F–H donor and does not form the same hydrogen-bond network by itself.
Noncovalent contacts can organize large biomolecules or polymers, between separate chains or between distant regions of one chain. Hydrogen bonds, dipoles, dispersion and charged sites influence folding and association. Changing one interaction can change shape and function without breaking the covalent backbone. For a mixed pair, inspect what each partner can actually offer rather than applying the forces of the pure substance automatically.
Let's work through it
Work it through
Compare pure ethanol, pure dimethyl ether and an ether–water mixture. Both small organic molecules have formula C2H6O, but ethanol has O–H and the ether does not.
- Both molecules have electrons and similar size, so both exhibit dispersion. Both are polar, so pure samples also have dipole–dipole contacts.
- Ethanol has an O–H donor and oxygen acceptor sites, so ethanol molecules can form intermolecular hydrogen bonds with one another. Ether molecules lack the qualifying donor, so they do not form such self hydrogen bonds.
- Water can donate an O–H hydrogen to ether’s oxygen lone pair. The ether–water pair therefore can hydrogen bond even though pure ether lacks self-donor contacts.
- A supported comparison of the similarly sized pure liquids predicts stronger net attraction and a higher boiling point for ethanol, all else comparable. The justification depends on both size and specific available contacts.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Do nonpolar molecules lack intermolecular forces?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Do nonpolar molecules lack intermolecular forces?
Worked answer and reason
No. Their fluctuating electron density produces dispersion interactions.
Zero permanent dipole does not mean zero temporary dipoles.
Retrieval 2
Which end of water points toward Mg2+?
Worked answer and reason
The oxygen δ− end.
Opposite signs attract in ion–dipole orientation.
Retrieval 3
Can an ether accept a hydrogen bond from water even without an O–H bond of its own?
Worked answer and reason
Yes. Ether oxygen has an acceptor site while water supplies the donor H.
Hydrogen bonding depends on the two partners, not on identical functional groups.
Retrieval 4
Can noncovalent contacts within one protein influence its shape?
Worked answer and reason
Yes. Distant chain regions can interact through hydrogen bonding, dispersion, dipoles or charged groups.
A single large molecule has spatially distinct regions that can contact one another.
Catch the tempting mistake
- Hydrogen bonding means any hydrogen atom attracts any other atom.
- The AP donor criterion requires H bonded to N, O or F and an appropriate N/O/F acceptor region.
- Dispersion is always the weakest total attraction in a sample.
- Large polarizable particles can have strong net dispersion.
- An O–H covalent bond breaks when water boils.
- Vaporization separates molecules by overcoming intermolecular contacts; intact water molecules retain covalent O–H bonds.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 1 · ap application · multiple
- Unit 3 problem 2 · ap synthesis · verbal
- Unit 3 problem 3 · ap synthesis · particle
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For the teacher
- Require a drawing of a favorable pair with partial charges and partner labels.
- Include mixed-pair tasks and a large-nonpolar comparison to challenge a force-name hierarchy.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.1.A
- Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: i. The molecules are of the same chemical species. ii. The molecules are of two different chemical species.
- 3.1.A.1
- London dispersion forces are a result of the Coulombic interactions between temporary, fluctuating dipoles. London dispersion forces are often the strongest net intermolecular force between large molecules. Dispersion forces increase with increasing contact area between molecules and with increasing polarizability of the molecules. The polarizability of a molecule increases with an increasing number of electrons in the molecule and the size of the electron cloud. It is enhanced by the presence of pi bonding. iii. The term “London dispersion forces” should not be used synonymously with the term “van der Waals forces.”
- 3.1.A.2
- The dipole moment of a polar molecule leads to additional interactions with other chemical species. i. Dipole-induced dipole interactions are present between a polar and nonpolar molecule. These forces are always attractive. The strength of these forces increases with the magnitude of the dipole of the polar molecule and with the polarizability of the nonpolar molecule. ii. Dipole-dipole interactions are present between polar molecules. The interaction strength depends on the magnitudes of the dipoles and their relative orientation. Interactions between polar molecules are typically greater than those between nonpolar molecules of comparable size because these interactions act in addition to London dispersion forces. iii. Ion-dipole forces of attraction are present between ions and polar molecules. These tend to be stronger than dipole-dipole forces.
- 3.1.A.3
- The relative strength and orientation dependence of dipole-dipole and ion-dipole forces can be understood qualitatively by considering the sign of the partial charges responsible for the molecular dipole moment, and how these partial charges interact with an ion or with an adjacent dipole.
- 3.1.A.4
- Hydrogen bonding is a strong type of intermolecular interaction that exists when hydrogen atoms covalently bonded to the highly electronegative atoms (N, O, and F) are attracted to the negative end of a dipole formed by the electronegative atom (N, O, and F) in a different molecule, or a different part of the same molecule.
- 3.1.A.5
- In large biomolecules, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §10.1, Intermolecular Forces — Dispersion, dipole and hydrogen-bonding comparisons and vapor-pressure reasoning.
Glossary and deeper reading · 11 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Molecular polarity
Molecular polarity concerns a molecule's overall separation of charge. Both bond polarity and molecular geometry determine whether individual bond dipoles cancel. For example, carbon dioxide has polar bonds but is nonpolar overall because its linear bond dipoles cancel.
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London dispersion forces
London dispersion forces arise from fluctuating electron distributions and the dipoles they induce in neighboring particles. They occur between all atoms and molecules, including nonpolar ones. For example, dispersion attracts neighboring argon atoms even though they have no permanent molecular dipoles.
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Hydrogen bond
Hydrogen bonding is a directional interaction involving hydrogen bonded to a sufficiently electronegative atom and an appropriate electron-rich acceptor. In the introductory model, N, O, and F commonly provide the relevant partners. For example, water molecules hydrogen bond when one molecule's O–H group interacts with another molecule's oxygen lone pair.
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Dipole–dipole forces
Dipole–dipole forces involve interactions between permanent molecular dipoles. Favorable orientations place relatively positive and negative regions near one another. For example, neighboring hydrogen chloride molecules can attract through their permanent dipoles.
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Polarizability
Polarizability describes how readily a particle's electron distribution can be distorted. Larger, more diffuse electron clouds are often more polarizable, strengthening dispersion interactions when other factors are comparable. For example, iodine is more polarizable than fluorine.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Ion–dipole force
An ion–dipole force is an interaction between an ion and a polar molecule. The molecule orients its oppositely charged region toward the ion. For example, water's oxygen end points toward a dissolved sodium cation.
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Covalent bond
A covalent bond involves shared electron density between atoms. The sharing can be approximately equal or unequal depending on the atoms and their environment. For example, the bond in a hydrogen molecule shares electrons between two identical atoms.
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Ether
An ether has an oxygen atom bonded to two carbon-containing groups. Its oxygen can accept hydrogen bonds, but an ether lacking an O–H group cannot donate one in the usual introductory model. For example, dimethyl ether has an oxygen joining two methyl groups.
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Polymer
A polymer is a large molecule built from many linked repeating units. Chain length, branching, and interactions between chains influence its material properties. For example, polyethylene contains long chains of repeating carbon-based units.
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Pure substance
A pure substance has one chemical identity and a fixed composition. It may be an element or a compound, so containing more than one element does not make it a mixture. For example, pure water contains hydrogen and oxygen in a fixed ratio.