Intramolecular forces hold atoms together inside a molecule—think covalent bonds (shared electrons) or ionic bonds—and determine molecular identity and bond energies. Intermolecular forces are the weaker, noncovalent attractions between separate molecules or particles: London dispersion (temporary fluctuating dipoles), dipole–dipole, dipole–induced dipole, ion–dipole, and hydrogen bonding (H bonded to N, O, or F interacting with a lone pair). Intramolecular >> intermolecular in strength; breaking intramolecular bonds changes chemical identity, while overcoming intermolecular forces changes phase or solubility. For AP Chem, you should connect molecular structure (size, electron count, pi bonds, polarity, geometry) to relative intermolecular strengths (dispersion increases with polarizability and contact area; H-bonding and ion–dipole are stronger) as called for in Topic 3.1 (CED 3.1.A). For a concise study guide and practice questions tied to this topic, see Fiveable’s Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and Unit 3 overview (https://library.fiveable.me/ap-chemistry/unit-3). For more practice, check Fiveable’s AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).

London dispersion forces come from tiny, momentary imbalances in an electron cloud that create instantaneous dipoles. Those temporary dipoles induce opposite dipoles in nearby molecules and the resulting Coulombic attractions (partial + next to partial −) pull molecules together. Bigger molecules have stronger dispersion forces because they have more electrons and a larger, more easily distorted electron cloud (greater polarizability), and they can touch across a larger contact area—both increase the instantaneous dipole strength. Extended pi systems (more delocalized electrons) also boost polarizability, so long, flat, or branched molecules show stronger LDF. On the AP exam this is Topic 3.1: explain how structure (size, electron count, contact area, pi bonds) controls dispersion strength (CED 3.1.A.1). For a focused review see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and for extra practice try the AP problem set collection (https://library.fiveable.me/practice/ap-chemistry).

Dipole-dipole forces are just the attraction between polar molecules—think of each molecule as a tiny bar magnet with a partially positive end (δ+) and a partially negative end (δ–). When two polar molecules get near each other, the δ+ end of one lines up near the δ– end of the other and they attract. These forces add on top of London dispersion forces, so polar molecules of similar size usually stick together more than nonpolar ones. Strength depends on the size of the dipoles (bigger partial charges → stronger attraction) and on orientation—the alignment matters. Dipole-dipole < ion-dipole and generally > dipole-induced-dipole for comparable molecules. This idea is in AP Topic 3.1 (CED 3.1.A.2b–c); it helps explain things like higher boiling points for polar liquids. For a quick review, check Fiveable’s Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).

Hydrogen bonding is stronger than ordinary dipole–dipole or dispersion forces for two main reasons. First, H bonded to a very electronegative atom (N, O, or F) creates a very large partial positive charge on H and a large partial negative on the N/O/F, so the Coulombic attraction between molecules is large (CED 3.1.A.4; keywords: electronegative atoms, partial charges). Second, hydrogen is just one proton with a tiny electron cloud, so the H can approach the lone pair on a neighboring N/O/F unusually closely—shorter distance means a much stronger interaction (Coulomb’s law, orientation dependence). That short distance also gives hydrogen bonds some partial covalent/directional character, making them stronger and more orientation-dependent than regular dipole–dipole interactions. You’ll see this show up in much higher boiling points and stronger association (important for AP-style reasoning questions). For a quick review, check the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Even nonpolar molecules have intermolecular forces because their electron clouds aren’t frozen—they fluctuate. At any instant the electrons can be unevenly distributed, creating a temporary (instantaneous) dipole in one molecule that induces a dipole in a neighbor. The resulting Coulombic attraction is called a London dispersion force (fluctuating/temporary dipoles). Dispersion strength increases with polarizability (bigger, more electrons, larger electron clouds or lots of π-bonding) and with contact area between molecules, so large nonpolar molecules can have sizable attractions. On the AP exam you should be able to explain dispersion forces in these CED terms (temporary dipoles, polarizability, contact area) and compare them to dipole-dipole or dipole-induced dipole interactions. Want a quick review or practice? Check the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and hit the practice problems page (https://library.fiveable.me/practice/ap-chemistry) for exam-style questions.

Short answer: van der Waals forces is a broad label for the weak, noncovalent attractions between molecules (it commonly refers to London dispersion, dipole–dipole, and dipole–induced dipole interactions). London dispersion forces are one specific type of van der Waals force: they arise from temporary, fluctuating dipoles (instantaneous electron-cloud distortions) and are present in every molecule. Key differences to remember (CED-aligned): - Scope: “van der Waals” = umbrella term; “London dispersion” = one mechanism under that umbrella. The CED explicitly warns not to use them synonymously. - Origin: dispersion = Coulombic attraction between instantaneous dipoles; dipole–dipole and dipole–induced are due to permanent or induced dipoles. - Strength trends: dispersion increases with polarizability (more electrons, bigger electron cloud, more pi bonds) and contact area, and for large molecules it can be the dominant intermolecular force. For more AP-aligned review, see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and practice questions (https://library.fiveable.me/practice/ap-chemistry).

Start by ranking the types of forces present between the two species—the strongest relevant force usually dominates. Quick checklist (in order): 1. Ion present? If yes, ion–dipole (or ion–ion) beats everything else. 2. Can either molecule hydrogen-bond? If one has H covalently bonded to N, O, or F and the other has a lone pair on N/O/F, hydrogen bonding is next-strongest. 3. Are both molecules polar? If so, dipole–dipole interactions add on top of dispersion. Larger dipole magnitude → stronger dipole–dipole. 4. If one is polar and the other nonpolar, dipole-induced dipole matters (stronger when polarizability is high). 5. Otherwise, London dispersion forces dominate—but remember dispersion gets much stronger with larger size, more electrons, greater contact area, or lots of pi bonding (so big nonpolar molecules can have the strongest net IMF). Also consider orientation dependence (dipole interactions depend on alignment) and polarizability (bigger electron clouds → stronger dispersion). For AP Topic 3.1 problems use this hierarchy and cite structure: look for ions → H-bond donors/acceptors → polarity → size. For a review, see Fiveable’s Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and more practice at (https://library.fiveable.me/practice/ap-chemistry).

Polarizability is how easily an atom or molecule’s electron cloud can be distorted to form a temporary (instantaneous) dipole. Bigger species with more electrons and larger, more diffuse electron clouds are more polarizable; pi bonds also make clouds easier to distort. That matters because London dispersion forces come from Coulombic attractions between these temporary dipoles. So, for molecules of similar shape, more polarizable = stronger dispersion forces = higher boiling point, greater surface contact, and stronger dipole-induced interactions with polar partners (CED 3.1.A.1–A.2). On the AP exam, use this idea to explain trends: e.g., I2 (large, polarizable) has stronger dispersion than Cl2; larger nonpolar molecules mix less with small polar ones because dipole-induced dipole strength depends on polarizability (CED language). For targeted review and practice problems on these ideas, see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and more practice questions (https://library.fiveable.me/practice/ap-chemistry).

Ion-dipole forces are electrostatic attractions between a full ion (like Na+ or Cl–) and the partial charge on a polar molecule (like H2O). They occur whenever an ionic species interacts with polar molecules—for example, when salt dissolves in water the Na+ is attracted to the oxygen end (δ–) of H2O and Cl– to the hydrogen ends (δ+). These interactions are orientation-dependent (how the dipole lines up with the ion) and are typically stronger than dipole–dipole forces because a full ionic charge interacts with a partial charge. Ion–dipole forces help explain solubility of ionic compounds in polar solvents and appear in CED Essential Knowledge 3.1.A.2.c and 3.1.A.3. For a quick review, see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01), and practice problems are at (https://library.fiveable.me/practice/ap-chemistry).

Dispersion (London) forces come from instantaneous fluctuations in an electron cloud that create temporary dipoles; those dipoles induce opposite dipoles in nearby molecules and attract each other (CED: London dispersion, fluctuating dipoles, polarizability). Contact area matters because larger surface contact lets more of those instantaneous charge fluctuations overlap—more opposite partial charges can line up across the two molecules at once. That increases the total Coulombic attraction summed over the contact region, so flattened or elongated molecules (larger contact area) have stronger dispersion forces than compact, spherical ones of the same mass. Polarizability and electron-cloud size also matter: bigger, more polarizable clouds amplify those fluctuations (CED: polarizability increases with electron number and pi bonds). This idea shows up on AP problems comparing boiling points or solubilities—review Topic 3.1 in the Fiveable study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and practice questions (https://library.fiveable.me/practice/ap-chemistry).

A polar molecule has a permanent dipole (partial + and −). When it comes near a nonpolar molecule, its electric field pulls the nonpolar molecule’s electrons a bit toward the positive or negative end, temporarily shifting that electron cloud. That temporary shift creates an induced dipole in the nonpolar molecule with the opposite sign near the polar end, so the two attract. These dipole-induced dipole interactions are always attractive and add on top of London dispersion forces. Their strength increases if the polar molecule has a larger dipole moment and if the nonpolar molecule is more polarizable (bigger, more electrons, or more diffuse electron cloud). This idea is in Topic 3.1 (CED 3.1.A.2.a)—review the Intermolecular Forces study guide on Fiveable for examples and diagrams (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01). For more practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).

Proteins and big biomolecules are held together mostly by noncovalent intermolecular forces described in the CED. Locally, hydrogen bonds (N–H and O–H to lone pairs on N/O) and dipole–dipole interactions stabilize secondary structures (α-helices, β-sheets). Oppositely charged side chains form ionic (ion–dipole/ion-pair) attractions. Over large surfaces, London dispersion forces become very important: bigger electron clouds and greater contact area (and pi systems in aromatic side chains) increase polarizability and net dispersion attraction. These forces act between different molecules and between distant regions of the same molecule to fold and maintain tertiary/quaternary structure. Orientation matters for dipoles and H-bonds; dispersive forces are less orientation-dependent but scale with size. For AP review, focus on how polarizability, contact area, dipoles, and H-bond donors/acceptors determine relative strengths (see Topic 3.1 CED). For a targeted study, check the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01), the unit overview (https://library.fiveable.me/ap-chemistry/unit-3), and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Hydrogen bonding only happens when H is covalently bonded to a very electronegative, small atom (N, O, or F) because two things must coincide: a large partial positive on H and a lone pair on a strongly electronegative acceptor. N, O, and F pull electron density away from H, making Hδ+ very exposed; their small size concentrates negative charge in lone pairs so the H···A distance is short and the Coulombic attraction is strong (see CED 3.1.A.4 and keywords: partial charges, electronegativity). Bigger or less electronegative atoms (like Cl, S) don’t create as large a Hδ+ or as tight an interaction, so you get only weaker dipole or dispersion forces. Consequences: H-bonding raises boiling points and solubility (important for comparing molecules on AP free-response/multiple-choice for Topic 3.1). For a focused review, see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Boiling point increases when intermolecular forces (IMFs) are stronger—more energy is needed to separate molecules. To predict which substance boils higher, compare IMFs using this checklist: 1. Type of IMF (strongest → weakest for neutral species): ion–dipole/ionic > hydrogen bonding (H bonded to N, O, F) > dipole–dipole > London dispersion. 2. For dispersion forces, bigger = stronger: higher molar mass, larger electron cloud, more pi-bonding, and greater contact area increase polarizability and dispersion strength. 3. If two molecules have the same IMFs, the one with greater molar mass or more surface area usually has the higher bp. 4. Polar molecules have dipole–dipole in addition to dispersion, so they often boil higher than similar-size nonpolar molecules. 5. Ionic compounds (or solutions with ions) have very high bps because of ion–dipole/ionic attractions. Use AP vocabulary: London dispersion, polarizability, dipole–dipole, ion–dipole, hydrogen bonding. For more examples and practice, see the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and thousands of practice problems (https://library.fiveable.me/practice/ap-chemistry).

Intermolecular forces (IMFs) are what make molecules stick to each other—they control macroscopic properties you care about. Stronger IMFs (hydrogen bonding, ion-dipole, dipole-dipole, and dispersion) raise boiling/melting points, increase viscosity and surface tension, and reduce vapor pressure. They determine solubility (like dissolving salt vs. oil), chromatographic separations, and how salts dissociate (ion-dipole). At the molecular level noncovalent IMFs control protein folding, enzyme–substrate binding, DNA base pairing, and drug-receptor affinity—so biological function and drug design depend on them. Key trends: dispersion increases with polarizability and contact area; dipole interactions depend on dipole magnitude and orientation; hydrogen bonds require H bonded to N, O, or F. On the AP exam Topic 3.1 is part of Unit 3 (18–22% weighting), so be fluent with these concepts and how structure affects IMF strength. For a focused review, check the Topic 3.1 study guide (https://library.fiveable.me/ap-chemistry/unit-3/intermolecular-forces/study-guide/aq9pSoNJoW70eCdben01) and practice problems (https://library.fiveable.me/practice/ap-chemistry).