Intermolecular Forces: London Dispersion, Dipole-Dipole, and Hydrogen Bonding

Covalent and ionic bonds hold atoms together within a molecule. Intermolecular forces are the weak electrostatic interactions that act between molecules — and they're what determine physical properties like melting point and boiling point. This subtopic covers the three main types of intermolecular force, from weakest to strongest: London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

Key Takeaways

• Intermolecular forces are weak electrostatic interactions between molecules (not within them) that determine physical properties like boiling and melting point.

• London dispersion forces are the weakest intermolecular force, arising from temporary, fluctuating electron distributions that induce matching dipoles in neighboring molecules. They're the only intermolecular force present in nonpolar molecules, and they strengthen with molecular size/polarizability.

• Dipole-dipole interactions occur only between molecules with a permanent dipole moment, from attraction between the positive end of one polar molecule and the negative end of another.

• Hydrogen bonds are an especially strong dipole-dipole interaction that requires hydrogen covalently bonded to nitrogen, oxygen, or fluorine; the resulting "naked proton" is strongly attracted to N, O, or F atoms on neighboring molecules. Hydrogen bonds are about 5-10% as strong as a covalent bond, but still much stronger than a typical dipole-dipole interaction.

• Hydrogen bonding causes unusually high boiling points relative to molecular weight — illustrated by water (100°C) versus the heavier but non-hydrogen-bonding H₂S (−60°C).

• Overall strength ranking: hydrogen bonds > dipole-dipole interactions > London dispersion forces, which directly predicts relative boiling and melting points.

What Are Intermolecular Forces?

Intermolecular forces are weak electrostatic interactions between atoms and compounds. Unlike the covalent and ionic bonds covered earlier in this chapter, which hold a molecule's own atoms together, intermolecular forces act between separate molecules — and they significantly influence physical properties such as melting and boiling points. There are three main types, in increasing order of strength: London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

London Dispersion Forces

London dispersion forces (also called dispersion forces or London forces) are the weakest intermolecular force, and the only one that occurs between non-polar molecules.

They arise from temporary fluctuations in electron distribution. Because electrons are always in motion, a molecule's electron density is never perfectly even at every instant — at any given moment, electrons can bunch up more on one side of a molecule than the other, creating a temporary dipole. That temporary dipole induces a similar, matching dipole in a neighboring molecule, and the brief attraction between the two is a London dispersion force.

Despite being weak, London dispersion forces play a crucial role in the properties and behavior of substances — they're the only intermolecular force available to nonpolar molecules, so they're what allows nonpolar substances to condense into liquids and solids at all.

MCAT Callout — London Dispersion Forces and Molecular Size: Because London dispersion forces come from fluctuating electron distributions, their strength scales with how many electrons a molecule has and how easily that electron cloud can be distorted (its polarizability). Larger, more electron-rich nonpolar molecules experience stronger London dispersion forces than smaller ones — which is why, for example, larger nonpolar molecules tend to have higher boiling points than smaller ones.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules — molecules with a permanent dipole moment, rather than the temporary one behind London dispersion forces.

These forces arise from the attraction between the positive end of one polar molecule and the negative end of a neighboring polar molecule. Because that attraction is between permanent charge separations rather than momentary ones, dipole-dipole interactions are stronger than London dispersion forces.

It's important to note that dipole-dipole interactions only occur in molecules with permanent dipoles. Non-polar molecules don't exhibit dipole-dipole interactions, but they can still experience London dispersion forces.

Hydrogen Bonds

Hydrogen bonds are a specific, unusually strong type of dipole-dipole interaction, and they're frequently tested on the MCAT.

A hydrogen bond occurs when hydrogen is covalently bonded to one of three highly electronegative atoms: nitrogen, oxygen, or fluorine. In that bond, the hydrogen atom carries only a small share of the electron density — it essentially acts as a "naked proton," with most of the shared electron density pulled toward the electronegative atom it's bonded to.

That positively charged hydrogen atom then interacts with the partial negative charge on a nearby fluorine, oxygen, or nitrogen atom in an adjacent molecule. This interaction is significantly stronger than a regular dipole-dipole interaction — though it's still only about 5-10% as strong as a covalent bond.

Substances that exhibit hydrogen bonding often have unusually high boiling points compared to similar-molecular-weight compounds that don't form hydrogen bonds, because breaking hydrogen bonds requires extra energy on top of overcoming the weaker dipole-dipole and London dispersion forces also present.

MCAT Callout — Water vs. Hydrogen Sulfide Boiling Points: A classic illustration of this effect is water (H₂O) versus hydrogen sulfide (H₂S). H₂S is the heavier molecule, but sulfur isn't electronegative enough to support hydrogen bonding — so H₂S relies only on weaker dipole-dipole and London dispersion forces, and boils at about −60°C. Water, by contrast, hydrogen-bonds extensively and boils at 100°C, over 150°C higher than its heavier "twin," despite water's lower molecular weight.

Ranking Intermolecular Forces and Why It Matters

Putting the three forces together, from strongest to weakest:

• Hydrogen bonds — strongest; requires H bonded to N, O, or F.

• Dipole-dipole interactions — intermediate; requires a permanent dipole moment.

• London dispersion forces — weakest; present in every molecule, polar or nonpolar.

This ranking matters directly for physical properties: the stronger the intermolecular forces holding a substance's molecules together, the more energy (and therefore the higher temperature) is needed to pull those molecules apart into a gas — so substances with hydrogen bonding tend to have the highest boiling and melting points relative to their molecular weight, and substances relying only on London dispersion forces tend to have the lowest.

Common MCAT Mistakes

• Treating intermolecular forces as bonds within a molecule. Intermolecular forces act between separate molecules; the covalent/ionic bonds holding a single molecule's own atoms together are a completely different, much stronger category of interaction.

• Assuming nonpolar molecules have no intermolecular forces at all. Every molecule, polar or nonpolar, experiences London dispersion forces — nonpolar molecules simply lack the additional dipole-dipole and hydrogen-bonding contributions available to polar molecules.

• Calling any strong dipole-dipole interaction a "hydrogen bond." A true hydrogen bond specifically requires hydrogen covalently bonded to nitrogen, oxygen, or fluorine — a strong dipole-dipole interaction between other polar groups doesn't qualify, no matter how strong it is.

• Predicting boiling point from molecular weight alone. Molecular weight matters, but the type of intermolecular force present can dominate — H₂O boils far higher than the heavier H₂S because only H₂O can hydrogen bond.

MCAT-Style Concept Check

Question: Which pair of molecules would be expected to exhibit hydrogen bonding between them?

• A) CH₄ and CH₄

• B) CO₂ and CO₂

• C) HF and HF

• D) CCl₄ and CCl₄

Answer: C

Explanation: Hydrogen bonding requires hydrogen covalently bonded directly to nitrogen, oxygen, or fluorine. HF has hydrogen bonded directly to fluorine, so HF molecules hydrogen bond with each other. CH₄, CO₂, and CCl₄ contain no H–N, H–O, or H–F bonds, so none of them can hydrogen bond — they rely on weaker London dispersion forces (all nonpolar) instead.

FAQ

What are the three types of intermolecular force, from weakest to strongest?

London dispersion forces (weakest, present in every molecule), dipole-dipole interactions (require a permanent dipole moment), and hydrogen bonds (strongest, require hydrogen bonded directly to nitrogen, oxygen, or fluorine).

Why do larger nonpolar molecules have higher boiling points than smaller ones?

Larger nonpolar molecules have more electrons and a more easily distorted (more polarizable) electron cloud, which strengthens the temporary dipoles behind London dispersion forces. Since London dispersion forces are the only intermolecular force nonpolar molecules experience, a stronger version of that force directly raises the boiling point.

What atoms does hydrogen need to be bonded to for hydrogen bonding to occur?

Nitrogen, oxygen, or fluorine. Hydrogen covalently bonded to one of these three highly electronegative atoms becomes a "naked proton" that's strongly attracted to N, O, or F atoms on neighboring molecules.

Why does water boil at a much higher temperature than hydrogen sulfide, even though H₂S is heavier?

Water's hydrogen atoms are bonded to oxygen, so water molecules hydrogen bond extensively with each other. Sulfur isn't electronegative enough to support hydrogen bonding, so H₂S relies only on weaker dipole-dipole and London dispersion forces — despite being the heavier molecule, it boils at a much lower temperature (−60°C vs. water's 100°C).