London forces: why do noble gas boiling points rise down the group, and are they always weaker than dipole-dipole?
Sara Fernandes11th Sample
21 Aug 2026 Β· 1 min read
Between football practice and guitar I got through the intermolecular forces section of States of Matter and there are two things I can't settle.
First, boiling points of noble gases go up as we go down the group, helium around 4 K and xenon around 165 K. NCERT says London forces (dispersion forces) are what hold them together, but I don't see why they would be stronger for bigger atoms when these atoms have no permanent dipole.
Second, my notes ranking says hydrogen bond > dipole-dipole > London. But I saw boiling points in a table: HCl about 188 K, HBr about 207 K, HI about 238 K. Dipole moments go down from HCl to HI, so the dipole-dipole forces should get weaker and the boiling point should drop. But it goes up! So are London forces sometimes stronger than dipole-dipole?
I also keep forgetting the distance dependence. NCERT gives an interaction energy proportional to for some of these and for others and I can't connect which is which. Please help me sort these out in a way I can write on a single page.
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[+][β]Neel Chatterjee12thSampleΒ· 1mo ago
London forces come from temporary dipoles. Electron clouds are always moving, so at any instant one side of an atom may have slightly more electron density, giving an instantaneous dipole which induces a dipole in the neighbouring atom. The bigger the atom, the larger and more loosely held its electron cloud (more polarisable), the easier it is to distort and the stronger the attraction. That's why boiling point climbs from He to Xe and Rn. The energy of this interaction falls off as , so it only matters at very short range.
[+][β]Sara Fernandes11thSampleΒ· 1mo agoAuthor
the instantaneous dipole explanation is the one I was missing. thanks neel!
[+][β]Aarav Kulkarni11thSampleΒ· 1mo ago
I thought London forces only act between nonpolar molecules like noble gases, and polar ones like HCl use dipole-dipole. Then HI shouldn't have any London forces?
[+][β]Vivek Mishra12thSampleΒ· 1mo ago
Aarav, London forces act between all atoms and molecules, polar or not. Polar molecules have dipole-dipole forces on top of that. In HCl, HBr, HI the molecules have many more electrons as we go down, so dispersion forces increase a lot, and that increase is more than the loss from the smaller dipole moment. So HI boils higher than HCl. So yes, London forces are not always weaker than dipole-dipole. HF is the odd one out because of hydrogen bonding, it boils much higher than HCl.
[+][β]Aarav Kulkarni11thSampleΒ· 1mo ago
okay, london forces are in everything, got it. Adding "even polar molecules" to my notes in red because I had it wrong
[+][β]Rahul DubeyDropperSampleΒ· 1mo ago
One page version of NCERT's distance dependence:
- Ion-dipole: energy falls as
- Dipole-dipole: for stationary polar molecules, for rotating ones (like in a liquid or gas)
- London (dispersion):
- Dipole-induced dipole:
Also the ranking is a rough guide, not a law. Size and polarisability can reverse things like HI vs HCl.
[+][β]Farhan ShaikhDropperSampleΒ· 1mo ago
Exam trick for boiling point comparison questions: first check for hydrogen bonding. If none, compare size or molar mass for same-type molecules. For isomers, compare shape: more surface contact means stronger London forces. n-pentane boils around 309 K while its branched isomer neopentane boils around 283 K, since the compact ball has less surface touching its neighbours.
