Intermolecular forces
What holds one molecule to the next — and what that does to a boiling point.
Scientific learning path
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The comparison that shows it
H2O
373.15K
Molar mass 18.02 · 10 e⁻
Hydrogen bonding
CH4
111.7K
Molar mass 16.04 · 10 e⁻
No hydrogen bonding
Methane and water have the same number of electrons and almost the same molar mass. Size cannot explain the 261 kelvin between their boiling points. Hydrogen bonding can.
Down group 16
- H2O373.15 K
- H2S212.8 K
- H2Se231.9 K
- H2Te271 K
Boiling point rises with molar mass from hydrogen sulfide downwards, exactly as dispersion forces predict. Water is the lightest of the four and should therefore boil lowest of all. It boils highest, by about 100 kelvin.
Description of this simulation
H2O has a molar mass of 18.02 and 10 electrons. The forces between its molecules are hydrogen bonding, dipole-dipole attraction and London dispersion. The strongest of them is hydrogen bonding. It boils at 373.15 kelvin.
H2O
Strongest force here: Hydrogen bonding
- Molar mass
- 18.02
- Electrons in the molecule
- 10
- Boiling point
- 373.15 K
Forces between these molecules
Hydrogen bondingPresent
Hydrogen is bonded directly to nitrogen, oxygen or fluorine, which is the arrangement that makes this unusually strong attraction possible.
Dipole-dipolePresent
The molecule has a net dipole, so neighbouring molecules attract each other end to end.
London dispersionPresent
Present in every molecule, polar or not. Electrons move, so an instantaneous imbalance is always available — and it grows with the size of the molecule.
The ranking is a guide, not an ordering of every real substance: dispersion in a large molecule can outweigh dipole-dipole attraction in a small one.
Where this came from
What this model shows — and what it simplifies
Educational ModelIn this introductory self-association model, London dispersion is present in every molecule; dipole–dipole attraction requires a permanent molecular dipole; and hydrogen bonding is identified when a hydrogen-bond donor has H directly bonded to N, O or F and another molecule provides a suitable lone-pair acceptor.
Where do we see this in real life?
Water boils 261 kelvin above methane despite being the smaller molecule. That difference is why there are oceans rather than an atmosphere of steam, and why life happens in a liquid.