Polarity and van der Waals forces
| English | Chinese | Pinyin |
|---|---|---|
| intermolecular force | 分子间力 | fèn zǐ jiān lì |
| dipole | 偶极 | ǒu jí |
| polar | 极性 | jí xìng |
| non-polar | 非极性 | fēi jí xìng |
| van der Waals' forces | 范德华力 | fàn dé huá lì |
| London dispersion | 伦敦色散 | lún dūn sè sàn |
Why some molecules stick
- Intermolecular forces 分子间力 act between molecules.
- They are much weaker than the bonds inside substances.
- They start with bond polarity.
Polarity and intermolecular forces lab
Classify molecules by the feature that controls attractions.
Compared with the bonds inside substances, intermolecular forces are:
Intermolecular forces (between molecules) are far weaker than ionic, covalent or metallic bonds within substances.
The weakest intermolecular forces, present between all molecules, are van der Waals (London) ______ forces.
Caused by temporary induced dipoles.
Polarity and dipoles 偶极
- When two atoms of different electronegativity bond, the electrons sit closer to the more electronegative one.
- This gives a dipole: one end slightly negative ($\delta-$), the other slightly positive ($\delta+$).
- If the dipoles don't cancel, the molecule is polar 极性; if they cancel by symmetry (like $\text{CO}_2$), it is non-polar 非极性.
Electronegativity rises across a period and falls down a group; fluorine is highest
A polar bond has:
Unequal sharing of electrons gives a dipole: δ− at the more electronegative end, δ+ at the other.
CO₂ is non-polar even though its bonds are polar because:
CO₂ is linear and symmetric, so the two bond dipoles point opposite ways and cancel.
Match each force to what it is.
Intermolecular forces, weakest to strongest: van der Waals, permanent dipole, hydrogen bond.
Van der Waals' forces 范德华力
- Van der Waals' forces is the general name for intermolecular forces. Two types:
- London dispersion 伦敦色散 (instantaneous dipole–induced dipole): brief dipoles from moving electrons; act between all molecules and get stronger with more electrons.
- permanent dipole–permanent dipole: between molecules that are always polar.
A momentary dipole induces one in a neighbour — the London dispersion force between all molecules
London dispersion forces get stronger when a molecule has:
More electrons give larger instantaneous dipoles, so London (dispersion) forces are stronger.
Bigger molecules, stronger forces
- Van der Waals forces get stronger as the number of electrons (molecular size) rises.
- This is why boiling points increase down a homologous series of alkanes.
You've got it
- intermolecular forces are weaker than the bonds inside substances
- a polar bond has $\delta-$ and $\delta+$ ends; a molecule is polar if its dipoles don't cancel ($\text{CO}_2$ cancels → non-polar)
- London dispersion acts between all molecules (stronger with more electrons); permanent dipole–dipole acts between polar molecules