Polarity and van der Waals forces
Introduced| English | Meaning |
|---|---|
| intermolecular force/ˌɪntəməˈlekjʊlə fɔːs/ | intermolecular force |
| dipole/ˈdaɪpəʊl/ | A pair or separation of opposite electric charges, producing a positive and a negative end. |
| polar/ˈpəʊlə/ | Describing a molecule or bond with an uneven distribution of electric charge that creates partial positive and negative regions. |
| non-polar/nɒn ˈpəʊlə/ | Having no significant separation of electric charge in a bond, or no overall permanent dipole in a molecule. |
| van der Waals' forces/ˈvændɜː wɑːlz ˈfɔːsɪz/ | Weak nonbonding interactions between atoms or molecules; for the nonpolar substances here, the attractions are London dispersion forces. |
| London dispersion/ˈlʌndn dɪˈspɜːʃn/ | London dispersion |
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:
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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.
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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:
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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:
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CO₂ is linear and symmetric, so the two bond dipoles point opposite ways and cancel.
Match each force to what it is.
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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:
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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