Van der Waal forces
- Weak, instantaneous forces between molecules caused by electron drift.
- These instaneous dipoles cause weak attractions between atoms/molecules.
- If electron density is not equally distributed, a dipole will occur.
- In turn, the negative pole will repel the electrons in a nearby atom or molecule, creating another dipole- the cycle repeats itself.
- Dipoles are caused by a difference in electronegativity between the two bonded atoms. e.g. H-Cl. The Cl will be negatively charged as the Chlorine is more electronegative.
- If two polar molecules are near eachother, their oppositely charged poles will attract.
- These are stronger than VDW forces.
- A hydrogen bonded to nitrogen, oxygen or flourine.
- and a lone pair.
The electronegativity difference is great, so the hydrogen is very positively charged. This means its very attracted the electron-dense-lone pair on the N, O or F.
They are the strongest of intermolecular forces, and explain why water is liquid at room temperatures.