2.2: Types of IMF
Types of IMFs, in order of strongest to weakest:
Ion-ion (aka Lattice Energy).
THESE ARE ALL JUST COULOMBIC ATTRACTIONS!
Not really an intermolecular force, but comes up in comparison to them often enough to be worth mentioning.
Not really a bond. Just a very strong dipole-dipole attraction. Must have a H bonded to a N, O, or F.
(note: a molecule with a N, O, or F but without a hydrogen on it can still be on the receiving end of a hydrogen bond from one that does)
Attraction between partial positive charges of one molecule and partial negatives of the next. Molecules must have a permanent dipole (see below).
Everything has these, but non-polar molecules only have these.
This is a temporary or induced dipole caused by electrons randomly 'sloshing' to one end of the molecule. This causes the neighboring molecules' electrons to respond the same way via Coulombic attraction.
Dispersion forces increase with size.
They can become greater than dipole attractions at large sizes, but you only ever make that determination based off experimental data. If you don't have data that says otherwise, the dipole forces are stronger.
Polar vs Non-polar compounds (permanent dipoles)
For a compound to have a permanent dipole (aka 'be polar'), two conditions need to be true:
1. Some of the bonds are polar
2. The dipoles of those bonds do not cancel each other out (this depends on shape)
Although it can be difficult to visualize, the dipoles of any of the base VSEPR shapes will cancel (linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral).
The only ways for dipoles to not cancel are:
1. Have a lone pair taking up one or two regions (H2O, NH3)
2. Have different atoms taking up one or more spots (CHCl3)
The dipoles in carbon dioxide directly oppose each other and cancel
The overall dipole of the molecule is the vector sum of the individual dipoles.
The molecule is non-polar, even though it has polar bonds.