 In the videos up until now, we have learned a bit about the bonds that hold atoms together, forming molecules. These are intramolecular forces, within molecules. But we know our world is made up of materials where many molecules are interacting, and so we need to learn more about the forces between molecules. These are inter-molecular forces. We consider three types of inter-molecular forces. Dipole-dipole interactions, hydrogen bonding, and dispersion forces. These three forces have the same foundation. The more negative part of a molecule is attracted to the more positive part of another molecule. They differ in how strong the negative and positive charges are, how permanent those charges are, and where on the molecule those charges reside. You could guess that the inter-molecular forces between these giant caffeine molecules are a bit different to those in the simple diatomic molecules that I sketched out here. First up, dipole-dipole interactions. These interactions occur when the molecules involved are polar, and so they have a permanent dipole, a more positive part and a more negative part. The molecules will arrange themselves so that the positive end of a polar molecule will attract the more negative end of the other molecule and influence its position. This arrangement reduces the potential energy of the material, which is a more favorable state for the material to be in. An example of this kind of inter-molecular bonding is hydrogen chloride. We usually draw dashed lines when we draw inter-molecular bond, as they are much weaker than the intramolecular bonds. Next up is hydrogen bonding. This is a special type of dipole-dipole bonding that occurs when there is hydrogen and one of the more electronegative elements present in a molecule. That is, fluorine, oxygen, or nitrogen. In these cases, the dipole over the covalent bond will be at a maximum because the hydrogen is very small and its partner in the bond is so electronegative. That means the electrons are spending the maximum possible time near the more electronegative element and spending most of their time away from the hydrogen in the bond. The effect of hydrogen bonding can be seen in an interesting and rather famous example of the boiling point of different compounds made of hydrogen and just one other element. In this plot, we're going to look at how the boiling point of a compound depends on the period of that other element. I'm going to talk about the compounds coloured in light blue, all of which are grouped six elements bonded with two hydrogen atoms. First looking at the right-hand side of this plot, we start with dihydrogen tellurium which has a boiling point near zero degrees. Moving one period up, we come to dihydrogen selenium which has a decreased boiling point of about minus 30 degrees. The next period up again takes us to dihydrogen sulfide which has a boiling point near minus 50 degrees. Now, if you had to guess what the boiling point is of the dihydrogen compound one period up would be, what would you guess? It would be sensible to follow this trend which gives you a boiling point of minus 100 degrees for dihydrogen monoxide. But it turns out that this is not a good prediction. We know that dihydrogen monoxide is water and actually has a boiling point of about positive 100 degrees. This is hydrogen bonding in action and we can see that it is present in hydrogen compounds with the fluorine and nitrogen as well. If we look at the trend for the group 4 elements however we can see that there's no hydrogen bonding present with carbon. That's because carbon is not nearly as electronegative as nitrogen, fluorine or oxygen and so we don't see the effect of hydrogen bonding. To end this video I'm going to leave you with this question on the screen which is about hydrogen bonding. I'll see you in the next video with the answer and an explanation.