 lecture I spoke a little bit about hydrogen bonds in terms of partial charges. You have the oxygen where you have the negative partial charge because it has stolen some electrons from the hydrogens. We talked about orbitals before and the reason why this happens we can describe this with orbitals too. All of this is due to the electron density around the atoms but if I were to draw this with orbitals what has actually happened is that the orbitals around the oxygen here they're in tetrahedral shape so that you kind of have one electron cloud pointing up here another one pointing up there as ears and then there are two of them as legs. What has then happened in a normal water molecule that for two of these orbitals those electrons have paired up with the hydrogens and that's why we have these larger rabbit ears here. So for two of those electron pairs there are literally formed pairs and they're formed stable bonds. Now a water is not reactive so we don't have unpaired electrons here but these pairs are so-called lone pairs. They're complete orbitals so it's not going to be super reactive that it's going to start binding to other things but you have a negative partial charge here due to this orbital and another negative partial charge due to this orbital. So that means that the entire oxygen here is going to be more negative while the hydrogens up here are more positive and as I mentioned last lecture this is a very strong effect. If we then take two of these atoms and bring them close together you're going to get this effect that this water down here is going to have a hydrogen here that is slightly positive and that's going to love to interact. Last time I said that it was interacting with the oxygen but it's not really interacting with the oxygen it's interacting with those lone electron pairs. We're going to need some sort of nomenclature to describe this because we're basically, this is a very generous water this water is basically taking his proton the hydrogen there and letting the other water molecule borrow his proton a bit to team up and become a partner to his very lonely electrons there. So we call this a donor a hydrogen bond donor and this would be a hydrogen bond acceptor and that is the hydrogen bond there's a UPAC standard that we're supposed to draw hydrogen bonds with three dots sorry UPAC if you're listening and I this is a very old illustration that I borrowed this is a strong interaction it's not as strong as a typical bond it's not as strong as a typical full-blown electrostatic interaction because remember we have a net zero charge here we have a net zero charge here I'm going to need to tell you what the interact it would be a bit absurd to introduce hydrogen bonds without telling you roughly how strong this is in the case of water you're talking about a few kcals maybe five kcals around 20 kilojoules per mole do you see that I made this mistake that I told you about a few so I just said five kcals bad Eric five kcals per mole but you're used by physicists now so when I said five kcals hopefully you assume that it was five kcals per mole so how do you determine which one is a donor and acceptor well this gets a little bit more complicated because what if you take that water atom assuming that I there might be a third water molecule here with two hydrogens this one is also interacting so this hydrogen would now form a hydrogen bond to that oxygen but in this case this is the acceptor and that is the donor so the acceptor and donor is not specific to the molecule in fact as you're going to see later is very common to have hydrogen bonds inside a single large molecule such as a protein the donor and acceptor criterion is around the specific hydrogen bonds so in this hydrogen bond that oxygen is the donor noticed it's the oxygen that's the donor not the hydrogen the oxygen donates its hydrogen or proton to this other acceptor that has the lone pair electrons but this in the sense of this particular water molecule that's also participating in this hydrogen bond in this hydrogen bond the same oxygen here acts as a donor and donates this proton to this oxygen as an acceptor so you need to understand the hydrogen bonds you need to understand the donor and acceptor and you need to I should be able to wake you up at three a.m. in the morning and you need to know what the energy of a hydrogen bond is seriously it's one of the most common questions I ask people in the exam and I'm simply flabbergasted that people haven't learned that by heart do yourself a favor look that up and train on that every day there is a reason why it's so important and there is a reason why it's important enough that you need to know it by heart