 So at close distance then atoms would form covalent bonds because by pairing up both Electrons would feel like they had a complete first shell with spin up and spin down If you now take these atoms and pull them further apart, that's not quite going to work that way But on the other hand we know that lots of atoms even such as noble gases that already have two electrons in there As well that have completely paired shells. They also tend to interact That's if you lower the temperature far enough even helium will eventually become liquid, although that's when it's really cold So one way or another these atoms have to interact if they don't interact they would prefer to be in gas phase And what happens with those atoms is that if we draw a nucleus here, too This is an atom and then we have some sort of Completely full electron shell around it Well that atom is happy. It doesn't really need anything So you have lots of plus charges here. I'll just put four pluses It's just a symbol and then we have lots of minus charges out here minus minus minus that would be a very special shell But again just an illustration The net charge of that system is plus minus zero no question about it But what if I take another small molecule here Such as the ones that you looked at the other day water and That's it's me intermediate distance here. I have my water oxygen and then I have two hydrogens You all remember now that we have these partial charges in this molecule because the oxygen stole the electrons from the hydrogens a bit and We can draw that rather than doing plus and minus signs you could think of that as having a dipole here, right? So we can have a fairly Sorry wrong direction a fairly large dipole in the direction from minus two plus But this and again this molecule two is neutral, but it's not neutral in the sense It's neutral in the sense that it doesn't have a net charge But it the charges are not equally distributed inside the molecule and that's what gives it its dipole Now if these molecules start to sense each other what's going to happen is that? Effectively we have a bit of minus charge here on the oxygen while the plus charges on the hydrogens are further away We have minus charges here too and plus charges But what we can then do is that as these molecules get closer. What if we change the representation here on the left a little bit? So that we still have this nucleus with plus plus plus plus But then we shift this electron cloud a little bit away. I'm exaggerating horribly here What this will now mean is that the negative charges here have moved further away from the water and this water again If I just draw the entire water as a blob here as Minus there and plus there those minus signs are now going to like that. It's plus signs closer here. Well that We avoid the repulsion and increase the interaction a little bit between them and another way of doing that if both of these pens are arrows You can say that you have one dipole here and you have one dipole here and they have now lined up a bit So this is going to create a weak attraction The net charge here is zero the net charge there is zero But they will still attract each other a little bit because the dipole here is inducing an effect in this molecule But the question wasn't whether a noble gas would interact with water, right? We know that noble gases per se start to interact Well, what if we had two of these molecules? let's just think of it and If you were at absolute zero nothing would move right and then we would likely have this as the best possible state You could imagine but at finite temperature atoms oscillate even electrons oscillate They oscillate much quicker that starts happening at like one Kelvin So at some point in time If instead of having a water here, I imagine having Another similar molecule here. I have my plus charges and Here I have my minus charges But now it's at high temperature so at some point this molecule is shaking a little bit The electrons is going in one way completely natural as the surface in a glass of water will shake a little bit if I'm carrying it around But this molecule now has the properties like the water hat up here, right? This is effectively a dipole. It's a temporary dipole. It will disappear in a second, but right for now. It is a dipole This second atom will respond to that because here we now have a net dipole This one is still plus minus zero. It doesn't have a dipole But if this molecule now does the same type of shift of the negative charges in that direction You will effectively under very short time have these dipoles line up again and again dipoles lining up so what we had up here was what we would call at an induced Dipole and This in this case is an induced dipole Dipole interaction This will happen for any atom even if it's a noble gas because they have electrons and even if they're not charged It's going to be very weak because what will happen a nono second later much if actually much less than that femtoseconds Probably these electrons will have shifted back and then a little point in time later they might have shifted in the opposite direction and then this molecule will have to adapt and What that's going to lead to is that you essentially have an interplay, right? Where these molecules start shifting around but on average at finite temperature, they tend to be aligned You can calculate the shape of that and it's a dispersion interaction and it will go as 1 over r to the power of 6 So it will be fake a very quickly at large distances and it's also a very weak interaction Which is the reason that noble gases do not really attract each other by a whole lot So these are essentially the two extremes. We have hydrogen bonds very strong electrostatic interactions And on the other hand an explanation why things will interact at very very long distances There are more interactions, but before we go into all the other interactions. I will Start to think about a complication that just happened here because this was not supposed to be a class on quantum chemistry, right? But everything we do here is about electrons and quantum