 Hello, this is Professor Nesheben. I'm here to talk to you about intermolecular interactions. So here this first slide, I'm showing this to you to define some terms. So intermolecular means that we're talking about maybe two different covalent molecules or two atoms or two ions, and another term that we haven't discussed is what an ion is. An ion is an atom or molecule that has an extra negative charge, in which case we call it an anion, or an extra positive charge, in which case we call it a cation. And the interaction between them means that we're talking about an attractive force, something that draws them together, or a repulsive force that pushes them apart, and sometimes a reorienting force that would cause a molecule to twist. Now the big divide that we can make, the big distinction is that there are two primary kinds of intermolecular interactions. One of them is called recalling surface effects, and the other one we're calling electrostatic effects. So the next topic then I just want to explore a little bit of surface effects. And here we talk about repulsion and stickiness. So first let's just talk about surfaces. For atoms we often talk about what's called the van der Waals radius, and in Spartan that's what the space filling model shows, and that's just the radius of one of these argon atoms. Now for molecules we've already constructed these molecular van der Waals surfaces. They're called, we've actually colored them with charge, in which case we call them electrostatic potential mapping. But the surface itself is called the van der Waals surface. Now the key idea there is that if you try to bring two molecules or atoms together closer than the van der Waals radius, or the surface would indicate if they start to overlap, then that's a surface repulsion, and that's what's being indicated here. So the idea is that if you start these two atoms closer than their van der Waals radius, and then just kind of imagine letting go, then what will happen is that they'll spread apart, so that's repulsion, and then they will arrive at this equilibrium point. On the other hand, if you start a little bit too far apart, then they will experience attractive forces and draw into each other, and once again reaching this equilibrium point. And so the equilibrium point is where the surface repulsion and the stickiness between these two atoms are in balance, and that's kind of where they like to be. Now other names for stickiness that you probably may have encountered, sometimes they're called London forces, sometimes London dispersion, induced dipole, induced dipole, van der Waals, we'll just call it stickiness. Now the second part of that big divide is what I called electrostatic interactions. Basically it's opposite charges attract, like you've encountered this, sometimes perhaps when you pull socks out of a clothes dryer, they stick together, that's because there's opposite charges on the two socks. So like charges that is positive, positive, or negative, negative charges tend to repel each other. So here we have an attractive electrostatic force between those two ions, and here we have a repulsive electrostatic force between same charged ions. Now another point about this though is that ions tend to reorient nearby molecules that are polar, and I just kind of want to show this to you, here's a chloride ion, it's negative, so it tends to tug on the positive part of that water molecule. On the other hand that same ion tends to push away the negative part of that polar water molecule, so it'll tend to like put a twisting torquing force on that water molecule to try to reorient it. Now as it turns out polar molecules also reorient nearby polar molecules that are also polar. So here's a polar molecule, another one just like it, but same deal though, here's the negative part of this water molecule which will tug on that positive part, the hydrogen of that other water molecule, pulling it toward it at the same time that it pushes away repels the negative part of that other water molecule. So once again we have a twisting or reorienting of that second molecule. The first case here, you might have run across this term before, it's called an ion-dipole interaction, and here it's a dipole-dipole interaction, but the same, the result is the same in this case, it's a reorienting of the second molecule. Now you can also get combination of electrostatic and surface effects. So for example, here I have an electrostatic, an attractive force electrostatic between those two ions, that cation, that anion, and if you just imagine that you had them in this position and then you let them go, well then they would of course be attracted to each other, but they won't dive into each other because now they encounter this van der Waals repulsion of the fact that these two things have their own van der Waals radii, and also once they get into that equilibrium point they start to stick to each other, so that's a kind of a combination of the effects that we're looking at. Here's another story, here we have that same water molecule trying to reorient the second water molecule through an electrostatic reorienting force, so here I've redrawn the second water molecule so that it's been reoriented, and it's kind of rotated around, and now we have that negative charge talking to that positive charge that's that are closest to each other, so they will tend to be attractive, and so that'll draw them in together, and then once they are together, now that they're at their equilibrium point they won't get any closer because they got the van der Waals surfaces that would have to overlap, and then of course they'll start to stick to each other, so another point I just want to emphasize, there is always sticking at the end of the day once they once molecules get pretty close together, okay another concept that's pretty important is called the range of the attraction, so all forces, the surface effects, electrostatic effects, they have a typical range, so here I'm just imagining that I've got sodium and chloride at their equilibrium point, I could imagine pulling them apart a little bit out almost to what we call the electrostatic attraction range, but if I then let them go they would just return back to the equilibrium point, so that distance you know where it's almost to the point where it's too far, but it still pulls itself back to its original equilibrium point, we would just call that distance between the chloride and the sodium to be you know the range of the attraction, you know as a counterpoint if I pull them apart even farther past the this thing that we're going to call the electrostatic range, now they're so far apart they don't even know each other is there, and so now we pull them apart entirely, now there's one more one more thought here it's called the strength of the attraction, so we'll just call the intermolecular bond energy, the energy that you would have to put in to pull that sodium away from that chloride, which of course are being attracted to each other by this ion-ion electrostatic and sticking attractions, okay I imagine I pull them apart entirely as it turns out that takes a lot of energy and we would say that the intermolecular bond energy, the energy required to pull them apart is you know maybe around a hundred kilojoules per mole, which is the unit we're going to use, on the other hand I've got this dipole dipole and sticking interactions and pull those two water molecules apart, turns out that takes a middling kind of energy, this is turns out to be around 25 kilojoules per mole, not as big as a hundred, and now I go back to my two argon atoms, those are only held by together by sticking attractions, there's no electrostatics going on, pull them apart entirely, turns out that's in the low range maybe you know one kilojoule per mole or something like that, these numbers all vary, and so to summarize the concept summary we talked about anions and cations, we've talked about attractive repulsive and reorienting forces, we talked about surface stickiness and how it arrives at an equilibrium point, and then we also talked about electrostatic attractions and repulsions, we touched on the range of interactions which is the distance that you can bring them out to and they would still fill each other but if you take them past that range then they don't see each other, and then finally the strength of the attractions that's how much energy it takes to pull two parts of it apart.