Molecular dipoles

In chemistry, we're going to see situations where a molecule, an entire molecule itself, might be neutral. But because of the differences in electronegativities and how the molecules are structured, you might have a partially positive charge on one side and a partially negative charge on another side.

The most famous example of that is a water molecule. A water molecule—these have three water molecules depicted here with the white oxygen molecule in the center and the two hydrogens right over here. This is the shape of the water molecule. Sometimes you'll see it with these electron pairs on the other end from the oxygen from those hydrogens.

But what's interesting, at least in the course of this discussion about a water molecule, is that even though the entire molecule is neutral, you have a total of 10 protons and 10 electrons in total—even though it's neutral, because of this shape and because the oxygen is more electronegative, you can end up with a more positive charge on one end and a more negative charge on the other end.

To reinforce this, we look at the periodic table. We know, in general, as we go to the top right, we get more and more electronegative. Oxygen is one of the most electronegative of the elements—it's a good bit more electronegative than hydrogen right over here.

In this situation, in this water molecule, the electrons in the covalent bond between the hydrogen and the oxygen are actually going to spend more time. They're going to spend more time around the oxygen. I could draw something like this; they're going to spend more time around here because oxygen likes to hog the electrons more than the hydrogen does.

What it would do is form a, you could say, a partial negative charge at the side of the molecule away from the hydrogens and a partial positive charge on the sides where the hydrogens are. This situation, where you have a separation of positive and negative charges within a molecule, is called a molecular dipole. Let me write that down: molecular dipole.

In a physics class, you might study the general idea of a dipole. This is an electric dipole, where you have a separation of your positive and negative charges. In this case, we call it a molecular dipole because it's, of course, across the short space of an actual molecule. This should not be confused with a magnetic dipole, which we might also talk about in other videos.

Now, in a water molecule, the dipole is a permanent dipole because we're not changing electronegativities here. The oxygen just likes to hog those electrons more. So this situation—let me write this—is a permanent dipole. Because of this permanent dipole among these water molecules, that's what allows these water molecules to be attracted to each other.

So this molecule right over here, this end, is going to be partially negative, this part n is going to be partially positive. This lowercase delta here is how we denote that partial charge, and those look like eights. Let me write them a little bit neater.

So, partially negative and partially positive right over here. This would be partially positive, partially positive, partially negative. Because of this permanent dipole of the water molecules, you can have attraction. The partially negative charge sides are going to be attracted to the partially positive charges, and these are the famous hydrogen bonds of water.

So that is H2O. That's going to be—yeah, I drew most of the attraction that you see over there. Now, all molecular dipoles don't have to be permanent. Here is a depiction of some methane molecules. Methane is CH4, and you see in methane that the carbon is bonded somewhat symmetrically.

You have this tetrahedral shape where the hydrogens are evenly distributed—they're getting as far away as they can from each other. It's a very symmetrical, three-dimensional shape. Also, carbon and hydrogen's electronegativities aren't that far apart. Carbon is a little bit more electronegative, but because they're close and there's this symmetry to the actual molecule, there isn't a permanent dipole in this situation.

However, you have to remember these electrons that are buzzing around between these bonds and around the nuclei of the different atoms—they aren't just staying in one place at any given time. You might have some of the electrons more on this side or more on this side of this molecule. You'll have a temporary partial negative charge; there may be a temporary partially positive charge.

That might be the exact same time that this molecule right over here has this molecule right over here—maybe its electrons happen to be more distributed more in that direction, so it's partially negative there and partially positive here. When you have just these random dipoles form at just a certain moment in time, just based on at that moment how the electrons are distributed, we call that a temporary dipole.

Temporary dipoles can create these temporary forces between molecules. We studied that in other videos: the famous London dispersion forces. Right at the moment that this one is partially negative and this one is partially positive, maybe that's right at the moment that this one is partially negative at this side and partially positive at this side.

So the electrons are spending a little bit more time here, and so they might be attracted to each other. Not only could it just be a temporary thing, it could also be induced. If this one just happens to be in that state and it's partially positive on this side, well, the electrons here on this molecule will be naturally attracted.

So, it might be an induced dipole, and you could have to be more explicit: induced dipoles because of an external force of the environment, like an electric field. You could have an induced dipole.

Let's say you have a molecule of H2O right over here, and let's say you have a molecule of neutral methane right over here. That neutral methane—you know its electrons are moving around, but it has no permanent dipole. But as it gets near this water molecule right over here, it has a permanent dipole where it has a positive charge here.

So maybe the methane molecule's electrons might be attracted more to that positive charge. In this situation, where you take something that otherwise would only maybe have temporary dipoles, because its electrons might be attracted to something or repelled from something here, you could have an induced dipole.

Dipoles actually don't just happen to be with multi-atom molecules. You can even have not permanent dipoles, but you could have induced or temporary dipoles from a single atom. Let's say we were to take an atom of argon. The noble gases are famously non-reactive, so let's say that this is one atom of argon.

Let's see, that's its electron cloud; let's say that's its nucleus there, and this is a huge oversimplification. You could imagine that just at some random point, the electrons are just a little bit more distributed on that side. You have this temporary dipole that would form, where you're more negative here and more positive right over here.

Of course, if this is next to something, they might be able to interact with each other. If this is maybe you could even induce this—it might not be random. It might be induced by a polar molecule nearby, something with a permanent dipole, or it might be induced by a magnetic field.

Well, I'll leave you there. But this idea of molecular dipoles, whether they're permanent, induced, or temporary, is a really neat idea because it actually explains a lot of the properties of molecules—their boiling points, their freezing points, how hard it is to get them to get into a gaseous state versus a liquid versus a solid state.

We'll see a lot of that in chemistry.