Molecular geometry (VSEPR theory)

0:00- [Instructor] A molecule of carbon dioxide is pretty much straight versus a molecule of water is bent. Why the difference? And more importantly, is there a way to predict what the shape looks like in three dimensions of any molecule? The answer is yes, by using a theory called, valence shell electron pair repulsion theory, or VSEPR for short.

0:19So in this video, VSEPR is going to help us predict the shapes of different molecules by using their Lewis structures. So let's begin. Let's directly start with an example. Say we have methane, its Lewis structure is given this way. Remember, Lewis structure does not give the shape of the molecule, that's what we wanna figure out.

0:36And so now, how does help us predict the shape of methane, for example? So VSEPR says, "Look at the central atom, which in this case is carbon, and count the number of electron groups around it". For example, the electrons that are participating in the bond, well, there is a group over here, there are a couple of electrons, so that's one group.

0:54So here's the second group, here's a third group, and here's a fourth group. So in this particular case, there are four electron groups around the central atom. A quick question could be, why do we say electron groups and not just bonds? Well as we will see, that it not just applies to bonds, it applies to other things as well.

1:09So think of it as electron groups and not just bonds, okay? And VSEPR says that these four groups of electrons are gonna repel each other, which makes sense, and then arrange themselves in such a way that they are as far away from each other as possible, as big angle that they can make as possible.

1:24Now, in two dimensions, if you had to do that, if you had to spread them out evenly and as far away as possible, and this is what it would look like, at a 90 degrees apart. But remember our molecules are in three dimensions. In three dimensions, they're gonna spread out like this, with the angle between them more than 90 degrees.

1:40Actually the angle turns out to be 109.5 degrees. The angle between any bonds over here would be 109.5 degrees. It's hard to see in the picture, but I have in front of me a model over here. And look, look at that, the angle between all the bonds are exactly the same.

1:58They are the farthest apart in all three dimensions. We given a name to this shape, we call it a tetrahedral geometry. We call it so because the vertices over here lie at the vertices of a tetrahedron. A tetrahedron is basically a pyramid with triangle base. Anyways, we have found the shape of methane.

2:16It's tetrahedral with carbon at the center and the four hydrogen atoms over here, and the born angle is gonna be 109.5 degrees. Let's go to the next one. Next up we have ammonia, whose Lewis structure looks like this. Remember, the nitrogen has a lone pair over here.

2:31But anyways, we have one, two, three bonds surrounding the central atom, right? But VSEPR says, "Mahesh, I don't care about the number of bonds, I care about the number of electron groups, okay?" And so if you count, we have one, two, three, and the lone pair of electron is also an electron group, right?

2:52Because VSEPR says, "I don't care whether the electron group is bonded or not bonded or whether it's in the lone pair, what matters is the number of groups, and there are four groups". So even in this case, there are four electron groups around the central atom.

3:05And this is the reason we say electron groups and not bonds, okay? By the way, the technical name is electron domains. So there are four electron domains, but you can think of this as electron groups. But anyways, the good news is since this is basically four electron groups, we already know what the shape is going to look like.

3:21Anytime we have four electron groups, the shape will always be tetrahedral, boom, got it. But there's a tiny difference, it's just that here, because there are only three actual bonds, only three of these will be actually hydrogen atoms. So for example, only three of these will be hydrogen atom, the fourth one is occupied by the lone pair.

3:41So this is something how we show it. And a small detail over here is because the lone pair electrons are not bonded, they are very close to the nucleus of the nitrogen, unlike the electrons over here which are shared between the nuclear of nitrogen and hydrogen.

3:55But anyways, because they are very close, they will have much stronger repulsion compared to normal single bonds. And therefore they're going to push on these bonds slightly more than usual single bonds do. And as a result, the angle between the bonds over here is gonna be less than 109.5 degrees, it happens to be somewhere close to 107.5 degrees.

4:18The actual number is not really important, but it makes sense why it is less than that actual tetrahedral bond angle, 109.5 degrees, right? But anyways, now what is the shape of ammonia? We might say it's tetrahedral, but that will not be accurate because there's no thing, there's no atom occupying this space, right?

4:38So the actual shape would be just this, this is how we show it. And we give a different name to it, we call this shape trigonal pyramidal. You can just basically think of this as a tetrahedral shape with one of the corners removed. Whatever we get is now like a tripod shaped trigonal pyramidal, okay?

4:56And so we distinguish between these two things, we call this the electron geometry because this is the shape that represents how the electron groups are oriented. But then when we get rid of the lone pair, we talk about only the atoms, only the bonds that are actually present, we call this the molecular geometry.

5:12When people are talking about the shape of a molecule, they're actually talking about the molecular geometry. The shape of ammonia is not tetrahedral, but trigonal pyramidal. But in the back of my head, it's kind of the same thing. It's basically a tetrahedral shape where there's nothing over here, so you get rid of it, and now you get this new shape, which looks like a tripod.

5:30So trigonal pyramidal, makes sense, right? But anyways, in the previous case, note that the electron geometry and the molecular geometry was the same because we only had bonds over here, there were no lone pairs. Onto the next one, water molecule, whose Lewis structure looks like this.

5:48Why don't you pause the video and think about what the shape is going to be. Think about the number of electron groups around it. Try to figure out the electron geometry and then the molecular geometry. Pause and try. All right, here we go. This time we see there are two bonds, but you probably know by now what VSEPR says, "I don't care about the number of bonds, count the number of electron groups".

6:08So I have one, two, three, four, because there are two lone pairs, there are still four groups. So I immediately know because there are four electron groups around the central atom and the central atom here is oxygen, I already know what that shape is gonna look like.

6:23Again, it's gonna be a tetrahedral shape. And just like before, this time there are only two of these are occupied by hydrogen, and the rest two would be your lone pair. So let's say these two are hydrogen, so we'll replace these two with lone pairs. And just like before, the lone pairs are going to cause more repulsion.

6:42So what will happen to this bond angle? It's gonna be smaller than 109.5 degrees. But here's a question for you, okay? In the previous case, in the ammonia, the bond angle was about 107.5, what do you think the bond angle over here is going to be? Is it going to be less than this number or more than that?

6:57What do you think? Well, I'm thinking that there are two lone pairs over here, so probably the push is gonna be stronger, and probably, as a result, the bond angle is gonna be much smaller, right? Guess what? That's exactly what happens. The bond angle actually becomes even smaller.

7:12Again, the exact number is not important, but look at where we've reached. Not only are we predicting the shape of the molecules, but we are even comparing and intuitively thinking about which bond angles must be bigger and smaller. That's pretty awesome, if you ask me.

7:26But anyways, just like before, this represents the electron geometry, and if you now want the molecular geometry, we'll just just get rid of these two, whatever now remains is the molecular shape of water. And look, that shape is just the bent shape. So this is the molecular geometry.

7:42Again, in the back of my head, it's basically tetrahedral because there are four groups, but because there's nothing occupying over here, I just get rid of it, and look what remains is that bent shape. There you have it, that's why the water molecule has this bent shape.

7:55Okay, so we got three done so far, let's do a couple of more. Next up, we'll look at formaldehyde, whose Lewis structure looks like this. Again, great idea to pause the video and see if you can predict what the geometric is gonna look like. It's fun to do that.

8:08Anyways, now I'm looking at this and I say, "Hmm, there's one, two, three and four bonds around the central atom". And again, Vesper is gonna tell me, "Mahesh, I don't care about the number of bonds, I only care about the number of groups, count the number of groups around the central atom".

8:24And I'm like, "Okay, okay, okay, okay", so there's group number one, it's the second group, and what about this? This is one entire group. Again, VSEPR doesn't care whether that group is single bonded electrons or it's a group of four electrons in a double bond or it's a lone pair, none of that matters, this is one group.

8:42So there's one, two, and three, there are three electron groups around the central atom this time. But wait a second, what about the lone pair? Shouldn't we also consider the lone pair? Remember VSEPR says, "Mahesh, around the central atom, the central atom is carbon, the lone pair is not on the carbon, not on the central atom, okay?"

9:00So let's not get confused by that, I don't care about any other atoms, VSEPR only cares about the central atom, all right? So we have three electron groups. And so now we have a geometry in which three groups are gonna try and be as far away from each other as possible.

9:14What does that geometry look like? That geometry is gonna look like this. It's like a fidget spinner, you can think of it, okay? Basically all of them will be in the same plane and they'll be 120 degrees apart. And so we call this the trigonal planar because they're all gonna stay in the same plane.

9:32Notice how it's different than what we got in ammonia, that was trigonal pyramidal. Again, remember for ammonia, we had tetrahedral electron geometry, so we got rid of one of them and we got trigonal pyramidal. Notice that these four are not in the same plane.

9:45If I keep it on the table, the blue central atom is protruding up. But look at your formaldehyde structure. This, these atoms are in the same plane. Can you see that? They're all in the same plane, that's why it is trigonal planar. So if you have three electron groups surrounding a central atom, you'll always get a trigonal planar.

10:06And by the way, if you're wondering, shouldn't this be a double bond? Yes, I'm not actually showing the molecule, I'm just showing the shape of the molecule. So yes, there will be a double bond over here, it'll be an oxygen here, hydrogen over here, and another hydrogen over here.

10:19Onto the next one, the last one we'll consider carbon dioxide. Lewis structure looks like this, okay? The central atom over here is carbon. How many groups of electrons around the carbon? I have one group of electrons. Again, notice, I don't care that it's a double bond and there are four electrons over here, it becomes one single group, and there's a second group.

10:39So there are two electron groups around the central atom. Again, do I care about these lone pairs? No, because that's not the central atom, only compare, only look at the central atom, all right? So what does a shape look like if I have two electron groups around the central atom?

10:53Well, they'll be as far away as possible. And if you have just two groups and they wanna be far away as possible, they'll just be opposite to each other, giving you a linear structure, with the bond angle of 180 degrees. And this now answers our original question as to why carbon dioxide is linear, because there are only two groups that are repelling each other versus when it comes to water, notice there were four groups.

11:14Even though there were two atoms, the oxygen was bonded to there, four groups because of the lone pair, that's why we had the tetrahedral electron geometry giving us the bent molecular geometry. So it answers our original question, isn't it? So here's a summary in one page.

11:28I know it looks daunting, but think about it logically, okay? So if you have four electron groups that is surrounding a central atom, then you'll get tetrahedral electron geometry. Now, if all of them happens to be four bonds and there are no lone pairs, that is the molecular geometry.

11:45But out of it, if there's one lone pair, get rid of that lone pair, what remains is trigonal pyramidal. Out of that tetrahedral, if there are two lone pairs, get rid of them and what remains is the bent geometry. And if you have three electron groups surrounding the central atom, then you'll get a trigonal planar electron geometry.

12:02And again, if all three are bonds, then that itself becomes the molecular geometry as well. And if you have two of them, well it just ends up becoming linear. I personally don't like to memorize these things over here or even refer to a table because all of this can be done logically.

12:17That was the whole point of VSEPR, right? So I will always like to figure out the central atom, count the number of electron groups, figure out what the electron geometry is going to be, and then get rid of all the lone pairs, and then finally arrive at the molecular geometry.