VSEPR Theory and Molecular Geometry

Hybridization describes orbitals, but molecular geometry describes the shape of the atom. 

Concept: Molecular Geometry Explained.

8m
Video Transcript

So now let's bring this all together, talking about molecular geometry and VSEPR theory.
So VSEPR theory is something you should have learned in gen chem and what it basically says is bond sites will repel each other as much as possible. We already hinted at this when we talked about the bond angles of different atoms.
I just want to draw a distinction. When I talk about hybridization, I'm always talking about the orbitals that are hybridizing, that are blending, so sp2, sp3, whatever. When I talk about geometry, what I'm going to be talking about is the actual shape of the molecule. What does it look like if I were to put it under a microscope, what kind of shape would it have?
Let's go ahead and start off with these first three molecules on – I'll just make them 1, 2 and 3. What I want you guys to do is use the hybridization summary chart and use the rules that I taught you about bond sites to figure out what kind of hybridization these three will have. So go ahead and pause the video, try to figure out the hybridization for all three.
Alright, so if you were using the rules correctly, what you would have noticed is that these are all sp3. Even though they look very different, they're all sp3 hybridized. The reason why is because they all have four bond sites. Some of them have four atoms, some of them have atoms and lone pairs, but that's still four bond sites total.
So now what I want to do is enter in the amount of lone pairs that each one has. As you can see, this first one doesn't have any lone pairs. Okay, so I'm just going to put zero. The second one has one lone pair. And the third one has two lone pairs. Oh my gosh, two.
I have one and two. And you can see that all these are sp3 hybridized, but they all look different and that's where the geometry comes in. Even though they're all sp3 hybridized, we want words that are going to distinguish something that looks like 1, 1 is very different looking from 3. We want ones that are going to describe those shapes. So kind of like I say that something looks a circle or a square, I want shape names that are going to relate to these.
The name that I'm going to use for zero lone pairs – does anyone know? Think I heard you say it. Tetrahedral. You've heard of this forever or at least since gen chem. The tetrahedral is the name given to the shape of four bond sites with zero lone pairs, so every bond site is an atom or is a bond.
Now if I have one lone pair, what I'm going to envision is that, think about for all these names, think about that I can't see the lone pairs. Remember that electrons are tiny, so imagine that I'm looking with a microscope and I can't see the lone pair at all. What is this going to look like?
Well, if you think about it, it kind of looks like a pyramid, right? Think about it. This is like the top of the pyramid. This is like Giza or whatever. So the full name is trigonal pyramidal or what sometimes is just called pyramidal. Trigonal pyramidal, but most of the time it's just called pyramidal.
All right guys, so that's just the name of the shape. Then finally, what if we have two lone pairs? That means there's even less that I can see, so when I'm just looking at this, all I see is something that is bent. So bent would be the name that I give to it when there's two lone pairs. Is that cool so far? Awesome.
So now what I want to do is move on to other types of hybridization.
So what if I have this atom right here and this atom right here. I want you guys to also pause the video and figure out what kind of hybridizations these two have. Let's call these 4 and 5. Figure out what their hybridizations would be.
All right, both of these, because they have three bond sites each, are going to be sp2. Let me point out how it's three. For the double bond O, I have basically one atom here, one atom here, one atom here. That's three. For the N, I have one atom here, one atom here and a lone pair there, which is also three. Get it? So they're both sp2.
But then they also look different. They don't look the same as each other. Notice that one of these has three atoms coming off of it, the other one only has two. So if I were to talk about lone pairs, the lone pairs would be zero here and one here.
And it turns out that these, oftentimes, will get different names as well. So the name of this first one, just so you guys know, if you have an sp2 is zero, is trigonal, by the way, this is the same way that you spell trigonal for trigonal pyramidal, so in case you wanted to spell it trigonal pyramidal, it would be this word, trigonal planar.
Trigonal planar is the name of it. The reason why is because when you have an sp2 hybridized atom, all of the groups are going to be on the same plane. So, in fact, if you thought about it, the way that I like to think of sp2 is kind of like a sand dollar, where imagine that this is like, you're walking on the beach and you see a sand dollar and you pick it up. That's basically the way trigonal planar looks, where you have those three lines, a line here, a line here, a line here and they're all on the same plane. It's just a very flat object. It has two sides. So that's trigonal planar. Does that make sense so far? Cool.
Now, what about if it has one lone pair. Now, this actually is a little bit controversial. And I've been looking for a universal answer to this question, but really it turns out that it just varies by professor. I'm just going to tell you this. Some professors are going to call this bent because really, if you take out that lone pair, it looks bent. Does that make sense so far? Some professors are going to say that that's a bent.
But then other professors are actually going to say both of these are trigonal planar. I'm going to put here TP. They're just going to say, “Hey, both of them are trigonal planar. Don't worry about the lone pairs.”
So I'm just going to teach it to you both ways and just tell you guys to be aware, to be mindful when your professor says that. Maybe you even want to ask them, “Would you consider this molecule trigonal planar or would you consider it bent?” And you can also see the answers to practice problems, however they answer them, that will tell you.
So I just wanted to let you know those are two different ways of looking at it. Technically, trigonal planar is always correct. If you say that that's trigonal planar, that should be correct, but bent is just a little bit more specific and some professors want you to be that specific. Cool?
So then finally, we have this last one, which is really easy. A carbon – I'm not even going to make you guys pause it – a carbon with two groups, that should be what? That should be sp. In this situation, I don't worry about lone pairs or not lone pairs. It's always just going to get the same name and that is linear. This is why it's very important, whenever you're drawing triple bonds, it's very, very important that you always draw triple bonds in a straight line.
In fact, if you draw your triple bond bent, like with a zigzag, you will get points taken off your test. The reason is because it makes it look like you don't know what you're doing. It makes it look like you have no clue that that's supposed to be a linear hybridization or a linear shape. So, many times when you see a triple bond written, you're going to see it like this, 1, 2, and then a triple bond like that. That means that's right over my head. But that means you're indicating that, “Hey, this has a bond angle of 180 degrees and it's linear.” Cool?

Note: Many professors refer to ALL sp2 hybridized atoms as “trigonal planar” even though that is technically not correct for atoms with lone pairs. Just go with the flow if that happens!

Example: Predict the hybridization and molecular geometry of the following selected atoms.

2m
Video Transcript

Alright, so for this first one what I noticed is that this oxygen, even though it looks like it only has two bond sites, we know that can't be the case because it has to fill its octet, so remember, this actually has two lone pairs. If you didn't draw those in, then you're going to be super confused.
That has two lone pairs so that means this is going to be sp3. And it's sp3 that has two lone pairs so that means that the name of this is bent. Is that cool?
Let's look at this next one. This next one looks like it only has two bond sites. I'm talking about this one right here. It looks like it only has two, but remember there's a hydrogen there. You have to draw that hydrogen because, if not, the octet wouldn't be completed.
Now if this is throwing you off, adding stuff, you're like, “Johnny, how would I know that there's a hydrogen? How would I know that there's a lone pair?” Go back and review what I was talking about with the octet rule and with bond line structures because that means that you're having a hard time interpreting a bond line structure. That means that you have to go back and practice.
The bond line structure means there's an H there, so now I know that I have three bond sites and there's no lone pairs, so this is just going to be sp2 and it's going to be trigonal planar. Cool?
Then we have our last one over here. Once again, it looks like it has two bond sites, but we know that can't be the case because then carbon would be very angry at us. It needs two hydrogens. How did we know that? From using bond line structures.
So we have four different groups here. They're all atoms. There's no lone pairs. Those are going to be sp3 and it's going to be tetrahedral. Easy, right? Once you learn it, it's like second nature, but at the beginning, it can be a little confusing.
So now, I hope that made sense to you guys and I want to do some practice problems. 

Problem: Determine the hybridization of the following selected atoms: 

3m

Problem: PRACTICE: Determine the hybridization and molecular geometry of the following selected atoms:

5m