Molecular Geometry - Online Tutor, Practice Problems & Exam Prep

VSEPR theory helps determine molecular geometry based on hybridization. Molecules with four bond sites and no lone pairs adopt a tetrahedral shape, while one lone pair results in a trigonal pyramidal shape. Molecules with three bond sites can be trigonal planar or bent, depending on lone pairs. For two bond sites, the shape is linear. Understanding these geometries is crucial for predicting molecular behavior and reactivity in organic synthesis, particularly in reactions involving alkenes and alkynes.

Molecular Geometry Explained.

So now let's bring this all together talking about molecular geometry and VSEPR theory. So, VSEPR theory is something that you should have learned in gen chem, and what 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 sp_₃, sp, 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? Alright. So let's go ahead and start off with these first three molecules. I'll just make them 1, 2, 3. And what I want you guys to do is use the hybridization summary chart and use the rules that I taught you of bond sites to figure out what kind of hybridization these 3 will have. So go ahead and pause the video. Try to figure out the hybridization for all 3.

Alright. So if you were using the rules correctly, what you would have noticed is that these are all sp₃ hybridized. Even though they look very different, they're all sp₃ hybridized. And the reason why is because they all have 4 bond sites. Some of them have 4 atoms. Some of them have atoms and lone pairs, but that's still 4 bond sites total. Alright. So now what I want to do is enter in the amount of lone pairs that each one has. So as you can see, this first one doesn't have any lone pairs. So I'm just going to put 0. The second one has one lone pair and the third one has 2 lone pairs. Oh my gosh, 2. Alright. So I have 1, 2 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 words that are going to describe those shapes. So the name that I'm going to use for 0 lone pairs, does anyone know? I think I heard you say it. Tetrahedral. You've heard of this forever. Well, at least since gen chem. The tetrahedral is the name given to the shape of 4 bond sites with 0 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 name of this 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. Alright guys, so that's just the name of the shape. And then finally, what if we have 2 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 get...

...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 2 would have. So let's call these 4, 5. Figure out what their hybridizations would be. Alright. So both of these, because they have 3 bond sites each, are going to be sp₂. Let me point out how it's 3. For the double bond O, I have basically 1 atom here, 1 atom here, 1 atom here. That's 3. For the n, I have 1 atom here, 1 atom here, and a lone pair there, which is also 3. 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 3 things, 3 atoms coming off of it. The other one only has 2. So if I were to talk about lone pairs, the lone pairs would be 0 here and 1 here. 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 with 0 is trigonal, by the way, this is the same way that you spell trigonal for trigonal pyramidal. In case you wanted to spell out 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 this 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. So 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? So some professors are going to say that that's bent, but then other professors are actually just going to say both of these are trigonal planar. I'm going to put here... Alright. They're just going to say, Hey, both of them are trigonal planar. Don't really worry about the lone pairs. Alright. 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? 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 the 2 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 2 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 off taken off your test. Okay? And 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 like a triple bond like that. And that means that's right over my head. But that means that you're indicating that hey, this has a bond angle of 180 degrees and it's linear. Cool?

Predict the hybridization and molecular geometry

All right. So for this first one, what I notice is that this oxygen, even though it looks like it only has 2 bond sites, we know that can't be the case because it has a full octet. So remember, this actually has 2 lone pairs. If you didn't draw those in, then you're going to be super confused. That has 2 lone pairs, so that means this is going to be sp3 and it's sp3 that has 2 lone pairs, so that means the name of this is bent. Is that cool? Let's look at this next one. This next one looks like it only has 2 bond sites. I'm talking about this one right here. It looks like it only has 2, but remember there's completed. Now, if this is throwing you off adding stuff, you're like Johnny, how would I know there's a hydrogen? How would I know if 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 you have to go back and practice. All right. So the bond line structure means there's an H there. So now I know that I have 3 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 2 bond sites, but we know that can't be the case because then carbon would be very angry at us. It needs 2 hydrogens. How did we know that? From using bond line structures. So we have 4 different groups here. They're all atoms. There's no lone pairs. So this is 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.

Determine the hybridization of the following selected atoms: 

• A. 1:sp3
  2:sp3
  3:sp2
• B. 1:sp2
  2:sp2
  3:sp3
• C. 1:sp3
  2:sp2
  3:sp2
• D. 1:sp3
  2:sp2
  3:sp3

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

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is VSEPR theory and how does it determine molecular geometry?

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs around a central atom. According to VSEPR theory, electron pairs (both bonding and lone pairs) will arrange themselves as far apart as possible to minimize repulsion. This arrangement determines the molecular geometry. For example, a molecule with four bonding pairs and no lone pairs will adopt a tetrahedral shape, while a molecule with three bonding pairs and one lone pair will form a trigonal pyramidal shape.

How do lone pairs affect the shape of a molecule?

Lone pairs of electrons occupy more space around the central atom than bonding pairs because lone pairs are only attracted to one nucleus, whereas bonding pairs are attracted to two. This increased repulsion from lone pairs causes the bond angles to adjust, leading to different molecular geometries. For instance, a molecule with four bond sites and no lone pairs is tetrahedral, but if one of those sites is a lone pair, the shape becomes trigonal pyramidal. If there are two lone pairs, the shape is bent.

What is the difference between sp² and sp³ hybridization?

sp² and sp³ hybridization refer to the mixing of atomic orbitals to form new hybrid orbitals. In sp² hybridization, one s orbital mixes with two p orbitals, resulting in three sp² hybrid orbitals arranged in a trigonal planar geometry with 120° bond angles. In sp³ hybridization, one s orbital mixes with three p orbitals, forming four sp³ hybrid orbitals arranged in a tetrahedral geometry with 109.5° bond angles. The type of hybridization affects the molecule's shape and bond angles.

What is the molecular geometry of a molecule with two bond sites and no lone pairs?

A molecule with two bond sites and no lone pairs adopts a linear geometry. In this arrangement, the two bond sites are positioned 180° apart, resulting in a straight-line shape. This is typical for molecules with sp hybridization, such as carbon dioxide (CO₂), where the central atom forms two double bonds with two other atoms, creating a linear structure.

Why is it important to draw triple bonds in a straight line?

It is important to draw triple bonds in a straight line because molecules with triple bonds exhibit sp hybridization, which results in a linear geometry with a bond angle of 180°. Drawing triple bonds in a straight line accurately represents the molecular geometry and ensures clarity in understanding the molecule's structure. Incorrectly drawing triple bonds as bent can lead to misunderstandings about the molecule's shape and properties, potentially resulting in points being deducted on exams or assignments.