VSEPR Theory Practice Problems (Advanced)
- Let's do some practice problems. We'll look at Lewis structures where the central atom is surrounded by either 5 or 6 things. And then, I'll talk you through the thought process for how to determine what the 3 dimensional vesper shape for the molecule is and what the bound angles are. Okay.
If you haven't already done it, you might want to watch Vesper videos part 2 and part 3. They talk about the background information for this stuff just so we're on the same page. Okay. So, here's our first Lewis structure. And, this is for xc02f2.
The first thing we want to ask ourselves, when we're figuring out the vesper structure is how many things are there around the central atom? So, xenon here is the central atom. There are these two double bonds to oxygen. Remember, in vesper, double bonds count for the same as single bonds. It's just a bond to an atom. Okay.
So, 1, 2, 3, 4, bonds and then a lone electron. So, there are going to be 5 total thins around this atom. So, number 2 asks what structure is this molecule based on. What I mean is, if something has 5 things around the central atom, it's based on a particular structure. It's going to be based on this shape here which is trigonal bipyramidal.
So, this is the shape, trigonal bipyramidal that we want to have in our minds. Okay. Now, what I mean by based on is that, if the 5 things surrounding xenon were all atoms, then, it would be a trigonal bipyramidal shape. But, they're not. Okay. They're not all 5 atoms.
So, number 3 here asks which atoms are replaced by lone electron pairs. So, in this molecule, there are 4 atoms and then 1 lone electron pair which means that 1 of the atoms in this trigonal bipyramidal molecule is going to be replaced by a lone electron pair.
Now, if you remember from what we said earlier, it's the equatorial atoms that get replaced by lone electron pairs first. So, that means that our final structure of this molecule is going to look like this where you can see here 1 of the equatorial atoms has been replaced by a lone electron pair. Okay. But, keep in mind that they're very similar.
This shape which is called the seesaw shape, is based on and related to the trigonal bipyramidal shape. It's just, it's missing one of these equatorial atoms and it's been replaced an unshared electron pair. So, this molecule here has the seesaw shape. Let's talk about angles really quickly. Okay.
In the trigonal bipyramidal there is a 180 degree angle between these two atoms here. And, if we look from the top down, there's 120 degrees between each of these atoms. Okay. Now, in the seesaw, we have this unshared electron pair here. And, as we said before, unshared electron pairs push harder against atoms because they want a little bit more space.
So, that means that these two atoms are going to get pushed closer together like this meaning that the angle between them is going to be less than 180 degrees. If we look at the top down view of this, we see the electron pairs here and they are pushing, pushing these atoms together, which means that, if we look at this top down view here, these atoms are getting pushed together.
So, the angle between them is less than 120 degrees. So, that's what the angles are for this, for this seesaw molecule here. brf3, what's the vesper shape? Okay. Step number 1, how many things are there around the central atom? There are 1, 2, 3 bonds and 1, 2 lone electron pairs. So, 5 total things. So then, what structure is this molecule based on?
Well, any molecule that has 5 things around the central atom is based on this trigonal bipyramidal shape. So now, question 3. Which electrons in this are going to be replaced by lone electron pairs? So, this has 5 atoms. But, brf3 only has 3 atoms then it has 2 unshared electron pairs. So, what it's going to look like is this right here.
Look at how similar this is to the trigonal bipyramidal shape. But, it's that 2 of these equatorial atoms, maybe say this one and this one, this one and this one, have been replaced by unshared electron pairs. Here are the unshared electron pairs, these two and these two. Okay?
So, what that causes is this T shaped molecule. This has a T shape. And, in terms of angles, we just looked at these angles a minute ago. And the T shaped is going to have an angle that is less than 180 degrees between these two atoms because these lone electron pairs are going to be pushing inward on these two atoms.
It's also going to mean the angle between these two bonds are less than 90 degrees. So, these are both less than 90 degrees. And then, this whole thing from here to here is going to be less than 180. So, that's what brf3 looks like, a T shaped molecule because 2 of these equatorial atoms from a molecule that it's based on have been replaced by lone electron pairs.
Okay. iof5, first thing we ask, how many things are there around the central atom? There are 1, 2, 3, 4, 5, 6 bonds, remember double bones single bonds, they count for the same, 6 bonds around this central atom. So, what molecule is it based on if there are 6 bonds, 6 atoms around the central atom? It is based on an octahedral, oh that was the wrong one. It's, there we go, that's my octahedral molecule. It's based on an octahedral molecule.
Now, which atoms are going to be replaced by lone electron pairs? They're not going to be because there are no lone electron pairs in this molecule. So, that means that every single one of these atoms is going to be in place here. And, my final structure is going to be octahedral. And, just to brush up on the angles. Every single one of these angles, in an octahedral, between any 2 bond, between any 2 bonds is going to be 90 degrees. They're all right angles.
So, this is real easy. 6 things, 6 atoms, no unshared electron pairs, and you have an octahedral molecule. icl4, okay, and the whole thing is surrounded by these brackets because it has a negative charge which means this whole clump of atoms has received an extra electron. It doesn't matter if it has a charge or not, we go through figuring out what the vesper shape is, we go through it exactly the same way. Don't be thrown off at all by this charge. It doesn't have any effect on the vesper. Okay.
Anyway, first thing we ask, how many things are there around the central atom? 1, 2, 3, 4, 5, 6.
6 total things. So, what structure is this going to be based on? It's going to be based on octahedral shape which looks like this which normally has 6 atoms around a central atom. Okay. Number 3, which atoms are going to be replaced by lone electron pairs?
Octahedral has 6 atoms. But, icl4 here has 2 of them replaced by lone electron pairs. And so, this is the structure that I'm going to end up getting where you can see that, if you make this comparison, the top and the bottom atoms, if you like to think about it that way, have been replaced by an unshared electron pair here an unshared electron pair here, I'm left with 4 round the middle here.
And, the shape that that gives me is square planar because these are all kind of plane, just like that. And, in terms of angles, my octahedral, as I said, has 90 degree angles between every bond. And the square planar, if we look at this top view down, it's really simple too, it's got 90 degrees between all of these bonds here. And that's because these unshared electrons push down, but, these electrons on the bottom push up.
So, there's no net movement of any of these atoms because they're being pushed up and down the same amount. So, they stay exactly in the center. And, there is a 90 degree angle between each set of those bonds. So, that's what we get, a square planar molecule when we take 2 of the atoms off of an octahedral molecule and replace them with lone electron pairs. sof4, okay, how many things are there around the central atom?
There are 1, 2, 3, 4, 5. Double bonds count the same as single bonds. 5 things around it. So, the structure that this molecule is going to be based on is the trigonal bipyramidal shape which is how you would arrange 5 things around a central atom. Now, which atoms are going to be replaced by lone electron pairs? None of them because there are no lone electron pairs in this Lewis structure.
So, that means that our sof4 is going to just look like this. It's going to be a full on regular trigonal bipyramidal molecule where these are 180 between these two bonds. These are 90 degrees between these two bonds. And, from the top down, each one of these bonds has a 120 degree angle between it. Bipyramidal for 5 atoms around a central atom. i3 and then the whole thing has a 1 minus charge.
So, how many things are there around the central atom? 1, 2, 3, 4, 5. Okay. How do we around 5 things around a central atom? It's going to be based on this pyramidal, on this trigonal bipyramidal shape that we just looked at a middle that we just looked at a minute ago. Okay. But now, which of the atoms from this 5 atom shape are going to be replaced by lone pairs?
Well, there are going to be 1, 2, 3 lone pairs instead of 3 atoms. So, this one's going to go, this one's going to go, and this one's going to go. And, here is the shape that we get left with. Okay? You can see this atom stays, this atom stays, obviously the central atom stays. Here they are. But now, if you look at it from the top down, these 3 unshared electron pairs have been replaced with 3 equatorial atoms in this trigonal bipyramidal structure. Okay.
So, this leaves us with a linear structure where the 3 atoms are all just I a row. Okay. And, the way this relates to what we saw before is, normally, we have our 180 degree angle between these two bonds. And, it turns out that, in a linear structure, we have the exact same thing because, sure, there are all of these unshared electrons. But, they're all pushing equally against this one and this one in all directions.
So, nothing gets bent up or down. They're all just bam, bam, bam in a straight line just like that, 180 degrees between them with 3 unshared electron pairs around the central atom on, replacing the equatorial atoms. Okay? Last one, tef5 and the whole thing has a negative charge. Okay. How many things around the central atom?
1, 2, 3, 4, 5 bonds and 1 lone electron pair, 6 things total. So, that means that the structure of this is going to be based on the octahedral molecule which is this right here, octahedral. Which atoms are replaced by lone electron pairs? So, instead of having 6 atoms, like I do in a regular octahedral, 1 of these atoms is going to be replaced by this lone electron pair.
So, that means that my structure is going to look like this. Check it out. I just cut its head off and replaced it with a lone electron pair. This atom is gone and it's been replaced by these electrons right here. Okay? So, this is going to give me the structure that we call square pyramidal because if you look at it like this, sits up, looks like this, kind of looks like a square pyramid.
So, really quickly, bond angles in a regular octahedral molecule are all 90 degrees. Bond angles are all 90 degrees. And, in the square pyramidal, we have these unshared electrons that are pushing down, they're pushing down on all of these atoms a little bit. So, that means that the bond between this and this, between this and this, this bond is going to be less than 90 degrees because these are getting pushed down. And, since these two are getting pushed down, they end up a little closer together which means that the bond between these two is a little less than 90 degrees as well.
So, take 1 of the 6 atoms away from an octahedral molecule and you end up with something that has a square pyramidal shape. Okay. So, that's how you go through and look at a Lewis Structure and figure out what its 3 dimensional vesper shape should be. A lot of people really freak out about vesper. And, they think there are all these different structures that they have to memorize. You know, they make the flash cards and they have all the bond angles and everything.
But, really, vesper's not that hard if you think about it along these lines like you're going through these steps here. You figure out how many things are around the central atom, what structure is that based on? Like, maybe it's based on the octahedral. And then, you just think about which of the atoms are going to get replaced when, with unshared electron pairs. And, that will tell you the structure you end up with.
Just 2 things to keep in mind. When you have a trigonal bipyramidal molecule, you always replace the equatorial atoms one at a time. So, if all 3 of them are replaced, you end up with this linear molecule. Other hand, when you have an octahedral molecule like this, the first atom you replace, well it doesn't really matter, but, it looks like it's at the top. And then, the second atom you replace, instead of being one of these, the second one is on the other side. And, you get this structure here where this has been replaced and this has been replaced.
But, anyway, you don't have to memorize a whole lot with vesper. All you have to do is understand, really well, how you start with the structure that the molecules are based on. And then, you just figure out which atoms to remove. And, always remember that unshared electron pairs push atoms closer together. So, if there's an unshared electron pair pushing atoms, their angle is going to be less than it is in the perfect structure that the molecule is based on. Okay. So, that's how you can figure out a vesper structure based on a Lewis structure.