Section 13.13b | Coconote

Welcome to section 13.13b. Alright, gentle people, we're going to go ahead and employ the ideas that we talked about in the last lecture. We're going to talk about VSEPR theory and we're going to see the ramifications of using VSEPR theory. So let's go ahead and remember our tenets. Electrons are negatively charged and things that are similarly charged are going to repel each other. Now remember, bonds and lone pairs are made out of electrons. So they're going to repel each other, and they're going to try to move away from each other as much as possible. Now the one stipulation you have to remember is that lone pairs are greedy. They are going to sweep out more area, so they are going to have a greater repulsion than bonds. So what we can do with VSEPR theory is we can count something called the steric number. Now the steric number is the amount of electron groups around the atom that I'm trying to determine the geometry of. So what that means is a single bond, a double bond, a triple bond, and lone pair electrons, all of these count as one steric number. We're going to treat all the bonds equally. It doesn't matter if it's a double, single, or triple. They're all going to count as one. Lone pairs count as one. The only caveat is they have greater repulsion. So let's go ahead and take a look at how this plays out. The first thing you need to do is go ahead and draw the Lewis dot structure. So let's take the molecule CO2. If I were to draw the Lewis dot structure, what you guys would draw is something like this. I'm going to look at the central atom, and I'm going to go ahead and count my electronic groups. So remember, a double bond counts as one. So this is one electronic group, two electronic groups. So this has a steric number of two. So what that means is there are two places of electrons that are around my central atom. And I want to go ahead and have these as far away from each other as possible. So if that's the case, if I take two things, the furthest I can move them apart from each other is 180 degrees. And so what we say is that the electrons adopt a linear geometry. So this is going to be the electronic geometry. And what I can also do is look at how the atoms are arranged. Well, what I see is all my atoms in a line. So this is also going to be the molecular geometry. All right, that takes care of steric number two. Let's go ahead and look at this next molecule, COCl2. So again, I'm going to go ahead. and draw out the Lewis dot structure. I'm going to go ahead and look at my central atom. In this case, it's carbon, and I'm going to count my electronic groups. A double bond counts as one, single bond counts as one, and another single bond counts as one. So there are three electronic groups around that carbon. So that means I have steric number three. So three electronic groups that want to spread out as far away from each other as possible. If that's the case, then my angles are going to be 120 degrees apart from each other. This gives us the electronic geometry of trigonal planar. Now, if there are no lone pairs, what I will see is I have three atoms that are evenly spaced away from each other. Each one is going to be 120 degrees apart from each other. So if I have a structure with no lone pairs and steric number three, we say that the molecular geometry is also going to be called trigonal planar. But let's go ahead and take a look at another molecule. Let's go ahead and draw the Lewis structure of ozone, something that we've already done before. Again, let's look at that central atom. We have one, two, and three. So we're still steric number three. We've got three groups of electrons that want to spread away from each other as far as possible. So in this structure, the electronic geometry is still called trigonal planar. However, if we want to look at how the atoms are arranged, I don't have an atom in this top space right here. So I can't call it a trigonal planar. when I'm talking about its molecular geometry. When I have a lone pair, it goes ahead and has a different name. The molecular geometry of something that is steric number 3 with one lone pair, well, that's going to be called the bent molecular geometry. What you should note is the angles are going to be slightly different for this molecule. While this guy over here... has angles of 120 degrees, we see that this lone pair is going to take up space and is going to push these bonds closer together. So when you report this angle, you're going to say that it is less than 120 degrees. So let's go ahead and look at this in model format. All right, gentle people, what you guys see here is we have our central atom, and then we have three bonds. to three other atoms around my central atom. So steric number three. What we'll see is the angle between the green, purple, and green sphere is 120 degrees. Notice that each one of these are equivalent. There is no top or bottom to this molecule. No matter how much I rotate it, each one of these green spheres are equivalent. You'll notice that this will that this is a flat molecule where all the atoms are in a single plane. Now what I can do is look at a molecule that's very similar. In this other molecule, I have one bond, two bonds, and this paddle is going to represent a lone pair. So again, one, two, and three, steric number three. But remember, this paddle right here is going to take up more space. It is going to push down on these two bonds, meaning that these two things are going to be tucked in tighter, and the angle is going to be less than 120 degrees. Now, I should note that your book says that this doesn't happen, but I'm letting you know that looking at the data for steric number 3, if you have a lone pair, the angle is going to be less than 120 degrees. It is. So make sure you make note of that. All right, let's take a look at our next group of atoms. So we've talked about this molecule CH4 before. So what I showed you is carbon in the center, the hydrogens around it. So this is my Lewis dot structure. And we can count our steric number. So one, two, three, four electronic groups around my central atom. So that means this is gonna be steric. number 4. For steric number 4, my electronic geometry is going to be called tetrahedral, and that's going to be true for anything with steric number 4. Now if there are no lone pairs, like for methane, then my molecular geometry is also called tetrahedral. Now, I talked about two other molecules the last time we talked about the tetrahedral geometry. I showed you NH3 with one lone pair and water with two lone pairs. Again, just to remind you, we can count electronic groups. For ammonia, 1, 2, 3, and 4 because of that lone pair. So again, steric number 4. For water... 1, 2, 3, and 4. So again, steric number 4. Now both of these have the electronic geometry of tetrahedral. However, when I talk about the molecular geometry, we call this structure with two lone pairs bent, and with one lone pair, we are going to call this molecular geometry trigonal pyramidal. Now instead of using models, what you guys can do is look on Gaucho Space and look at a simulation. What you guys will see is a link to the Vesper Theory visualization. Go ahead and click on that link. Next, go ahead and click on Models. What you guys will be able to do is build a molecule. You guys can go ahead and click on... molecule geometry, which is really the molecular geometry, and the electronic geometry, and it will tell you the names. So let's go ahead and build methane. For methane, I have a central atom, one bond, two bonds, three... and four and what you guys will see electronic geometry tetrahedral molecular geometry tetrahedral again to remind you this is a perfectly symmetrical molecule there is no top or bottom to these things now i want you to be careful when using this program i would avoid using the show bond angles well it does a good job with things with no lone pairs, it does have bugs to it where it mistakes the angles when there are lone pairs. So for this structure, it's fine. I prefer that you guys keep it off. So let's go ahead and show you what ammonia looks like. So I'm going to replace one of these bonds with a lone pair. So again, 1, 2, 3, 4 is my steric number. My molecular geometry, trigonal pyramidal, my electronic geometry is still called tetrahedral. We can also do water, so that's going to have two lone pairs on there. So my electronic geometry is still considered tetrahedral, and my molecular geometry now is considered bent. Again, to try to highlight the difference between these two, the electronic geometry is telling you the rough. idea of where your electronic groups are. And so what you can see is my electronic groups, the lone pairs and the bonds, are roughly arranged in a tetrahedral fashion. The molecular geometry is only concerned about atoms. If I look at just my atoms and don't look at my lone pairs, you can see that this is a bent picture. And that's why we call this geometry bent. All right, let's move on to the next geometry. So let's take a look at PCL5. So for PCL5, this is something that expands its octet. So there are five chlorines around the phosphorus. Now I've emitted the lone pairs on the chlorine because I don't really care about them to assess the geometry of this molecule. I only care about the central atom. I can count the electronic groups around that central atom. 1, 2, 3, 4, and 5. So this is steric number 5. The electronic geometry is called trigonal bipyramidal. So let's take a look at the model to kind of explain how this molecule is arranged. Alright, general people. For this structure, what I had to do was I had to use different colors to represent... some of these chlorines. So in the middle, we have our central atom, phosphorus. It is in purple. Now what you guys will see is that some of the chlorines I've represented as these green spheres and some of the chlorines I've represented as these white spheres. And the reason I did this is because this, so this structure is the only geometry that actually has a top and bottom. So not... all these sites are equivalent to each other. So what you shall see is the green spheres represent what is called the equatorial position. The equatorial positions wrap around the molecule. There are three equatorial positions. If I put the molecule on its side, what you will see is the equatorial positions are 120 degrees away from each other. So each one of those green spheres makes up an angle of 120 with that purple sphere in the center. Now the other position that I have are these white spheres. Now the white spheres, these are considered the axial position. The axial positions are 180 degrees away from each other. If I were to measure the angle from this white sphere on top to this white sphere on the bottom, That would be 180 degrees. Now the angle between the white sphere, the purple, and one of these green spheres, well that's going to be 90 degrees. What's special about this geometry is that the green spheres, or the equatorial positions, they have a lot more space. What you can see is that the green spheres are only 90 degrees from two atoms, the white spheres. However, the white spheres, they are in a more crowded location. They are 90 degrees away from three atoms. So what happens is there's more space in the equatorial positions. So let's go ahead and think about what happens when I take a bond away and I replace it by a lone pair. So again, in this molecule, what you will see is there are five bonds. So steric number five. So what I did now is I replaced one of the bonds with a lone pair. So what I have is... four bonds, one, two, three, and four, and a lone pair. So I'm still steric number five. You'll notice because the lone pair sweeps out more area, it wants to be in a space with a lot of room. So it'll take one of the equatorial positions. So the electronic geometry of this is still considered trigonal bipyramidal. However, the molecular... geometry of this one is called seesaw and you guys can kind of see that this makes a little bit of a seesaw going back and forth to describe the angles on here what you would say is that the angles are less than the traditional angles if it didn't have a lone pair meaning the angles are less than 90 less than 120 and less than 180 and that's because the lone pair is going to push everything and pinch everything away from it that includes the axial positions and the equatorial position so it's going to so it's going to push things back now let's say that i take another bond off and replace it by a lone pair so again my lone pairs are going to be represented by the paddles and so i have two paddles on there And I have three bonds, one, two, and three. So this is still steric number five. The lone pairs are occupying the equatorial position. And so this is still going to have the electronic geometry of trigonal bipyramidal. However, the molecular geometry of this one is called the T-shaped. You guys can kind of see. This makes a T if I just look at my atoms. Now it's not a perfect T because remember, these lone pairs are going to be greedy and they're going to push bonds back. So what that means is that this is going to be slightly bent like this and so my angles are going to be less than 90 degrees. So let's go ahead and take a look at the last structure for SN5. So in this case, I have three lone pairs around my molecule. Again, this is going to be steric number 5. Again, they are going to take the equatorial positions. I still have two things that are bonded, one on top, one on bottom, so this is steric number 5. Again, my electronic geometry is trigonal bipyramidal, but let's just focus on the atom. don't pay attention to the paddles what you guys can see is that all my atoms are in a row so that means that my molecular geometry is called linear the angle that I have between the white purple and white is a hundred and eighty degrees alright let's cover our last steric number let's look at this molecule SF six so this is going to be a sulfur with six fluorines around it. Again, I have an expanded octet. So if I count the electronic groups around that sulfur, one, two, three, four, five, and six. So this is steric number six. What you guys will see is that this structure is called the octahedral. So let's go back to our visualizer and take a look at this molecule. All right, general people, I want to go ahead and put six things around my central atom. So what we have is an electronic geometry called called octahedral, and my molecular geometry is octahedral. The angles for this one are all 90 degrees. Again, this is a purely symmetric molecule. There is no top and bottom to this molecule. They are all equivalent. You guys can envision this as like your Cartesian coordinate graph, where I have an x-axis, a y-axis, and a z-axis. Now let's go ahead and see what happens when we take away a bond and have a lone pair. Again, this is still considered steric number 6. I have 5 bonds plus a lone pair. So again, my electronic geometry is considered octahedral, but now the molecular geometry, if I were to look at just the atoms here, what I have is something called the square pyramidal. Here, all my angles are going to be less than 90 degrees. So let's go ahead and talk about something that has four bonds and two lone pairs. Now what you'll notice is that when I put that second lone pair on there, that second lone pair goes 180 degrees from that other lone pair. Or in other words, if I have a lone pair, or... The second lone pair is going to go on the other side. And this is always going to be the case. And that's because the lone pairs want the most space around them. And so if I have two lone pairs, they want to be on opposite sides so they can sweep out the most areas without interfering with each other. Now, again, this is going to be steric number six. Two lone pairs, four bonds. So my electronic geometry... is going to be octahedral. Now, the molecular geometry, if I just look at the atoms, what you guys will see is everything is in the same plane. And so this is called a square planar complex because you guys can see that it makes kind of a square in this plane. All right, gentle people, here is a summary of all my structures. What I want you guys to know is these five steric numbers. 2, 3, 4, 5, and 6. I want you guys to know the electronic geometry, and I want you guys to know the molecular geometry. I want you guys to also know the angles. For things with lone pairs, all you have to say is less than what the angle would be if there wasn't a lone pair. For example... if you have a tetrahedral structure, 109.5. But if I ask you what's the angle in a trigonal pyramidal or a bent structure, what you guys would say is this is less than 109.5. You guys can see all of the relevant angles listed on the electronic geometry. Anything that is not marked, you guys can just say it is less than what you guys see on that. on those structures without lone pairs with the same name as the electronic geometry. All right, gentle people, let's go ahead and practice one of these. Tell me what the molecular structure of SF4 is. All right, gentle people, let's go ahead and make our table. We have a sulfur and a fluorine making up this molecule. The valence of sulfur is 6. The valence of fluorine is 7. There's only one sulfur in here. There are four fluorines in this molecule. So I do my multiplication out, and I want a picture with 34 electrons. I'll start out with my sulfur in the center. I'm going to put my fluorines radiating out. I'm going to go ahead and fill the octet out. So this would be the structure with all the octets fulfilled, but let's see if I used enough electrons. So I've 8, 16, 24, 32 electrons in this picture. That's not enough electrons. I needed a picture with 34 electrons. So if I didn't use enough electrons, I'm going to dump the extra electrons onto my central atom, providing that the central atom can expand its octet, and sulfur can. So I'm going to put a lone pair. on my sulfur, and now I have 34 electrons, which is exactly what I wanted. So let's go ahead and do our steric number. I have four bonds plus a lone pair electron. This gets me five electronic groups. So this is going to be steric number five. with one lone pair. So steric number five is in this orange box. And if I look at this picture, the one with one lone pair is the seesaw. So what I can write is that my electronic geometry, because it's SN5, well that's going to be trigonal bipyramidal. But because it has one lone pair, My molecular geometry is going to be called seesaw. Well, I hope that made sense, and remember to stay safe, Chem 1A.

Now remember, bonds and lone pairs are made out of electrons. So they're going to repel each other, and they're going to try to move away from each other as much as possible. Now the one stipulation you have to remember is that lone pairs are greedy. They are going to sweep out more area, so they are going to have a greater repulsion than bonds.

So what we can do with VSEPR theory is we can count something called the steric number. Now the steric number is the amount of electron groups around the atom that I'm trying to determine the geometry of. So what that means is a single bond, a double bond, a triple bond, and lone pair electrons, all of these count as one steric number.

We're going to treat all the bonds equally. It doesn't matter if it's a double, single, or triple. They're all going to count as one.

Lone pairs count as one. The only caveat is they have greater repulsion. So let's go ahead and take a look at how this plays out. The first thing you need to do is go ahead and draw the Lewis dot structure.

So let's take the molecule CO2. If I were to draw the Lewis dot structure, what you guys would draw is something like this. I'm going to look at the central atom, and I'm going to go ahead and count my electronic groups. So remember, a double bond counts as one.

So this is one electronic group, two electronic groups. So this has a steric number of two. So what that means is there are two places of electrons that are around my central atom.

And I want to go ahead and have these as far away from each other as possible. So if that's the case, if I take two things, the furthest I can move them apart from each other is 180 degrees. And so what we say is that the electrons adopt a linear geometry.

So this is going to be the electronic geometry. And what I can also do is look at how the atoms are arranged. Well, what I see is all my atoms in a line. So this is also going to be the molecular geometry. All right, that takes care of steric number two.

Let's go ahead and look at this next molecule, COCl2. So again, I'm going to go ahead. and draw out the Lewis dot structure. I'm going to go ahead and look at my central atom. In this case, it's carbon, and I'm going to count my electronic groups.

A double bond counts as one, single bond counts as one, and another single bond counts as one. So there are three electronic groups around that carbon. So that means I have steric number three. So three electronic groups that want to spread out as far away from each other as possible. If that's the case, then my angles are going to be 120 degrees apart from each other.

This gives us the electronic geometry of trigonal planar. Now, if there are no lone pairs, what I will see is I have three atoms that are evenly spaced away from each other. Each one is going to be 120 degrees apart from each other. So if I have a structure with no lone pairs and steric number three, we say that the molecular geometry is also going to be called trigonal planar.

But let's go ahead and take a look at another molecule. Let's go ahead and draw the Lewis structure of ozone, something that we've already done before. Again, let's look at that central atom. We have one, two, and three.

So we're still steric number three. We've got three groups of electrons that want to spread away from each other as far as possible. So in this structure, the electronic geometry is still called trigonal planar. However, if we want to look at how the atoms are arranged, I don't have an atom in this top space right here.

So I can't call it a trigonal planar. when I'm talking about its molecular geometry. When I have a lone pair, it goes ahead and has a different name.

The molecular geometry of something that is steric number 3 with one lone pair, well, that's going to be called the bent molecular geometry. What you should note is the angles are going to be slightly different for this molecule. While this guy over here... has angles of 120 degrees, we see that this lone pair is going to take up space and is going to push these bonds closer together. So when you report this angle, you're going to say that it is less than 120 degrees.

So let's go ahead and look at this in model format. All right, gentle people, what you guys see here is we have our central atom, and then we have three bonds. to three other atoms around my central atom. So steric number three.

What we'll see is the angle between the green, purple, and green sphere is 120 degrees. Notice that each one of these are equivalent. There is no top or bottom to this molecule.

No matter how much I rotate it, each one of these green spheres are equivalent. You'll notice that this will that this is a flat molecule where all the atoms are in a single plane. Now what I can do is look at a molecule that's very similar.

In this other molecule, I have one bond, two bonds, and this paddle is going to represent a lone pair. So again, one, two, and three, steric number three. But remember, this paddle right here is going to take up more space. It is going to push down on these two bonds, meaning that these two things are going to be tucked in tighter, and the angle is going to be less than 120 degrees.

Now, I should note that your book says that this doesn't happen, but I'm letting you know that looking at the data for steric number 3, if you have a lone pair, the angle is going to be less than 120 degrees. It is. So make sure you make note of that.

All right, let's take a look at our next group of atoms. So we've talked about this molecule CH4 before. So what I showed you is carbon in the center, the hydrogens around it.

So this is my Lewis dot structure. And we can count our steric number. So one, two, three, four electronic groups around my central atom. So that means this is gonna be steric. number 4. For steric number 4, my electronic geometry is going to be called tetrahedral, and that's going to be true for anything with steric number 4. Now if there are no lone pairs, like for methane, then my molecular geometry is also called tetrahedral.

Now, I talked about two other molecules the last time we talked about the tetrahedral geometry. I showed you NH3 with one lone pair and water with two lone pairs. Again, just to remind you, we can count electronic groups.

For ammonia, 1, 2, 3, and 4 because of that lone pair. So again, steric number 4. For water... 1, 2, 3, and 4. So again, steric number 4. Now both of these have the electronic geometry of tetrahedral. However, when I talk about the molecular geometry, we call this structure with two lone pairs bent, and with one lone pair, we are going to call this molecular geometry trigonal pyramidal. Now instead of using models, what you guys can do is look on Gaucho Space and look at a simulation.

What you guys will see is a link to the Vesper Theory visualization. Go ahead and click on that link. Next, go ahead and click on Models. What you guys will be able to do is build a molecule. You guys can go ahead and click on...

molecule geometry, which is really the molecular geometry, and the electronic geometry, and it will tell you the names. So let's go ahead and build methane. For methane, I have a central atom, one bond, two bonds, three... and four and what you guys will see electronic geometry tetrahedral molecular geometry tetrahedral again to remind you this is a perfectly symmetrical molecule there is no top or bottom to these things now i want you to be careful when using this program i would avoid using the show bond angles well it does a good job with things with no lone pairs, it does have bugs to it where it mistakes the angles when there are lone pairs. So for this structure, it's fine.

I prefer that you guys keep it off. So let's go ahead and show you what ammonia looks like. So I'm going to replace one of these bonds with a lone pair.

So again, 1, 2, 3, 4 is my steric number. My molecular geometry, trigonal pyramidal, my electronic geometry is still called tetrahedral. We can also do water, so that's going to have two lone pairs on there.

So my electronic geometry is still considered tetrahedral, and my molecular geometry now is considered bent. Again, to try to highlight the difference between these two, the electronic geometry is telling you the rough. idea of where your electronic groups are. And so what you can see is my electronic groups, the lone pairs and the bonds, are roughly arranged in a tetrahedral fashion.

The molecular geometry is only concerned about atoms. If I look at just my atoms and don't look at my lone pairs, you can see that this is a bent picture. And that's why we call this geometry bent. All right, let's move on to the next geometry.

So let's take a look at PCL5. So for PCL5, this is something that expands its octet. So there are five chlorines around the phosphorus.

Now I've emitted the lone pairs on the chlorine because I don't really care about them to assess the geometry of this molecule. I only care about the central atom. I can count the electronic groups around that central atom. 1, 2, 3, 4, and 5. So this is steric number 5. The electronic geometry is called trigonal bipyramidal. So let's take a look at the model to kind of explain how this molecule is arranged.

Alright, general people. For this structure, what I had to do was I had to use different colors to represent... some of these chlorines. So in the middle, we have our central atom, phosphorus. It is in purple.

Now what you guys will see is that some of the chlorines I've represented as these green spheres and some of the chlorines I've represented as these white spheres. And the reason I did this is because this, so this structure is the only geometry that actually has a top and bottom. So not... all these sites are equivalent to each other.

So what you shall see is the green spheres represent what is called the equatorial position. The equatorial positions wrap around the molecule. There are three equatorial positions. If I put the molecule on its side, what you will see is the equatorial positions are 120 degrees away from each other.

So each one of those green spheres makes up an angle of 120 with that purple sphere in the center. Now the other position that I have are these white spheres. Now the white spheres, these are considered the axial position.

The axial positions are 180 degrees away from each other. If I were to measure the angle from this white sphere on top to this white sphere on the bottom, That would be 180 degrees. Now the angle between the white sphere, the purple, and one of these green spheres, well that's going to be 90 degrees. What's special about this geometry is that the green spheres, or the equatorial positions, they have a lot more space.

What you can see is that the green spheres are only 90 degrees from two atoms, the white spheres. However, the white spheres, they are in a more crowded location. They are 90 degrees away from three atoms. So what happens is there's more space in the equatorial positions.

So let's go ahead and think about what happens when I take a bond away and I replace it by a lone pair. So again, in this molecule, what you will see is there are five bonds. So steric number five.

So what I did now is I replaced one of the bonds with a lone pair. So what I have is... four bonds, one, two, three, and four, and a lone pair.

So I'm still steric number five. You'll notice because the lone pair sweeps out more area, it wants to be in a space with a lot of room. So it'll take one of the equatorial positions. So the electronic geometry of this is still considered trigonal bipyramidal. However, the molecular...

geometry of this one is called seesaw and you guys can kind of see that this makes a little bit of a seesaw going back and forth to describe the angles on here what you would say is that the angles are less than the traditional angles if it didn't have a lone pair meaning the angles are less than 90 less than 120 and less than 180 and that's because the lone pair is going to push everything and pinch everything away from it that includes the axial positions and the equatorial position so it's going to so it's going to push things back now let's say that i take another bond off and replace it by a lone pair so again my lone pairs are going to be represented by the paddles and so i have two paddles on there And I have three bonds, one, two, and three. So this is still steric number five. The lone pairs are occupying the equatorial position. And so this is still going to have the electronic geometry of trigonal bipyramidal. However, the molecular geometry of this one is called the T-shaped.

You guys can kind of see. This makes a T if I just look at my atoms. Now it's not a perfect T because remember, these lone pairs are going to be greedy and they're going to push bonds back. So what that means is that this is going to be slightly bent like this and so my angles are going to be less than 90 degrees. So let's go ahead and take a look at the last structure for SN5.

So in this case, I have three lone pairs around my molecule. Again, this is going to be steric number 5. Again, they are going to take the equatorial positions. I still have two things that are bonded, one on top, one on bottom, so this is steric number 5. Again, my electronic geometry is trigonal bipyramidal, but let's just focus on the atom. don't pay attention to the paddles what you guys can see is that all my atoms are in a row so that means that my molecular geometry is called linear the angle that I have between the white purple and white is a hundred and eighty degrees alright let's cover our last steric number let's look at this molecule SF six so this is going to be a sulfur with six fluorines around it. Again, I have an expanded octet.

So if I count the electronic groups around that sulfur, one, two, three, four, five, and six. So this is steric number six. What you guys will see is that this structure is called the octahedral.

So let's go back to our visualizer and take a look at this molecule. All right, general people, I want to go ahead and put six things around my central atom. So what we have is an electronic geometry called called octahedral, and my molecular geometry is octahedral.

The angles for this one are all 90 degrees. Again, this is a purely symmetric molecule. There is no top and bottom to this molecule. They are all equivalent.

You guys can envision this as like your Cartesian coordinate graph, where I have an x-axis, a y-axis, and a z-axis. Now let's go ahead and see what happens when we take away a bond and have a lone pair. Again, this is still considered steric number 6. I have 5 bonds plus a lone pair.

So again, my electronic geometry is considered octahedral, but now the molecular geometry, if I were to look at just the atoms here, what I have is something called the square pyramidal. Here, all my angles are going to be less than 90 degrees. So let's go ahead and talk about something that has four bonds and two lone pairs.

Now what you'll notice is that when I put that second lone pair on there, that second lone pair goes 180 degrees from that other lone pair. Or in other words, if I have a lone pair, or... The second lone pair is going to go on the other side. And this is always going to be the case. And that's because the lone pairs want the most space around them.

And so if I have two lone pairs, they want to be on opposite sides so they can sweep out the most areas without interfering with each other. Now, again, this is going to be steric number six. Two lone pairs, four bonds.

So my electronic geometry... is going to be octahedral. Now, the molecular geometry, if I just look at the atoms, what you guys will see is everything is in the same plane. And so this is called a square planar complex because you guys can see that it makes kind of a square in this plane. All right, gentle people, here is a summary of all my structures.

What I want you guys to know is these five steric numbers. 2, 3, 4, 5, and 6. I want you guys to know the electronic geometry, and I want you guys to know the molecular geometry. I want you guys to also know the angles.

For things with lone pairs, all you have to say is less than what the angle would be if there wasn't a lone pair. For example... if you have a tetrahedral structure, 109.5.

But if I ask you what's the angle in a trigonal pyramidal or a bent structure, what you guys would say is this is less than 109.5. You guys can see all of the relevant angles listed on the electronic geometry. Anything that is not marked, you guys can just say it is less than what you guys see on that.

on those structures without lone pairs with the same name as the electronic geometry. All right, gentle people, let's go ahead and practice one of these. Tell me what the molecular structure of SF4 is.

All right, gentle people, let's go ahead and make our table. We have a sulfur and a fluorine making up this molecule. The valence of sulfur is 6. The valence of fluorine is 7. There's only one sulfur in here. There are four fluorines in this molecule.

So I do my multiplication out, and I want a picture with 34 electrons. I'll start out with my sulfur in the center. I'm going to put my fluorines radiating out.

I'm going to go ahead and fill the octet out. So this would be the structure with all the octets fulfilled, but let's see if I used enough electrons. So I've 8, 16, 24, 32 electrons in this picture.

That's not enough electrons. I needed a picture with 34 electrons. So if I didn't use enough electrons, I'm going to dump the extra electrons onto my central atom, providing that the central atom can expand its octet, and sulfur can. So I'm going to put a lone pair.

on my sulfur, and now I have 34 electrons, which is exactly what I wanted. So let's go ahead and do our steric number. I have four bonds plus a lone pair electron.

This gets me five electronic groups. So this is going to be steric number five. with one lone pair.

So steric number five is in this orange box. And if I look at this picture, the one with one lone pair is the seesaw. So what I can write is that my electronic geometry, because it's SN5, well that's going to be trigonal bipyramidal.

But because it has one lone pair, My molecular geometry is going to be called seesaw. Well, I hope that made sense, and remember to stay safe, Chem 1A.

Transcript for:Section 13.13b

Transcript for:
Section 13.13b