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Chemistry: Molecular Shapes for Steric Numbers 2-4

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  • Type: Video Tutorial
  • Length: 10:27
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  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Molecular Geometry and Bonding Theory (11 lessons, $18.81)
Chemistry: Molecular Geometry and the VSEPR Theory (4 lessons, $7.92)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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So now you can take a Lewis dot structure and determine the steric number. And what we're going to see is that there's a particular atom in a Lewis dot structure that we have to focus on, but before we look at a particular example, I want to point out that if we have a simple diatomic molecule--this could be O2, N2, or NO or anything like that--we say that this molecule is linear because two points determine a line, and really, all we're focused on is the relative positions of the atoms, and so any diatomic molecule, we would describe as linear.
But once we go to a triatomic molecule it turns out we're going to have some options and we're going to see those options in this lecture. Let's start at the beginning, and the beginning would be steric number 2. Here's an example of steric number 2. We focus on the central atom in a particular Lewis dot structure, so this you can think of as the hinge point. What goes on at carbon is going to determine what this shape of this molecule is going to be. We don't really care about what's going on on these outer atoms. So this is steric number 2 because there's one bond from the carbon to the oxygen on the right and one bond from the carbon to the oxygen on the left. Molecules that are steric number 2 are linear. So here's the model of linear steric number 2 molecule. This would be the carbon, and each of these would be the oxygens.
This bond angle is a 180 degrees, and if you think back to the balloons, two balloons are going to want to get as far apart from each other as possible, and to do that they will be on opposite sides of the central atom, and so that bond angle here is going to be 180 degrees. Remember that there are 360 degrees in a complete circle.
Now, if we look at another molecule. Here's nitrous oxide, laughing gas, or dinitrogen monoxide, you can see that we have two resonance structures, one that has a double bond between the two nitrogens and a double bond between the nitrogen and the oxygen. Here's one that has a triple bond between the two nitrogens and a single bond between the nitrogen and the oxygen, but the steric number is still 2, and so the prediction about the geometry is exactly the same. This is a linear molecule, and it doesn't matter which resonance structure you look at, you're still going to conclude the nitrous oxide should be a linear molecule. Again, it'll look something like this, except this atom and this atom will be nitrogen and this one will be oxygen.
When we go to steric number 3--borane, BH3. You'll remember that borane is one of those molecules where we don't satisfy the octet rule, but we won't worry about that. The steric number is 3 because we have one bond to each of the three hydrogens from boron. Molecules with steric number 3 are built on the molecule of the equilateral triangle, and we describe this geometry as trigonal planar, and the reason why it's trigonal planar is, you can see, first of all, that it's trigonal, and then if you look at it end on, you'll see that it's planar. In other words, the central atom is in the same plane as that described by the three other atoms, and we'll see an example later on where that's not true so that you can see the distinction. So this model is a molecule of the borane molecule, and this bond angle is 120 degrees. It's 120 degrees because these angles are all equivalent, and remember, again, 360 degrees is the total number of degrees in a circle.
So this is the borane molecule, but it turns out that there are lots of other molecules that have steric number 3. Here's an example. This is the formaldehyde molecule, and here we have a double bond between the carbon and the oxygen, and having a double bond between carbon and oxygen doesn't affect the steric number--in other words, it's still steric number 3. And so for the most part we would describe as being trigonal planar. Now, is it exactly trigonal planar like the borane molecule? Borane is exactly trigonal planar. The formaldehyde molecule, there are going to be some slight distortions. Although you probably aren't going to be responsible for this--it turns out that the oxygen, as your intuition might say, does sort of take up more space. And so what that does is this bond angle is a little bit larger than 120 degrees, and this one gets squished down a bit. But the model that we're going to use is that it's on the trigonal plane, and this bond angle is going to be really close to 120 degrees.
Now, what happens when we have steric number 3, but we have a lone pair? An example of that would be the sulfur dioxide molecule. You can see that it has steric number 3 because we have a sulfur oxygen bond, another sulfur oxygen bond, and a lone pair, so that's steric number 3. How do we get to the sulfur dioxide model? We take the trigonal plane and we pull out one of the atoms. Remember, you don't see lone pairs. When we're talking about the shape of a molecule we're talking about the relative positions of the nuclei. So this is a model of the sulfur dioxide molecule. Now recall that it doesn't matter what resonance structure you look at, the steric number is still the same, and that's good, because we want to not predict a different geometry based on the resonance structure because you have to consider all of the valid resonance structures, and it's reassuring that they give you the same prediction.
So here's the sulfur dioxide molecule, and what's this angle? Well, we're still looking at exactly the same model we looked at for borane, so this bond angle here is going to be 120 degrees. Again, is it going to be exactly 120 degrees? It turns out that it's not exactly. As a rule of thumb you can think of lone pairs as occupying slightly more space than bonding pairs, and so this gets squished down a little bit, so this bond angle might be, I don't know, 117, 118, something smaller than 120, but still pretty close to 120.
Finally, let's look at steric number 4, and here are the examples for steric number 4. The one that is exactly perfectly having all bond angles exactly the same would be a molecule like methane. The steric number 4 we build on a model of what is known as a tetrahedron, and this is a tetrahedron, and you can see that all of the positions are exactly the same. So this is a very highly symmetric molecule. Now, the bond angles are the white, black, white bond angle here, is 109.5 and that's probably a number you've never heard before. It just turns out that there is this special angle for the tetrahedron called the "tetrahedral angle," which 109.5. And you'll just have to remember that.
So, again, all of the hydrogens are equivalent and they're all equi-distant from each other, and all of the bond angles are exactly equivalent, and this is a perfect tetrahedron. Now, when we go to ammonia, which is also steric number 4, but it has one lone pair, all we do is pull out one of the atoms, and this is a model of the ammonia molecule, and this molecule is trigonal pyramidal. Why is it trigonal pyramidal in contrast to trigonal planar? Let me bring in the trigonal planar molecule again. Here was our trigonal planar molecule and you can see that the black ball and the three white balls are all in the same plane; whereas, in the trigonal pyramidal structure the white ball and the black balls are not in the same plane. And so this is the ammonia molecule. Why is the ammonia molecule trigonal pyramidal? It's because of that lone pair. That lone pair is up here and it impacts the shape, but when we describe the molecule we're only talking about the relative positions of the nuclei, and so this is the trigonal pyramidal shape.
The bond angle--remember, we built this from the model of the tetrahedron, so this bond angle here is going to be very close to 109.5. Again, it's not exactly 109.5. I think it's 107, and the way we might arrive at that is because the lone pair, again, takes up a little more space so it squishes down, makes the hydrogens a little closer to each other than they might have been in the methane molecule.
And when we go to steric number 4 with two lone pairs... Again, all we do is pull out one of the atoms, and you might want to get yourself a set of these models so that you can play around with this and see these relationships yourself. This is steric number 4 with two lone pairs. What's an example of that? Well, water is an example of steric number 4 with two lone pairs. So water is a bent molecule, and this bent angle, this angle for water is close to 109.5, because again, we built it on the tetrahedral model, but because these lone pairs--again, you can think of them as taking up a little bit of extra space--this bond angle is close to 104.
Again, having pi bonds, having double bonds and things like that, that doesn't change the steric number, so sulfur still has steric number 4, and so sulfate anion or dianion is going to be a tetrahedral molecule as well, a tetrahedral ion as well. And let me conclude this lecture by showing you the three possibilities for triatomic molecules and show you that you get three different possible shapes for those three different steric numbers.
Remember when we have steric number 2, and that's going to be linear. For steric number 3, which is this one, sulfur dioxide, for instance, it's also called bent, and its bond angle is 120 degrees, and then here's water, and it's also called bent, but this bond angle is 109.5, or approximately 109.5. So you can see that if you know the steric number and if you can identify what bond angle you're interested in, you focus on the atom that is at the vertex or the hinge point of that bond angle, and from the steric number, and knowing what model you're supposed to be working from, you can get a very good prediction of what the bond angle is supposed to be, and that's a really powerful tool. It also allows you to predict what the molecular shape is, and again, molecular shape strongly impacts things like reactivity.
Molecular Geometry and Bonding Theory
Molecular Geometry and the VSEPR Theory
Molecular Shapes for Steric Number 2-4 Page [1 of 2]

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