WEBVTT 00:00:03.639 --> 00:00:08.520 Molecules aren't flat. They're three dimensional, and that has implications for their physical 00:00:08.520 --> 00:00:14.500 and chemical properties. For example, X-ray crystallography labs at MIT determine the 00:00:14.500 --> 00:00:20.599 3D shapes of protein molecules to design drugs that will fit into these proteins. In this 00:00:20.599 --> 00:00:26.050 video, you'll learn about an empirical model chemists use to predict a molecule's 3D shape 00:00:26.050 --> 00:00:28.589 from its Lewis structure. 00:00:28.589 --> 00:00:32.720 This video is part of the Representations video series. Information can be represented 00:00:32.720 --> 00:00:38.809 in words, through mathematical symbols, graphically, or in 3-D models. Representations are used 00:00:38.809 --> 00:00:44.879 to develop a deeper and more flexible understanding of objects, systems, and processes. 00:00:44.879 --> 00:00:49.829 Hi. My name is Cathy Drennan and I am a professor in the chemistry department at MIT. I hope 00:00:49.829 --> 00:00:53.539 you have been enjoying your general chemistry course at SUTD. 00:00:53.539 --> 00:00:58.500 After watching this video, you will be able to use the VSEPR model to predict 3D molecular 00:00:58.500 --> 00:01:03.089 structures from 2D Lewis structures and... ...discuss some of the assumptions of the 00:01:03.089 --> 00:01:06.880 VSEPR model. Before watching this video, you should be able to draw Lewis structures for 00:01:06.880 --> 00:01:07.759 simple molecules. 00:01:07.759 --> 00:01:08.240 We can determine molecular shape experimentally or predict it with varying degrees of accuracy 00:01:08.240 --> 00:01:08.409 using empirical and theoretical models. 00:01:08.409 --> 00:01:08.850 The model that will be introduced in this video, the Valence-Shell Electron-Pair Repulsion 00:01:08.850 --> 00:01:09.659 Model, or the VSEPR model, is an empirical model, but works quite well for most simple 00:01:09.659 --> 00:01:09.810 Because it is an empirical model, the VSEPR model was constructed by looking for patterns 00:01:09.810 --> 00:01:13.130 The VSEPR model, which builds off of Lewis theory, is based on the idea that regions 00:01:13.130 --> 00:01:17.729 of high electron density repel one another. Let's take a look at the Lewis structure for 00:01:17.729 --> 00:01:24.600 NF3 to see what that means. The VSEPR model focuses on the central atoms of molecules... 00:01:24.600 --> 00:01:30.680 ...and assumes that bonding atoms... ...and lone pairs on the central atom are spaced 00:01:30.680 --> 00:01:35.590 as far apart as possible from each other to minimize electron repulsions, but at the same 00:01:35.590 --> 00:01:41.789 time are equidistant from the central atom. The VSEPR model attributes lone pairs of electrons 00:01:41.789 --> 00:01:46.539 with having a stronger repulsion than bonded electrons. And finally, the VSEPR model treats 00:01:46.539 --> 00:01:51.829 multiple bonds between 2 atoms as a single region of electron density. 00:01:51.829 --> 00:01:57.399 Let's apply the VSEPR model to a few molecules. For example, if we look at nitrous oxide, 00:01:57.399 --> 00:02:02.570 we see that there are... ...2 atoms bonded to the central atom and zero lone pairs of 00:02:02.570 --> 00:02:07.789 electrons on the central atom. To minimize electron repulsions, we will maximize the 00:02:07.789 --> 00:02:13.400 distance between the two bonded atoms by placing them along a line on either side of the central 00:02:13.400 --> 00:02:19.310 N atom. We can also see that if we placed one of the bonded atoms elsewhere, we would 00:02:19.310 --> 00:02:25.090 not have maximized the distance between the bonded atoms. We would describe nitrous oxide 00:02:25.090 --> 00:02:32.090 as having a linear geometry. In a linear molecule, the bond angle between atoms is 180 degrees. 00:02:33.250 --> 00:02:38.870 Note that when we examined the Lewis structure for nitrous oxide, the triple bonded nitrogen 00:02:38.870 --> 00:02:43.319 was treated the same as the single bonded oxygen atom. Chapter Break Now, if we followed 00:02:43.319 --> 00:02:49.110 a similar procedure for sulfur trioxide, ... we see that there are 3 atoms bonded to the central 00:02:49.110 --> 00:02:54.610 atom and zero lone pairs on the central atom. To maximize the distance of the bonding atoms 00:02:54.610 --> 00:02:59.769 from each other, we will space them 120 degrees apart around the central atom... ...on the 00:02:59.769 --> 00:03:06.769 vertices of an equilateral triangle. We would describe sulfur trioxide as a trigonal planar 00:03:06.890 --> 00:03:13.890 molecule... ...with bond angles of 120 degrees. Chapter Break Now let's take a look at SO2. 00:03:14.879 --> 00:03:20.159 Notice that you can draw resonance structures for SO2, but because VSEPR doesn't distinguish 00:03:20.159 --> 00:03:24.459 b/w single and multiple bonds, we can look at any resonance structure when predicting 00:03:24.459 --> 00:03:30.709 geometry. So let's just focus on one of these structures. What do you think the geometry 00:03:30.709 --> 00:03:36.060 of this molecule will be? Pause the video, draw a picture or construct the molecule using 00:03:36.060 --> 00:03:41.329 a molecule kit, and continue playing the video to see if you are correct. 00:03:41.329 --> 00:03:48.329 Remember, the VSEPR model assumes that bonding atoms AND lone pairs on the central atom are 00:03:52.299 --> 00:03:58.790 spaced as far apart as possible. On SO2 we have two atoms bonded to the central atom 00:03:58.790 --> 00:04:05.439 and one lone pair on the central atom. So, we have 3 regions of electron density total. 00:04:05.439 --> 00:04:10.519 Think about how you would space these 3 regions as far apart as possible. By placing them 00:04:10.519 --> 00:04:16.640 on the vertices of an equilateral triangle. On our model, this (pointing at electron cloud) 00:04:16.640 --> 00:04:22.100 represents the lone pair of electrons. While we have to think about lone pairs of electrons 00:04:22.100 --> 00:04:28.200 when predicting geometry, the naming convention actually ignores them. So, if we were to name 00:04:28.200 --> 00:04:32.409 this shape, we would focus on the shape that is determined by the atoms. 00:04:32.409 --> 00:04:39.390 We call this shape "bent". It's tempting to say that the bond angle will be 120 degrees, 00:04:39.390 --> 00:04:44.230 but bond angles in molecules with lone pairs on the central atom have been observed to 00:04:44.230 --> 00:04:50.600 be smaller than expected. One possible explanation is that a lone pair can spread over a larger 00:04:50.600 --> 00:04:56.500 region than bonded electrons causing the bonded atoms to move farther from the lone pair and 00:04:56.500 --> 00:05:01.860 closer to each other, compressing the bond angle. The VSEPR model accounts for this by 00:05:01.860 --> 00:05:07.950 saying that lone pairs repel more strongly than bonded electrons. The result is that 00:05:07.950 --> 00:05:13.550 the bond angle on sulfur dioxide will be less than 120 degrees, but VSEPR theory cannot 00:05:13.550 --> 00:05:16.520 tell us what the precise bond angle will be. 00:05:16.520 --> 00:05:23.520 Chapter Break Let's do another example - CH3Cl (chloromethane, a.k.a. methyl chloride). What 00:05:24.680 --> 00:05:29.310 do you think the geometry of this molecule will be? Pause the video, draw a picture or 00:05:29.310 --> 00:05:33.870 construct the molecule using a molecule kit, and continue playing the video to see if you 00:05:33.870 --> 00:05:40.870 are correct. How many atoms are bonded to the central atom? 4 How many lone pairs are 00:05:45.530 --> 00:05:52.530 on the central atom? 0 If we are only thinking in 2-dimensions, it's tempting to place the 00:05:52.750 --> 00:05:57.480 4 regions of electron density at 90 degree angles to each other, ...but this would be 00:05:57.480 --> 00:06:03.710 incorrect. Remember, we're working in 3-dimensions. To maximize the distance of 4 regions of electron 00:06:03.710 --> 00:06:09.280 density, we can think about them lying at the 4 corners of a tetrahedron. It may help 00:06:09.280 --> 00:06:14.020 to review what a tetrahedron looks like. A tetrahedron is a pyramid constructed from 00:06:14.020 --> 00:06:20.590 4 equilateral triangles. It has a triangular base and sides made from triangles. This results 00:06:20.590 --> 00:06:27.590 in 4 corners, or vertices. We imagine our central atom to be at the center of the tetrahedron. 00:06:29.180 --> 00:06:35.630 In a tetrahedral geometry, the bond angle between bonding atoms is 109.5. This is larger 00:06:35.630 --> 00:06:41.650 than the 90 degrees predicted by putting all 4 regions in the same plane The correct model 00:06:41.650 --> 00:06:48.650 of CH3Cl would look like this. This shape is aptly named a tetrahedron. Chapter Break 00:06:48.990 --> 00:06:54.810 Let's do another example -- nitrogen trifluoride. What do you think the geometry of this molecule 00:06:54.810 --> 00:06:59.670 will be? Pause the video, draw a picture or construct the molecule using a molecule kit, 00:06:59.670 --> 00:07:06.670 and continue playing the video to see if you are correct. How many atoms are bonded to 00:07:09.180 --> 00:07:16.180 the central atom? 3 How many lone pairs are on the central atom? 1 Again, we have to make 00:07:16.500 --> 00:07:21.910 sure we count the lone pair of electrons. Again, to maximize the distance of 4 regions 00:07:21.910 --> 00:07:27.080 of electron density, we can think about them lying at the 4 corners of a tetrahedron with 00:07:27.080 --> 00:07:32.680 the central atom at the center of the tetrahedron. If we build this with our kit, it looks like 00:07:32.680 --> 00:07:39.500 this. As we said earlier, the nomenclature that is used to describe molecular shapes 00:07:39.500 --> 00:07:43.840 only focuses on the positions of atoms. So, if we ignore the lone pair, we're left with 00:07:43.840 --> 00:07:50.840 a shape called a trigonal pyramid. Because the trigonal pyramid is based off of the tetrahedron, 00:07:51.030 --> 00:07:56.830 we would normally predict the bond angles to be 109.5 degrees. However, the lone pair 00:07:56.830 --> 00:08:02.590 will repel the bonded electrons more strongly, causing a compression of bond angles. The 00:08:02.590 --> 00:08:08.330 bond angles in a trigonal pyramid will be less than 109.5 degrees. Chapter Break Now 00:08:08.330 --> 00:08:13.680 I encourage you to predict the geometry of sulfur hexafluoride. Pause the video, draw 00:08:13.680 --> 00:08:18.440 a picture or construct the molecule using a molecule kit, and continue playing the video 00:08:18.440 --> 00:08:25.440 to see if you are correct. How many atoms are bonded to the central atom? 6 How many 00:08:30.050 --> 00:08:37.049 lone pairs are on the central atom? 0 To maximize the distance of 6 regions of electron density, 00:08:37.260 --> 00:08:43.760 they lie at the vertices of an octahedron. Let's take a closer look at the octahedron. 00:08:43.760 --> 00:08:50.760 An octahedron is composed of 8 triangles and has 8 sides, but only 6 vertices. If we constructed 00:09:01.830 --> 00:09:08.830 sulfur hexafluoride with our kit, it would look like this. This shape is called an octahedron. 00:09:09.830 --> 00:09:15.640 The bond angle in an octahedral geometry is 90 degrees. Chapter Break Let's try one that's 00:09:15.640 --> 00:09:21.590 a little different—bromine pentafluoride. Pause the video, draw a picture or construct 00:09:21.590 --> 00:09:28.590 the molecule using a molecule kit, and continue playing the video to see if you are correct. 00:09:31.830 --> 00:09:38.830 How many atoms are bonded to the central atom? 5 How many lone pairs are on the central atom? 00:09:38.860 --> 00:09:45.710 1 Again, to maximize the distance of 6 regions of electron density, they lie at the vertices 00:09:45.710 --> 00:09:52.710 of an octahedron. Our model would look like this. Remember, we only name geometric shapes 00:09:52.780 --> 00:09:57.960 in chemistry based on the position of atoms, so we ignore the lone pair. This shape is 00:09:57.960 --> 00:10:04.960 called a square pyramid. Because the square pyramid is based off of the octahedral geometry, 00:10:05.120 --> 00:10:09.310 we would normally predict the bond angles to be 90 degrees. However, the lone pair will 00:10:09.310 --> 00:10:15.030 repel the bonded electrons more strongly, causing a compression of bond angles. The 00:10:15.030 --> 00:10:18.610 bond angles in a square pyramid will be less than 90 degrees. 00:10:18.610 --> 00:10:24.980 Chapter break These were just a few examples to help get you started with visualizing molecules 00:10:24.980 --> 00:10:27.500 in 3D. There are other variations of these base geometries listed in your textbook that 00:10:27.500 --> 00:10:32.370 you should make sure you are familiar with. The VSEPR model is a simple, empirical model 00:10:32.370 --> 00:10:37.140 that works well for predicting the geometry of most simple molecules, but it's important 00:10:37.140 --> 00:10:42.970 to remember that as is true for all models, it is based on some assumptions that limit 00:10:42.970 --> 00:10:48.200 its range of validity. As you progress in your studies and begin to look at more complex 00:10:48.200 --> 00:10:52.840 molecules such as proteins, you will find that you will need to use more complicated 00:10:52.840 --> 00:10:59.370 models in order to generate 3D representations of these molecules. In this video, you learned 00:10:59.370 --> 00:11:06.370 that the VSEPR model can be used to help understand a simple molecule's 3D structure. In order 00:11:06.420 --> 00:11:11.460 to do this, you need to start with the Lewis structure for the molecule of interest and 00:11:11.460 --> 00:11:17.150 count how many atoms are bonded to the central atom and how many lone pairs are on the central 00:11:17.150 --> 00:11:24.150 atom. Then, you need to think about how you can position those regions as far apart from 00:11:24.190 --> 00:11:29.990 one another as possible. This is where thinking about polyhedra is useful because the vertices 00:11:29.990 --> 00:11:35.550 of polyhedra are at a maximum distance from each other. Finally, you should remember that 00:11:35.550 --> 00:11:42.550 the naming convention for molecular geometries focuses on the locations of the atoms only. 00:11:42.780 --> 00:11:48.000 Because of the simplicity of the VSEPR model, it is an easy way for you to learn how to 00:11:48.000 --> 00:11:52.740 translate 2D representation of molecules into 3D. With some practice, visualizing molecules 00:11:52.740 --> 00:11:59.740 in 3D can become second nature to you.