1 00:00:03,639 --> 00:00:08,520 Molecules aren't flat. They're three dimensional, and that has implications for their physical 2 00:00:08,520 --> 00:00:14,500 and chemical properties. For example, X-ray crystallography labs at MIT determine the 3 00:00:14,500 --> 00:00:20,599 3D shapes of protein molecules to design drugs that will fit into these proteins. In this 4 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 5 00:00:26,050 --> 00:00:28,589 from its Lewis structure. 6 00:00:28,589 --> 00:00:32,720 This video is part of the Representations video series. Information can be represented 7 00:00:32,720 --> 00:00:38,809 in words, through mathematical symbols, graphically, or in 3-D models. Representations are used 8 00:00:38,809 --> 00:00:44,879 to develop a deeper and more flexible understanding of objects, systems, and processes. 9 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 10 00:00:49,829 --> 00:00:53,539 you have been enjoying your general chemistry course at SUTD. 11 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 12 00:00:58,500 --> 00:01:03,089 structures from 2D Lewis structures and... ...discuss some of the assumptions of the 13 00:01:03,089 --> 00:01:06,880 VSEPR model. Before watching this video, you should be able to draw Lewis structures for 14 00:01:06,880 --> 00:01:07,759 simple molecules. 15 00:01:07,759 --> 00:01:08,240 We can determine molecular shape experimentally or predict it with varying degrees of accuracy 16 00:01:08,240 --> 00:01:08,409 using empirical and theoretical models. 17 00:01:08,409 --> 00:01:08,850 The model that will be introduced in this video, the Valence-Shell Electron-Pair Repulsion 18 00:01:08,850 --> 00:01:09,659 Model, or the VSEPR model, is an empirical model, but works quite well for most simple 19 00:01:09,659 --> 00:01:09,810 Because it is an empirical model, the VSEPR model was constructed by looking for patterns 20 00:01:09,810 --> 00:01:13,130 The VSEPR model, which builds off of Lewis theory, is based on the idea that regions 21 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 22 00:01:17,729 --> 00:01:24,600 NF3 to see what that means. The VSEPR model focuses on the central atoms of molecules... 23 00:01:24,600 --> 00:01:30,680 ...and assumes that bonding atoms... ...and lone pairs on the central atom are spaced 24 00:01:30,680 --> 00:01:35,590 as far apart as possible from each other to minimize electron repulsions, but at the same 25 00:01:35,590 --> 00:01:41,789 time are equidistant from the central atom. The VSEPR model attributes lone pairs of electrons 26 00:01:41,789 --> 00:01:46,539 with having a stronger repulsion than bonded electrons. And finally, the VSEPR model treats 27 00:01:46,539 --> 00:01:51,829 multiple bonds between 2 atoms as a single region of electron density. 28 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, 29 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 30 00:02:02,570 --> 00:02:07,789 electrons on the central atom. To minimize electron repulsions, we will maximize the 31 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 32 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 33 00:02:19,310 --> 00:02:25,090 not have maximized the distance between the bonded atoms. We would describe nitrous oxide 34 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. 35 00:02:33,250 --> 00:02:38,870 Note that when we examined the Lewis structure for nitrous oxide, the triple bonded nitrogen 36 00:02:38,870 --> 00:02:43,319 was treated the same as the single bonded oxygen atom. Chapter Break Now, if we followed 37 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 38 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 39 00:02:54,610 --> 00:02:59,769 from each other, we will space them 120 degrees apart around the central atom... ...on the 40 00:02:59,769 --> 00:03:06,769 vertices of an equilateral triangle. We would describe sulfur trioxide as a trigonal planar 41 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. 42 00:03:14,879 --> 00:03:20,159 Notice that you can draw resonance structures for SO2, but because VSEPR doesn't distinguish 43 00:03:20,159 --> 00:03:24,459 b/w single and multiple bonds, we can look at any resonance structure when predicting 44 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 45 00:03:30,709 --> 00:03:36,060 of this molecule will be? Pause the video, draw a picture or construct the molecule using 46 00:03:36,060 --> 00:03:41,329 a molecule kit, and continue playing the video to see if you are correct. 47 00:03:41,329 --> 00:03:48,329 Remember, the VSEPR model assumes that bonding atoms AND lone pairs on the central atom are 48 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 49 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. 50 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 51 00:04:10,519 --> 00:04:16,640 on the vertices of an equilateral triangle. On our model, this (pointing at electron cloud) 52 00:04:16,640 --> 00:04:22,100 represents the lone pair of electrons. While we have to think about lone pairs of electrons 53 00:04:22,100 --> 00:04:28,200 when predicting geometry, the naming convention actually ignores them. So, if we were to name 54 00:04:28,200 --> 00:04:32,409 this shape, we would focus on the shape that is determined by the atoms. 55 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, 56 00:04:39,390 --> 00:04:44,230 but bond angles in molecules with lone pairs on the central atom have been observed to 57 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 58 00:04:50,600 --> 00:04:56,500 region than bonded electrons causing the bonded atoms to move farther from the lone pair and 59 00:04:56,500 --> 00:05:01,860 closer to each other, compressing the bond angle. The VSEPR model accounts for this by 60 00:05:01,860 --> 00:05:07,950 saying that lone pairs repel more strongly than bonded electrons. The result is that 61 00:05:07,950 --> 00:05:13,550 the bond angle on sulfur dioxide will be less than 120 degrees, but VSEPR theory cannot 62 00:05:13,550 --> 00:05:16,520 tell us what the precise bond angle will be. 63 00:05:16,520 --> 00:05:23,520 Chapter Break Let's do another example - CH3Cl (chloromethane, a.k.a. methyl chloride). What 64 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 65 00:05:29,310 --> 00:05:33,870 construct the molecule using a molecule kit, and continue playing the video to see if you 66 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 67 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 68 00:05:52,750 --> 00:05:57,480 4 regions of electron density at 90 degree angles to each other, ...but this would be 69 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 70 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 71 00:06:09,280 --> 00:06:14,020 to review what a tetrahedron looks like. A tetrahedron is a pyramid constructed from 72 00:06:14,020 --> 00:06:20,590 4 equilateral triangles. It has a triangular base and sides made from triangles. This results 73 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. 74 00:06:29,180 --> 00:06:35,630 In a tetrahedral geometry, the bond angle between bonding atoms is 109.5. This is larger 75 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 76 00:06:41,650 --> 00:06:48,650 of CH3Cl would look like this. This shape is aptly named a tetrahedron. Chapter Break 77 00:06:48,990 --> 00:06:54,810 Let's do another example -- nitrogen trifluoride. What do you think the geometry of this molecule 78 00:06:54,810 --> 00:06:59,670 will be? Pause the video, draw a picture or construct the molecule using a molecule kit, 79 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 80 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 81 00:07:16,500 --> 00:07:21,910 sure we count the lone pair of electrons. Again, to maximize the distance of 4 regions 82 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 83 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 84 00:07:32,680 --> 00:07:39,500 this. As we said earlier, the nomenclature that is used to describe molecular shapes 85 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 86 00:07:43,840 --> 00:07:50,840 a shape called a trigonal pyramid. Because the trigonal pyramid is based off of the tetrahedron, 87 00:07:51,030 --> 00:07:56,830 we would normally predict the bond angles to be 109.5 degrees. However, the lone pair 88 00:07:56,830 --> 00:08:02,590 will repel the bonded electrons more strongly, causing a compression of bond angles. The 89 00:08:02,590 --> 00:08:08,330 bond angles in a trigonal pyramid will be less than 109.5 degrees. Chapter Break Now 90 00:08:08,330 --> 00:08:13,680 I encourage you to predict the geometry of sulfur hexafluoride. Pause the video, draw 91 00:08:13,680 --> 00:08:18,440 a picture or construct the molecule using a molecule kit, and continue playing the video 92 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 93 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, 94 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. 95 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 96 00:09:01,830 --> 00:09:08,830 sulfur hexafluoride with our kit, it would look like this. This shape is called an octahedron. 97 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 98 00:09:15,640 --> 00:09:21,590 a little different—bromine pentafluoride. Pause the video, draw a picture or construct 99 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. 100 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? 101 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 102 00:09:45,710 --> 00:09:52,710 of an octahedron. Our model would look like this. Remember, we only name geometric shapes 103 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 104 00:09:57,960 --> 00:10:04,960 called a square pyramid. Because the square pyramid is based off of the octahedral geometry, 105 00:10:05,120 --> 00:10:09,310 we would normally predict the bond angles to be 90 degrees. However, the lone pair will 106 00:10:09,310 --> 00:10:15,030 repel the bonded electrons more strongly, causing a compression of bond angles. The 107 00:10:15,030 --> 00:10:18,610 bond angles in a square pyramid will be less than 90 degrees. 108 00:10:18,610 --> 00:10:24,980 Chapter break These were just a few examples to help get you started with visualizing molecules 109 00:10:24,980 --> 00:10:27,500 in 3D. There are other variations of these base geometries listed in your textbook that 110 00:10:27,500 --> 00:10:32,370 you should make sure you are familiar with. The VSEPR model is a simple, empirical model 111 00:10:32,370 --> 00:10:37,140 that works well for predicting the geometry of most simple molecules, but it's important 112 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 113 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 114 00:10:48,200 --> 00:10:52,840 molecules such as proteins, you will find that you will need to use more complicated 115 00:10:52,840 --> 00:10:59,370 models in order to generate 3D representations of these molecules. In this video, you learned 116 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 117 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 118 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 119 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 120 00:11:24,190 --> 00:11:29,990 one another as possible. This is where thinking about polyhedra is useful because the vertices 121 00:11:29,990 --> 00:11:35,550 of polyhedra are at a maximum distance from each other. Finally, you should remember that 122 00:11:35,550 --> 00:11:42,550 the naming convention for molecular geometries focuses on the locations of the atoms only. 123 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 124 00:11:48,000 --> 00:11:52,740 translate 2D representation of molecules into 3D. With some practice, visualizing molecules 125 00:11:52,740 --> 00:11:59,740 in 3D can become second nature to you.