WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.780 Commons license. 00:00:03.780 --> 00:00:06.020 Your support will help MIT OpenCourseWare 00:00:06.020 --> 00:00:10.080 continue to offer high quality educational resources for free. 00:00:10.080 --> 00:00:12.670 To make a donation or to view additional materials 00:00:12.670 --> 00:00:16.580 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.580 --> 00:00:27.820 at ocw.mit.edu 00:00:27.820 --> 00:00:28.910 PROFESSOR: OK. 00:00:28.910 --> 00:00:31.205 All right, let us take ten more seconds. 00:00:49.350 --> 00:00:51.190 All right, 92%. 00:00:51.190 --> 00:00:52.090 I like it. 00:00:54.910 --> 00:00:58.170 That's great, I told you there'd be a formal charge question, 00:00:58.170 --> 00:01:00.450 and there was, and you learned it. 00:01:00.450 --> 00:01:02.710 Awesome, that's what I like. 00:01:02.710 --> 00:01:07.730 All right, that was carbon had four, no lone pairs. 00:01:07.730 --> 00:01:12.260 So, 4 minus 0 minus 1/2 of eight bonding electrons, 00:01:12.260 --> 00:01:12.940 which is four. 00:01:12.940 --> 00:01:16.300 So, 4 minus 4 is 0. 00:01:16.300 --> 00:01:20.620 OK, so today we're having a clicker competition. 00:01:20.620 --> 00:01:24.370 And problem set 4 will be posted later today. 00:01:24.370 --> 00:01:29.640 And today's goal is for all recitations, but recitation 12 00:01:29.640 --> 00:01:33.720 to try to unseat recitation 12. 00:01:33.720 --> 00:01:36.130 And the goal of recitation 12 is, of course, 00:01:36.130 --> 00:01:42.380 to win an unprecedented third week in a row. 00:01:42.380 --> 00:01:45.270 So, that's the goal for everyone today. 00:01:45.270 --> 00:01:50.910 Now, remember recitations that win a multiple weeks will 00:01:50.910 --> 00:01:53.910 be in the playoffs for the clicker competition 00:01:53.910 --> 00:01:57.174 at the end of the semester for a specially designed t-shirt. 00:01:59.780 --> 00:02:02.800 OK, more on clicker competitions actually later today. 00:02:05.450 --> 00:02:08.350 So, if we can get settled in. 00:02:08.350 --> 00:02:09.900 I know it's exciting. 00:02:09.900 --> 00:02:13.390 And you are allowed to talk to your recitations, 00:02:13.390 --> 00:02:16.510 other recitation members during the clicker questions, that 00:02:16.510 --> 00:02:18.410 is allowed. 00:02:18.410 --> 00:02:21.707 But I'll need to cover a little material first. 00:02:21.707 --> 00:02:24.040 So today, we're going to talk about shapes of molecules, 00:02:24.040 --> 00:02:28.070 and I brought some molecules with me today to help me out. 00:02:28.070 --> 00:02:30.880 And we're going to be talking about VSEPR theory. 00:02:30.880 --> 00:02:35.420 So, why is shapes of molecules important? 00:02:35.420 --> 00:02:39.230 So shape, which we can also call geometry. 00:02:39.230 --> 00:02:41.830 So, you're going to ask about the geometry of the molecule, 00:02:41.830 --> 00:02:44.290 you are asking what is it's shape. 00:02:44.290 --> 00:02:46.830 It's particularly important in chemistry 00:02:46.830 --> 00:02:49.680 because shape can dictate properties 00:02:49.680 --> 00:02:51.610 of a particular molecule. 00:02:51.610 --> 00:02:53.710 It can tell you about-- or dictate 00:02:53.710 --> 00:02:57.440 melting points, or boiling points, or reactivity. 00:02:57.440 --> 00:02:59.480 And I'm a biological chemist, so I 00:02:59.480 --> 00:03:03.600 care a lot about shapes of molecules because in biology 00:03:03.600 --> 00:03:05.280 shape is really important. 00:03:05.280 --> 00:03:08.000 So, you have enzymes in your body catalyzing reactions. 00:03:08.000 --> 00:03:09.970 And for those enzymes to work, they 00:03:09.970 --> 00:03:12.630 are specially designed to react with one 00:03:12.630 --> 00:03:15.820 particular kind of molecule and not any molecule in the cell. 00:03:15.820 --> 00:03:18.010 So, they're designed to recognize 00:03:18.010 --> 00:03:20.010 the shape of that molecule. 00:03:20.010 --> 00:03:23.100 So, biochemistry really works by shape recognition, 00:03:23.100 --> 00:03:25.830 so shape is very important. 00:03:25.830 --> 00:03:30.120 So, there's a lot of ways to get information about shape. 00:03:30.120 --> 00:03:33.200 But there's one very simple theory 00:03:33.200 --> 00:03:35.910 that does exceedingly well in predicting 00:03:35.910 --> 00:03:37.760 the shapes of small molecules. 00:03:37.760 --> 00:03:42.930 And that is called the Valence Shell Electron Pair Repulsion 00:03:42.930 --> 00:03:46.550 theory, which is known as VSEPR. 00:03:46.550 --> 00:03:51.430 And is also known as the V-S-E-P-R theory. 00:03:51.430 --> 00:03:57.350 I will call it VSEPR because it is really hard to say V-S-E-P-R 00:03:57.350 --> 00:03:58.950 theory. 00:03:58.950 --> 00:04:01.100 So, VSEPR is the topic today and this 00:04:01.100 --> 00:04:02.920 is based on Lewis structures. 00:04:02.920 --> 00:04:05.620 So, this is highly exciting because you just 00:04:05.620 --> 00:04:08.040 finished a problem set that had you 00:04:08.040 --> 00:04:10.250 draw lots of Lewis structures. 00:04:10.250 --> 00:04:13.530 And now on the next problem set, you 00:04:13.530 --> 00:04:16.740 can draw more Lewis structures and then 00:04:16.740 --> 00:04:18.690 tell us about his shape. 00:04:18.690 --> 00:04:20.950 So, that's very exciting. 00:04:20.950 --> 00:04:23.590 So, you're going to retain all of the knowledge 00:04:23.590 --> 00:04:25.500 that you've gained in the last problem set 00:04:25.500 --> 00:04:27.930 and continue on with that problems. 00:04:27.930 --> 00:04:32.590 All right, so this is a very simple theory. 00:04:32.590 --> 00:04:35.820 And it's based on the idea that valence shell-- 00:04:35.820 --> 00:04:40.820 valence electron pairs repel each other. 00:04:40.820 --> 00:04:42.900 Electrons repel each other. 00:04:42.900 --> 00:04:45.810 They're negatively charged, they repel each other. 00:04:45.810 --> 00:04:49.900 That is the heart of this theory, very simple. 00:04:49.900 --> 00:04:52.050 And as many of you may have gathered, 00:04:52.050 --> 00:04:55.020 I love it one very simple theories 00:04:55.020 --> 00:04:57.730 explain a lot of stuff. 00:04:57.730 --> 00:04:59.120 I enjoy that. 00:04:59.120 --> 00:05:01.620 Gets it, it doesn't get it right 100% of the time, 00:05:01.620 --> 00:05:04.310 I'm OK with that, I'm good with about 90%. 00:05:04.310 --> 00:05:07.900 So, this works really pretty well. 00:05:07.900 --> 00:05:10.170 So, again we're talking about the geometry 00:05:10.170 --> 00:05:12.570 around a central atom. 00:05:12.570 --> 00:05:15.650 And the idea is that the atoms are 00:05:15.650 --> 00:05:18.320 lone pairs around that central atom are going 00:05:18.320 --> 00:05:21.880 to move in such a way, attain such a shape, such 00:05:21.880 --> 00:05:25.150 that the repulsion is minimized. 00:05:25.150 --> 00:05:28.990 So, this is all about minimizing repulsion, minimizing stress. 00:05:28.990 --> 00:05:32.020 Again, I'm a big fan of minimizing stress, 00:05:32.020 --> 00:05:35.230 so I also like VSEPR for that reason as well. 00:05:35.230 --> 00:05:37.880 AUDIENCE: [INAUDIBLE] 00:05:37.880 --> 00:05:39.750 PROFESSOR: Lewis structures? 00:05:39.750 --> 00:05:40.720 What causes stress? 00:05:40.720 --> 00:05:41.460 Problem sets. 00:05:41.460 --> 00:05:46.280 Oh, but they build character. 00:05:46.280 --> 00:05:52.390 All right, VSEPR Very simple nomenclature, 00:05:52.390 --> 00:05:54.060 again I like simple nomenclature. 00:05:54.060 --> 00:05:57.230 A is the central atom. 00:05:57.230 --> 00:05:59.520 Maybe should be C, but atom is there, 00:05:59.520 --> 00:06:04.800 so central atom is A. X is the bonding atom, 00:06:04.800 --> 00:06:08.700 and E is the lone pair of electrons. 00:06:08.700 --> 00:06:10.810 So, E lone pair electrons. 00:06:10.810 --> 00:06:16.140 X is whatever bonding atom, and A, A is in the center. 00:06:16.140 --> 00:06:18.650 A is the central atom. 00:06:18.650 --> 00:06:23.890 One more term you need to know for VSEPR and that's steric 00:06:23.890 --> 00:06:26.420 number, . 00:06:26.420 --> 00:06:30.080 And that's used to predict geometries. 00:06:30.080 --> 00:06:33.700 So, what is steric number equal? 00:06:33.700 --> 00:06:36.260 Steric number is the number of atoms 00:06:36.260 --> 00:06:42.160 bonded to the central atom, plus the number of lone pair 00:06:42.160 --> 00:06:42.700 electrons. 00:06:42.700 --> 00:06:47.310 And you count one pair as one. 00:06:47.310 --> 00:06:50.800 And so, you want to note, when considering 00:06:50.800 --> 00:06:53.640 VSEPR double triple bonds are all-- 00:06:53.640 --> 00:06:56.102 and single bonds all the same. 00:06:56.102 --> 00:06:58.560 So, you don't have to worry about double triple bonds right 00:06:58.560 --> 00:06:59.550 now. 00:06:59.550 --> 00:07:01.390 They're all counts the same. 00:07:01.390 --> 00:07:03.180 It's only the number of bonded atoms 00:07:03.180 --> 00:07:05.160 and the number of loan pairs. 00:07:05.160 --> 00:07:07.300 So, let's look at some examples of this. 00:07:07.300 --> 00:07:12.250 So, we have our central atom A, bonded to two bonding atoms 00:07:12.250 --> 00:07:15.480 X, with one lone pair of electrons. 00:07:15.480 --> 00:07:18.130 So, if you were asked what is the formula for this, 00:07:18.130 --> 00:07:24.320 the VSEPR formula, it would be AX2E, as shown there. 00:07:24.320 --> 00:07:26.930 And then you might be asked to steric number. 00:07:26.930 --> 00:07:30.440 And the steric number in this case would be what? 00:07:30.440 --> 00:07:31.726 Three. 00:07:31.726 --> 00:07:33.980 We have two bonding atoms. 00:07:33.980 --> 00:07:35.860 One lone pair of electrons. 00:07:35.860 --> 00:07:39.940 Again, the lone pair just counts as one. 00:07:39.940 --> 00:07:43.590 So, I could also draw this structure with a double bond 00:07:43.590 --> 00:07:45.890 to one of the Xs. 00:07:45.890 --> 00:07:52.302 And this would have the equation AX2E, exactly the same formula. 00:07:52.302 --> 00:07:54.760 Because we don't care about the double bond in the formula, 00:07:54.760 --> 00:07:56.490 only the number of bonding atoms. 00:07:56.490 --> 00:07:58.320 Only the number of lone pairs. 00:07:58.320 --> 00:08:01.460 And this would have an SN number of what? 00:08:01.460 --> 00:08:03.360 Three, right. 00:08:03.360 --> 00:08:07.970 Because it only matters about the lone pairs and the atoms. 00:08:07.970 --> 00:08:09.580 That's all we care about here. 00:08:09.580 --> 00:08:12.190 We don't care about the double bonds, triple bonds, whatever, 00:08:12.190 --> 00:08:13.356 we'll care about them later. 00:08:13.356 --> 00:08:15.020 But for right now for geometry, we're 00:08:15.020 --> 00:08:18.010 only caring about bonded atoms and lone pairs. 00:08:21.090 --> 00:08:23.830 Some of you will be very happy after doing the last problem 00:08:23.830 --> 00:08:30.410 set to know, that you can apply VSEPR theory to all 00:08:30.410 --> 00:08:32.789 the resonance structures that you may come up with when 00:08:32.789 --> 00:08:34.760 you're doing Lewis structures. 00:08:34.760 --> 00:08:38.190 So, if a molecule has one or more resonance structures 00:08:38.190 --> 00:08:39.440 that's OK. 00:08:39.440 --> 00:08:41.730 VSEPR can be applied to any one of them. 00:08:44.900 --> 00:08:47.610 Also, if there's more than one central atom, 00:08:47.610 --> 00:08:51.260 you need to consider those atoms separately. 00:08:51.260 --> 00:08:54.110 So, in this case, you would be asked 00:08:54.110 --> 00:08:57.060 about the geometry around the carbon and the geometry 00:08:57.060 --> 00:09:01.400 around the oxygen. But a lot of the examples we have today 00:09:01.400 --> 00:09:04.200 just have one central atom. 00:09:04.200 --> 00:09:07.450 All right, so that's just a little introduction to VSEPR 00:09:07.450 --> 00:09:10.680 Now, there's two cases that we're going to consider today, 00:09:10.680 --> 00:09:13.450 one are molecules without lone pairs 00:09:13.450 --> 00:09:15.530 and one are molecules with lone pairs. 00:09:15.530 --> 00:09:17.460 Without lone pairs is a little bit easier 00:09:17.460 --> 00:09:18.840 than with lone pairs. 00:09:18.840 --> 00:09:21.550 So, let's start there. 00:09:21.550 --> 00:09:23.750 And we have a nice table for you. 00:09:23.750 --> 00:09:26.820 I'll tell you that these lecture notes are of high value. 00:09:26.820 --> 00:09:29.340 I've had people who've taken this course want to come back, 00:09:29.340 --> 00:09:31.620 it's a nice summary of all these shapes. 00:09:31.620 --> 00:09:35.650 So, you want to keep this in a nice, secure location. 00:09:35.650 --> 00:09:37.420 A highly desirable notes. 00:09:37.420 --> 00:09:40.410 OK, so let's look at a formula type. 00:09:40.410 --> 00:09:43.820 The simplest we have AX2 here. 00:09:43.820 --> 00:09:46.360 So, that has a SN number of 2, because it 00:09:46.360 --> 00:09:48.380 has two bonded atoms. 00:09:48.380 --> 00:09:50.890 And it has this molecular shape. 00:09:50.890 --> 00:09:53.910 And I brought an example here. 00:09:53.910 --> 00:09:57.860 And this is, of course, a linear molecule and therefore 00:09:57.860 --> 00:10:00.530 the angle-- and here we're talking about the angle 00:10:00.530 --> 00:10:01.850 around the central atom. 00:10:01.850 --> 00:10:08.030 So, from this black atom to that black atom, we'd have 180. 00:10:08.030 --> 00:10:12.010 So, that's the simplest linear molecule. 00:10:12.010 --> 00:10:13.910 So, it's got a little more complicated 00:10:13.910 --> 00:10:17.890 and add three bonding atoms. 00:10:17.890 --> 00:10:21.237 So, here we have a case of AX3. 00:10:23.820 --> 00:10:25.930 It has a SN number of 3. 00:10:25.930 --> 00:10:28.760 There are three things bonded to the central atom. 00:10:28.760 --> 00:10:31.690 And its shape is trigonal planar. 00:10:31.690 --> 00:10:34.880 Trigonal should be remembered, it looks like a triangle. 00:10:34.880 --> 00:10:37.140 And you should also remember that it's planar. 00:10:37.140 --> 00:10:39.860 You can hold it this way and see all of the atoms 00:10:39.860 --> 00:10:41.170 are in one plane. 00:10:41.170 --> 00:10:42.620 And I'm making some of this point 00:10:42.620 --> 00:10:44.880 because later on the exam, when you're asked 00:10:44.880 --> 00:10:46.560 to name geometries of things. 00:10:46.560 --> 00:10:49.020 People come up with all sorts of crazy things. 00:10:49.020 --> 00:10:53.600 So, if I spend a little time doing my demonstration here 00:10:53.600 --> 00:10:57.480 of the shapes of molecules, it will pay off later in the exam. 00:10:57.480 --> 00:10:58.860 It'll sear in your brain. 00:10:58.860 --> 00:11:00.060 It will be hard to forget. 00:11:00.060 --> 00:11:01.187 Trigonal planar. 00:11:01.187 --> 00:11:02.020 What are the angles? 00:11:04.041 --> 00:11:04.540 120. 00:11:06.352 --> 00:11:07.260 Right. 00:11:07.260 --> 00:11:08.680 OK, let's move on. 00:11:08.680 --> 00:11:11.900 We'll have four atoms. 00:11:11.900 --> 00:11:13.680 SN number of 4, AX4. 00:11:16.390 --> 00:11:19.740 So, this is our tetrahedral geometry. 00:11:19.740 --> 00:11:23.510 And I'm going to just note, as it says down here, 00:11:23.510 --> 00:11:27.490 that when you have a thick arrow coming out 00:11:27.490 --> 00:11:32.800 at you, like this bond and a thin one going back, 00:11:32.800 --> 00:11:35.590 that means the one coming out is coming straight out 00:11:35.590 --> 00:11:36.460 towards you. 00:11:36.460 --> 00:11:39.420 The dashed line is going back into the screen. 00:11:39.420 --> 00:11:43.660 And the two that are not thickened or dashed 00:11:43.660 --> 00:11:45.010 are in the plane. 00:11:45.010 --> 00:11:48.520 So, if I hold it like this, we have two atoms in the plane, 00:11:48.520 --> 00:11:51.160 one coming out, one going back. 00:11:51.160 --> 00:11:55.030 And so, that's how-- if you see that drawn that way, 00:11:55.030 --> 00:11:58.530 and you will, you can think about it in three dimensions. 00:11:58.530 --> 00:12:01.366 All right, so what's the angle of tetrahedral? 00:12:03.930 --> 00:12:07.920 109.5. 00:12:07.920 --> 00:12:10.809 So, a lot of people have already familiar with this is great, 00:12:10.809 --> 00:12:12.850 if you're not, you will learn it quickly when you 00:12:12.850 --> 00:12:14.354 do the problems. 00:12:14.354 --> 00:12:15.520 All right, we'll keep going. 00:12:15.520 --> 00:12:17.570 We have five. 00:12:17.570 --> 00:12:19.570 And you move over here. 00:12:19.570 --> 00:12:20.955 So, a SN number of 5. 00:12:20.955 --> 00:12:23.350 We have AX5. 00:12:23.350 --> 00:12:25.470 And here is our shape. 00:12:25.470 --> 00:12:28.720 So, this is trigonal bipyramidal. 00:12:28.720 --> 00:12:32.590 So, if you think about the shape of your trigonal planar, 00:12:32.590 --> 00:12:36.950 you see along here, we have our trigonal planar. 00:12:36.950 --> 00:12:41.250 So, we have our trigonal shape, but we also now have an atom 00:12:41.250 --> 00:12:43.030 above and an atom below. 00:12:43.030 --> 00:12:46.830 And that forms a kind of a pyramid on top 00:12:46.830 --> 00:12:48.840 and a pyramid on the bottom. 00:12:48.840 --> 00:12:52.160 So, it's trigonal bipyramidal. 00:12:52.160 --> 00:12:55.050 And there are two sets of angles now. 00:12:55.050 --> 00:12:59.037 What are the angles in the equatorial? 00:12:59.037 --> 00:12:59.537 120. 00:13:01.690 --> 00:13:04.630 I love when people yell things out, it's awesome. 00:13:04.630 --> 00:13:08.060 Angle from the axial to the equatorial? 00:13:08.060 --> 00:13:08.560 90. 00:13:08.560 --> 00:13:11.940 Awesome. 00:13:11.940 --> 00:13:13.410 All right. 00:13:13.410 --> 00:13:19.670 And one more basic shape group, and that's this one here. 00:13:19.670 --> 00:13:24.850 We have six atoms bonded to the central atom, AX6. 00:13:24.850 --> 00:13:27.110 And we have this shape. 00:13:27.110 --> 00:13:30.270 So, now I'm holding it, two axial atoms. 00:13:30.270 --> 00:13:33.850 Two atoms coming out towards you, two going back. 00:13:33.850 --> 00:13:35.330 Octahedral geometry. 00:13:35.330 --> 00:13:37.210 And what are all the angles here? 00:13:37.210 --> 00:13:40.620 90, awesome. 00:13:40.620 --> 00:13:41.517 All right. 00:13:44.930 --> 00:13:47.000 So, let's look at some examples. 00:13:47.000 --> 00:13:49.880 Hopefully, everyone got the 90 and can write it down. 00:13:49.880 --> 00:13:52.550 If not, I'm sure someone will yell it out again for you. 00:13:52.550 --> 00:13:55.060 All right, so let's look at some examples. 00:13:55.060 --> 00:13:57.760 We were talking about CO2, and you calculated 00:13:57.760 --> 00:14:00.820 the formal charge on it for me. 00:14:00.820 --> 00:14:03.070 So, we have AX2. 00:14:03.070 --> 00:14:04.780 SN number of 2. 00:14:04.780 --> 00:14:09.830 This is linear and our angle is 180. 00:14:09.830 --> 00:14:13.570 And now we can explain why this is non-polar. 00:14:13.570 --> 00:14:15.790 We learned before that we had polar bonds 00:14:15.790 --> 00:14:17.720 where we had difference of electronegativity 00:14:17.720 --> 00:14:19.660 between the carbon and the oxygen, 00:14:19.660 --> 00:14:21.090 which would make a polar bonds. 00:14:21.090 --> 00:14:24.310 You have two polar bonds, so why is this not a polar molecule? 00:14:24.310 --> 00:14:27.240 And it's not a polar molecule because this oxygen is pulling 00:14:27.240 --> 00:14:29.550 this way, this one's pulling that way, 00:14:29.550 --> 00:14:32.470 and that makes it non-planar because there's no net dipole. 00:14:32.470 --> 00:14:35.141 So, shape is important for this. 00:14:35.141 --> 00:14:36.890 All right, let's look at the next example. 00:14:36.890 --> 00:14:39.950 We've seen a lot of these examples before. 00:14:39.950 --> 00:14:42.090 We're talking about Lewis structure. 00:14:42.090 --> 00:14:44.202 What was boron an example of? 00:14:47.298 --> 00:14:48.670 Incomplete octet. 00:14:48.670 --> 00:14:49.570 That's right. 00:14:49.570 --> 00:14:54.090 So, here's an example, it has three things bound to it. 00:14:54.090 --> 00:14:58.560 And it has the shape of trigonal planar. 00:14:58.560 --> 00:15:01.670 And the middle atom is incomplete octet. 00:15:01.670 --> 00:15:02.910 But it's OK. 00:15:02.910 --> 00:15:07.540 Boron and which other one are OK being incomplete octets? 00:15:07.540 --> 00:15:09.900 Aluminum, right. 00:15:09.900 --> 00:15:12.074 So, the angles are 120 here. 00:15:14.740 --> 00:15:15.950 Now, we move on. 00:15:15.950 --> 00:15:18.040 We have this molecule CH4. 00:15:18.040 --> 00:15:20.880 Can someone tell me what this is? 00:15:20.880 --> 00:15:22.600 Methane. 00:15:22.600 --> 00:15:26.400 So, we have AX4, SN number of 4. 00:15:26.400 --> 00:15:31.630 And we have our tetrahedral shape and our angles of? 00:15:31.630 --> 00:15:33.040 109.5. 00:15:33.040 --> 00:15:34.180 Right. 00:15:34.180 --> 00:15:36.290 So, methane we should all care about. 00:15:36.290 --> 00:15:38.460 Greenhouse gas, but also some people 00:15:38.460 --> 00:15:42.550 believe will be the salivation to our biofuels and energy 00:15:42.550 --> 00:15:43.070 problem. 00:15:43.070 --> 00:15:45.050 We will see if that's true or not. 00:15:45.050 --> 00:15:47.940 OK, another example over here. 00:15:47.940 --> 00:15:51.950 So we have phosphorus in the middle and five chlorines. 00:15:51.950 --> 00:15:56.990 So, what is this an example of, in terms of Lewis structures? 00:15:56.990 --> 00:16:03.310 Yep, so this is trigonal bipyramidal or bipyramidal. 00:16:03.310 --> 00:16:05.900 I think both are right, I don't know. 00:16:05.900 --> 00:16:09.390 And so we have our 120 and our 90. 00:16:09.390 --> 00:16:11.800 And this was an example we had in Lewis structures 00:16:11.800 --> 00:16:14.960 or something very similar of an expanded octet. 00:16:14.960 --> 00:16:16.250 So, we have more. 00:16:16.250 --> 00:16:18.710 We have five bonds around the phosphorus 00:16:18.710 --> 00:16:22.840 and that's OK because it's N equals three or greater. 00:16:22.840 --> 00:16:26.200 So, we have an example again of an expanded octet. 00:16:26.200 --> 00:16:29.300 And if we keep going, we'll have another expanded octet. 00:16:29.300 --> 00:16:32.190 So, here we have AX6. 00:16:32.190 --> 00:16:34.630 So, we have six bonds around the sulfur. 00:16:34.630 --> 00:16:36.470 Sulfur is OK with that. 00:16:36.470 --> 00:16:39.160 And what is the geometry here? 00:16:39.160 --> 00:16:41.720 Octahedral angles? 00:16:41.720 --> 00:16:43.496 90. 00:16:43.496 --> 00:16:46.860 Yep, all right, so we have several examples here 00:16:46.860 --> 00:16:53.290 of things with expanded octets and also deficient octets. 00:16:53.290 --> 00:16:56.800 All right, so this is pretty straightforward. 00:16:56.800 --> 00:16:58.620 People get a lot of points on exams, 00:16:58.620 --> 00:17:01.370 except when they come up with all sorts of weird kind 00:17:01.370 --> 00:17:02.880 of shapes that don't exist. 00:17:02.880 --> 00:17:05.819 But most of the time you can learn this and it's great. 00:17:05.819 --> 00:17:09.050 With lone pairs it's a little more complicated, but also 00:17:09.050 --> 00:17:10.319 much more fun. 00:17:10.319 --> 00:17:12.960 All right, so let's talk now about what happens 00:17:12.960 --> 00:17:14.070 when we have lone pairs. 00:17:17.750 --> 00:17:25.190 So, electrons in bonds are hanging out in their bond 00:17:25.190 --> 00:17:28.210 and they're not really doing much, but being in their bond. 00:17:28.210 --> 00:17:32.810 So, they have less spatial distribution than loan pairs. 00:17:32.810 --> 00:17:39.360 Meaning that, electrons in bonds take up less space. 00:17:39.360 --> 00:17:44.490 And electrons in lone pairs, they can be anywhere. 00:17:44.490 --> 00:17:46.420 They're not restricted to their bond, 00:17:46.420 --> 00:17:49.450 so they take up more space and therefore, 00:17:49.450 --> 00:17:51.710 cause more repulsion. 00:17:51.710 --> 00:17:53.870 So, the whole idea of when you're 00:17:53.870 --> 00:17:56.140 talking about VSEPR with lone pairs 00:17:56.140 --> 00:17:59.130 is that you're thinking about electron pair repulsion. 00:17:59.130 --> 00:18:01.970 And if you have a lone pair, that's 00:18:01.970 --> 00:18:04.900 going to give you more repulsion than bonded electrons 00:18:04.900 --> 00:18:08.730 because lone pair electrons can take up more space. 00:18:08.730 --> 00:18:11.210 So, it's a very simple idea, but it actually 00:18:11.210 --> 00:18:14.630 works to explain a lot of stuff and again, the geometry 00:18:14.630 --> 00:18:18.580 re-arranges to minimize that repulsion. 00:18:18.580 --> 00:18:22.470 So, when we're talking about repulsive forces then, 00:18:22.470 --> 00:18:28.260 we go from the most repulsion, lone pair, lone pair. 00:18:28.260 --> 00:18:30.830 That's like two messy roommates living together, that's 00:18:30.830 --> 00:18:33.150 very repulsive situation. 00:18:33.150 --> 00:18:35.750 Lone pair, bonding pair. 00:18:35.750 --> 00:18:38.890 And then bonding pair, bonding pair is the least repulsive. 00:18:38.890 --> 00:18:42.860 To neat roommates that usually works out quite well. 00:18:42.860 --> 00:18:45.050 So, if we keep this in mind, we can now 00:18:45.050 --> 00:18:49.640 predict shapes of molecules based on this repulsion. 00:18:49.640 --> 00:18:53.710 So, we can rationalize shapes based on VSEPR theory. 00:18:57.150 --> 00:19:02.940 So, now we can think about AX4E has a seesaw shape. 00:19:02.940 --> 00:19:06.230 Which of these two shapes is seesaw? 00:19:06.230 --> 00:19:12.170 So, we have one case-- I'll make this one-- where 00:19:12.170 --> 00:19:17.100 we have the lone pair in an axial position. 00:19:17.100 --> 00:19:19.910 And we have another case where we have the lone pair 00:19:19.910 --> 00:19:23.140 in an equatorial position. 00:19:23.140 --> 00:19:28.530 So, when you have it in the axial position, 00:19:28.530 --> 00:19:34.380 here, you have three bonding pairs of bonding electrons 00:19:34.380 --> 00:19:34.930 pretty close. 00:19:34.930 --> 00:19:37.120 They're 90 degrees away from each other. 00:19:37.120 --> 00:19:40.830 So, there are three sets of bonding electrons 00:19:40.830 --> 00:19:44.620 that will be repelled pretty strongly. 00:19:44.620 --> 00:19:46.260 Now, clicker question. 00:19:46.260 --> 00:19:50.780 Think about what's going to be true here. 00:19:50.780 --> 00:19:54.250 How many things with an equatorial lone pair 00:19:54.250 --> 00:19:55.772 will be repelled strongly? 00:20:14.710 --> 00:20:16.908 Very repulsive, those lone pairs. 00:20:19.660 --> 00:20:20.720 OK. 00:20:20.720 --> 00:20:23.150 So, that was the answer we're going for two. 00:20:23.150 --> 00:20:26.660 Because there are two sets of bonding electrons here 00:20:26.660 --> 00:20:28.400 that are 90 degrees away. 00:20:28.400 --> 00:20:32.570 There are two that are 120, but 120 is bigger than 90. 00:20:32.570 --> 00:20:34.640 So, there are two that are close whereas, 00:20:34.640 --> 00:20:40.190 with this geometry there three sets of bonding electrons 00:20:40.190 --> 00:20:41.960 that are 90 degrees away. 00:20:41.960 --> 00:20:46.520 So, it turns out that the equatorial lone pair 00:20:46.520 --> 00:20:47.820 is more favorable. 00:20:47.820 --> 00:20:51.430 It has a little bit more room in the equatorial area 00:20:51.430 --> 00:20:53.190 to spread out. 00:20:53.190 --> 00:20:56.980 And so, this is the shape that we find. 00:20:56.980 --> 00:21:00.040 We never see this shape, does not exist. 00:21:00.040 --> 00:21:03.050 We'll just take that away, it doesn't exist. 00:21:03.050 --> 00:21:05.560 And this is called seesaw. 00:21:05.560 --> 00:21:09.170 And let me demonstrate to you why that is the case. 00:21:13.350 --> 00:21:14.415 May I'll do it this way. 00:21:18.080 --> 00:21:22.170 So, how many of you had seesaws in a playground? 00:21:22.170 --> 00:21:23.580 Quite a number. 00:21:23.580 --> 00:21:26.390 Seesaws are not considered that safe. 00:21:26.390 --> 00:21:28.460 You know, the heavy kid sits on it 00:21:28.460 --> 00:21:31.090 and keeps you up in the air for days or whatever, 00:21:31.090 --> 00:21:33.360 until your mom comes and gets you from the playground. 00:21:33.360 --> 00:21:36.560 Or gets off suddenly, and the seesaw flips, 00:21:36.560 --> 00:21:38.510 and you go flying in the air. 00:21:38.510 --> 00:21:42.440 But studies show actually, that dangerous playground equipment 00:21:42.440 --> 00:21:47.460 builds neural networks and is good for cognitive development. 00:21:47.460 --> 00:21:50.520 So, I think we could do a survey here and see 00:21:50.520 --> 00:21:52.800 if there's a correlation between how many of you 00:21:52.800 --> 00:21:55.010 played with seesaws when you were a kid. 00:21:55.010 --> 00:21:57.370 And whether you ended up as an MIT student or not. 00:21:57.370 --> 00:21:59.760 That could be interesting, but now none of you 00:21:59.760 --> 00:22:02.530 will ever forget that this shape is seesaw, right? 00:22:02.530 --> 00:22:04.000 You will remember this forever? 00:22:04.000 --> 00:22:04.500 OK. 00:22:06.700 --> 00:22:07.310 All right. 00:22:07.310 --> 00:22:13.900 So, this idea also is responsible 00:22:13.900 --> 00:22:18.940 for a T-shaped molecule. 00:22:18.940 --> 00:22:20.450 So, we'll add another one. 00:22:20.450 --> 00:22:21.920 And I think this is a little harder 00:22:21.920 --> 00:22:24.990 to rationalize why it wouldn't go in a different place, 00:22:24.990 --> 00:22:25.770 but it does. 00:22:25.770 --> 00:22:27.130 This is how it goes. 00:22:27.130 --> 00:22:30.070 Both the lone pairs are an equatorial positions, 00:22:30.070 --> 00:22:33.820 and we get something that looks like a T. So, if you have 00:22:37.410 --> 00:22:43.430 AX3E2 SN 5 number, it's a T-shaped molecule. 00:22:43.430 --> 00:22:48.430 We can also think about our core octahedral geometry. 00:22:48.430 --> 00:22:51.670 And if you have two lone pairs, so here's 00:22:51.670 --> 00:22:55.370 our octahedral with six bonding atoms. 00:22:55.370 --> 00:22:58.720 And if you have two lone pairs and four bonding atoms. 00:22:58.720 --> 00:23:02.660 If you have AX4E2, it has this shape. 00:23:02.660 --> 00:23:04.880 The lone pairs go on opposite sides. 00:23:04.880 --> 00:23:08.090 And now they have their repelling the bonding electrons 00:23:08.090 --> 00:23:11.600 here, but they're far away from each other, which is favorable. 00:23:11.600 --> 00:23:14.040 And this is called square planar. 00:23:14.040 --> 00:23:17.680 So, it's square and it's planar. 00:23:17.680 --> 00:23:19.170 Square planar. 00:23:19.170 --> 00:23:22.390 OK, the two lone pairs are far apart 00:23:22.390 --> 00:23:25.740 on the opposite sides of the bond. 00:23:25.740 --> 00:23:28.400 So, in addition to predicting these shapes, which 00:23:28.400 --> 00:23:33.460 work pretty well, we can also think about the geometries. 00:23:33.460 --> 00:23:37.230 And when you get lone pairs, you find deviations 00:23:37.230 --> 00:23:39.550 from your standard geometries. 00:23:39.550 --> 00:23:41.900 So, let's look at some examples. 00:23:41.900 --> 00:23:45.780 So, with molecules with a lone pairs, such as NH3, 00:23:45.780 --> 00:23:48.840 the angles tend to be smaller. 00:23:48.840 --> 00:23:52.960 So, we saw methane before, it was 109.5. 00:23:52.960 --> 00:23:56.200 You yelled that nicely out for me. 00:23:56.200 --> 00:23:58.780 These are both SN 4. 00:23:58.780 --> 00:24:00.110 This is AX4. 00:24:00.110 --> 00:24:05.880 This is AX3E, but it's still an SN 4 system. 00:24:05.880 --> 00:24:09.120 So, it looks like this, we have-- here we have methane, 00:24:09.120 --> 00:24:10.740 here we have NH3. 00:24:10.740 --> 00:24:14.350 And now, we want to think about which would be more repulsive, 00:24:14.350 --> 00:24:16.970 the bonded electrons or the lone pair 00:24:16.970 --> 00:24:20.320 electrons in terms of the geometries around. 00:24:20.320 --> 00:24:26.080 So, if we have hydrogen carbon hydrogen angle of 109.5 here, 00:24:26.080 --> 00:24:28.270 then here with nitrogen we're thinking 00:24:28.270 --> 00:24:30.840 about hydrogen nitrogen hydrogen. 00:24:30.840 --> 00:24:37.080 And this lone pair is pushing on those bonded electrons, 00:24:37.080 --> 00:24:38.790 and it's taken up a lot of space. 00:24:38.790 --> 00:24:40.620 This is the messy roommate. 00:24:40.620 --> 00:24:43.720 The messy roommate has a lot of stuff 00:24:43.720 --> 00:24:46.190 that's spreading out all over your room. 00:24:46.190 --> 00:24:50.110 And so instead of having 109.5 amount of space, 00:24:50.110 --> 00:24:54.700 you now have 106.7 amount of space in your room. 00:24:54.700 --> 00:24:57.110 Because it's just-- the messy roommates just 00:24:57.110 --> 00:24:59.310 spreading out all over the place, 00:24:59.310 --> 00:25:05.000 pushing down on those bonds and the bond's contract. 00:25:05.000 --> 00:25:06.500 All right. 00:25:06.500 --> 00:25:09.910 Now, let's go back to trends in the periodic table 00:25:09.910 --> 00:25:11.820 for a minute. 00:25:11.820 --> 00:25:15.460 We learned that atomic size increases 00:25:15.460 --> 00:25:20.270 as we go down the periodic table because what increases? 00:25:20.270 --> 00:25:24.240 N. Principal quantum number N increases, so atomic size 00:25:24.240 --> 00:25:26.480 increases as we go down. 00:25:26.480 --> 00:25:30.510 Think of this as a messy roommate having more stuff. 00:25:32.890 --> 00:25:39.160 So, the lone pairs now occupy a larger volume. 00:25:39.160 --> 00:25:43.730 Still messy, but now messy with a lot more stuff. 00:25:43.730 --> 00:25:48.010 And this stuff impedes in your space. 00:25:48.010 --> 00:25:53.030 So, the angles tend to be even smaller between the bonded 00:25:53.030 --> 00:25:54.590 atoms. 00:25:54.590 --> 00:25:59.690 So, we had NH3 before at 106.7. 00:25:59.690 --> 00:26:02.180 But nitrogen is up here. 00:26:02.180 --> 00:26:06.430 Phosphorus is below it, so it has a bigger atomic size 00:26:06.430 --> 00:26:10.640 and it has lone pairs that occupy larger volumes. 00:26:10.640 --> 00:26:16.790 So, this is what happens. [EXPLOSION SOUND EFFECT] 00:26:16.790 --> 00:26:19.070 Some of you may have experienced that. 00:26:19.070 --> 00:26:22.250 That is a messy roommate with a lot of stuff. 00:26:22.250 --> 00:26:24.420 Now, if you have a roommate that's very neat, 00:26:24.420 --> 00:26:26.280 they're putting their clothes in drawers. 00:26:26.280 --> 00:26:29.440 When the clothes are in drawers, just like electrons and bonds, 00:26:29.440 --> 00:26:31.270 they don't really go anyplace. 00:26:31.270 --> 00:26:33.980 But when the messy roommate does not put their clothes-- 00:26:33.980 --> 00:26:36.450 when they're lone pair clothes-- then 00:26:36.450 --> 00:26:37.917 they go and this is what happens. 00:26:39.080 --> 00:26:43.690 OK, so we can predict now by looking at periodic trends, 00:26:43.690 --> 00:26:48.180 and thinking about how much room those lone pair electrons 00:26:48.180 --> 00:26:52.030 are taking up, we can think about and predict the angle 00:26:52.030 --> 00:26:53.550 between the bonded atoms. 00:26:55.900 --> 00:26:56.560 All right. 00:26:56.560 --> 00:27:01.766 So, this is pretty cool, but can it save the world? 00:27:07.200 --> 00:27:10.620 What do you think, can VSEPR save the world? 00:27:10.620 --> 00:27:11.610 Do something important? 00:27:11.610 --> 00:27:16.085 Of course it can, it's part of chemistry. 00:27:16.085 --> 00:27:19.500 So let's think about a pressing problem in the world right now 00:27:19.500 --> 00:27:21.580 and how VSEPR can address it. 00:27:21.580 --> 00:27:25.380 And if you listen to NPR or open a newspaper these days, 00:27:25.380 --> 00:27:29.900 you're hearing about a lot of car bombs, explosions, 00:27:29.900 --> 00:27:33.090 things are not good in the Middle East. 00:27:33.090 --> 00:27:34.690 We're hearing about all sorts of-- I 00:27:34.690 --> 00:27:38.000 heard on the news about bombs going off at a school 00:27:38.000 --> 00:27:39.920 and then when parents rushed toward the school 00:27:39.920 --> 00:27:41.350 more bombs went off. 00:27:41.350 --> 00:27:44.310 I mean just really horrible stories. 00:27:44.310 --> 00:27:48.560 And even when the conflict is over, a lot of times 00:27:48.560 --> 00:27:52.040 those improvised explosives are still there. 00:27:52.040 --> 00:27:54.300 They still exist in the countries. 00:27:54.300 --> 00:27:57.630 And it's estimated by the UN, by the United Nations, 00:27:57.630 --> 00:28:03.000 that landmines kill 15,000 to 20,000 people, mostly children, 00:28:03.000 --> 00:28:05.920 women, elderly who are out in the farm fields 00:28:05.920 --> 00:28:08.600 trying to grow food for their family 00:28:08.600 --> 00:28:11.220 and they step on something and blow up. 00:28:11.220 --> 00:28:14.410 So, how do you find these explosive devices? 00:28:14.410 --> 00:28:16.570 How do you find explosive devices 00:28:16.570 --> 00:28:21.010 that are actively being used now in dangerous parts 00:28:21.010 --> 00:28:21.700 of the world? 00:28:21.700 --> 00:28:24.210 And how do you find the explosive devices left 00:28:24.210 --> 00:28:27.110 behind when the war is over? 00:28:27.110 --> 00:28:30.080 So, if you are Stephanie, you use 00:28:30.080 --> 00:28:34.640 VSEPR That's what you do to find those explosives. 00:28:34.640 --> 00:28:37.750 So, in her own words now I'm going to tell you-- 00:28:37.750 --> 00:28:42.990 or she's going to tell you, why VSEPR what she calls V-S-E-P-R, 00:28:42.990 --> 00:28:47.335 because she can say that better than I can, to find explosives. 00:28:47.335 --> 00:28:49.210 STEPHANIE SYDLIK: My name is Stephanie Sydlik 00:28:49.210 --> 00:28:52.900 and I am a graduate student in Tim Swager's research 00:28:52.900 --> 00:28:54.400 group at MIT. 00:28:54.400 --> 00:28:56.760 The research that were perhaps the most well-known for 00:28:56.760 --> 00:29:00.077 is for sensing explosives, such as TNT. [EXPLOSION SOUND 00:29:00.077 --> 00:29:05.810 EFFECT] New, bigger, better explosives have been developed. 00:29:05.810 --> 00:29:09.680 And two of these are RDX and PETN. 00:29:09.680 --> 00:29:13.530 And these kind of have more bang for your buck, if you will. 00:29:13.530 --> 00:29:17.760 Unfortunately, they have an even lower vapor pressure than TNT. 00:29:17.760 --> 00:29:20.610 Which means there's even less molecules 00:29:20.610 --> 00:29:24.300 of the explosive in the air and it makes them even harder 00:29:24.300 --> 00:29:25.410 to detect. 00:29:25.410 --> 00:29:27.040 The dogs that they would send out 00:29:27.040 --> 00:29:31.570 are actually sensing cyclohexanone and acetone, 00:29:31.570 --> 00:29:33.620 which are molecules that are used 00:29:33.620 --> 00:29:37.080 in the purification of these two explosives. 00:29:37.080 --> 00:29:41.520 Both cyclohexanone and acetone have a carbonyl in them. 00:29:41.520 --> 00:29:44.350 And this carbonyl then can interact 00:29:44.350 --> 00:29:47.180 with a group known as the urea. 00:29:47.180 --> 00:29:51.640 We have two nitrogens connected to a carbon that 00:29:51.640 --> 00:29:55.320 has a double bond to an oxygen between them. 00:29:55.320 --> 00:29:57.840 And these nitrogens have hydrogens on it, 00:29:57.840 --> 00:30:02.440 that can hydrogen bond with that acetone or the cyclohexanone 00:30:02.440 --> 00:30:03.690 that we're looking for. 00:30:03.690 --> 00:30:07.890 What happens is that the lone pair from the carbonyl 00:30:07.890 --> 00:30:11.260 reaches out and grabs those hydrogens and pulls them away 00:30:11.260 --> 00:30:12.650 from the nitrogens. 00:30:12.650 --> 00:30:15.580 And this makes the nitrogen hydrogen bond 00:30:15.580 --> 00:30:18.260 more lone pair-like. 00:30:18.260 --> 00:30:20.370 As it becomes more lone pair-like, 00:30:20.370 --> 00:30:24.930 we see more repulsion between the lone pair-like bond 00:30:24.930 --> 00:30:27.480 of electrons and the neighboring bonds. 00:30:27.480 --> 00:30:30.650 And by the V-S-E-P-R theory, we know that this is going 00:30:30.650 --> 00:30:33.800 to cause the bond angle to become larger between 00:30:33.800 --> 00:30:37.530 the nitrogen hydrogen bond and the accompanying bonds 00:30:37.530 --> 00:30:41.030 and smaller between the other bonds around the nitrogen. 00:30:41.030 --> 00:30:44.720 And this causes large scale changes in the polymer. 00:30:44.720 --> 00:30:47.270 So, we can see differences in the way in the large scale 00:30:47.270 --> 00:30:49.930 the polymer interacts with light as fluorescents, 00:30:49.930 --> 00:30:52.260 so it will start to glow. 00:30:52.260 --> 00:30:55.810 Or, a refractive index change, which is also a different way 00:30:55.810 --> 00:30:57.960 the polymer interacts with light. 00:30:57.960 --> 00:31:00.560 We have instruments that can very easily measure 00:31:00.560 --> 00:31:02.590 both fluorescents and refractive index. 00:31:02.590 --> 00:31:04.990 And with these very easy signals, 00:31:04.990 --> 00:31:08.850 we now know that our acetone or cyclohexanone 00:31:08.850 --> 00:31:12.150 and therefore, the explosive is there. 00:31:12.150 --> 00:31:14.620 For soldiers, this is a really big deal. 00:31:14.620 --> 00:31:17.770 In Iraq and Afghanistan, there are minefields 00:31:17.770 --> 00:31:22.590 and improvised explosive devices almost everywhere. 00:31:22.590 --> 00:31:24.170 And the soldiers over there really 00:31:24.170 --> 00:31:25.730 have to watch where they step. 00:31:25.730 --> 00:31:27.760 So, if we can come up with a handheld device, 00:31:27.760 --> 00:31:30.570 and we have in the past come up with some. 00:31:30.570 --> 00:31:33.180 And I'm hoping that my technology might in the future 00:31:33.180 --> 00:31:35.600 also go towards these types of devices 00:31:35.600 --> 00:31:38.990 that will be attached to a robot and sent out 00:31:38.990 --> 00:31:43.530 to sniff out the area before the soldiers go there. 00:31:43.530 --> 00:31:46.190 You can really save a lot of the soldiers lives as well. 00:31:46.190 --> 00:31:49.680 It's very cool to do the hands on work in the chemistry 00:31:49.680 --> 00:31:53.860 laboratory, and then know that what you've done at your bench 00:31:53.860 --> 00:31:56.620 will then one day be actually used by someone 00:31:56.620 --> 00:31:58.560 and potentially save their life. 00:31:58.560 --> 00:32:00.610 PROFESSOR: OK. 00:32:00.610 --> 00:32:06.000 So, back to VSEPR and lone pairs. 00:32:06.000 --> 00:32:07.770 Let's look at some examples and think 00:32:07.770 --> 00:32:10.260 about the shapes we've seen some of these already, 00:32:10.260 --> 00:32:12.170 but let's look at some more. 00:32:12.170 --> 00:32:19.100 So, now we can have AX2E, SN number of 3. 00:32:19.100 --> 00:32:22.600 So, two bonded atoms one lone pair. 00:32:22.600 --> 00:32:27.870 This has bent geometry, but again think 00:32:27.870 --> 00:32:29.780 about this lone pair. 00:32:29.780 --> 00:32:31.530 Now, we're going to talk about the angles. 00:32:31.530 --> 00:32:33.810 And we're talking about angles in this class, 00:32:33.810 --> 00:32:36.560 we're talking about the angles between the bonded atoms. 00:32:36.560 --> 00:32:38.682 So, the angle from the lone pair down 00:32:38.682 --> 00:32:40.890 is going to be bigger because the lone pair is really 00:32:40.890 --> 00:32:42.830 repulsive, but we're going to be thinking 00:32:42.830 --> 00:32:45.134 about the angle between the atoms you see. 00:32:45.134 --> 00:32:46.550 So, when we ask about angle, we're 00:32:46.550 --> 00:32:50.400 asking about this angle between one bonded atom, central atom, 00:32:50.400 --> 00:32:52.090 and the other bonded atom. 00:32:52.090 --> 00:32:56.080 So, this lone pair doesn't look that repulsive. 00:32:56.080 --> 00:33:00.800 So, I just want you to sort of think more about this. 00:33:00.800 --> 00:33:03.950 So keep this in mind. 00:33:03.950 --> 00:33:07.440 This is really more what a messy roommate is all about. 00:33:07.440 --> 00:33:13.360 So, if this is your lone pair pressing down on those bonds, 00:33:13.360 --> 00:33:15.660 what do you expect the angle is going to be? 00:33:17.840 --> 00:33:19.960 What's the angle in the normal case first? 00:33:23.542 --> 00:33:24.940 [BALLOON POP] 00:33:24.940 --> 00:33:25.705 So, we have 120. 00:33:28.060 --> 00:33:29.500 And now think-- and actually it's 00:33:29.500 --> 00:33:32.166 a clicker question, think about what the answer is going to be. 00:33:32.166 --> 00:33:33.290 We talked about the normal. 00:33:40.790 --> 00:33:42.730 All right, 10 more seconds. 00:33:52.760 --> 00:33:55.960 All right, yes, less than 120. 00:33:55.960 --> 00:33:57.720 So, you don't know exactly, you can't 00:33:57.720 --> 00:34:02.180 say "Oh, that's going to be 118.5 or something." 00:34:02.180 --> 00:34:03.660 But you can say less than 120. 00:34:03.660 --> 00:34:05.890 That's how you would express it. 00:34:05.890 --> 00:34:07.980 And I'll just remind myself to say, 00:34:07.980 --> 00:34:10.449 if you like these model kits and want your own, 00:34:10.449 --> 00:34:13.630 some toothpicks and gum drops can create 00:34:13.630 --> 00:34:15.469 some awesome VSEPR model kits. 00:34:15.469 --> 00:34:16.969 And we'll try to bring some of these 00:34:16.969 --> 00:34:19.510 into recitation for people who want 00:34:19.510 --> 00:34:24.390 to have gum drops and toothpicks for making models. 00:34:24.390 --> 00:34:24.890 OK, 00:34:27.230 --> 00:34:29.159 let's keep going. 00:34:29.159 --> 00:34:32.960 So, now we have our tetrahedral based system 00:34:35.630 --> 00:34:36.130 AX3E 00:34:39.810 --> 00:34:45.480 and an SN number of 4 based on tetrahedral. 00:34:45.480 --> 00:34:50.460 And so here, now, we have trigonal pyramidal. 00:34:50.460 --> 00:34:53.080 So, we have a bunch, this is why it's confusing. 00:34:53.080 --> 00:34:56.850 It's not bi-pyramidal, there's only one pyramid here 00:34:56.850 --> 00:34:58.600 and it looks like a triangle. 00:34:58.600 --> 00:35:01.120 So, trigonal pyramidal. 00:35:01.120 --> 00:35:03.540 And now what are the angles going to be? 00:35:03.540 --> 00:35:07.180 And you can just yell this one out. 00:35:07.180 --> 00:35:10.370 Yeah, 109.5. 00:35:10.370 --> 00:35:11.840 And now let's keep going. 00:35:11.840 --> 00:35:13.426 And we have another clicker question. 00:35:24.561 --> 00:35:25.685 All right, 10 more seconds. 00:35:41.201 --> 00:35:41.700 OK. 00:35:44.330 --> 00:35:47.050 So the trick here was to think about the parent 00:35:47.050 --> 00:35:50.910 geometry of the system. 00:35:50.910 --> 00:35:52.960 And so this is the parent geometry 00:35:52.960 --> 00:35:55.340 is the tetrahedral system. 00:35:55.340 --> 00:35:59.790 And we know that because it has a SN number of 4. 00:35:59.790 --> 00:36:02.710 And so, when you have a SN number of 4, 00:36:02.710 --> 00:36:05.710 then it's going to be less than 109.5. 00:36:05.710 --> 00:36:08.500 And this is called a bent geometry. 00:36:08.500 --> 00:36:12.070 So, again you can think about it within terms of those-- 00:36:12.070 --> 00:36:15.220 if you have some whole cans, those lone pairs 00:36:15.220 --> 00:36:19.650 are pressing down on the bonds and compressing them. 00:36:19.650 --> 00:36:22.190 So, it's less than 109.5. 00:36:22.190 --> 00:36:25.340 I just teach chemistry because I like to buy whole cans 00:36:25.340 --> 00:36:28.870 and have a justification for it, is really the bottom line. 00:36:28.870 --> 00:36:34.240 OK, so if we keep going now, we have our friend seesaw, 00:36:34.240 --> 00:36:37.110 which you're never going to forget. 00:36:37.110 --> 00:36:41.960 And I'll rebuild my-- oops wrong one, 00:36:41.960 --> 00:36:46.110 rebuild my seesaw over here. 00:36:46.110 --> 00:36:51.230 So, now what are the angles here? 00:36:51.230 --> 00:36:52.170 There's two of them. 00:36:52.170 --> 00:36:57.160 Think about the equatorial angles. 00:36:57.160 --> 00:36:59.795 Yep, I'm hearing it, less than 120 00:36:59.795 --> 00:37:02.420 and the axial would be less than 90. 00:37:02.420 --> 00:37:05.540 So, the lone pair is pressing down both on the 90 00:37:05.540 --> 00:37:06.840 and on that 120. 00:37:06.840 --> 00:37:10.030 Probably more repulsive for the 90, but all you have to do 00:37:10.030 --> 00:37:12.250 is say less than for both of them. 00:37:12.250 --> 00:37:14.890 So, the trick is just think about the parent geometry. 00:37:14.890 --> 00:37:17.310 What are the angles in the parent geometry? 00:37:17.310 --> 00:37:18.740 And then, it's less than. 00:37:22.580 --> 00:37:28.890 And if we keep going with this, we had our T-shaped molecule, 00:37:28.890 --> 00:37:30.120 as well. 00:37:30.120 --> 00:37:35.760 So, when we added another lone pair to our SN 5 case. 00:37:35.760 --> 00:37:37.770 And what's the angle now going to be 00:37:37.770 --> 00:37:40.480 with those two lone pairs? 00:37:40.480 --> 00:37:43.950 Less than 90. 00:37:43.950 --> 00:37:45.330 Right. 00:37:45.330 --> 00:37:47.540 All right. 00:37:47.540 --> 00:37:52.170 So, there so many possibilities for lone pairs. 00:37:54.810 --> 00:37:59.930 If we add yet another lone pair to the system, 00:37:59.930 --> 00:38:00.850 what's my geometry? 00:38:03.134 --> 00:38:06.790 So, this is now going to be linear geometry. 00:38:06.790 --> 00:38:10.580 And what's the angle going to be? 00:38:10.580 --> 00:38:12.350 Yeah, it's just going to be 180. 00:38:12.350 --> 00:38:14.550 So, we don't have a less then here 00:38:14.550 --> 00:38:16.830 because whatever way it would bend 00:38:16.830 --> 00:38:20.030 it would be just moving toward more repulsive loan pairs. 00:38:20.030 --> 00:38:22.630 So, there's no way you can minimize 00:38:22.630 --> 00:38:24.340 the repulsion in this case. 00:38:24.340 --> 00:38:26.736 So, it's just going to be a linear molecule. 00:38:31.360 --> 00:38:36.310 So, now we're going to move into our SN six category. 00:38:36.310 --> 00:38:37.930 And we're going to talk about a shape 00:38:37.930 --> 00:38:40.640 that we haven't talked about yet. 00:38:40.640 --> 00:38:44.260 So, based on this what happens? 00:38:44.260 --> 00:38:47.780 We have our parent geometry of octahedral. 00:38:47.780 --> 00:38:52.180 And so this has six bonded atoms, SN 6. 00:38:52.180 --> 00:38:55.970 But now we're going to take off one of the bonds-- put it 00:38:55.970 --> 00:38:57.510 the same as the figure-- we're going 00:38:57.510 --> 00:39:01.810 to take off one of the bonds and put on the lone pair. 00:39:01.810 --> 00:39:05.420 And this is called square pyramidal 00:39:05.420 --> 00:39:09.100 because you have this square here in your axial. 00:39:09.100 --> 00:39:10.900 It looks like a square. 00:39:10.900 --> 00:39:15.870 But when you consider you have an atom on top coming down 00:39:15.870 --> 00:39:19.980 to these four atoms on the side, that again 00:39:19.980 --> 00:39:21.240 looks like a pyramid. 00:39:21.240 --> 00:39:24.030 So, this is square pyramidal. 00:39:24.030 --> 00:39:25.870 And what angle do you think you're 00:39:25.870 --> 00:39:30.320 going to have here for these bonded atoms? 00:39:30.320 --> 00:39:33.740 Yeah, that will be less than 90. 00:39:33.740 --> 00:39:37.230 All right, so we can keep going and we saw this one before. 00:39:37.230 --> 00:39:40.580 If we take off another bonded electron 00:39:40.580 --> 00:39:42.600 and put it in a second lone pair. 00:39:42.600 --> 00:39:44.170 As we saw before those lone parents 00:39:44.170 --> 00:39:46.540 want to be a far apart from each other as they can. 00:39:46.540 --> 00:39:49.690 So, one goes on top, one goes on the bottom. 00:39:49.690 --> 00:39:52.990 And this was our square planar geometry 00:39:52.990 --> 00:39:56.000 because it is so square and it's planar. 00:39:56.000 --> 00:39:58.600 Now, what do you think the angles are? 00:39:58.600 --> 00:39:59.310 90. 00:39:59.310 --> 00:40:00.120 Right. 00:40:00.120 --> 00:40:02.670 There's no where to escape when you 00:40:02.670 --> 00:40:05.950 have a messy roommate on top and a messy roommate on the bottom. 00:40:05.950 --> 00:40:08.570 If you're in a triple between messy roommates, 00:40:08.570 --> 00:40:11.010 there's just nothing you can do. 00:40:11.010 --> 00:40:13.940 And you can't minimize the repulsion at all. 00:40:13.940 --> 00:40:15.970 You just have to live with it. 00:40:15.970 --> 00:40:18.750 OK, so if we keep going again and now 00:40:18.750 --> 00:40:21.899 we're going to add another lone pair. 00:40:21.899 --> 00:40:23.440 And it really doesn't matter where we 00:40:23.440 --> 00:40:25.380 put that, it's all equivalent. 00:40:25.380 --> 00:40:29.640 And it comes up with the shape, that's T-shaped again 00:40:29.640 --> 00:40:32.860 that makes sense when you look at the structure. 00:40:32.860 --> 00:40:35.670 And now what do you think the angles are? 00:40:35.670 --> 00:40:38.770 Less than 90, and you would be correct in that. 00:40:38.770 --> 00:40:43.080 All right, so if we put on yet one more lone pair 00:40:43.080 --> 00:40:48.620 and take off a bonded electron or take off one more bonded 00:40:48.620 --> 00:40:49.900 atom and put on an electron. 00:40:49.900 --> 00:40:52.475 What's our geometry? 00:40:52.475 --> 00:40:54.570 All right, so we have this structure, 00:40:54.570 --> 00:40:57.860 it's linear 180 no place to go. 00:40:57.860 --> 00:41:00.840 So, we keep going far enough to come back to linear a lot. 00:41:03.810 --> 00:41:07.760 Now, let's just look at a couple more-- some real life 00:41:07.760 --> 00:41:12.510 examples of molecules and think about their geometries 00:41:12.510 --> 00:41:13.280 and shapes. 00:41:13.280 --> 00:41:15.960 I have a couple more clicker questions, 00:41:15.960 --> 00:41:19.550 so let's start with our friend water. 00:41:19.550 --> 00:41:22.170 And I had water here. 00:41:22.170 --> 00:41:25.280 So, now let's think about what the formula 00:41:25.280 --> 00:41:27.070 type is going to be for water. 00:41:27.070 --> 00:41:28.617 And what's our formula type? 00:41:32.040 --> 00:41:35.140 AX Yup. 00:41:35.140 --> 00:41:42.090 AX2E2, two bonded atoms, two lone pairs SN number of 4. 00:41:42.090 --> 00:41:45.716 And do you remember what this geometry is called? 00:41:45.716 --> 00:41:47.300 Bent, yeah. 00:41:47.300 --> 00:41:50.870 And this explains then what we talked about before 00:41:50.870 --> 00:41:52.950 that this is a polar molecule. 00:41:52.950 --> 00:41:56.820 So, these are polar bonds between the oxygen 00:41:56.820 --> 00:41:59.900 and the hydrogen. But in this case, 00:41:59.900 --> 00:42:02.360 it's not a linear molecule it's a bent molecule, 00:42:02.360 --> 00:42:04.180 so it creates a net dipole. 00:42:04.180 --> 00:42:06.200 And so that makes it a polar molecule. 00:42:06.200 --> 00:42:10.000 And if water had any other shape and was not a polar molecule, 00:42:10.000 --> 00:42:12.460 then life would be entirely different 00:42:12.460 --> 00:42:14.860 because water is the solvent of life. 00:42:14.860 --> 00:42:19.060 So, this shape pretty important for anything. 00:42:19.060 --> 00:42:22.910 Actually, they researched on what medical doctors thought 00:42:22.910 --> 00:42:25.260 was the most important topic to learn 00:42:25.260 --> 00:42:27.920 as an undergraduate in premed education. 00:42:27.920 --> 00:42:31.824 And the number one topic that was most important was water. 00:42:31.824 --> 00:42:33.240 There, and you just learned about. 00:42:33.240 --> 00:42:37.760 It OK, so we keep going now and you 00:42:37.760 --> 00:42:40.790 have to answer a lot of things about this on a clicker 00:42:40.790 --> 00:42:41.290 question. 00:42:59.960 --> 00:43:01.877 All right, just 10 more seconds. 00:43:01.877 --> 00:43:03.210 You got to finish your hand out. 00:43:23.540 --> 00:43:27.750 All right, so let's go take a look at this one 00:43:27.750 --> 00:43:32.080 and fill it in. 00:43:32.080 --> 00:43:38.710 So, we have our AX4, which is a SN 5 00:43:38.710 --> 00:43:42.800 system because we have one E. And then 00:43:42.800 --> 00:43:47.190 we have our seesaw geometry. 00:43:47.190 --> 00:43:49.810 All right, so let's just fill in the rest 00:43:49.810 --> 00:43:53.310 here and see who's won the clicker competition. 00:43:53.310 --> 00:43:57.820 So, for the next system we have Br. 00:43:57.820 --> 00:44:02.530 And now it's expanded, so we have AX3E2 SN 5. 00:44:02.530 --> 00:44:05.520 We've added another lone pair. 00:44:05.520 --> 00:44:08.630 And now this makes a T-shape, but it's 00:44:08.630 --> 00:44:12.040 kind of a little benty shape because of the repulsion. 00:44:12.040 --> 00:44:12.960 We come down. 00:44:12.960 --> 00:44:16.280 We have AX2E5 SN 5. 00:44:16.280 --> 00:44:19.430 We've added three more with xenon. 00:44:19.430 --> 00:44:21.570 Xenon is expanded here. 00:44:21.570 --> 00:44:23.640 We have our linear shape. 00:44:23.640 --> 00:44:28.570 We also have xenon with four things bound. 00:44:28.570 --> 00:44:30.510 And if we did the Lewis structure of that, 00:44:30.510 --> 00:44:33.720 we'd realize there's lone pairs on the top and the bottom. 00:44:33.720 --> 00:44:36.800 AX4E2 SN 6. 00:44:36.800 --> 00:44:40.110 And this is our square planar geometry. 00:44:40.110 --> 00:44:44.790 So, you can predict a lot about just doing a Lewis structure 00:44:44.790 --> 00:44:47.000 or thinking about where the lone pairs. 00:44:47.000 --> 00:44:49.020 You can predict geometries. 00:44:49.020 --> 00:44:51.064 And let's see who has won today. 00:44:55.710 --> 00:44:56.738 We have an upset. 00:44:59.810 --> 00:45:00.768 All right, Sam. 00:45:02.958 --> 00:45:06.260 All right, see everybody on Monday.