1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,080 continue to offer high quality educational resources for free. 5 00:00:10,080 --> 00:00:12,670 To make a donation or to view additional materials 6 00:00:12,670 --> 00:00:16,580 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,580 --> 00:00:27,820 at ocw.mit.edu 8 00:00:27,820 --> 00:00:28,910 PROFESSOR: OK. 9 00:00:28,910 --> 00:00:31,205 All right, let us take ten more seconds. 10 00:00:49,350 --> 00:00:51,190 All right, 92%. 11 00:00:51,190 --> 00:00:52,090 I like it. 12 00:00:54,910 --> 00:00:58,170 That's great, I told you there'd be a formal charge question, 13 00:00:58,170 --> 00:01:00,450 and there was, and you learned it. 14 00:01:00,450 --> 00:01:02,710 Awesome, that's what I like. 15 00:01:02,710 --> 00:01:07,730 All right, that was carbon had four, no lone pairs. 16 00:01:07,730 --> 00:01:12,260 So, 4 minus 0 minus 1/2 of eight bonding electrons, 17 00:01:12,260 --> 00:01:12,940 which is four. 18 00:01:12,940 --> 00:01:16,300 So, 4 minus 4 is 0. 19 00:01:16,300 --> 00:01:20,620 OK, so today we're having a clicker competition. 20 00:01:20,620 --> 00:01:24,370 And problem set 4 will be posted later today. 21 00:01:24,370 --> 00:01:29,640 And today's goal is for all recitations, but recitation 12 22 00:01:29,640 --> 00:01:33,720 to try to unseat recitation 12. 23 00:01:33,720 --> 00:01:36,130 And the goal of recitation 12 is, of course, 24 00:01:36,130 --> 00:01:42,380 to win an unprecedented third week in a row. 25 00:01:42,380 --> 00:01:45,270 So, that's the goal for everyone today. 26 00:01:45,270 --> 00:01:50,910 Now, remember recitations that win a multiple weeks will 27 00:01:50,910 --> 00:01:53,910 be in the playoffs for the clicker competition 28 00:01:53,910 --> 00:01:57,174 at the end of the semester for a specially designed t-shirt. 29 00:01:59,780 --> 00:02:02,800 OK, more on clicker competitions actually later today. 30 00:02:05,450 --> 00:02:08,350 So, if we can get settled in. 31 00:02:08,350 --> 00:02:09,900 I know it's exciting. 32 00:02:09,900 --> 00:02:13,390 And you are allowed to talk to your recitations, 33 00:02:13,390 --> 00:02:16,510 other recitation members during the clicker questions, that 34 00:02:16,510 --> 00:02:18,410 is allowed. 35 00:02:18,410 --> 00:02:21,707 But I'll need to cover a little material first. 36 00:02:21,707 --> 00:02:24,040 So today, we're going to talk about shapes of molecules, 37 00:02:24,040 --> 00:02:28,070 and I brought some molecules with me today to help me out. 38 00:02:28,070 --> 00:02:30,880 And we're going to be talking about VSEPR theory. 39 00:02:30,880 --> 00:02:35,420 So, why is shapes of molecules important? 40 00:02:35,420 --> 00:02:39,230 So shape, which we can also call geometry. 41 00:02:39,230 --> 00:02:41,830 So, you're going to ask about the geometry of the molecule, 42 00:02:41,830 --> 00:02:44,290 you are asking what is it's shape. 43 00:02:44,290 --> 00:02:46,830 It's particularly important in chemistry 44 00:02:46,830 --> 00:02:49,680 because shape can dictate properties 45 00:02:49,680 --> 00:02:51,610 of a particular molecule. 46 00:02:51,610 --> 00:02:53,710 It can tell you about-- or dictate 47 00:02:53,710 --> 00:02:57,440 melting points, or boiling points, or reactivity. 48 00:02:57,440 --> 00:02:59,480 And I'm a biological chemist, so I 49 00:02:59,480 --> 00:03:03,600 care a lot about shapes of molecules because in biology 50 00:03:03,600 --> 00:03:05,280 shape is really important. 51 00:03:05,280 --> 00:03:08,000 So, you have enzymes in your body catalyzing reactions. 52 00:03:08,000 --> 00:03:09,970 And for those enzymes to work, they 53 00:03:09,970 --> 00:03:12,630 are specially designed to react with one 54 00:03:12,630 --> 00:03:15,820 particular kind of molecule and not any molecule in the cell. 55 00:03:15,820 --> 00:03:18,010 So, they're designed to recognize 56 00:03:18,010 --> 00:03:20,010 the shape of that molecule. 57 00:03:20,010 --> 00:03:23,100 So, biochemistry really works by shape recognition, 58 00:03:23,100 --> 00:03:25,830 so shape is very important. 59 00:03:25,830 --> 00:03:30,120 So, there's a lot of ways to get information about shape. 60 00:03:30,120 --> 00:03:33,200 But there's one very simple theory 61 00:03:33,200 --> 00:03:35,910 that does exceedingly well in predicting 62 00:03:35,910 --> 00:03:37,760 the shapes of small molecules. 63 00:03:37,760 --> 00:03:42,930 And that is called the Valence Shell Electron Pair Repulsion 64 00:03:42,930 --> 00:03:46,550 theory, which is known as VSEPR. 65 00:03:46,550 --> 00:03:51,430 And is also known as the V-S-E-P-R theory. 66 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 67 00:03:57,350 --> 00:03:58,950 theory. 68 00:03:58,950 --> 00:04:01,100 So, VSEPR is the topic today and this 69 00:04:01,100 --> 00:04:02,920 is based on Lewis structures. 70 00:04:02,920 --> 00:04:05,620 So, this is highly exciting because you just 71 00:04:05,620 --> 00:04:08,040 finished a problem set that had you 72 00:04:08,040 --> 00:04:10,250 draw lots of Lewis structures. 73 00:04:10,250 --> 00:04:13,530 And now on the next problem set, you 74 00:04:13,530 --> 00:04:16,740 can draw more Lewis structures and then 75 00:04:16,740 --> 00:04:18,690 tell us about his shape. 76 00:04:18,690 --> 00:04:20,950 So, that's very exciting. 77 00:04:20,950 --> 00:04:23,590 So, you're going to retain all of the knowledge 78 00:04:23,590 --> 00:04:25,500 that you've gained in the last problem set 79 00:04:25,500 --> 00:04:27,930 and continue on with that problems. 80 00:04:27,930 --> 00:04:32,590 All right, so this is a very simple theory. 81 00:04:32,590 --> 00:04:35,820 And it's based on the idea that valence shell-- 82 00:04:35,820 --> 00:04:40,820 valence electron pairs repel each other. 83 00:04:40,820 --> 00:04:42,900 Electrons repel each other. 84 00:04:42,900 --> 00:04:45,810 They're negatively charged, they repel each other. 85 00:04:45,810 --> 00:04:49,900 That is the heart of this theory, very simple. 86 00:04:49,900 --> 00:04:52,050 And as many of you may have gathered, 87 00:04:52,050 --> 00:04:55,020 I love it one very simple theories 88 00:04:55,020 --> 00:04:57,730 explain a lot of stuff. 89 00:04:57,730 --> 00:04:59,120 I enjoy that. 90 00:04:59,120 --> 00:05:01,620 Gets it, it doesn't get it right 100% of the time, 91 00:05:01,620 --> 00:05:04,310 I'm OK with that, I'm good with about 90%. 92 00:05:04,310 --> 00:05:07,900 So, this works really pretty well. 93 00:05:07,900 --> 00:05:10,170 So, again we're talking about the geometry 94 00:05:10,170 --> 00:05:12,570 around a central atom. 95 00:05:12,570 --> 00:05:15,650 And the idea is that the atoms are 96 00:05:15,650 --> 00:05:18,320 lone pairs around that central atom are going 97 00:05:18,320 --> 00:05:21,880 to move in such a way, attain such a shape, such 98 00:05:21,880 --> 00:05:25,150 that the repulsion is minimized. 99 00:05:25,150 --> 00:05:28,990 So, this is all about minimizing repulsion, minimizing stress. 100 00:05:28,990 --> 00:05:32,020 Again, I'm a big fan of minimizing stress, 101 00:05:32,020 --> 00:05:35,230 so I also like VSEPR for that reason as well. 102 00:05:35,230 --> 00:05:37,880 AUDIENCE: [INAUDIBLE] 103 00:05:37,880 --> 00:05:39,750 PROFESSOR: Lewis structures? 104 00:05:39,750 --> 00:05:40,720 What causes stress? 105 00:05:40,720 --> 00:05:41,460 Problem sets. 106 00:05:41,460 --> 00:05:46,280 Oh, but they build character. 107 00:05:46,280 --> 00:05:52,390 All right, VSEPR Very simple nomenclature, 108 00:05:52,390 --> 00:05:54,060 again I like simple nomenclature. 109 00:05:54,060 --> 00:05:57,230 A is the central atom. 110 00:05:57,230 --> 00:05:59,520 Maybe should be C, but atom is there, 111 00:05:59,520 --> 00:06:04,800 so central atom is A. X is the bonding atom, 112 00:06:04,800 --> 00:06:08,700 and E is the lone pair of electrons. 113 00:06:08,700 --> 00:06:10,810 So, E lone pair electrons. 114 00:06:10,810 --> 00:06:16,140 X is whatever bonding atom, and A, A is in the center. 115 00:06:16,140 --> 00:06:18,650 A is the central atom. 116 00:06:18,650 --> 00:06:23,890 One more term you need to know for VSEPR and that's steric 117 00:06:23,890 --> 00:06:26,420 number, . 118 00:06:26,420 --> 00:06:30,080 And that's used to predict geometries. 119 00:06:30,080 --> 00:06:33,700 So, what is steric number equal? 120 00:06:33,700 --> 00:06:36,260 Steric number is the number of atoms 121 00:06:36,260 --> 00:06:42,160 bonded to the central atom, plus the number of lone pair 122 00:06:42,160 --> 00:06:42,700 electrons. 123 00:06:42,700 --> 00:06:47,310 And you count one pair as one. 124 00:06:47,310 --> 00:06:50,800 And so, you want to note, when considering 125 00:06:50,800 --> 00:06:53,640 VSEPR double triple bonds are all-- 126 00:06:53,640 --> 00:06:56,102 and single bonds all the same. 127 00:06:56,102 --> 00:06:58,560 So, you don't have to worry about double triple bonds right 128 00:06:58,560 --> 00:06:59,550 now. 129 00:06:59,550 --> 00:07:01,390 They're all counts the same. 130 00:07:01,390 --> 00:07:03,180 It's only the number of bonded atoms 131 00:07:03,180 --> 00:07:05,160 and the number of loan pairs. 132 00:07:05,160 --> 00:07:07,300 So, let's look at some examples of this. 133 00:07:07,300 --> 00:07:12,250 So, we have our central atom A, bonded to two bonding atoms 134 00:07:12,250 --> 00:07:15,480 X, with one lone pair of electrons. 135 00:07:15,480 --> 00:07:18,130 So, if you were asked what is the formula for this, 136 00:07:18,130 --> 00:07:24,320 the VSEPR formula, it would be AX2E, as shown there. 137 00:07:24,320 --> 00:07:26,930 And then you might be asked to steric number. 138 00:07:26,930 --> 00:07:30,440 And the steric number in this case would be what? 139 00:07:30,440 --> 00:07:31,726 Three. 140 00:07:31,726 --> 00:07:33,980 We have two bonding atoms. 141 00:07:33,980 --> 00:07:35,860 One lone pair of electrons. 142 00:07:35,860 --> 00:07:39,940 Again, the lone pair just counts as one. 143 00:07:39,940 --> 00:07:43,590 So, I could also draw this structure with a double bond 144 00:07:43,590 --> 00:07:45,890 to one of the Xs. 145 00:07:45,890 --> 00:07:52,302 And this would have the equation AX2E, exactly the same formula. 146 00:07:52,302 --> 00:07:54,760 Because we don't care about the double bond in the formula, 147 00:07:54,760 --> 00:07:56,490 only the number of bonding atoms. 148 00:07:56,490 --> 00:07:58,320 Only the number of lone pairs. 149 00:07:58,320 --> 00:08:01,460 And this would have an SN number of what? 150 00:08:01,460 --> 00:08:03,360 Three, right. 151 00:08:03,360 --> 00:08:07,970 Because it only matters about the lone pairs and the atoms. 152 00:08:07,970 --> 00:08:09,580 That's all we care about here. 153 00:08:09,580 --> 00:08:12,190 We don't care about the double bonds, triple bonds, whatever, 154 00:08:12,190 --> 00:08:13,356 we'll care about them later. 155 00:08:13,356 --> 00:08:15,020 But for right now for geometry, we're 156 00:08:15,020 --> 00:08:18,010 only caring about bonded atoms and lone pairs. 157 00:08:21,090 --> 00:08:23,830 Some of you will be very happy after doing the last problem 158 00:08:23,830 --> 00:08:30,410 set to know, that you can apply VSEPR theory to all 159 00:08:30,410 --> 00:08:32,789 the resonance structures that you may come up with when 160 00:08:32,789 --> 00:08:34,760 you're doing Lewis structures. 161 00:08:34,760 --> 00:08:38,190 So, if a molecule has one or more resonance structures 162 00:08:38,190 --> 00:08:39,440 that's OK. 163 00:08:39,440 --> 00:08:41,730 VSEPR can be applied to any one of them. 164 00:08:44,900 --> 00:08:47,610 Also, if there's more than one central atom, 165 00:08:47,610 --> 00:08:51,260 you need to consider those atoms separately. 166 00:08:51,260 --> 00:08:54,110 So, in this case, you would be asked 167 00:08:54,110 --> 00:08:57,060 about the geometry around the carbon and the geometry 168 00:08:57,060 --> 00:09:01,400 around the oxygen. But a lot of the examples we have today 169 00:09:01,400 --> 00:09:04,200 just have one central atom. 170 00:09:04,200 --> 00:09:07,450 All right, so that's just a little introduction to VSEPR 171 00:09:07,450 --> 00:09:10,680 Now, there's two cases that we're going to consider today, 172 00:09:10,680 --> 00:09:13,450 one are molecules without lone pairs 173 00:09:13,450 --> 00:09:15,530 and one are molecules with lone pairs. 174 00:09:15,530 --> 00:09:17,460 Without lone pairs is a little bit easier 175 00:09:17,460 --> 00:09:18,840 than with lone pairs. 176 00:09:18,840 --> 00:09:21,550 So, let's start there. 177 00:09:21,550 --> 00:09:23,750 And we have a nice table for you. 178 00:09:23,750 --> 00:09:26,820 I'll tell you that these lecture notes are of high value. 179 00:09:26,820 --> 00:09:29,340 I've had people who've taken this course want to come back, 180 00:09:29,340 --> 00:09:31,620 it's a nice summary of all these shapes. 181 00:09:31,620 --> 00:09:35,650 So, you want to keep this in a nice, secure location. 182 00:09:35,650 --> 00:09:37,420 A highly desirable notes. 183 00:09:37,420 --> 00:09:40,410 OK, so let's look at a formula type. 184 00:09:40,410 --> 00:09:43,820 The simplest we have AX2 here. 185 00:09:43,820 --> 00:09:46,360 So, that has a SN number of 2, because it 186 00:09:46,360 --> 00:09:48,380 has two bonded atoms. 187 00:09:48,380 --> 00:09:50,890 And it has this molecular shape. 188 00:09:50,890 --> 00:09:53,910 And I brought an example here. 189 00:09:53,910 --> 00:09:57,860 And this is, of course, a linear molecule and therefore 190 00:09:57,860 --> 00:10:00,530 the angle-- and here we're talking about the angle 191 00:10:00,530 --> 00:10:01,850 around the central atom. 192 00:10:01,850 --> 00:10:08,030 So, from this black atom to that black atom, we'd have 180. 193 00:10:08,030 --> 00:10:12,010 So, that's the simplest linear molecule. 194 00:10:12,010 --> 00:10:13,910 So, it's got a little more complicated 195 00:10:13,910 --> 00:10:17,890 and add three bonding atoms. 196 00:10:17,890 --> 00:10:21,237 So, here we have a case of AX3. 197 00:10:23,820 --> 00:10:25,930 It has a SN number of 3. 198 00:10:25,930 --> 00:10:28,760 There are three things bonded to the central atom. 199 00:10:28,760 --> 00:10:31,690 And its shape is trigonal planar. 200 00:10:31,690 --> 00:10:34,880 Trigonal should be remembered, it looks like a triangle. 201 00:10:34,880 --> 00:10:37,140 And you should also remember that it's planar. 202 00:10:37,140 --> 00:10:39,860 You can hold it this way and see all of the atoms 203 00:10:39,860 --> 00:10:41,170 are in one plane. 204 00:10:41,170 --> 00:10:42,620 And I'm making some of this point 205 00:10:42,620 --> 00:10:44,880 because later on the exam, when you're asked 206 00:10:44,880 --> 00:10:46,560 to name geometries of things. 207 00:10:46,560 --> 00:10:49,020 People come up with all sorts of crazy things. 208 00:10:49,020 --> 00:10:53,600 So, if I spend a little time doing my demonstration here 209 00:10:53,600 --> 00:10:57,480 of the shapes of molecules, it will pay off later in the exam. 210 00:10:57,480 --> 00:10:58,860 It'll sear in your brain. 211 00:10:58,860 --> 00:11:00,060 It will be hard to forget. 212 00:11:00,060 --> 00:11:01,187 Trigonal planar. 213 00:11:01,187 --> 00:11:02,020 What are the angles? 214 00:11:04,041 --> 00:11:04,540 120. 215 00:11:06,352 --> 00:11:07,260 Right. 216 00:11:07,260 --> 00:11:08,680 OK, let's move on. 217 00:11:08,680 --> 00:11:11,900 We'll have four atoms. 218 00:11:11,900 --> 00:11:13,680 SN number of 4, AX4. 219 00:11:16,390 --> 00:11:19,740 So, this is our tetrahedral geometry. 220 00:11:19,740 --> 00:11:23,510 And I'm going to just note, as it says down here, 221 00:11:23,510 --> 00:11:27,490 that when you have a thick arrow coming out 222 00:11:27,490 --> 00:11:32,800 at you, like this bond and a thin one going back, 223 00:11:32,800 --> 00:11:35,590 that means the one coming out is coming straight out 224 00:11:35,590 --> 00:11:36,460 towards you. 225 00:11:36,460 --> 00:11:39,420 The dashed line is going back into the screen. 226 00:11:39,420 --> 00:11:43,660 And the two that are not thickened or dashed 227 00:11:43,660 --> 00:11:45,010 are in the plane. 228 00:11:45,010 --> 00:11:48,520 So, if I hold it like this, we have two atoms in the plane, 229 00:11:48,520 --> 00:11:51,160 one coming out, one going back. 230 00:11:51,160 --> 00:11:55,030 And so, that's how-- if you see that drawn that way, 231 00:11:55,030 --> 00:11:58,530 and you will, you can think about it in three dimensions. 232 00:11:58,530 --> 00:12:01,366 All right, so what's the angle of tetrahedral? 233 00:12:03,930 --> 00:12:07,920 109.5. 234 00:12:07,920 --> 00:12:10,809 So, a lot of people have already familiar with this is great, 235 00:12:10,809 --> 00:12:12,850 if you're not, you will learn it quickly when you 236 00:12:12,850 --> 00:12:14,354 do the problems. 237 00:12:14,354 --> 00:12:15,520 All right, we'll keep going. 238 00:12:15,520 --> 00:12:17,570 We have five. 239 00:12:17,570 --> 00:12:19,570 And you move over here. 240 00:12:19,570 --> 00:12:20,955 So, a SN number of 5. 241 00:12:20,955 --> 00:12:23,350 We have AX5. 242 00:12:23,350 --> 00:12:25,470 And here is our shape. 243 00:12:25,470 --> 00:12:28,720 So, this is trigonal bipyramidal. 244 00:12:28,720 --> 00:12:32,590 So, if you think about the shape of your trigonal planar, 245 00:12:32,590 --> 00:12:36,950 you see along here, we have our trigonal planar. 246 00:12:36,950 --> 00:12:41,250 So, we have our trigonal shape, but we also now have an atom 247 00:12:41,250 --> 00:12:43,030 above and an atom below. 248 00:12:43,030 --> 00:12:46,830 And that forms a kind of a pyramid on top 249 00:12:46,830 --> 00:12:48,840 and a pyramid on the bottom. 250 00:12:48,840 --> 00:12:52,160 So, it's trigonal bipyramidal. 251 00:12:52,160 --> 00:12:55,050 And there are two sets of angles now. 252 00:12:55,050 --> 00:12:59,037 What are the angles in the equatorial? 253 00:12:59,037 --> 00:12:59,537 120. 254 00:13:01,690 --> 00:13:04,630 I love when people yell things out, it's awesome. 255 00:13:04,630 --> 00:13:08,060 Angle from the axial to the equatorial? 256 00:13:08,060 --> 00:13:08,560 90. 257 00:13:08,560 --> 00:13:11,940 Awesome. 258 00:13:11,940 --> 00:13:13,410 All right. 259 00:13:13,410 --> 00:13:19,670 And one more basic shape group, and that's this one here. 260 00:13:19,670 --> 00:13:24,850 We have six atoms bonded to the central atom, AX6. 261 00:13:24,850 --> 00:13:27,110 And we have this shape. 262 00:13:27,110 --> 00:13:30,270 So, now I'm holding it, two axial atoms. 263 00:13:30,270 --> 00:13:33,850 Two atoms coming out towards you, two going back. 264 00:13:33,850 --> 00:13:35,330 Octahedral geometry. 265 00:13:35,330 --> 00:13:37,210 And what are all the angles here? 266 00:13:37,210 --> 00:13:40,620 90, awesome. 267 00:13:40,620 --> 00:13:41,517 All right. 268 00:13:44,930 --> 00:13:47,000 So, let's look at some examples. 269 00:13:47,000 --> 00:13:49,880 Hopefully, everyone got the 90 and can write it down. 270 00:13:49,880 --> 00:13:52,550 If not, I'm sure someone will yell it out again for you. 271 00:13:52,550 --> 00:13:55,060 All right, so let's look at some examples. 272 00:13:55,060 --> 00:13:57,760 We were talking about CO2, and you calculated 273 00:13:57,760 --> 00:14:00,820 the formal charge on it for me. 274 00:14:00,820 --> 00:14:03,070 So, we have AX2. 275 00:14:03,070 --> 00:14:04,780 SN number of 2. 276 00:14:04,780 --> 00:14:09,830 This is linear and our angle is 180. 277 00:14:09,830 --> 00:14:13,570 And now we can explain why this is non-polar. 278 00:14:13,570 --> 00:14:15,790 We learned before that we had polar bonds 279 00:14:15,790 --> 00:14:17,720 where we had difference of electronegativity 280 00:14:17,720 --> 00:14:19,660 between the carbon and the oxygen, 281 00:14:19,660 --> 00:14:21,090 which would make a polar bonds. 282 00:14:21,090 --> 00:14:24,310 You have two polar bonds, so why is this not a polar molecule? 283 00:14:24,310 --> 00:14:27,240 And it's not a polar molecule because this oxygen is pulling 284 00:14:27,240 --> 00:14:29,550 this way, this one's pulling that way, 285 00:14:29,550 --> 00:14:32,470 and that makes it non-planar because there's no net dipole. 286 00:14:32,470 --> 00:14:35,141 So, shape is important for this. 287 00:14:35,141 --> 00:14:36,890 All right, let's look at the next example. 288 00:14:36,890 --> 00:14:39,950 We've seen a lot of these examples before. 289 00:14:39,950 --> 00:14:42,090 We're talking about Lewis structure. 290 00:14:42,090 --> 00:14:44,202 What was boron an example of? 291 00:14:47,298 --> 00:14:48,670 Incomplete octet. 292 00:14:48,670 --> 00:14:49,570 That's right. 293 00:14:49,570 --> 00:14:54,090 So, here's an example, it has three things bound to it. 294 00:14:54,090 --> 00:14:58,560 And it has the shape of trigonal planar. 295 00:14:58,560 --> 00:15:01,670 And the middle atom is incomplete octet. 296 00:15:01,670 --> 00:15:02,910 But it's OK. 297 00:15:02,910 --> 00:15:07,540 Boron and which other one are OK being incomplete octets? 298 00:15:07,540 --> 00:15:09,900 Aluminum, right. 299 00:15:09,900 --> 00:15:12,074 So, the angles are 120 here. 300 00:15:14,740 --> 00:15:15,950 Now, we move on. 301 00:15:15,950 --> 00:15:18,040 We have this molecule CH4. 302 00:15:18,040 --> 00:15:20,880 Can someone tell me what this is? 303 00:15:20,880 --> 00:15:22,600 Methane. 304 00:15:22,600 --> 00:15:26,400 So, we have AX4, SN number of 4. 305 00:15:26,400 --> 00:15:31,630 And we have our tetrahedral shape and our angles of? 306 00:15:31,630 --> 00:15:33,040 109.5. 307 00:15:33,040 --> 00:15:34,180 Right. 308 00:15:34,180 --> 00:15:36,290 So, methane we should all care about. 309 00:15:36,290 --> 00:15:38,460 Greenhouse gas, but also some people 310 00:15:38,460 --> 00:15:42,550 believe will be the salivation to our biofuels and energy 311 00:15:42,550 --> 00:15:43,070 problem. 312 00:15:43,070 --> 00:15:45,050 We will see if that's true or not. 313 00:15:45,050 --> 00:15:47,940 OK, another example over here. 314 00:15:47,940 --> 00:15:51,950 So we have phosphorus in the middle and five chlorines. 315 00:15:51,950 --> 00:15:56,990 So, what is this an example of, in terms of Lewis structures? 316 00:15:56,990 --> 00:16:03,310 Yep, so this is trigonal bipyramidal or bipyramidal. 317 00:16:03,310 --> 00:16:05,900 I think both are right, I don't know. 318 00:16:05,900 --> 00:16:09,390 And so we have our 120 and our 90. 319 00:16:09,390 --> 00:16:11,800 And this was an example we had in Lewis structures 320 00:16:11,800 --> 00:16:14,960 or something very similar of an expanded octet. 321 00:16:14,960 --> 00:16:16,250 So, we have more. 322 00:16:16,250 --> 00:16:18,710 We have five bonds around the phosphorus 323 00:16:18,710 --> 00:16:22,840 and that's OK because it's N equals three or greater. 324 00:16:22,840 --> 00:16:26,200 So, we have an example again of an expanded octet. 325 00:16:26,200 --> 00:16:29,300 And if we keep going, we'll have another expanded octet. 326 00:16:29,300 --> 00:16:32,190 So, here we have AX6. 327 00:16:32,190 --> 00:16:34,630 So, we have six bonds around the sulfur. 328 00:16:34,630 --> 00:16:36,470 Sulfur is OK with that. 329 00:16:36,470 --> 00:16:39,160 And what is the geometry here? 330 00:16:39,160 --> 00:16:41,720 Octahedral angles? 331 00:16:41,720 --> 00:16:43,496 90. 332 00:16:43,496 --> 00:16:46,860 Yep, all right, so we have several examples here 333 00:16:46,860 --> 00:16:53,290 of things with expanded octets and also deficient octets. 334 00:16:53,290 --> 00:16:56,800 All right, so this is pretty straightforward. 335 00:16:56,800 --> 00:16:58,620 People get a lot of points on exams, 336 00:16:58,620 --> 00:17:01,370 except when they come up with all sorts of weird kind 337 00:17:01,370 --> 00:17:02,880 of shapes that don't exist. 338 00:17:02,880 --> 00:17:05,819 But most of the time you can learn this and it's great. 339 00:17:05,819 --> 00:17:09,050 With lone pairs it's a little more complicated, but also 340 00:17:09,050 --> 00:17:10,319 much more fun. 341 00:17:10,319 --> 00:17:12,960 All right, so let's talk now about what happens 342 00:17:12,960 --> 00:17:14,070 when we have lone pairs. 343 00:17:17,750 --> 00:17:25,190 So, electrons in bonds are hanging out in their bond 344 00:17:25,190 --> 00:17:28,210 and they're not really doing much, but being in their bond. 345 00:17:28,210 --> 00:17:32,810 So, they have less spatial distribution than loan pairs. 346 00:17:32,810 --> 00:17:39,360 Meaning that, electrons in bonds take up less space. 347 00:17:39,360 --> 00:17:44,490 And electrons in lone pairs, they can be anywhere. 348 00:17:44,490 --> 00:17:46,420 They're not restricted to their bond, 349 00:17:46,420 --> 00:17:49,450 so they take up more space and therefore, 350 00:17:49,450 --> 00:17:51,710 cause more repulsion. 351 00:17:51,710 --> 00:17:53,870 So, the whole idea of when you're 352 00:17:53,870 --> 00:17:56,140 talking about VSEPR with lone pairs 353 00:17:56,140 --> 00:17:59,130 is that you're thinking about electron pair repulsion. 354 00:17:59,130 --> 00:18:01,970 And if you have a lone pair, that's 355 00:18:01,970 --> 00:18:04,900 going to give you more repulsion than bonded electrons 356 00:18:04,900 --> 00:18:08,730 because lone pair electrons can take up more space. 357 00:18:08,730 --> 00:18:11,210 So, it's a very simple idea, but it actually 358 00:18:11,210 --> 00:18:14,630 works to explain a lot of stuff and again, the geometry 359 00:18:14,630 --> 00:18:18,580 re-arranges to minimize that repulsion. 360 00:18:18,580 --> 00:18:22,470 So, when we're talking about repulsive forces then, 361 00:18:22,470 --> 00:18:28,260 we go from the most repulsion, lone pair, lone pair. 362 00:18:28,260 --> 00:18:30,830 That's like two messy roommates living together, that's 363 00:18:30,830 --> 00:18:33,150 very repulsive situation. 364 00:18:33,150 --> 00:18:35,750 Lone pair, bonding pair. 365 00:18:35,750 --> 00:18:38,890 And then bonding pair, bonding pair is the least repulsive. 366 00:18:38,890 --> 00:18:42,860 To neat roommates that usually works out quite well. 367 00:18:42,860 --> 00:18:45,050 So, if we keep this in mind, we can now 368 00:18:45,050 --> 00:18:49,640 predict shapes of molecules based on this repulsion. 369 00:18:49,640 --> 00:18:53,710 So, we can rationalize shapes based on VSEPR theory. 370 00:18:57,150 --> 00:19:02,940 So, now we can think about AX4E has a seesaw shape. 371 00:19:02,940 --> 00:19:06,230 Which of these two shapes is seesaw? 372 00:19:06,230 --> 00:19:12,170 So, we have one case-- I'll make this one-- where 373 00:19:12,170 --> 00:19:17,100 we have the lone pair in an axial position. 374 00:19:17,100 --> 00:19:19,910 And we have another case where we have the lone pair 375 00:19:19,910 --> 00:19:23,140 in an equatorial position. 376 00:19:23,140 --> 00:19:28,530 So, when you have it in the axial position, 377 00:19:28,530 --> 00:19:34,380 here, you have three bonding pairs of bonding electrons 378 00:19:34,380 --> 00:19:34,930 pretty close. 379 00:19:34,930 --> 00:19:37,120 They're 90 degrees away from each other. 380 00:19:37,120 --> 00:19:40,830 So, there are three sets of bonding electrons 381 00:19:40,830 --> 00:19:44,620 that will be repelled pretty strongly. 382 00:19:44,620 --> 00:19:46,260 Now, clicker question. 383 00:19:46,260 --> 00:19:50,780 Think about what's going to be true here. 384 00:19:50,780 --> 00:19:54,250 How many things with an equatorial lone pair 385 00:19:54,250 --> 00:19:55,772 will be repelled strongly? 386 00:20:14,710 --> 00:20:16,908 Very repulsive, those lone pairs. 387 00:20:19,660 --> 00:20:20,720 OK. 388 00:20:20,720 --> 00:20:23,150 So, that was the answer we're going for two. 389 00:20:23,150 --> 00:20:26,660 Because there are two sets of bonding electrons here 390 00:20:26,660 --> 00:20:28,400 that are 90 degrees away. 391 00:20:28,400 --> 00:20:32,570 There are two that are 120, but 120 is bigger than 90. 392 00:20:32,570 --> 00:20:34,640 So, there are two that are close whereas, 393 00:20:34,640 --> 00:20:40,190 with this geometry there three sets of bonding electrons 394 00:20:40,190 --> 00:20:41,960 that are 90 degrees away. 395 00:20:41,960 --> 00:20:46,520 So, it turns out that the equatorial lone pair 396 00:20:46,520 --> 00:20:47,820 is more favorable. 397 00:20:47,820 --> 00:20:51,430 It has a little bit more room in the equatorial area 398 00:20:51,430 --> 00:20:53,190 to spread out. 399 00:20:53,190 --> 00:20:56,980 And so, this is the shape that we find. 400 00:20:56,980 --> 00:21:00,040 We never see this shape, does not exist. 401 00:21:00,040 --> 00:21:03,050 We'll just take that away, it doesn't exist. 402 00:21:03,050 --> 00:21:05,560 And this is called seesaw. 403 00:21:05,560 --> 00:21:09,170 And let me demonstrate to you why that is the case. 404 00:21:13,350 --> 00:21:14,415 May I'll do it this way. 405 00:21:18,080 --> 00:21:22,170 So, how many of you had seesaws in a playground? 406 00:21:22,170 --> 00:21:23,580 Quite a number. 407 00:21:23,580 --> 00:21:26,390 Seesaws are not considered that safe. 408 00:21:26,390 --> 00:21:28,460 You know, the heavy kid sits on it 409 00:21:28,460 --> 00:21:31,090 and keeps you up in the air for days or whatever, 410 00:21:31,090 --> 00:21:33,360 until your mom comes and gets you from the playground. 411 00:21:33,360 --> 00:21:36,560 Or gets off suddenly, and the seesaw flips, 412 00:21:36,560 --> 00:21:38,510 and you go flying in the air. 413 00:21:38,510 --> 00:21:42,440 But studies show actually, that dangerous playground equipment 414 00:21:42,440 --> 00:21:47,460 builds neural networks and is good for cognitive development. 415 00:21:47,460 --> 00:21:50,520 So, I think we could do a survey here and see 416 00:21:50,520 --> 00:21:52,800 if there's a correlation between how many of you 417 00:21:52,800 --> 00:21:55,010 played with seesaws when you were a kid. 418 00:21:55,010 --> 00:21:57,370 And whether you ended up as an MIT student or not. 419 00:21:57,370 --> 00:21:59,760 That could be interesting, but now none of you 420 00:21:59,760 --> 00:22:02,530 will ever forget that this shape is seesaw, right? 421 00:22:02,530 --> 00:22:04,000 You will remember this forever? 422 00:22:04,000 --> 00:22:04,500 OK. 423 00:22:06,700 --> 00:22:07,310 All right. 424 00:22:07,310 --> 00:22:13,900 So, this idea also is responsible 425 00:22:13,900 --> 00:22:18,940 for a T-shaped molecule. 426 00:22:18,940 --> 00:22:20,450 So, we'll add another one. 427 00:22:20,450 --> 00:22:21,920 And I think this is a little harder 428 00:22:21,920 --> 00:22:24,990 to rationalize why it wouldn't go in a different place, 429 00:22:24,990 --> 00:22:25,770 but it does. 430 00:22:25,770 --> 00:22:27,130 This is how it goes. 431 00:22:27,130 --> 00:22:30,070 Both the lone pairs are an equatorial positions, 432 00:22:30,070 --> 00:22:33,820 and we get something that looks like a T. So, if you have 433 00:22:37,410 --> 00:22:43,430 AX3E2 SN 5 number, it's a T-shaped molecule. 434 00:22:43,430 --> 00:22:48,430 We can also think about our core octahedral geometry. 435 00:22:48,430 --> 00:22:51,670 And if you have two lone pairs, so here's 436 00:22:51,670 --> 00:22:55,370 our octahedral with six bonding atoms. 437 00:22:55,370 --> 00:22:58,720 And if you have two lone pairs and four bonding atoms. 438 00:22:58,720 --> 00:23:02,660 If you have AX4E2, it has this shape. 439 00:23:02,660 --> 00:23:04,880 The lone pairs go on opposite sides. 440 00:23:04,880 --> 00:23:08,090 And now they have their repelling the bonding electrons 441 00:23:08,090 --> 00:23:11,600 here, but they're far away from each other, which is favorable. 442 00:23:11,600 --> 00:23:14,040 And this is called square planar. 443 00:23:14,040 --> 00:23:17,680 So, it's square and it's planar. 444 00:23:17,680 --> 00:23:19,170 Square planar. 445 00:23:19,170 --> 00:23:22,390 OK, the two lone pairs are far apart 446 00:23:22,390 --> 00:23:25,740 on the opposite sides of the bond. 447 00:23:25,740 --> 00:23:28,400 So, in addition to predicting these shapes, which 448 00:23:28,400 --> 00:23:33,460 work pretty well, we can also think about the geometries. 449 00:23:33,460 --> 00:23:37,230 And when you get lone pairs, you find deviations 450 00:23:37,230 --> 00:23:39,550 from your standard geometries. 451 00:23:39,550 --> 00:23:41,900 So, let's look at some examples. 452 00:23:41,900 --> 00:23:45,780 So, with molecules with a lone pairs, such as NH3, 453 00:23:45,780 --> 00:23:48,840 the angles tend to be smaller. 454 00:23:48,840 --> 00:23:52,960 So, we saw methane before, it was 109.5. 455 00:23:52,960 --> 00:23:56,200 You yelled that nicely out for me. 456 00:23:56,200 --> 00:23:58,780 These are both SN 4. 457 00:23:58,780 --> 00:24:00,110 This is AX4. 458 00:24:00,110 --> 00:24:05,880 This is AX3E, but it's still an SN 4 system. 459 00:24:05,880 --> 00:24:09,120 So, it looks like this, we have-- here we have methane, 460 00:24:09,120 --> 00:24:10,740 here we have NH3. 461 00:24:10,740 --> 00:24:14,350 And now, we want to think about which would be more repulsive, 462 00:24:14,350 --> 00:24:16,970 the bonded electrons or the lone pair 463 00:24:16,970 --> 00:24:20,320 electrons in terms of the geometries around. 464 00:24:20,320 --> 00:24:26,080 So, if we have hydrogen carbon hydrogen angle of 109.5 here, 465 00:24:26,080 --> 00:24:28,270 then here with nitrogen we're thinking 466 00:24:28,270 --> 00:24:30,840 about hydrogen nitrogen hydrogen. 467 00:24:30,840 --> 00:24:37,080 And this lone pair is pushing on those bonded electrons, 468 00:24:37,080 --> 00:24:38,790 and it's taken up a lot of space. 469 00:24:38,790 --> 00:24:40,620 This is the messy roommate. 470 00:24:40,620 --> 00:24:43,720 The messy roommate has a lot of stuff 471 00:24:43,720 --> 00:24:46,190 that's spreading out all over your room. 472 00:24:46,190 --> 00:24:50,110 And so instead of having 109.5 amount of space, 473 00:24:50,110 --> 00:24:54,700 you now have 106.7 amount of space in your room. 474 00:24:54,700 --> 00:24:57,110 Because it's just-- the messy roommates just 475 00:24:57,110 --> 00:24:59,310 spreading out all over the place, 476 00:24:59,310 --> 00:25:05,000 pushing down on those bonds and the bond's contract. 477 00:25:05,000 --> 00:25:06,500 All right. 478 00:25:06,500 --> 00:25:09,910 Now, let's go back to trends in the periodic table 479 00:25:09,910 --> 00:25:11,820 for a minute. 480 00:25:11,820 --> 00:25:15,460 We learned that atomic size increases 481 00:25:15,460 --> 00:25:20,270 as we go down the periodic table because what increases? 482 00:25:20,270 --> 00:25:24,240 N. Principal quantum number N increases, so atomic size 483 00:25:24,240 --> 00:25:26,480 increases as we go down. 484 00:25:26,480 --> 00:25:30,510 Think of this as a messy roommate having more stuff. 485 00:25:32,890 --> 00:25:39,160 So, the lone pairs now occupy a larger volume. 486 00:25:39,160 --> 00:25:43,730 Still messy, but now messy with a lot more stuff. 487 00:25:43,730 --> 00:25:48,010 And this stuff impedes in your space. 488 00:25:48,010 --> 00:25:53,030 So, the angles tend to be even smaller between the bonded 489 00:25:53,030 --> 00:25:54,590 atoms. 490 00:25:54,590 --> 00:25:59,690 So, we had NH3 before at 106.7. 491 00:25:59,690 --> 00:26:02,180 But nitrogen is up here. 492 00:26:02,180 --> 00:26:06,430 Phosphorus is below it, so it has a bigger atomic size 493 00:26:06,430 --> 00:26:10,640 and it has lone pairs that occupy larger volumes. 494 00:26:10,640 --> 00:26:16,790 So, this is what happens. [EXPLOSION SOUND EFFECT] 495 00:26:16,790 --> 00:26:19,070 Some of you may have experienced that. 496 00:26:19,070 --> 00:26:22,250 That is a messy roommate with a lot of stuff. 497 00:26:22,250 --> 00:26:24,420 Now, if you have a roommate that's very neat, 498 00:26:24,420 --> 00:26:26,280 they're putting their clothes in drawers. 499 00:26:26,280 --> 00:26:29,440 When the clothes are in drawers, just like electrons and bonds, 500 00:26:29,440 --> 00:26:31,270 they don't really go anyplace. 501 00:26:31,270 --> 00:26:33,980 But when the messy roommate does not put their clothes-- 502 00:26:33,980 --> 00:26:36,450 when they're lone pair clothes-- then 503 00:26:36,450 --> 00:26:37,917 they go and this is what happens. 504 00:26:39,080 --> 00:26:43,690 OK, so we can predict now by looking at periodic trends, 505 00:26:43,690 --> 00:26:48,180 and thinking about how much room those lone pair electrons 506 00:26:48,180 --> 00:26:52,030 are taking up, we can think about and predict the angle 507 00:26:52,030 --> 00:26:53,550 between the bonded atoms. 508 00:26:55,900 --> 00:26:56,560 All right. 509 00:26:56,560 --> 00:27:01,766 So, this is pretty cool, but can it save the world? 510 00:27:07,200 --> 00:27:10,620 What do you think, can VSEPR save the world? 511 00:27:10,620 --> 00:27:11,610 Do something important? 512 00:27:11,610 --> 00:27:16,085 Of course it can, it's part of chemistry. 513 00:27:16,085 --> 00:27:19,500 So let's think about a pressing problem in the world right now 514 00:27:19,500 --> 00:27:21,580 and how VSEPR can address it. 515 00:27:21,580 --> 00:27:25,380 And if you listen to NPR or open a newspaper these days, 516 00:27:25,380 --> 00:27:29,900 you're hearing about a lot of car bombs, explosions, 517 00:27:29,900 --> 00:27:33,090 things are not good in the Middle East. 518 00:27:33,090 --> 00:27:34,690 We're hearing about all sorts of-- I 519 00:27:34,690 --> 00:27:38,000 heard on the news about bombs going off at a school 520 00:27:38,000 --> 00:27:39,920 and then when parents rushed toward the school 521 00:27:39,920 --> 00:27:41,350 more bombs went off. 522 00:27:41,350 --> 00:27:44,310 I mean just really horrible stories. 523 00:27:44,310 --> 00:27:48,560 And even when the conflict is over, a lot of times 524 00:27:48,560 --> 00:27:52,040 those improvised explosives are still there. 525 00:27:52,040 --> 00:27:54,300 They still exist in the countries. 526 00:27:54,300 --> 00:27:57,630 And it's estimated by the UN, by the United Nations, 527 00:27:57,630 --> 00:28:03,000 that landmines kill 15,000 to 20,000 people, mostly children, 528 00:28:03,000 --> 00:28:05,920 women, elderly who are out in the farm fields 529 00:28:05,920 --> 00:28:08,600 trying to grow food for their family 530 00:28:08,600 --> 00:28:11,220 and they step on something and blow up. 531 00:28:11,220 --> 00:28:14,410 So, how do you find these explosive devices? 532 00:28:14,410 --> 00:28:16,570 How do you find explosive devices 533 00:28:16,570 --> 00:28:21,010 that are actively being used now in dangerous parts 534 00:28:21,010 --> 00:28:21,700 of the world? 535 00:28:21,700 --> 00:28:24,210 And how do you find the explosive devices left 536 00:28:24,210 --> 00:28:27,110 behind when the war is over? 537 00:28:27,110 --> 00:28:30,080 So, if you are Stephanie, you use 538 00:28:30,080 --> 00:28:34,640 VSEPR That's what you do to find those explosives. 539 00:28:34,640 --> 00:28:37,750 So, in her own words now I'm going to tell you-- 540 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, 541 00:28:42,990 --> 00:28:47,335 because she can say that better than I can, to find explosives. 542 00:28:47,335 --> 00:28:49,210 STEPHANIE SYDLIK: My name is Stephanie Sydlik 543 00:28:49,210 --> 00:28:52,900 and I am a graduate student in Tim Swager's research 544 00:28:52,900 --> 00:28:54,400 group at MIT. 545 00:28:54,400 --> 00:28:56,760 The research that were perhaps the most well-known for 546 00:28:56,760 --> 00:29:00,077 is for sensing explosives, such as TNT. [EXPLOSION SOUND 547 00:29:00,077 --> 00:29:05,810 EFFECT] New, bigger, better explosives have been developed. 548 00:29:05,810 --> 00:29:09,680 And two of these are RDX and PETN. 549 00:29:09,680 --> 00:29:13,530 And these kind of have more bang for your buck, if you will. 550 00:29:13,530 --> 00:29:17,760 Unfortunately, they have an even lower vapor pressure than TNT. 551 00:29:17,760 --> 00:29:20,610 Which means there's even less molecules 552 00:29:20,610 --> 00:29:24,300 of the explosive in the air and it makes them even harder 553 00:29:24,300 --> 00:29:25,410 to detect. 554 00:29:25,410 --> 00:29:27,040 The dogs that they would send out 555 00:29:27,040 --> 00:29:31,570 are actually sensing cyclohexanone and acetone, 556 00:29:31,570 --> 00:29:33,620 which are molecules that are used 557 00:29:33,620 --> 00:29:37,080 in the purification of these two explosives. 558 00:29:37,080 --> 00:29:41,520 Both cyclohexanone and acetone have a carbonyl in them. 559 00:29:41,520 --> 00:29:44,350 And this carbonyl then can interact 560 00:29:44,350 --> 00:29:47,180 with a group known as the urea. 561 00:29:47,180 --> 00:29:51,640 We have two nitrogens connected to a carbon that 562 00:29:51,640 --> 00:29:55,320 has a double bond to an oxygen between them. 563 00:29:55,320 --> 00:29:57,840 And these nitrogens have hydrogens on it, 564 00:29:57,840 --> 00:30:02,440 that can hydrogen bond with that acetone or the cyclohexanone 565 00:30:02,440 --> 00:30:03,690 that we're looking for. 566 00:30:03,690 --> 00:30:07,890 What happens is that the lone pair from the carbonyl 567 00:30:07,890 --> 00:30:11,260 reaches out and grabs those hydrogens and pulls them away 568 00:30:11,260 --> 00:30:12,650 from the nitrogens. 569 00:30:12,650 --> 00:30:15,580 And this makes the nitrogen hydrogen bond 570 00:30:15,580 --> 00:30:18,260 more lone pair-like. 571 00:30:18,260 --> 00:30:20,370 As it becomes more lone pair-like, 572 00:30:20,370 --> 00:30:24,930 we see more repulsion between the lone pair-like bond 573 00:30:24,930 --> 00:30:27,480 of electrons and the neighboring bonds. 574 00:30:27,480 --> 00:30:30,650 And by the V-S-E-P-R theory, we know that this is going 575 00:30:30,650 --> 00:30:33,800 to cause the bond angle to become larger between 576 00:30:33,800 --> 00:30:37,530 the nitrogen hydrogen bond and the accompanying bonds 577 00:30:37,530 --> 00:30:41,030 and smaller between the other bonds around the nitrogen. 578 00:30:41,030 --> 00:30:44,720 And this causes large scale changes in the polymer. 579 00:30:44,720 --> 00:30:47,270 So, we can see differences in the way in the large scale 580 00:30:47,270 --> 00:30:49,930 the polymer interacts with light as fluorescents, 581 00:30:49,930 --> 00:30:52,260 so it will start to glow. 582 00:30:52,260 --> 00:30:55,810 Or, a refractive index change, which is also a different way 583 00:30:55,810 --> 00:30:57,960 the polymer interacts with light. 584 00:30:57,960 --> 00:31:00,560 We have instruments that can very easily measure 585 00:31:00,560 --> 00:31:02,590 both fluorescents and refractive index. 586 00:31:02,590 --> 00:31:04,990 And with these very easy signals, 587 00:31:04,990 --> 00:31:08,850 we now know that our acetone or cyclohexanone 588 00:31:08,850 --> 00:31:12,150 and therefore, the explosive is there. 589 00:31:12,150 --> 00:31:14,620 For soldiers, this is a really big deal. 590 00:31:14,620 --> 00:31:17,770 In Iraq and Afghanistan, there are minefields 591 00:31:17,770 --> 00:31:22,590 and improvised explosive devices almost everywhere. 592 00:31:22,590 --> 00:31:24,170 And the soldiers over there really 593 00:31:24,170 --> 00:31:25,730 have to watch where they step. 594 00:31:25,730 --> 00:31:27,760 So, if we can come up with a handheld device, 595 00:31:27,760 --> 00:31:30,570 and we have in the past come up with some. 596 00:31:30,570 --> 00:31:33,180 And I'm hoping that my technology might in the future 597 00:31:33,180 --> 00:31:35,600 also go towards these types of devices 598 00:31:35,600 --> 00:31:38,990 that will be attached to a robot and sent out 599 00:31:38,990 --> 00:31:43,530 to sniff out the area before the soldiers go there. 600 00:31:43,530 --> 00:31:46,190 You can really save a lot of the soldiers lives as well. 601 00:31:46,190 --> 00:31:49,680 It's very cool to do the hands on work in the chemistry 602 00:31:49,680 --> 00:31:53,860 laboratory, and then know that what you've done at your bench 603 00:31:53,860 --> 00:31:56,620 will then one day be actually used by someone 604 00:31:56,620 --> 00:31:58,560 and potentially save their life. 605 00:31:58,560 --> 00:32:00,610 PROFESSOR: OK. 606 00:32:00,610 --> 00:32:06,000 So, back to VSEPR and lone pairs. 607 00:32:06,000 --> 00:32:07,770 Let's look at some examples and think 608 00:32:07,770 --> 00:32:10,260 about the shapes we've seen some of these already, 609 00:32:10,260 --> 00:32:12,170 but let's look at some more. 610 00:32:12,170 --> 00:32:19,100 So, now we can have AX2E, SN number of 3. 611 00:32:19,100 --> 00:32:22,600 So, two bonded atoms one lone pair. 612 00:32:22,600 --> 00:32:27,870 This has bent geometry, but again think 613 00:32:27,870 --> 00:32:29,780 about this lone pair. 614 00:32:29,780 --> 00:32:31,530 Now, we're going to talk about the angles. 615 00:32:31,530 --> 00:32:33,810 And we're talking about angles in this class, 616 00:32:33,810 --> 00:32:36,560 we're talking about the angles between the bonded atoms. 617 00:32:36,560 --> 00:32:38,682 So, the angle from the lone pair down 618 00:32:38,682 --> 00:32:40,890 is going to be bigger because the lone pair is really 619 00:32:40,890 --> 00:32:42,830 repulsive, but we're going to be thinking 620 00:32:42,830 --> 00:32:45,134 about the angle between the atoms you see. 621 00:32:45,134 --> 00:32:46,550 So, when we ask about angle, we're 622 00:32:46,550 --> 00:32:50,400 asking about this angle between one bonded atom, central atom, 623 00:32:50,400 --> 00:32:52,090 and the other bonded atom. 624 00:32:52,090 --> 00:32:56,080 So, this lone pair doesn't look that repulsive. 625 00:32:56,080 --> 00:33:00,800 So, I just want you to sort of think more about this. 626 00:33:00,800 --> 00:33:03,950 So keep this in mind. 627 00:33:03,950 --> 00:33:07,440 This is really more what a messy roommate is all about. 628 00:33:07,440 --> 00:33:13,360 So, if this is your lone pair pressing down on those bonds, 629 00:33:13,360 --> 00:33:15,660 what do you expect the angle is going to be? 630 00:33:17,840 --> 00:33:19,960 What's the angle in the normal case first? 631 00:33:23,542 --> 00:33:24,940 [BALLOON POP] 632 00:33:24,940 --> 00:33:25,705 So, we have 120. 633 00:33:28,060 --> 00:33:29,500 And now think-- and actually it's 634 00:33:29,500 --> 00:33:32,166 a clicker question, think about what the answer is going to be. 635 00:33:32,166 --> 00:33:33,290 We talked about the normal. 636 00:33:40,790 --> 00:33:42,730 All right, 10 more seconds. 637 00:33:52,760 --> 00:33:55,960 All right, yes, less than 120. 638 00:33:55,960 --> 00:33:57,720 So, you don't know exactly, you can't 639 00:33:57,720 --> 00:34:02,180 say "Oh, that's going to be 118.5 or something." 640 00:34:02,180 --> 00:34:03,660 But you can say less than 120. 641 00:34:03,660 --> 00:34:05,890 That's how you would express it. 642 00:34:05,890 --> 00:34:07,980 And I'll just remind myself to say, 643 00:34:07,980 --> 00:34:10,449 if you like these model kits and want your own, 644 00:34:10,449 --> 00:34:13,630 some toothpicks and gum drops can create 645 00:34:13,630 --> 00:34:15,469 some awesome VSEPR model kits. 646 00:34:15,469 --> 00:34:16,969 And we'll try to bring some of these 647 00:34:16,969 --> 00:34:19,510 into recitation for people who want 648 00:34:19,510 --> 00:34:24,390 to have gum drops and toothpicks for making models. 649 00:34:24,390 --> 00:34:24,890 OK, 650 00:34:27,230 --> 00:34:29,159 let's keep going. 651 00:34:29,159 --> 00:34:32,960 So, now we have our tetrahedral based system 652 00:34:35,630 --> 00:34:36,130 AX3E 653 00:34:39,810 --> 00:34:45,480 and an SN number of 4 based on tetrahedral. 654 00:34:45,480 --> 00:34:50,460 And so here, now, we have trigonal pyramidal. 655 00:34:50,460 --> 00:34:53,080 So, we have a bunch, this is why it's confusing. 656 00:34:53,080 --> 00:34:56,850 It's not bi-pyramidal, there's only one pyramid here 657 00:34:56,850 --> 00:34:58,600 and it looks like a triangle. 658 00:34:58,600 --> 00:35:01,120 So, trigonal pyramidal. 659 00:35:01,120 --> 00:35:03,540 And now what are the angles going to be? 660 00:35:03,540 --> 00:35:07,180 And you can just yell this one out. 661 00:35:07,180 --> 00:35:10,370 Yeah, 109.5. 662 00:35:10,370 --> 00:35:11,840 And now let's keep going. 663 00:35:11,840 --> 00:35:13,426 And we have another clicker question. 664 00:35:24,561 --> 00:35:25,685 All right, 10 more seconds. 665 00:35:41,201 --> 00:35:41,700 OK. 666 00:35:44,330 --> 00:35:47,050 So the trick here was to think about the parent 667 00:35:47,050 --> 00:35:50,910 geometry of the system. 668 00:35:50,910 --> 00:35:52,960 And so this is the parent geometry 669 00:35:52,960 --> 00:35:55,340 is the tetrahedral system. 670 00:35:55,340 --> 00:35:59,790 And we know that because it has a SN number of 4. 671 00:35:59,790 --> 00:36:02,710 And so, when you have a SN number of 4, 672 00:36:02,710 --> 00:36:05,710 then it's going to be less than 109.5. 673 00:36:05,710 --> 00:36:08,500 And this is called a bent geometry. 674 00:36:08,500 --> 00:36:12,070 So, again you can think about it within terms of those-- 675 00:36:12,070 --> 00:36:15,220 if you have some whole cans, those lone pairs 676 00:36:15,220 --> 00:36:19,650 are pressing down on the bonds and compressing them. 677 00:36:19,650 --> 00:36:22,190 So, it's less than 109.5. 678 00:36:22,190 --> 00:36:25,340 I just teach chemistry because I like to buy whole cans 679 00:36:25,340 --> 00:36:28,870 and have a justification for it, is really the bottom line. 680 00:36:28,870 --> 00:36:34,240 OK, so if we keep going now, we have our friend seesaw, 681 00:36:34,240 --> 00:36:37,110 which you're never going to forget. 682 00:36:37,110 --> 00:36:41,960 And I'll rebuild my-- oops wrong one, 683 00:36:41,960 --> 00:36:46,110 rebuild my seesaw over here. 684 00:36:46,110 --> 00:36:51,230 So, now what are the angles here? 685 00:36:51,230 --> 00:36:52,170 There's two of them. 686 00:36:52,170 --> 00:36:57,160 Think about the equatorial angles. 687 00:36:57,160 --> 00:36:59,795 Yep, I'm hearing it, less than 120 688 00:36:59,795 --> 00:37:02,420 and the axial would be less than 90. 689 00:37:02,420 --> 00:37:05,540 So, the lone pair is pressing down both on the 90 690 00:37:05,540 --> 00:37:06,840 and on that 120. 691 00:37:06,840 --> 00:37:10,030 Probably more repulsive for the 90, but all you have to do 692 00:37:10,030 --> 00:37:12,250 is say less than for both of them. 693 00:37:12,250 --> 00:37:14,890 So, the trick is just think about the parent geometry. 694 00:37:14,890 --> 00:37:17,310 What are the angles in the parent geometry? 695 00:37:17,310 --> 00:37:18,740 And then, it's less than. 696 00:37:22,580 --> 00:37:28,890 And if we keep going with this, we had our T-shaped molecule, 697 00:37:28,890 --> 00:37:30,120 as well. 698 00:37:30,120 --> 00:37:35,760 So, when we added another lone pair to our SN 5 case. 699 00:37:35,760 --> 00:37:37,770 And what's the angle now going to be 700 00:37:37,770 --> 00:37:40,480 with those two lone pairs? 701 00:37:40,480 --> 00:37:43,950 Less than 90. 702 00:37:43,950 --> 00:37:45,330 Right. 703 00:37:45,330 --> 00:37:47,540 All right. 704 00:37:47,540 --> 00:37:52,170 So, there so many possibilities for lone pairs. 705 00:37:54,810 --> 00:37:59,930 If we add yet another lone pair to the system, 706 00:37:59,930 --> 00:38:00,850 what's my geometry? 707 00:38:03,134 --> 00:38:06,790 So, this is now going to be linear geometry. 708 00:38:06,790 --> 00:38:10,580 And what's the angle going to be? 709 00:38:10,580 --> 00:38:12,350 Yeah, it's just going to be 180. 710 00:38:12,350 --> 00:38:14,550 So, we don't have a less then here 711 00:38:14,550 --> 00:38:16,830 because whatever way it would bend 712 00:38:16,830 --> 00:38:20,030 it would be just moving toward more repulsive loan pairs. 713 00:38:20,030 --> 00:38:22,630 So, there's no way you can minimize 714 00:38:22,630 --> 00:38:24,340 the repulsion in this case. 715 00:38:24,340 --> 00:38:26,736 So, it's just going to be a linear molecule. 716 00:38:31,360 --> 00:38:36,310 So, now we're going to move into our SN six category. 717 00:38:36,310 --> 00:38:37,930 And we're going to talk about a shape 718 00:38:37,930 --> 00:38:40,640 that we haven't talked about yet. 719 00:38:40,640 --> 00:38:44,260 So, based on this what happens? 720 00:38:44,260 --> 00:38:47,780 We have our parent geometry of octahedral. 721 00:38:47,780 --> 00:38:52,180 And so this has six bonded atoms, SN 6. 722 00:38:52,180 --> 00:38:55,970 But now we're going to take off one of the bonds-- put it 723 00:38:55,970 --> 00:38:57,510 the same as the figure-- we're going 724 00:38:57,510 --> 00:39:01,810 to take off one of the bonds and put on the lone pair. 725 00:39:01,810 --> 00:39:05,420 And this is called square pyramidal 726 00:39:05,420 --> 00:39:09,100 because you have this square here in your axial. 727 00:39:09,100 --> 00:39:10,900 It looks like a square. 728 00:39:10,900 --> 00:39:15,870 But when you consider you have an atom on top coming down 729 00:39:15,870 --> 00:39:19,980 to these four atoms on the side, that again 730 00:39:19,980 --> 00:39:21,240 looks like a pyramid. 731 00:39:21,240 --> 00:39:24,030 So, this is square pyramidal. 732 00:39:24,030 --> 00:39:25,870 And what angle do you think you're 733 00:39:25,870 --> 00:39:30,320 going to have here for these bonded atoms? 734 00:39:30,320 --> 00:39:33,740 Yeah, that will be less than 90. 735 00:39:33,740 --> 00:39:37,230 All right, so we can keep going and we saw this one before. 736 00:39:37,230 --> 00:39:40,580 If we take off another bonded electron 737 00:39:40,580 --> 00:39:42,600 and put it in a second lone pair. 738 00:39:42,600 --> 00:39:44,170 As we saw before those lone parents 739 00:39:44,170 --> 00:39:46,540 want to be a far apart from each other as they can. 740 00:39:46,540 --> 00:39:49,690 So, one goes on top, one goes on the bottom. 741 00:39:49,690 --> 00:39:52,990 And this was our square planar geometry 742 00:39:52,990 --> 00:39:56,000 because it is so square and it's planar. 743 00:39:56,000 --> 00:39:58,600 Now, what do you think the angles are? 744 00:39:58,600 --> 00:39:59,310 90. 745 00:39:59,310 --> 00:40:00,120 Right. 746 00:40:00,120 --> 00:40:02,670 There's no where to escape when you 747 00:40:02,670 --> 00:40:05,950 have a messy roommate on top and a messy roommate on the bottom. 748 00:40:05,950 --> 00:40:08,570 If you're in a triple between messy roommates, 749 00:40:08,570 --> 00:40:11,010 there's just nothing you can do. 750 00:40:11,010 --> 00:40:13,940 And you can't minimize the repulsion at all. 751 00:40:13,940 --> 00:40:15,970 You just have to live with it. 752 00:40:15,970 --> 00:40:18,750 OK, so if we keep going again and now 753 00:40:18,750 --> 00:40:21,899 we're going to add another lone pair. 754 00:40:21,899 --> 00:40:23,440 And it really doesn't matter where we 755 00:40:23,440 --> 00:40:25,380 put that, it's all equivalent. 756 00:40:25,380 --> 00:40:29,640 And it comes up with the shape, that's T-shaped again 757 00:40:29,640 --> 00:40:32,860 that makes sense when you look at the structure. 758 00:40:32,860 --> 00:40:35,670 And now what do you think the angles are? 759 00:40:35,670 --> 00:40:38,770 Less than 90, and you would be correct in that. 760 00:40:38,770 --> 00:40:43,080 All right, so if we put on yet one more lone pair 761 00:40:43,080 --> 00:40:48,620 and take off a bonded electron or take off one more bonded 762 00:40:48,620 --> 00:40:49,900 atom and put on an electron. 763 00:40:49,900 --> 00:40:52,475 What's our geometry? 764 00:40:52,475 --> 00:40:54,570 All right, so we have this structure, 765 00:40:54,570 --> 00:40:57,860 it's linear 180 no place to go. 766 00:40:57,860 --> 00:41:00,840 So, we keep going far enough to come back to linear a lot. 767 00:41:03,810 --> 00:41:07,760 Now, let's just look at a couple more-- some real life 768 00:41:07,760 --> 00:41:12,510 examples of molecules and think about their geometries 769 00:41:12,510 --> 00:41:13,280 and shapes. 770 00:41:13,280 --> 00:41:15,960 I have a couple more clicker questions, 771 00:41:15,960 --> 00:41:19,550 so let's start with our friend water. 772 00:41:19,550 --> 00:41:22,170 And I had water here. 773 00:41:22,170 --> 00:41:25,280 So, now let's think about what the formula 774 00:41:25,280 --> 00:41:27,070 type is going to be for water. 775 00:41:27,070 --> 00:41:28,617 And what's our formula type? 776 00:41:32,040 --> 00:41:35,140 AX Yup. 777 00:41:35,140 --> 00:41:42,090 AX2E2, two bonded atoms, two lone pairs SN number of 4. 778 00:41:42,090 --> 00:41:45,716 And do you remember what this geometry is called? 779 00:41:45,716 --> 00:41:47,300 Bent, yeah. 780 00:41:47,300 --> 00:41:50,870 And this explains then what we talked about before 781 00:41:50,870 --> 00:41:52,950 that this is a polar molecule. 782 00:41:52,950 --> 00:41:56,820 So, these are polar bonds between the oxygen 783 00:41:56,820 --> 00:41:59,900 and the hydrogen. But in this case, 784 00:41:59,900 --> 00:42:02,360 it's not a linear molecule it's a bent molecule, 785 00:42:02,360 --> 00:42:04,180 so it creates a net dipole. 786 00:42:04,180 --> 00:42:06,200 And so that makes it a polar molecule. 787 00:42:06,200 --> 00:42:10,000 And if water had any other shape and was not a polar molecule, 788 00:42:10,000 --> 00:42:12,460 then life would be entirely different 789 00:42:12,460 --> 00:42:14,860 because water is the solvent of life. 790 00:42:14,860 --> 00:42:19,060 So, this shape pretty important for anything. 791 00:42:19,060 --> 00:42:22,910 Actually, they researched on what medical doctors thought 792 00:42:22,910 --> 00:42:25,260 was the most important topic to learn 793 00:42:25,260 --> 00:42:27,920 as an undergraduate in premed education. 794 00:42:27,920 --> 00:42:31,824 And the number one topic that was most important was water. 795 00:42:31,824 --> 00:42:33,240 There, and you just learned about. 796 00:42:33,240 --> 00:42:37,760 It OK, so we keep going now and you 797 00:42:37,760 --> 00:42:40,790 have to answer a lot of things about this on a clicker 798 00:42:40,790 --> 00:42:41,290 question. 799 00:42:59,960 --> 00:43:01,877 All right, just 10 more seconds. 800 00:43:01,877 --> 00:43:03,210 You got to finish your hand out. 801 00:43:23,540 --> 00:43:27,750 All right, so let's go take a look at this one 802 00:43:27,750 --> 00:43:32,080 and fill it in. 803 00:43:32,080 --> 00:43:38,710 So, we have our AX4, which is a SN 5 804 00:43:38,710 --> 00:43:42,800 system because we have one E. And then 805 00:43:42,800 --> 00:43:47,190 we have our seesaw geometry. 806 00:43:47,190 --> 00:43:49,810 All right, so let's just fill in the rest 807 00:43:49,810 --> 00:43:53,310 here and see who's won the clicker competition. 808 00:43:53,310 --> 00:43:57,820 So, for the next system we have Br. 809 00:43:57,820 --> 00:44:02,530 And now it's expanded, so we have AX3E2 SN 5. 810 00:44:02,530 --> 00:44:05,520 We've added another lone pair. 811 00:44:05,520 --> 00:44:08,630 And now this makes a T-shape, but it's 812 00:44:08,630 --> 00:44:12,040 kind of a little benty shape because of the repulsion. 813 00:44:12,040 --> 00:44:12,960 We come down. 814 00:44:12,960 --> 00:44:16,280 We have AX2E5 SN 5. 815 00:44:16,280 --> 00:44:19,430 We've added three more with xenon. 816 00:44:19,430 --> 00:44:21,570 Xenon is expanded here. 817 00:44:21,570 --> 00:44:23,640 We have our linear shape. 818 00:44:23,640 --> 00:44:28,570 We also have xenon with four things bound. 819 00:44:28,570 --> 00:44:30,510 And if we did the Lewis structure of that, 820 00:44:30,510 --> 00:44:33,720 we'd realize there's lone pairs on the top and the bottom. 821 00:44:33,720 --> 00:44:36,800 AX4E2 SN 6. 822 00:44:36,800 --> 00:44:40,110 And this is our square planar geometry. 823 00:44:40,110 --> 00:44:44,790 So, you can predict a lot about just doing a Lewis structure 824 00:44:44,790 --> 00:44:47,000 or thinking about where the lone pairs. 825 00:44:47,000 --> 00:44:49,020 You can predict geometries. 826 00:44:49,020 --> 00:44:51,064 And let's see who has won today. 827 00:44:55,710 --> 00:44:56,738 We have an upset. 828 00:44:59,810 --> 00:45:00,768 All right, Sam. 829 00:45:02,958 --> 00:45:06,260 All right, see everybody on Monday.