1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,830 Commons license. 3 00:00:03,830 --> 00:00:06,850 Your support will help MIT OpenCourseWare continue to 4 00:00:06,850 --> 00:00:10,520 offer high-quality educational resources for free. 5 00:00:10,520 --> 00:00:13,390 To make a donation or view additional materials from 6 00:00:13,390 --> 00:00:17,490 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:17,490 --> 00:00:18,740 ocw.mit.edu. 8 00:00:22,290 --> 00:00:23,450 PROFESSOR: OK. 9 00:00:23,450 --> 00:00:25,150 Settle down. 10 00:00:25,150 --> 00:00:25,890 Settle down. 11 00:00:25,890 --> 00:00:28,770 The weekend doesn't start until after the lecture. 12 00:00:28,770 --> 00:00:30,740 First we learn. 13 00:00:30,740 --> 00:00:31,030 All right. 14 00:00:31,030 --> 00:00:34,920 So couple of announcements, the big one being the reminder 15 00:00:34,920 --> 00:00:38,490 about that little celebration next Wednesday, coverage 16 00:00:38,490 --> 00:00:41,500 through the end of the lecture on Monday, there'll be no 17 00:00:41,500 --> 00:00:46,160 weekly quiz next Tuesday, and Hilary wanted me to announce 18 00:00:46,160 --> 00:00:49,200 that there's a job opening for someone who's in class 19 00:00:49,200 --> 00:00:52,690 anyways, and wants to-- if you take decent notes and your 20 00:00:52,690 --> 00:00:56,250 handwriting is legible, you want to make $9 an hour, go 21 00:00:56,250 --> 00:00:57,630 see Hilary. 22 00:00:57,630 --> 00:00:57,980 All right. 23 00:00:57,980 --> 00:00:59,060 Let's get into the lesson. 24 00:00:59,060 --> 00:01:00,200 Got a lot to cover today. 25 00:01:00,200 --> 00:01:04,940 Last day, we were looking at the formation of molecular 26 00:01:04,940 --> 00:01:09,120 orbitals by linear combination of atomic orbitals into 27 00:01:09,120 --> 00:01:10,050 molecular orbitals. 28 00:01:10,050 --> 00:01:14,710 So we had our SLI six-letter initialization, and we saw the 29 00:01:14,710 --> 00:01:17,800 formation of sigma and pi molecular orbitals. 30 00:01:17,800 --> 00:01:21,620 And the way you distinguish between sigma and pi is based 31 00:01:21,620 --> 00:01:23,610 on electron distribution. 32 00:01:23,610 --> 00:01:27,210 Specifically, in a sigma molecular orbital, you have 33 00:01:27,210 --> 00:01:30,920 continuous electron density between the nuclei. 34 00:01:30,920 --> 00:01:32,410 That is to say, no holiday. 35 00:01:32,410 --> 00:01:37,350 So I've drawn a sigma orbital here where I've taken two P 36 00:01:37,350 --> 00:01:42,140 orbitals lying on their sides, so they're bonding 37 00:01:42,140 --> 00:01:45,590 longitudinally and when the bond is formed, the one 38 00:01:45,590 --> 00:01:49,360 nitrogen nucleus is here and the other nitrogen nucleus is 39 00:01:49,360 --> 00:01:52,020 here and you can see that there's continuous electron 40 00:01:52,020 --> 00:01:55,620 density, no holidays between the two nuclei. 41 00:01:55,620 --> 00:01:57,660 So this would be called a sigma bond. 42 00:01:57,660 --> 00:02:01,270 The pi molecular orbital, on the other hand, has a nodal 43 00:02:01,270 --> 00:02:04,270 plane containing both nuclei. 44 00:02:04,270 --> 00:02:08,430 So since the nuclei are in a nodal plane, that means that 45 00:02:08,430 --> 00:02:11,650 you've got to have two lobes, one above the plane and one 46 00:02:11,650 --> 00:02:14,440 below the plane, and that's shown here. 47 00:02:14,440 --> 00:02:17,670 This is the example of nitrogen, where it forms both 48 00:02:17,670 --> 00:02:21,930 sigma and pi bonds and to form the pi bonds, you take the 49 00:02:21,930 --> 00:02:23,390 lateral smearing. 50 00:02:23,390 --> 00:02:25,340 And so this is what I call the 88. 51 00:02:25,340 --> 00:02:27,670 If you take the 88, you're going to make a pi bond. 52 00:02:27,670 --> 00:02:32,310 So that's lateral smearing of the p atomic orbitals. 53 00:02:32,310 --> 00:02:36,240 And the last thing I want to make sure that you appreciate 54 00:02:36,240 --> 00:02:40,230 is that when you form multiple bonds, you have to 55 00:02:40,230 --> 00:02:42,280 have sigma and pi. 56 00:02:42,280 --> 00:02:44,780 You can only form one sigma bond because you have no 57 00:02:44,780 --> 00:02:47,890 holidays between the two nuclei so now you can't go and 58 00:02:47,890 --> 00:02:51,110 invade that same space with a second bond. 59 00:02:51,110 --> 00:02:54,310 So that means you're going to have to get beyond that zone 60 00:02:54,310 --> 00:02:56,360 and that means you have to go into pi. 61 00:02:56,360 --> 00:02:58,020 So if we've got only bond, it's a sigma. 62 00:02:58,020 --> 00:03:00,310 If you've got two, it's a sigma and a pi. 63 00:03:00,310 --> 00:03:02,530 If it's three, it's a sigma and two pis. 64 00:03:02,530 --> 00:03:04,080 That's what we saw in the case of nitrogen. 65 00:03:04,080 --> 00:03:07,510 I didn't complete it here, but there's two pi bonds because 66 00:03:07,510 --> 00:03:08,530 this is a triple bond. 67 00:03:08,530 --> 00:03:10,490 So there's a sigma and there's two pi's. 68 00:03:10,490 --> 00:03:13,880 One pi is shown here, and that's vertical to the board, 69 00:03:13,880 --> 00:03:17,180 and the other pi is the sigma x, which would be orthogonal 70 00:03:17,180 --> 00:03:19,330 coming in and out of the plane of the board. 71 00:03:19,330 --> 00:03:22,950 So those smear give us two of these and then we've got the 72 00:03:22,950 --> 00:03:24,040 sigma bond. 73 00:03:24,040 --> 00:03:28,630 And we even extended this to hybrid systems, and towards 74 00:03:28,630 --> 00:03:30,900 the end of the lecture, we looked at ethylene, and I just 75 00:03:30,900 --> 00:03:33,120 wanted to come back to that for a second because I raced 76 00:03:33,120 --> 00:03:34,050 through it there. 77 00:03:34,050 --> 00:03:38,540 And so we wanted to look at C2H4, which is ethylene. 78 00:03:41,840 --> 00:03:47,010 And in the case of C2H4, we recognized that when we went 79 00:03:47,010 --> 00:03:51,110 through the Lewis structure, we had a molecule that looked 80 00:03:51,110 --> 00:03:55,410 like this, and the thing to note here is that we have a 81 00:03:55,410 --> 00:03:56,770 double bond. 82 00:03:56,770 --> 00:03:59,710 So once you see this double bond, then that means I'm 83 00:03:59,710 --> 00:04:02,160 going to need a mix of sigma and pi. 84 00:04:02,160 --> 00:04:07,450 Now earlier we had studied methane-- 85 00:04:07,450 --> 00:04:14,240 methane, which was CH4, and to get methane, we hybridize sp3. 86 00:04:14,240 --> 00:04:19,170 sp3, which gave us four identical struts coming off 87 00:04:19,170 --> 00:04:20,280 the carbon. 88 00:04:20,280 --> 00:04:22,910 Well, we recognized that wouldn't work here because we 89 00:04:22,910 --> 00:04:26,350 needed to make a sigma and a pi so the gambit here was to 90 00:04:26,350 --> 00:04:30,820 reserve a p orbital. 91 00:04:30,820 --> 00:04:32,875 Reserve a p orbital. 92 00:04:32,875 --> 00:04:35,300 Don't combine all of the three p orbitals. 93 00:04:35,300 --> 00:04:41,410 Combine only two of the p's and that gave us sp2 plus a p. 94 00:04:41,410 --> 00:04:45,170 And the box notation for that looked like this. 95 00:04:45,170 --> 00:04:48,660 Here's the p orbital that's not been hybridized. 96 00:04:48,660 --> 00:04:52,500 These are the sp2 orbitals that are a mix of S, and I've 97 00:04:52,500 --> 00:04:54,080 taken two of the p's and I'm leaving 98 00:04:54,080 --> 00:04:56,130 one of the p's unmixed. 99 00:04:56,130 --> 00:04:59,110 And now I've got four electrons to put in here and 100 00:04:59,110 --> 00:05:00,550 that's where I ran out of gas last day. 101 00:05:00,550 --> 00:05:02,280 I have to put the fourth electron in here. 102 00:05:02,280 --> 00:05:03,820 These are still degenerate. 103 00:05:03,820 --> 00:05:05,710 So now I've got still the capability of 104 00:05:05,710 --> 00:05:07,230 forming four bonds. 105 00:05:07,230 --> 00:05:09,460 You can see-- whenever you see carbon, there's an 106 00:05:09,460 --> 00:05:10,210 interesting-- 107 00:05:10,210 --> 00:05:12,940 just something to train your eye, if you see carbon in a 108 00:05:12,940 --> 00:05:15,320 structure, you need four sticks coming out of the 109 00:05:15,320 --> 00:05:17,980 carbon, or else there's a spelling error. 110 00:05:17,980 --> 00:05:19,050 So look here. 111 00:05:19,050 --> 00:05:21,150 This is carbon with four hydrogens. 112 00:05:21,150 --> 00:05:22,140 One, two, three, four. 113 00:05:22,140 --> 00:05:24,890 This is carbon one, two, three, four. 114 00:05:24,890 --> 00:05:26,710 Here's a carbon one, two, three, four. 115 00:05:26,710 --> 00:05:28,730 It's always got four struts. 116 00:05:28,730 --> 00:05:29,950 That's not what's different here. 117 00:05:29,950 --> 00:05:32,150 It's the arrangement of the struts. 118 00:05:32,150 --> 00:05:37,340 These are all four going at 109 degrees in three space. 119 00:05:37,340 --> 00:05:40,860 This one is going to have three of them identical, 120 00:05:40,860 --> 00:05:44,650 because we studied last day the principle that equal 121 00:05:44,650 --> 00:05:52,070 energies, equal bond energies, imply-- 122 00:05:52,070 --> 00:05:53,500 I want to get the spelling here. 123 00:05:53,500 --> 00:06:00,240 Equal bond energies imply equal spatial disposition. 124 00:06:00,240 --> 00:06:03,210 Equal spatial disposition of those bonds. 125 00:06:03,210 --> 00:06:06,100 So if we have four bonds of equal energy, we have the 126 00:06:06,100 --> 00:06:06,890 tetrahedron. 127 00:06:06,890 --> 00:06:09,890 If we have three bonds of equal energy, how do I take 128 00:06:09,890 --> 00:06:13,650 three sticks coming out of a central carbon and make them 129 00:06:13,650 --> 00:06:16,310 uniformly distributed in space? 130 00:06:16,310 --> 00:06:18,360 They're going to lie in a plane and they're going to be 131 00:06:18,360 --> 00:06:21,250 120 degrees apart. 132 00:06:21,250 --> 00:06:24,070 That's the consequence of having this structure. 133 00:06:24,070 --> 00:06:26,700 All of this follow and why did we get to this? 134 00:06:26,700 --> 00:06:28,280 Octet. 135 00:06:28,280 --> 00:06:29,020 So here we are. 136 00:06:29,020 --> 00:06:33,400 Here's the three of them and here the fourth bond is p, and 137 00:06:33,400 --> 00:06:37,880 it's normal to the plane of the board, and now we can take 138 00:06:37,880 --> 00:06:41,490 one of these and bond it to another one, bring them in 139 00:06:41,490 --> 00:06:42,600 along here. 140 00:06:42,600 --> 00:06:47,010 This is an sp2 and an sp2 and they bond to 141 00:06:47,010 --> 00:06:48,540 make a sigma bond. 142 00:06:48,540 --> 00:06:49,110 How do I know? 143 00:06:49,110 --> 00:06:50,850 Look, no holidays. 144 00:06:50,850 --> 00:06:52,740 Continuous electron density. 145 00:06:52,740 --> 00:06:55,490 And now in the plane normal to the board, I have lateral 146 00:06:55,490 --> 00:06:57,920 smearing, and I make a pi. 147 00:06:57,920 --> 00:06:59,170 So I've got a sigma and a pi. 148 00:06:59,170 --> 00:07:03,010 That gives me the double bond and let me just show you the 149 00:07:03,010 --> 00:07:06,650 cartoons here because we've got some nice artistic 150 00:07:06,650 --> 00:07:08,770 renderings. 151 00:07:08,770 --> 00:07:09,390 So here we are. 152 00:07:09,390 --> 00:07:13,350 This is the top view of the three sp2's and then the p's 153 00:07:13,350 --> 00:07:14,630 are vertical. 154 00:07:14,630 --> 00:07:17,640 so we're going to move along the z axis, bring two of these 155 00:07:17,640 --> 00:07:23,260 together-- there's the smearing of the sp2's on axis. 156 00:07:23,260 --> 00:07:27,460 So this is a sigma bond, No electron density holidays from 157 00:07:27,460 --> 00:07:30,730 one nucleus to the other, and now we're going to smear the 158 00:07:30,730 --> 00:07:38,120 lobes of the p orbitals of the unmixed carbon. 159 00:07:38,120 --> 00:07:40,690 That gives us the double bond and there we are. 160 00:07:40,690 --> 00:07:42,530 This is the same thing taken from your book. 161 00:07:42,530 --> 00:07:46,670 This is the sigma orbital and then chemists like to do this. 162 00:07:46,670 --> 00:07:48,560 You see this sort of slashed line? 163 00:07:48,560 --> 00:07:50,430 It sort of indicates it's receding. 164 00:07:50,430 --> 00:07:54,970 It's going into the plane and then these are coming out. 165 00:07:54,970 --> 00:07:57,340 This hydrogen is coming out at us and that other one's going 166 00:07:57,340 --> 00:08:00,250 way, way in the back and this is in the plane and there's 167 00:08:00,250 --> 00:08:04,830 the lobes colored is they irritate me always. 168 00:08:04,830 --> 00:08:07,840 By the way, they've chosen to make this the z-axis, which I 169 00:08:07,840 --> 00:08:09,080 don't like. 170 00:08:09,080 --> 00:08:12,950 The z-axis should be the axis along which they bond. 171 00:08:12,950 --> 00:08:14,620 And then here's acetylene. 172 00:08:14,620 --> 00:08:18,910 Acetylene is a compound that C2H2. 173 00:08:18,910 --> 00:08:21,650 C2H2, and I'm not going to go through this in class. 174 00:08:21,650 --> 00:08:24,650 You can try it when you have nothing else to do. 175 00:08:24,650 --> 00:08:28,170 and so it's got a triple bond. 176 00:08:28,170 --> 00:08:31,800 it's got a triple bond between the carbons and then hydrogens 177 00:08:31,800 --> 00:08:32,490 on either side. 178 00:08:32,490 --> 00:08:35,150 Again, look at the carbon: one, two, three, four. 179 00:08:35,150 --> 00:08:39,400 But now we're going to have to have two lateral smears, so 180 00:08:39,400 --> 00:08:43,320 that means I'm only going to take one of the p orbitals and 181 00:08:43,320 --> 00:08:45,170 leave two in reserve. 182 00:08:45,170 --> 00:08:47,890 so when I do that, I'm going to end up with this, starting 183 00:08:47,890 --> 00:08:49,030 with the box notation. 184 00:08:49,030 --> 00:08:55,680 Here's carbon as it would exist without any 185 00:08:55,680 --> 00:08:56,970 hybridization. 186 00:08:56,970 --> 00:08:59,500 An S and three p orbitals. 187 00:08:59,500 --> 00:09:00,130 And what I want to do-- 188 00:09:00,130 --> 00:09:04,460 I want to have two pi's and a sigma, so I'm going to just 189 00:09:04,460 --> 00:09:08,910 capture one of these and reserve two p's as unmixed, so 190 00:09:08,910 --> 00:09:10,450 that will give me this. 191 00:09:10,450 --> 00:09:13,230 This is one S and one p, so this is called sp 192 00:09:13,230 --> 00:09:14,270 hybridization. 193 00:09:14,270 --> 00:09:18,690 And then I've got the two p's unmixed, so now I've got one, 194 00:09:18,690 --> 00:09:20,900 two, three, four. 195 00:09:20,900 --> 00:09:23,090 are both orthogonal to one another and 196 00:09:23,090 --> 00:09:24,300 these two are what? 197 00:09:24,300 --> 00:09:26,790 I've got two struts coming off the carbon. 198 00:09:26,790 --> 00:09:30,170 How do I get two struts coming off a carbon symmetrically 199 00:09:30,170 --> 00:09:32,740 disposed in space? 200 00:09:32,740 --> 00:09:33,940 That's it. 201 00:09:33,940 --> 00:09:36,140 So look at the shape of the molecule. 202 00:09:36,140 --> 00:09:37,290 You see it here. 203 00:09:37,290 --> 00:09:38,190 There's the-- 204 00:09:38,190 --> 00:09:42,220 the sigma orbital is here and then we have vertical and then 205 00:09:42,220 --> 00:09:43,600 out on the plane. 206 00:09:43,600 --> 00:09:48,390 And so you first would say, wow, I understand that. 207 00:09:48,390 --> 00:09:49,800 That looks a lot like nitrogen. 208 00:09:49,800 --> 00:09:51,850 And then, of course, I looked at that, and I said, what 209 00:09:51,850 --> 00:09:52,930 about the right-hand rule? 210 00:09:52,930 --> 00:09:55,790 And according to the right-hand rule, the z is 211 00:09:55,790 --> 00:09:56,690 going in this direction. 212 00:09:56,690 --> 00:10:00,520 It's a little more subtle, but I pick up on stuff like that. 213 00:10:00,520 --> 00:10:01,480 I can't help it. 214 00:10:01,480 --> 00:10:01,860 OK. 215 00:10:01,860 --> 00:10:04,350 So this is good. 216 00:10:04,350 --> 00:10:08,220 So what I've done here is gone through a number of instances 217 00:10:08,220 --> 00:10:10,650 where we start with electronic structure. 218 00:10:10,650 --> 00:10:13,010 sometimes we hybridize, sometimes we don't. 219 00:10:13,010 --> 00:10:14,770 But what follows here? 220 00:10:14,770 --> 00:10:18,660 When we start with the octet rule and we start playing with 221 00:10:18,660 --> 00:10:22,110 the energetics and forming the orbitals, can you see that 222 00:10:22,110 --> 00:10:25,740 there's a connection between electronic structure, which 223 00:10:25,740 --> 00:10:29,840 then dictates the bond disposition in space, which 224 00:10:29,840 --> 00:10:34,170 ultimately dictates the architecture of the molecule? 225 00:10:34,170 --> 00:10:35,390 Molecular architecture. 226 00:10:35,390 --> 00:10:40,920 So there's a way of doing this There's a way of doing this 227 00:10:40,920 --> 00:10:42,170 systematically. 228 00:10:47,090 --> 00:10:48,160 I just got ahead of myself. 229 00:10:48,160 --> 00:10:50,980 Before we get to that, I wanted to just pause here for 230 00:10:50,980 --> 00:10:55,430 a second and reflect upon the properties of the covalent 231 00:10:55,430 --> 00:10:59,840 bond as contrasted to the properties of the ionic bond. 232 00:10:59,840 --> 00:11:10,140 So let me get to that first. Properties of covalent bond. 233 00:11:10,140 --> 00:11:10,480 OK. 234 00:11:10,480 --> 00:11:14,220 The first property I want to highlight is the fact that the 235 00:11:14,220 --> 00:11:16,130 bond is saturated. 236 00:11:16,130 --> 00:11:18,790 Remember when we said that the ionic bond was saturated? 237 00:11:18,790 --> 00:11:19,380 So what does it mean? 238 00:11:19,380 --> 00:11:20,400 Saturated? 239 00:11:20,400 --> 00:11:23,745 It means that there's two atoms only. 240 00:11:26,860 --> 00:11:29,670 Once you've got the electron pair between the two atoms, 241 00:11:29,670 --> 00:11:32,440 the bond is formed and that's the end of the story, whereas 242 00:11:32,440 --> 00:11:35,630 with ionic bonding, you can keep by electrostatic 243 00:11:35,630 --> 00:11:37,780 attraction piling on and piling on. 244 00:11:37,780 --> 00:11:40,160 So this is the concept of saturation. 245 00:11:40,160 --> 00:11:43,610 So we have the shared electron pair. 246 00:11:43,610 --> 00:11:45,570 Shared electron pair and that's all 247 00:11:45,570 --> 00:11:47,180 there is to the story. 248 00:11:47,180 --> 00:11:51,650 The other thing about the covalent bond is that it is 249 00:11:51,650 --> 00:11:52,900 directional. 250 00:11:55,900 --> 00:11:58,455 And so the bond spatial arrangement-- 251 00:12:03,180 --> 00:12:05,110 these are the positioners. 252 00:12:05,110 --> 00:12:08,620 You can't put atoms where there are no bonds and since 253 00:12:08,620 --> 00:12:10,940 the bonds have a direct spatial arrangement, then 254 00:12:10,940 --> 00:12:15,560 that's going to inform molecular architecture. 255 00:12:15,560 --> 00:12:17,245 Dictates molecular architecture. 256 00:12:21,440 --> 00:12:22,855 So what I'd like to do next-- 257 00:12:25,900 --> 00:12:28,320 so for example, here's one that we know. 258 00:12:28,320 --> 00:12:29,120 You've seen this one. 259 00:12:29,120 --> 00:12:30,040 This is methane. 260 00:12:30,040 --> 00:12:32,450 This is sp3 hybridization. 261 00:12:32,450 --> 00:12:36,800 So we learn that methane is tetrahedronal. 262 00:12:36,800 --> 00:12:41,985 We say that the four corners of the hydrogen positions form 263 00:12:41,985 --> 00:12:42,830 a tetrahedron. 264 00:12:42,830 --> 00:12:44,780 So we call this molecule tetrahedral. 265 00:12:47,410 --> 00:12:51,260 So I could look at other molecules that have sp3 266 00:12:51,260 --> 00:12:52,110 hybridization. 267 00:12:52,110 --> 00:12:55,240 So if I look at something like titanium tetrachloride, which 268 00:12:55,240 --> 00:12:59,550 we met earlier to make titanium, this is titanium in 269 00:12:59,550 --> 00:13:00,820 the center and the four chlorines. 270 00:13:00,820 --> 00:13:03,190 It's the same molecular shape. 271 00:13:03,190 --> 00:13:06,420 If you see leaded gasoline, the compound in unleaded 272 00:13:06,420 --> 00:13:08,820 gasoline is tetraethyl lead. 273 00:13:08,820 --> 00:13:11,580 Lead at the center, ethyl's at the four corners of the 274 00:13:11,580 --> 00:13:12,230 tetrahedron. 275 00:13:12,230 --> 00:13:14,920 So all of these-- and same over here. 276 00:13:14,920 --> 00:13:20,380 This one here, sp2, this means the molecule is planar. 277 00:13:20,380 --> 00:13:22,970 The carbons and the hydrogens lie in a plane. 278 00:13:22,970 --> 00:13:25,500 The whole molecule lies in the plane of the board. 279 00:13:25,500 --> 00:13:29,500 If I give you any molecule that's got sp3 hybridization 280 00:13:29,500 --> 00:13:32,900 and has one, two, three, four, five, six atoms, in other 281 00:13:32,900 --> 00:13:35,380 words, all the bonds have atoms, the molecular is going 282 00:13:35,380 --> 00:13:38,950 to be a planar molecule with this, the 120 degrees. 283 00:13:38,950 --> 00:13:43,930 So we've got a grand scheme here that allows us to codify 284 00:13:43,930 --> 00:13:49,100 this behavior and the connection between molecular 285 00:13:49,100 --> 00:13:56,950 structure and the electronics arrangement and it's written 286 00:13:56,950 --> 00:14:00,610 up here under the term Valence-Shell 287 00:14:00,610 --> 00:14:02,740 Electron-Pair-Repulsion Model. 288 00:14:08,400 --> 00:14:09,460 Let's write that down. 289 00:14:09,460 --> 00:14:11,440 I'm going to go through some examples. 290 00:14:11,440 --> 00:14:13,860 Valence-Shell-- 291 00:14:13,860 --> 00:14:15,090 hyphenated-- 292 00:14:15,090 --> 00:14:16,340 Electron-Pair-Repulsion. 293 00:14:22,380 --> 00:14:30,270 That's the model, and it goes by the acronym VSEPR, which is 294 00:14:30,270 --> 00:14:31,700 a five-letter acronym. 295 00:14:31,700 --> 00:14:35,790 This is not an initialization because chemists reverse 296 00:14:35,790 --> 00:14:38,760 these, and they call it VSEPR, because the V and the S is 297 00:14:38,760 --> 00:14:41,650 kind of hard to pronounce unless you have Slavic blood. 298 00:14:41,650 --> 00:14:49,380 So it's called VSEPR, and that is the scheme that's up here. 299 00:14:49,380 --> 00:14:52,000 So I'm going to go through some examples and show you how 300 00:14:52,000 --> 00:14:56,490 you can start with electronic structure and figure out what 301 00:14:56,490 --> 00:14:59,060 the molecular architecture is. 302 00:14:59,060 --> 00:15:02,170 So I'm going to follow this. 303 00:15:02,170 --> 00:15:03,830 So first thing we're going to do is 304 00:15:03,830 --> 00:15:04,960 write the Lewis structure. 305 00:15:04,960 --> 00:15:07,470 And the example I want to start with is sulfur 306 00:15:07,470 --> 00:15:08,270 hexafluoride. 307 00:15:08,270 --> 00:15:09,710 I want to-- 308 00:15:09,710 --> 00:15:12,570 so the question is, what is the molecular architecture of 309 00:15:12,570 --> 00:15:13,610 sulfur hexafluoride? 310 00:15:13,610 --> 00:15:15,320 Can we predict the shape? 311 00:15:15,320 --> 00:15:17,590 And this is an interesting molecule. 312 00:15:17,590 --> 00:15:18,640 It's very dense. 313 00:15:18,640 --> 00:15:21,480 It's about five times-- it's a gas at room temperature. 314 00:15:21,480 --> 00:15:24,050 It's about five times the density of air. 315 00:15:24,050 --> 00:15:28,760 It's got an atomic mass of about 150, and it's used in 316 00:15:28,760 --> 00:15:29,800 the light metal industry. 317 00:15:29,800 --> 00:15:33,010 It's used as a blanket gas for casting of aluminum and 318 00:15:33,010 --> 00:15:35,460 magnesium to keep the air away from the liquid metal. 319 00:15:35,460 --> 00:15:37,730 Otherwise, the metal would oxidize. 320 00:15:37,730 --> 00:15:41,310 But unfortunately, the sulfur fluorine bond is a really good 321 00:15:41,310 --> 00:15:45,020 infrared absorber, and when this stuff leaks out, it's a 322 00:15:45,020 --> 00:15:47,250 fantastic greenhouse gas trapper. 323 00:15:47,250 --> 00:15:49,835 It's got a greenhouse gas coefficient that's, I don't 324 00:15:49,835 --> 00:15:52,990 know, something like 10,000 times that of CO2. 325 00:15:52,990 --> 00:15:57,290 So there's a lot of conversation 326 00:15:57,290 --> 00:15:58,610 about banning this. 327 00:15:58,610 --> 00:16:01,300 If we ban this, we're going to have to find a substitute. 328 00:16:01,300 --> 00:16:04,250 There is no readily available substitute. 329 00:16:04,250 --> 00:16:06,830 And what that'll mean is the cost of aluminum castings and 330 00:16:06,830 --> 00:16:10,880 magnesium castings will go up, which means that they won't be 331 00:16:10,880 --> 00:16:12,040 used in automobiles. 332 00:16:12,040 --> 00:16:14,790 Instead, cheaper steel will be used in automobiles, which 333 00:16:14,790 --> 00:16:17,490 means the mass of the automobile will go up, fuel 334 00:16:17,490 --> 00:16:20,390 economy will go down, tail pipe emissions will go up and 335 00:16:20,390 --> 00:16:22,300 greenhouse gas emissions will go up. 336 00:16:22,300 --> 00:16:26,120 So how do we get ourselves out of this situation? 337 00:16:26,120 --> 00:16:29,640 Do we ban SF6 or not? 338 00:16:29,640 --> 00:16:32,500 How do you go about thinking about that problem? 339 00:16:32,500 --> 00:16:35,720 Well, if you're in Washington today, it's the lobbyists with 340 00:16:35,720 --> 00:16:37,350 the biggest bag of money that comes in. 341 00:16:37,350 --> 00:16:39,400 That's what's going to dictate policy. 342 00:16:39,400 --> 00:16:42,840 But if we lived in an ideal world, we'd have people like 343 00:16:42,840 --> 00:16:45,470 you who understand the chemistry and what would be 344 00:16:45,470 --> 00:16:47,160 the analysis? 345 00:16:47,160 --> 00:16:51,480 You'd look at all the SF6 emitted from the cast shops in 346 00:16:51,480 --> 00:16:56,490 a given year and compare that to the increase in tail pipe 347 00:16:56,490 --> 00:17:01,010 CO2 emissions if you replaced the aluminum with steel, and 348 00:17:01,010 --> 00:17:03,060 then you figure out what the trade off is. 349 00:17:03,060 --> 00:17:06,410 And maybe what you'd do is set up a policy where you would 350 00:17:06,410 --> 00:17:08,480 gradually phase this out with-- 351 00:17:08,480 --> 00:17:09,560 heaven forbid-- 352 00:17:09,560 --> 00:17:12,270 solid research support to give us what we need. 353 00:17:12,270 --> 00:17:15,720 That way we get clean air and everything else. 354 00:17:15,720 --> 00:17:16,040 OK. 355 00:17:16,040 --> 00:17:17,900 So that's what we would like. 356 00:17:17,900 --> 00:17:20,100 Now let's look at the Lewis structure of this thing 357 00:17:20,100 --> 00:17:21,540 because that's the first thing we're supposed to do 358 00:17:21,540 --> 00:17:23,000 according to VSEPR. 359 00:17:23,000 --> 00:17:28,460 So I'm going to put sulfur here and I know sulfur's got-- 360 00:17:28,460 --> 00:17:29,710 it's 3s2 3p4. 361 00:17:33,060 --> 00:17:35,140 So it's got six valence electrons. 362 00:17:35,140 --> 00:17:38,750 I'm going to put them symmetrically around the 363 00:17:38,750 --> 00:17:41,300 sulfur, and then I'm going to bring in the fluorine. 364 00:17:41,300 --> 00:17:44,170 I'll bring in one fluorine, and I know it's got seven 365 00:17:44,170 --> 00:17:49,030 valence electrons: one, two, three, four, five, six, seven. 366 00:17:49,030 --> 00:17:52,090 And the same thing with the other fluorines. 367 00:17:52,090 --> 00:17:54,880 So I'm going to put one, and then for the electron pair, 368 00:17:54,880 --> 00:17:56,740 I'm just going to draw a line. 369 00:17:56,740 --> 00:17:57,250 OK. 370 00:17:57,250 --> 00:17:58,050 You're smart. 371 00:17:58,050 --> 00:17:58,840 You can figure that out. 372 00:17:58,840 --> 00:18:00,220 That's just shorthand. 373 00:18:00,220 --> 00:18:00,380 OK. 374 00:18:00,380 --> 00:18:01,530 Here's another fluorine. 375 00:18:01,530 --> 00:18:02,720 One, two, three. 376 00:18:02,720 --> 00:18:05,610 So there's two, four, six, seven. 377 00:18:05,610 --> 00:18:09,480 There's one, two, three, four, five, six, seven. 378 00:18:09,480 --> 00:18:12,670 One, two, three, four, five, six, seven. 379 00:18:12,670 --> 00:18:17,660 One, two, three, four, five, six, seven. 380 00:18:17,660 --> 00:18:18,430 Beautiful. 381 00:18:18,430 --> 00:18:18,760 OK. 382 00:18:18,760 --> 00:18:21,950 So now what I want to do is identify all 383 00:18:21,950 --> 00:18:23,820 of the bonding orbitals. 384 00:18:23,820 --> 00:18:28,380 So I see around the sulfur, I've got one, two, three, 385 00:18:28,380 --> 00:18:35,120 four, five, six, six bonding orbitals, six bonding domains, 386 00:18:35,120 --> 00:18:37,730 if you like. 387 00:18:37,730 --> 00:18:39,790 Well, that's 12 electrons. 388 00:18:39,790 --> 00:18:41,150 I said octet stability. 389 00:18:41,150 --> 00:18:43,730 Looks like sulfur took that idea and ran with it. 390 00:18:43,730 --> 00:18:48,080 And it turns out that this does happen on occasion in 391 00:18:48,080 --> 00:18:51,470 elements that are very electronegative. 392 00:18:51,470 --> 00:18:55,090 Very, very strong non-metals will form what is known as an 393 00:18:55,090 --> 00:18:56,450 expanded octet. 394 00:18:59,680 --> 00:19:03,500 And here we've got-- 395 00:19:03,500 --> 00:19:05,280 here's a list of elements. 396 00:19:05,280 --> 00:19:06,530 [MICROPHONE ADJUSTMENT] 397 00:19:12,320 --> 00:19:14,432 PROFESSOR: We need a high-tech tie clip. 398 00:19:17,780 --> 00:19:20,280 Pull out a little more, maybe it's constrained. 399 00:19:20,280 --> 00:19:21,770 It's tethered. 400 00:19:21,770 --> 00:19:22,925 the bond is too tight. 401 00:19:22,925 --> 00:19:24,480 That's what it is. 402 00:19:24,480 --> 00:19:26,080 It's not flexible. 403 00:19:26,080 --> 00:19:28,140 It's covalent, you see. 404 00:19:28,140 --> 00:19:28,450 All right. 405 00:19:28,450 --> 00:19:32,020 So here we are with the elements, and the ones shown 406 00:19:32,020 --> 00:19:37,620 in the in the grayish-blue here are the ones that on 407 00:19:37,620 --> 00:19:42,270 occasion might form an expanded octet. 408 00:19:42,270 --> 00:19:47,720 Not always, but if you do see 10 or 12 electrons around one 409 00:19:47,720 --> 00:19:49,660 of those elements, don't be alarmed. 410 00:19:49,660 --> 00:19:52,920 If you see 10 or 12 electrons around the potassium, that's 411 00:19:52,920 --> 00:19:54,160 probably a mistake. 412 00:19:54,160 --> 00:19:54,480 OK. 413 00:19:54,480 --> 00:19:59,040 So we have six bonding domains and what else do we have? 414 00:19:59,040 --> 00:20:04,760 We have six bonding domains, and we have zero non-bonding 415 00:20:04,760 --> 00:20:11,090 domains, and in total, we have six electron domains, in other 416 00:20:11,090 --> 00:20:16,590 words, six electron pair systems 417 00:20:16,590 --> 00:20:18,310 around the central sulfur. 418 00:20:18,310 --> 00:20:22,090 And on the basis of the total of bonding and non-bonding, 419 00:20:22,090 --> 00:20:24,805 this is what dictates the skeletal structure. 420 00:20:28,240 --> 00:20:30,440 In this case, the skeletal structure is trivial. 421 00:20:30,440 --> 00:20:33,580 It's the same as the atomic structure because there are no 422 00:20:33,580 --> 00:20:35,670 non-bonding domains. 423 00:20:35,670 --> 00:20:37,800 And so I've got six of them. 424 00:20:37,800 --> 00:20:38,760 They're equivalent. 425 00:20:38,760 --> 00:20:42,510 So what's that going to mean in terms of shape? 426 00:20:42,510 --> 00:20:48,240 Well, I'll put the sulfur in the center and I've got one, 427 00:20:48,240 --> 00:20:51,660 two, three, four, five, six. 428 00:20:51,660 --> 00:20:55,980 So two in one plane crossways and then two vertical. 429 00:20:55,980 --> 00:20:57,050 And then I put the fluorines-- 430 00:20:57,050 --> 00:21:01,570 one, two, three, four, five, six. 431 00:21:01,570 --> 00:21:05,130 So there's the molecular architecture and it's termed 432 00:21:05,130 --> 00:21:07,440 octahedral. 433 00:21:07,440 --> 00:21:09,330 I know at first this throws you 434 00:21:09,330 --> 00:21:10,820 because you see six struts. 435 00:21:10,820 --> 00:21:12,200 Where do you get the octahedral? 436 00:21:12,200 --> 00:21:15,390 Well, if we connect the four fluorines along with the 437 00:21:15,390 --> 00:21:18,500 sulfur, they lie in a plane, and if we were to pitch a tent 438 00:21:18,500 --> 00:21:21,940 here, we could pitch a tent to the upper fluorine, and we'd 439 00:21:21,940 --> 00:21:23,800 end up with four faces. 440 00:21:23,800 --> 00:21:25,270 So this is Greek. 441 00:21:25,270 --> 00:21:26,570 Eight faces. 442 00:21:26,570 --> 00:21:28,850 So there's four above and four below. 443 00:21:28,850 --> 00:21:30,870 I know it's sort of like a Jasper Johns painting. 444 00:21:30,870 --> 00:21:31,850 You say, octahedral. 445 00:21:31,850 --> 00:21:35,530 You hear octa, you're thinking eight and you're seeing six 446 00:21:35,530 --> 00:21:37,660 fluorines or seven. 447 00:21:37,660 --> 00:21:38,880 Anything but eight. 448 00:21:38,880 --> 00:21:41,740 So this is anything but eight, but it's eight faces. 449 00:21:41,740 --> 00:21:43,970 Last thing-- let's look at the energetics. 450 00:21:43,970 --> 00:21:46,380 To hybridize like this, we needed to do something 451 00:21:46,380 --> 00:21:52,690 different because we know that the base structure of sulfur 452 00:21:52,690 --> 00:21:54,240 would be 3s2 3p4. 453 00:21:57,450 --> 00:22:00,550 So this is 3s and this is 3p. 454 00:22:00,550 --> 00:22:06,440 So that's 3s2, and then 3p4 would be one, two, three-- 455 00:22:06,440 --> 00:22:06,930 whoops! 456 00:22:06,930 --> 00:22:09,210 I violated the polyexclusion there. 457 00:22:09,210 --> 00:22:09,610 OK. 458 00:22:09,610 --> 00:22:10,630 There we go. 459 00:22:10,630 --> 00:22:12,720 So I can't form six bonds with this. 460 00:22:12,720 --> 00:22:14,330 I only have two bonds. 461 00:22:14,330 --> 00:22:16,360 So you say, well, okay, let's make sp3. 462 00:22:16,360 --> 00:22:20,950 Well, if I make sp3, sp3 will give me this. 463 00:22:20,950 --> 00:22:23,310 So now I got one, two, three, four, and then I'm going to 464 00:22:23,310 --> 00:22:24,390 put five, six. 465 00:22:24,390 --> 00:22:26,040 I'm still down with two bonds. 466 00:22:26,040 --> 00:22:28,810 I need to have six unpaired electrons. 467 00:22:28,810 --> 00:22:31,590 So I've already consumed my s's and my p's. 468 00:22:31,590 --> 00:22:34,850 Where else can I go shopping for orbitals? 469 00:22:34,850 --> 00:22:36,710 I go to the orbital store and the orbital 470 00:22:36,710 --> 00:22:38,550 store is over here. 471 00:22:38,550 --> 00:22:40,800 Here's 3d. 472 00:22:40,800 --> 00:22:42,470 One, two, three, four. 473 00:22:42,470 --> 00:22:45,520 So I've got one, two, three, four, five d orbitals. 474 00:22:45,520 --> 00:22:48,420 So what I'm going to do is, I'm going to take the s, I'm 475 00:22:48,420 --> 00:22:51,880 going to take all three of the p and I'm going to lop off two 476 00:22:51,880 --> 00:22:53,400 of the d's. 477 00:22:53,400 --> 00:22:57,860 So now I'm going to have one, two, three, four, five, six 478 00:22:57,860 --> 00:23:04,160 and I'll leave three d's by themselves 479 00:23:04,160 --> 00:23:05,485 and here's now sp3d2. 480 00:23:09,420 --> 00:23:11,130 And now I've got six electrons-- 481 00:23:11,130 --> 00:23:15,190 one, two, three, four, five, six and now I have six equal 482 00:23:15,190 --> 00:23:18,300 energy states, which means I have to have six struts 483 00:23:18,300 --> 00:23:24,020 equally disposed in space and everything makes sense. 484 00:23:24,020 --> 00:23:25,060 All right. 485 00:23:25,060 --> 00:23:27,086 Is this polar or nonpolar? 486 00:23:27,086 --> 00:23:30,710 Is this molecule polar or nonpolar? 487 00:23:30,710 --> 00:23:32,800 Nonpolar, good. 488 00:23:32,800 --> 00:23:34,030 Let's look at it carefully. 489 00:23:34,030 --> 00:23:34,900 What do we have? 490 00:23:34,900 --> 00:23:37,230 I've got sulfur fluorine. 491 00:23:37,230 --> 00:23:40,030 Whenever you see fluorine, that's the biggest electron 492 00:23:40,030 --> 00:23:43,880 hog on the Periodic Table so you know you're going to have 493 00:23:43,880 --> 00:23:46,390 a dipole moment on the bond. 494 00:23:46,390 --> 00:23:48,690 But I have six such bonds and they're 495 00:23:48,690 --> 00:23:50,730 symmetrically disposed in space. 496 00:23:50,730 --> 00:23:53,970 All of the fluorines lie on the surface of a sphere so the 497 00:23:53,970 --> 00:23:58,130 center of local negative charge is right on top of the 498 00:23:58,130 --> 00:24:01,110 sulfur nucleus, which is where the center of locally 499 00:24:01,110 --> 00:24:02,540 positive charge is. 500 00:24:02,540 --> 00:24:07,730 No electron displacement, no net dipole moment. 501 00:24:07,730 --> 00:24:08,770 OK. 502 00:24:08,770 --> 00:24:10,150 This is working. 503 00:24:10,150 --> 00:24:11,640 Let's try another one. 504 00:24:11,640 --> 00:24:11,940 Let's see. 505 00:24:11,940 --> 00:24:12,520 What do I have here? 506 00:24:12,520 --> 00:24:15,450 I got bromide pentafluoride. 507 00:24:18,520 --> 00:24:19,950 So start-- 508 00:24:19,950 --> 00:24:24,140 let's go back to the VSPER rules. 509 00:24:24,140 --> 00:24:25,630 There we go. 510 00:24:25,630 --> 00:24:25,900 OK. 511 00:24:25,900 --> 00:24:26,760 VSPER rules. 512 00:24:26,760 --> 00:24:29,950 So I put bromine in the center and it's got seven valence 513 00:24:29,950 --> 00:24:30,760 electrons-- 514 00:24:30,760 --> 00:24:35,060 one, two, three, four, five, six-- 515 00:24:35,060 --> 00:24:37,230 and I'll put the seventh up here. 516 00:24:37,230 --> 00:24:38,660 And I'm going to bring in the fluorines-- 517 00:24:38,660 --> 00:24:44,780 one, two, three, four, five and just as before, there's 518 00:24:44,780 --> 00:24:48,890 one, two, three, four, five, six, seven. 519 00:24:48,890 --> 00:24:53,020 So you get a bond with each fluorine. 520 00:24:53,020 --> 00:24:54,270 A bond with each fluorine. 521 00:24:59,430 --> 00:25:00,180 All right. 522 00:25:00,180 --> 00:25:03,410 So what do we have now? 523 00:25:03,410 --> 00:25:05,220 How many electron domains? 524 00:25:05,220 --> 00:25:08,360 One, two, three, four, five, six. 525 00:25:08,360 --> 00:25:12,450 Six electron domains as before, but one of them's 526 00:25:12,450 --> 00:25:13,870 nonbonding. 527 00:25:13,870 --> 00:25:16,650 I get to use the colored chalk again. 528 00:25:16,650 --> 00:25:16,960 All right. 529 00:25:16,960 --> 00:25:22,280 So this is one nonbonding domain and 530 00:25:22,280 --> 00:25:24,870 five bonding domains. 531 00:25:24,870 --> 00:25:27,910 One, two, three, four, five. 532 00:25:27,910 --> 00:25:31,110 One bond to each of the fluorines, five bonding, and 533 00:25:31,110 --> 00:25:36,370 the total is six electron domains, so that means 534 00:25:36,370 --> 00:25:39,020 octahedral structure. 535 00:25:39,020 --> 00:25:40,850 The total number of electron domains 536 00:25:40,850 --> 00:25:42,270 dictates skeletal structure. 537 00:25:42,270 --> 00:25:45,050 So man said put six. 538 00:25:45,050 --> 00:25:47,120 So I put the bromine here, and I go, one, two, 539 00:25:47,120 --> 00:25:49,550 three, four, five, six. 540 00:25:49,550 --> 00:25:51,140 That's the skeletal structure. 541 00:25:51,140 --> 00:25:53,620 Those are the bonds and space. 542 00:25:53,620 --> 00:25:56,530 Now I know one of those is a nonbonding domain. 543 00:25:56,530 --> 00:25:58,330 So where do I put it? 544 00:25:58,330 --> 00:25:59,390 Well, they're all equivalent. 545 00:25:59,390 --> 00:26:00,510 It doesn't make any difference. 546 00:26:00,510 --> 00:26:03,260 Nobody knows where the real x is in this world. 547 00:26:03,260 --> 00:26:07,110 So just for grins and chuckles, I'll make-- 548 00:26:07,110 --> 00:26:10,460 I'll put the nonbonding pair down here. 549 00:26:10,460 --> 00:26:14,590 And then I put the fluorines at the other struts. 550 00:26:14,590 --> 00:26:16,830 And so now what's the shape of the molecule? 551 00:26:16,830 --> 00:26:18,650 You don't see these electrons. 552 00:26:18,650 --> 00:26:21,530 And in fact, this is only done by chemistry professors, the 553 00:26:21,530 --> 00:26:22,510 other ones, not me. 554 00:26:22,510 --> 00:26:25,180 I only do this to let you know what the books are doing 555 00:26:25,180 --> 00:26:27,340 because we all know that those nonbonding 556 00:26:27,340 --> 00:26:29,550 electrons are in here. 557 00:26:29,550 --> 00:26:31,990 They're right up tucked against the bromine. 558 00:26:31,990 --> 00:26:34,260 So if you walked in the room, what do you see? 559 00:26:34,260 --> 00:26:37,760 The bromine and the four fluorines lie in a plane, and 560 00:26:37,760 --> 00:26:40,500 then you've got this thing sticking up. 561 00:26:40,500 --> 00:26:44,600 So the molecule is a pyramid with a square base. 562 00:26:44,600 --> 00:26:46,130 So it's called square pyramidal. 563 00:26:53,270 --> 00:26:57,700 And how about polar or nonpolar? 564 00:26:57,700 --> 00:26:59,270 Is there a net dipole moment? 565 00:26:59,270 --> 00:27:01,310 Yes or no? 566 00:27:01,310 --> 00:27:05,070 Yes, because we got the-- in the plane with the bromine, 567 00:27:05,070 --> 00:27:06,430 everything is centered, right? 568 00:27:06,430 --> 00:27:08,360 The fluorines pull out, but they all pull out 569 00:27:08,360 --> 00:27:09,900 symmetrically, but then there's the 570 00:27:09,900 --> 00:27:11,250 fluorine pulling up. 571 00:27:11,250 --> 00:27:13,700 So this is polar. 572 00:27:13,700 --> 00:27:19,870 This is polar and it's got a dipole setup like this. 573 00:27:19,870 --> 00:27:21,180 Good. 574 00:27:21,180 --> 00:27:21,760 All right. 575 00:27:21,760 --> 00:27:23,450 Let's do one more. 576 00:27:23,450 --> 00:27:24,490 I'm having so much fun. 577 00:27:24,490 --> 00:27:27,190 I'm going to do as many as I can until I run out of time. 578 00:27:27,190 --> 00:27:29,745 This is the best part because colored chalk. 579 00:27:29,745 --> 00:27:30,990 That's the end. 580 00:27:30,990 --> 00:27:31,290 All right. 581 00:27:31,290 --> 00:27:32,380 Let's do this one. 582 00:27:32,380 --> 00:27:34,670 Tetrafluoroiodate. 583 00:27:34,670 --> 00:27:37,190 IF4 minus. 584 00:27:37,190 --> 00:27:39,750 This is cool because it's got a net charge, but it's a 585 00:27:39,750 --> 00:27:43,630 covalent anion. 586 00:27:43,630 --> 00:27:44,600 So same thing. 587 00:27:44,600 --> 00:27:45,740 Lewis structure. 588 00:27:45,740 --> 00:27:49,390 So we're going to put iodine first. Iodine and it's just 589 00:27:49,390 --> 00:27:50,210 like bromine. 590 00:27:50,210 --> 00:27:55,530 It's got one, two, three, four, five, six-- 591 00:27:55,530 --> 00:27:58,670 I'm going to put a seventh electron here because I'm 592 00:27:58,670 --> 00:28:00,010 anticipating. 593 00:28:00,010 --> 00:28:01,130 Just to mix things up. 594 00:28:01,130 --> 00:28:03,380 See, last time I put the seventh electron up. 595 00:28:03,380 --> 00:28:06,440 I could put the seventh electron down, but it's got a 596 00:28:06,440 --> 00:28:07,620 net charge. 597 00:28:07,620 --> 00:28:10,070 How'd it get the net charge? 598 00:28:10,070 --> 00:28:11,990 By accepting an electron. 599 00:28:11,990 --> 00:28:14,140 So that electron's got to be here, too. 600 00:28:14,140 --> 00:28:16,350 So I'm going to indicates that that extra 601 00:28:16,350 --> 00:28:18,580 electron's here as well. 602 00:28:18,580 --> 00:28:22,200 So iodine's got eight already. 603 00:28:22,200 --> 00:28:23,365 So we're off to a roaring start. 604 00:28:23,365 --> 00:28:27,300 It's got eight and it hasn't even started bonding yet. 605 00:28:27,300 --> 00:28:28,720 So we know where we're going here. 606 00:28:28,720 --> 00:28:31,220 We're going to expanded octet land. 607 00:28:31,220 --> 00:28:31,490 All right. 608 00:28:31,490 --> 00:28:32,740 So we need four more fluorine. 609 00:28:32,740 --> 00:28:34,480 I'm going to put four fluorines-- 610 00:28:34,480 --> 00:28:37,700 one, two, three, four-- 611 00:28:37,700 --> 00:28:51,520 and as before, so we've got bonds. 612 00:28:51,520 --> 00:28:55,180 One, two, three, four. 613 00:28:55,180 --> 00:28:58,830 And we've still got some other orbitals lying next to the 614 00:28:58,830 --> 00:29:01,510 iodine and they bond to nothing. 615 00:29:01,510 --> 00:29:07,390 So we have two nonbonding, and we have four bonding for a 616 00:29:07,390 --> 00:29:10,960 total of six electron domains. 617 00:29:10,960 --> 00:29:12,925 Again, octahedral structure. 618 00:29:15,580 --> 00:29:16,830 Octahedral skeleton. 619 00:29:20,960 --> 00:29:22,780 So I'm going to put iodine here. 620 00:29:22,780 --> 00:29:26,810 One, two, three, four, five, six. 621 00:29:26,810 --> 00:29:29,250 All right. 622 00:29:29,250 --> 00:29:31,510 Now comes the part-- see, we said-- 623 00:29:31,510 --> 00:29:34,160 this VSPER consists of valence-shell. 624 00:29:34,160 --> 00:29:37,050 I've been doing a lot of valence-shell thought here, 625 00:29:37,050 --> 00:29:39,670 but I haven't said anything about electron-pair repulsion. 626 00:29:39,670 --> 00:29:41,450 Now I'm going to show you how electron-pair 627 00:29:41,450 --> 00:29:43,120 repulsion comes into play. 628 00:29:43,120 --> 00:29:46,020 So I can put four fluorines around here in 629 00:29:46,020 --> 00:29:47,460 two different ways. 630 00:29:47,460 --> 00:29:52,290 One way is to put the four fluorines into play 631 00:29:52,290 --> 00:29:55,000 and then I have the-- 632 00:29:55,000 --> 00:29:57,000 these are called lone pairs. 633 00:29:57,000 --> 00:29:57,850 Why are they lone? 634 00:29:57,850 --> 00:30:00,030 Because they don't bond to a second. 635 00:30:00,030 --> 00:30:04,040 So sometimes chemists call these lone pairs. 636 00:30:04,040 --> 00:30:05,490 They're all alone. 637 00:30:05,490 --> 00:30:08,280 It's Friday and all alone. 638 00:30:08,280 --> 00:30:09,920 So here they are. 639 00:30:09,920 --> 00:30:12,830 So that's one way to put them, but there's another way to put 640 00:30:12,830 --> 00:30:15,660 them, and that's this one here. 641 00:30:15,660 --> 00:30:18,700 I'm going to go way over to the other side, and 642 00:30:18,700 --> 00:30:20,290 here's the other way. 643 00:30:20,290 --> 00:30:24,660 The other way to put them is to put the pairs in the same 644 00:30:24,660 --> 00:30:30,480 plane next to each other, because if I put the pairs 645 00:30:30,480 --> 00:30:32,000 opposite each other, that's equivalent. 646 00:30:32,000 --> 00:30:32,710 It doesn't matter. 647 00:30:32,710 --> 00:30:35,430 It just-- whether on the x-axis or y-axis. 648 00:30:35,430 --> 00:30:39,910 So if you think about this as a globe, you think about the 649 00:30:39,910 --> 00:30:46,250 molecule as a globe, so in this case over there, I put 650 00:30:46,250 --> 00:30:49,690 all the fluorines on the equatorial position. 651 00:30:49,690 --> 00:30:54,180 You can think of this plane at the waistline as the equator. 652 00:30:54,180 --> 00:30:57,710 So these are all at the equatorial position, and the 653 00:30:57,710 --> 00:31:00,900 electron pairs are at the poles. 654 00:31:00,900 --> 00:31:03,540 So the lone pairs are at polar positions. 655 00:31:03,540 --> 00:31:06,420 In this case, the lone pairs are lying 656 00:31:06,420 --> 00:31:08,790 in equatorial positions. 657 00:31:08,790 --> 00:31:11,110 Now what do you know about electrons when 658 00:31:11,110 --> 00:31:13,400 they get close together? 659 00:31:13,400 --> 00:31:14,310 They repel. 660 00:31:14,310 --> 00:31:16,190 And it's a really strong repulsion. 661 00:31:16,190 --> 00:31:17,780 That's the Bourne exponent. 662 00:31:17,780 --> 00:31:21,230 Electron-pair repulsion is really strong and it'll jack 663 00:31:21,230 --> 00:31:22,620 the energy way up. 664 00:31:22,620 --> 00:31:26,110 So if you've got a choice of putting electrons here close 665 00:31:26,110 --> 00:31:30,000 together or here farther apart, which one is going to 666 00:31:30,000 --> 00:31:32,160 be the lower energy condition? 667 00:31:32,160 --> 00:31:35,810 This, because this minimizes electron-pair repulsion, and 668 00:31:35,810 --> 00:31:38,020 in fact, this is the one that's favored. 669 00:31:38,020 --> 00:31:43,010 So by using valence-shell thought, bonding, nonbonding 670 00:31:43,010 --> 00:31:45,770 skeletal structure, and then finally electron-pair 671 00:31:45,770 --> 00:31:48,410 repulsion, you conclude that this is the correct form, and 672 00:31:48,410 --> 00:31:51,210 so you don't see these electron pairs because we know 673 00:31:51,210 --> 00:31:53,080 they're really tucked in tight here. 674 00:31:53,080 --> 00:31:55,080 What do you see if you do a spectroscopic 675 00:31:55,080 --> 00:31:56,270 analysis of this thing? 676 00:31:56,270 --> 00:31:58,260 All five atoms lie in a plane. 677 00:31:58,260 --> 00:32:01,610 So this molecule is planar. 678 00:32:01,610 --> 00:32:05,790 It's planar, just as the sp2 ethylene was planar, but this 679 00:32:05,790 --> 00:32:08,980 is planar for a different reason. 680 00:32:08,980 --> 00:32:10,230 Is this polar or nonpolar? 681 00:32:13,680 --> 00:32:15,380 See, it's negative. 682 00:32:15,380 --> 00:32:16,450 It's not negative. 683 00:32:16,450 --> 00:32:19,130 If I were a cation, I'd be drawn in like this. 684 00:32:19,130 --> 00:32:21,580 So is it polar or nonpolar? 685 00:32:21,580 --> 00:32:23,590 Where's the net displacement? 686 00:32:23,590 --> 00:32:26,260 The fluorines are pulling away from the iodine, but they're 687 00:32:26,260 --> 00:32:27,840 pulling away symmetrically. 688 00:32:27,840 --> 00:32:28,960 So there's no net dipole moment. 689 00:32:28,960 --> 00:32:31,000 It's cool, huh? 690 00:32:31,000 --> 00:32:34,650 No net dipole moment, but net negative charge. 691 00:32:34,650 --> 00:32:35,560 I love it. 692 00:32:35,560 --> 00:32:37,160 It just-- 693 00:32:37,160 --> 00:32:38,340 it's the best. OK. 694 00:32:38,340 --> 00:32:41,610 So now let's look at some others. 695 00:32:41,610 --> 00:32:43,480 We've looked at CF4. 696 00:32:43,480 --> 00:32:45,900 We know CF4. 697 00:32:45,900 --> 00:32:48,640 CF4, you could do this one in your head. 698 00:32:48,640 --> 00:32:50,480 This would be just like methane, right? 699 00:32:50,480 --> 00:32:52,690 This would be tetrahedral. 700 00:32:52,690 --> 00:32:53,790 I'm not even going to write that out. 701 00:32:53,790 --> 00:32:58,460 This tetrahedral and it would be sp3 hybridization and we 702 00:32:58,460 --> 00:33:00,990 just did IF4 minus. 703 00:33:00,990 --> 00:33:02,560 See the stoichiometry. 704 00:33:02,560 --> 00:33:04,420 It's four fluorines. 705 00:33:04,420 --> 00:33:07,510 Four fluorines with something and it's not the same 706 00:33:07,510 --> 00:33:08,260 architecture. 707 00:33:08,260 --> 00:33:10,610 So stoichiometry doesn't dictate molecular 708 00:33:10,610 --> 00:33:12,180 architecture. 709 00:33:12,180 --> 00:33:13,460 Very important. 710 00:33:13,460 --> 00:33:14,920 You can say, well, it's a net charge. 711 00:33:14,920 --> 00:33:17,590 A net charge doesn't do-- it's this. 712 00:33:17,590 --> 00:33:18,840 This is square planar. 713 00:33:23,200 --> 00:33:24,290 Yeah, so I'm going to add that. 714 00:33:24,290 --> 00:33:26,710 It's not only planar, it's planar and it's a square, 715 00:33:26,710 --> 00:33:28,980 whereas the other one is trigonal planar. 716 00:33:28,980 --> 00:33:32,440 So this is square planar. 717 00:33:32,440 --> 00:33:36,160 So I'm going to do another F4, just to show you, make the 718 00:33:36,160 --> 00:33:38,640 point that this thing doesn't-- 719 00:33:38,640 --> 00:33:41,010 so I'm going to do sulfur hexafluoride. 720 00:33:41,010 --> 00:33:44,480 You say, gee, sulfur hexafluoride. 721 00:33:44,480 --> 00:33:47,870 I bet it's just like carbon because carbon, sulfur-- 722 00:33:47,870 --> 00:33:49,070 no. 723 00:33:49,070 --> 00:33:51,230 Let's look carefully. 724 00:33:51,230 --> 00:33:55,720 So I'm going to put the sulfur here and we're going to put-- 725 00:33:55,720 --> 00:33:57,160 how many electrons? 726 00:33:57,160 --> 00:34:01,710 One, two, three, four, five, six. 727 00:34:01,710 --> 00:34:02,520 OK. 728 00:34:02,520 --> 00:34:07,450 And then we're going to bring in the fluorines, one, two, 729 00:34:07,450 --> 00:34:08,890 three, four. 730 00:34:08,890 --> 00:34:10,800 So each of the fluorines is going to form a bond. 731 00:34:10,800 --> 00:34:11,800 Do I have to do this? 732 00:34:11,800 --> 00:34:13,990 Can I just do this? 733 00:34:13,990 --> 00:34:14,880 Yeah, that's enough. 734 00:34:14,880 --> 00:34:17,230 I'm not-- 735 00:34:17,230 --> 00:34:18,430 I'm raising-- 736 00:34:18,430 --> 00:34:20,170 we're accelerating now. 737 00:34:20,170 --> 00:34:23,720 Now I'm going to stop talking and I'll just think about it 738 00:34:23,720 --> 00:34:24,970 and you'll get it. 739 00:34:27,300 --> 00:34:27,640 All right. 740 00:34:27,640 --> 00:34:28,650 So here we go. 741 00:34:28,650 --> 00:34:29,620 Bonding domains. 742 00:34:29,620 --> 00:34:31,980 One with the fluorine, two with the fluorines, three with 743 00:34:31,980 --> 00:34:33,630 the fluorine, four with the fluorine and there's two 744 00:34:33,630 --> 00:34:35,670 electrons, and they'll just sit together in 745 00:34:35,670 --> 00:34:37,540 a nonbonding domain. 746 00:34:37,540 --> 00:34:38,460 So how many domains? 747 00:34:38,460 --> 00:34:40,660 One, two, three, four, five. 748 00:34:40,660 --> 00:34:43,180 Five electron domains. 749 00:34:43,180 --> 00:34:49,380 Four bonding, one nonbonding It all adds up. 750 00:34:49,380 --> 00:34:50,040 Good. 751 00:34:50,040 --> 00:34:52,020 But now I got to put five struts. 752 00:34:52,020 --> 00:34:54,910 This gives me the skeletal structure. 753 00:34:54,910 --> 00:34:56,915 As before, skeletal structure. 754 00:34:59,960 --> 00:35:01,150 So I've got to-- 755 00:35:01,150 --> 00:35:03,790 how do I put five struts around something 756 00:35:03,790 --> 00:35:07,340 uniformly in space? 757 00:35:07,340 --> 00:35:11,760 One, two and I'll put three in the plane. 758 00:35:11,760 --> 00:35:12,790 Three in the plane. 759 00:35:12,790 --> 00:35:16,790 So three at the equator and two at the poles. 760 00:35:16,790 --> 00:35:18,076 That's the skeletal structure. 761 00:35:20,620 --> 00:35:23,900 This is called a trigonal bipyramid. 762 00:35:23,900 --> 00:35:27,570 Because I can make a pyramid above, right? 763 00:35:27,570 --> 00:35:31,280 I got a tent with three corners. 764 00:35:31,280 --> 00:35:36,270 So that's a pyramid with three faces. 765 00:35:36,270 --> 00:35:37,520 So that's called trigonal. 766 00:35:40,350 --> 00:35:41,100 It's trigonal-- 767 00:35:41,100 --> 00:35:41,680 three-- 768 00:35:41,680 --> 00:35:44,390 and then I've got it above and below. 769 00:35:44,390 --> 00:35:47,530 So that's trigonal bipyramid. 770 00:35:47,530 --> 00:35:49,312 See, you're going to learn all these new words. 771 00:35:49,312 --> 00:35:51,650 When you go out tonight, you're going to be able to 772 00:35:51,650 --> 00:35:56,900 impress people to the point they'll walk away from you. 773 00:35:56,900 --> 00:35:57,260 All right. 774 00:35:57,260 --> 00:36:02,623 So now let's start putting the fluorines-- 775 00:36:05,170 --> 00:36:06,440 again, we've got choices here. 776 00:36:06,440 --> 00:36:07,250 They're not equivalent. 777 00:36:07,250 --> 00:36:11,770 So one possibility is to put the lone pair at the equator 778 00:36:11,770 --> 00:36:16,400 and then the fluorines go here, and the other 779 00:36:16,400 --> 00:36:21,410 possibility is to put the-- 780 00:36:21,410 --> 00:36:21,860 let's see. 781 00:36:21,860 --> 00:36:23,680 I've got room up there. 782 00:36:23,680 --> 00:36:25,880 The other possibility is-- 783 00:36:25,880 --> 00:36:28,390 let's make the trigonal bipyramid-- 784 00:36:28,390 --> 00:36:32,770 is to put the lone pair at a polar position and put all the 785 00:36:32,770 --> 00:36:37,760 fluorines in the equatorial plane. 786 00:36:37,760 --> 00:36:39,760 And so now we have to think about this for a second and 787 00:36:39,760 --> 00:36:44,170 try to convince ourselves which situation has greater 788 00:36:44,170 --> 00:36:46,480 electron-pair repulsion. 789 00:36:46,480 --> 00:36:49,510 So the way to think about it is, in which position does 790 00:36:49,510 --> 00:36:53,430 that lone pair come into more contact than otherwise? 791 00:36:53,430 --> 00:36:54,760 The other thing to know is-- 792 00:36:54,760 --> 00:36:57,260 I didn't say this earlier, but it just occurred to me-- is 793 00:36:57,260 --> 00:37:00,200 that if you've got a choice of bonding electrons versus 794 00:37:00,200 --> 00:37:02,390 nonbonding electrons, nonbonding 795 00:37:02,390 --> 00:37:04,210 electrons want more room. 796 00:37:04,210 --> 00:37:05,140 Why? 797 00:37:05,140 --> 00:37:06,550 Because they're not tightly confined. 798 00:37:06,550 --> 00:37:09,460 When you get something in a bond, it's on axis between the 799 00:37:09,460 --> 00:37:13,330 two atoms, and so things are much more tightly confined. 800 00:37:13,330 --> 00:37:15,150 So these things want a lot of room. 801 00:37:15,150 --> 00:37:17,490 So if I'm sitting here, I interact with this fluorine 802 00:37:17,490 --> 00:37:18,420 and this fluorine-- 803 00:37:18,420 --> 00:37:20,510 these two are far away. 804 00:37:20,510 --> 00:37:23,700 If I'm here, I interact with three fluorines. 805 00:37:23,700 --> 00:37:28,420 So it turns out the equatorial position is favored. 806 00:37:28,420 --> 00:37:29,890 Give more space nonbonding-- 807 00:37:29,890 --> 00:37:32,600 place nonbonding domains at equatorial positions in 808 00:37:32,600 --> 00:37:34,390 trigonal bipyramid. 809 00:37:34,390 --> 00:37:35,640 That's what number five means. 810 00:37:39,330 --> 00:37:41,840 If somebody asked you to test the microphone, instead of 811 00:37:41,840 --> 00:37:44,630 saying number nine, number nine or one, two, three, say, 812 00:37:44,630 --> 00:37:47,910 place nonbonding domains at equatorial positions and a 813 00:37:47,910 --> 00:37:50,260 trigonal bipyramid. 814 00:37:50,260 --> 00:37:52,580 And they'll ask you to step away from the microphone. 815 00:37:52,580 --> 00:37:54,310 OK. 816 00:37:54,310 --> 00:37:58,000 So this structure here is, in fact, the 817 00:37:58,000 --> 00:37:58,850 shape of the molecule. 818 00:37:58,850 --> 00:38:00,230 You don't see this. 819 00:38:00,230 --> 00:38:05,260 And this molecule is called seesaw, like a teeter totter. 820 00:38:05,260 --> 00:38:06,800 You remember, when you were kids. 821 00:38:06,800 --> 00:38:07,990 You don't remember that. 822 00:38:07,990 --> 00:38:08,290 OK. 823 00:38:08,290 --> 00:38:11,680 So how about polar versus nonpolar? 824 00:38:14,890 --> 00:38:15,550 It's obvious. 825 00:38:15,550 --> 00:38:18,790 If it's asymmetric and you've got the fluorines and the 826 00:38:18,790 --> 00:38:22,260 non-pair, this is definitely polar. 827 00:38:22,260 --> 00:38:24,600 And where's the net dipole? 828 00:38:24,600 --> 00:38:27,340 Where's the negative side? 829 00:38:27,340 --> 00:38:30,800 Well, along this axis, it's pulled right to the center. 830 00:38:30,800 --> 00:38:33,570 So along this axis, clearly the fluorines are pulling so 831 00:38:33,570 --> 00:38:36,700 you have something that looks like this. 832 00:38:36,700 --> 00:38:38,830 And how about the energetics? 833 00:38:38,830 --> 00:38:40,140 How do we get five of these? 834 00:38:40,140 --> 00:38:43,350 If we've got five struts, it means we must have five 835 00:38:43,350 --> 00:38:46,690 hybridized orbitals of equal energy. 836 00:38:46,690 --> 00:38:51,230 Well, we got six before and before that, we had four. 837 00:38:51,230 --> 00:38:52,800 Before that, we had three. 838 00:38:52,800 --> 00:38:53,980 Well, now we're going to get five. 839 00:38:53,980 --> 00:38:57,230 So how do we make five orbitals? 840 00:38:57,230 --> 00:38:59,180 So in this case I start-- 841 00:38:59,180 --> 00:39:08,370 I want s, p, d, and I want five. 842 00:39:08,370 --> 00:39:12,190 So one plus three is four plus one is five and 843 00:39:12,190 --> 00:39:14,660 that gives me sp3d. 844 00:39:17,790 --> 00:39:22,560 Gives me five and then, what do we got? 845 00:39:22,560 --> 00:39:24,480 One, two, three. 846 00:39:24,480 --> 00:39:26,050 It's good. 847 00:39:26,050 --> 00:39:26,730 OK. 848 00:39:26,730 --> 00:39:29,210 Now-- 849 00:39:29,210 --> 00:39:30,750 so we've covered a lot. 850 00:39:30,750 --> 00:39:31,050 All right. 851 00:39:31,050 --> 00:39:33,330 So some parting comments. 852 00:39:33,330 --> 00:39:35,200 We've been sticking other things-- what we could do is 853 00:39:35,200 --> 00:39:39,230 we could do this one. 854 00:39:39,230 --> 00:39:41,670 We could take carbon and I'm going to go now sp3. 855 00:39:44,470 --> 00:39:46,330 only instead of putting hydrogens, I'm going to put 856 00:39:46,330 --> 00:39:48,040 carbons here. 857 00:39:48,040 --> 00:39:49,190 So what happens now? 858 00:39:49,190 --> 00:39:51,450 One, two, three, four. 859 00:39:51,450 --> 00:39:52,870 I'll put another carbon here. 860 00:39:52,870 --> 00:39:55,620 One, two, three, four, et cetera. 861 00:39:55,620 --> 00:40:03,530 So this is sp3, all carbon, and this is called diamond. 862 00:40:03,530 --> 00:40:06,180 This is called diamond sp3. 863 00:40:06,180 --> 00:40:08,930 So these are all sigma bonds, all sigma 864 00:40:08,930 --> 00:40:12,490 bonds everywhere, sp3. 865 00:40:12,490 --> 00:40:15,733 And these are very, very strong bonds and these are 866 00:40:15,733 --> 00:40:18,660 very tight bonds-- and one of the properties of diamond is 867 00:40:18,660 --> 00:40:24,400 that it has a very high refractive index 868 00:40:24,400 --> 00:40:25,690 because high energy-- 869 00:40:25,690 --> 00:40:27,790 and these are tight, tight bonds. 870 00:40:27,790 --> 00:40:28,975 So the electrons aren't easily excited. 871 00:40:28,975 --> 00:40:31,410 It's transparent, divisible light, which means that when 872 00:40:31,410 --> 00:40:33,880 light shines on this, there's no expectation, 873 00:40:33,880 --> 00:40:35,500 but there is bending. 874 00:40:35,500 --> 00:40:38,060 So it's very, very high refractive index and a high 875 00:40:38,060 --> 00:40:41,170 refractive index is what makes this such a troublesome 876 00:40:41,170 --> 00:40:43,270 compound because it concentrates light. 877 00:40:43,270 --> 00:40:46,530 So you can imagine there's somebody in a dimly lit cafe, 878 00:40:46,530 --> 00:40:49,690 and she's sitting over in the corner with a candle in front, 879 00:40:49,690 --> 00:40:52,130 and she's wearing a diamond stud, and it takes those few 880 00:40:52,130 --> 00:40:53,510 candellas of energy. 881 00:40:53,510 --> 00:40:56,030 It concentrates, it shoots it in the room, and some 882 00:40:56,030 --> 00:40:59,210 unsuspecting fellow gets hit in the eye, is attracted to 883 00:40:59,210 --> 00:41:01,610 the person wearing the earrings, and that's when the 884 00:41:01,610 --> 00:41:04,060 trouble begins. 885 00:41:04,060 --> 00:41:07,850 So this is very, very dangerous, very dangerous, and 886 00:41:07,850 --> 00:41:11,690 it's a consequence of these strong sigma bonds. 887 00:41:11,690 --> 00:41:13,770 But there's another way. 888 00:41:13,770 --> 00:41:16,260 There's another way that we can bond carbon. 889 00:41:16,260 --> 00:41:19,550 We can make sp2 hybrids. 890 00:41:19,550 --> 00:41:22,280 So if we make sp2 hybrids, that now looks like this. 891 00:41:22,280 --> 00:41:26,500 The carbon lies in the plane, 120 degrees, which means we're 892 00:41:26,500 --> 00:41:32,330 going to end up with something that looks like this, et 893 00:41:32,330 --> 00:41:33,650 cetera, et cetera. 894 00:41:33,650 --> 00:41:36,100 And this is graphite. 895 00:41:36,100 --> 00:41:38,150 This is graphite. 896 00:41:38,150 --> 00:41:41,400 And now we have a mix of sigma bonds in the plane that are 897 00:41:41,400 --> 00:41:42,090 very strong. 898 00:41:42,090 --> 00:41:45,280 In fact, they're every bit as strong as these bonds, but 899 00:41:45,280 --> 00:41:45,820 there's a fourth. 900 00:41:45,820 --> 00:41:47,370 I said, you always have to find four 901 00:41:47,370 --> 00:41:48,230 struts off the carbons. 902 00:41:48,230 --> 00:41:49,910 So where's the fourth strut? 903 00:41:49,910 --> 00:41:52,460 It's the pi bond that comes out of the board. 904 00:41:52,460 --> 00:41:55,710 So there's a pi bond here and a pi bond here and a pi bond 905 00:41:55,710 --> 00:41:57,680 here and what we're going to learn later is that these 906 00:41:57,680 --> 00:41:58,930 things are delocalized. 907 00:42:02,100 --> 00:42:06,000 Because of the spacing here, the electrons can form a bond 908 00:42:06,000 --> 00:42:09,150 here that then forms a bond here, but it can actually 909 00:42:09,150 --> 00:42:11,710 resonate in such a way that the electrons can move 910 00:42:11,710 --> 00:42:15,100 everywhere throughout the crystal of graphite and this 911 00:42:15,100 --> 00:42:18,860 is what gives graphite its electrical conductivity, 912 00:42:18,860 --> 00:42:20,750 whereas diamond is a strict insulator. 913 00:42:20,750 --> 00:42:24,370 So these are very strong bonds in the plane, but not between 914 00:42:24,370 --> 00:42:26,800 planes, which gives graphite its lubricity. 915 00:42:26,800 --> 00:42:29,430 It's a dry-lock lubricant and so on. 916 00:42:29,430 --> 00:42:30,170 OK. 917 00:42:30,170 --> 00:42:31,870 So this is very good. 918 00:42:31,870 --> 00:42:36,590 So we have a mix of sigma and pi bonds. 919 00:42:39,650 --> 00:42:45,860 And it's an absorber and it's dark. 920 00:42:45,860 --> 00:42:49,310 It's an absorber so it appears black, whereas this diamond is 921 00:42:49,310 --> 00:42:50,580 transparent to visible light. 922 00:42:54,650 --> 00:42:57,860 And if you look in your book, this is all categorized for 923 00:42:57,860 --> 00:43:00,420 you, everything that we've done here with VSPER. 924 00:43:00,420 --> 00:43:00,780 All right. 925 00:43:00,780 --> 00:43:02,210 Look at this. 926 00:43:02,210 --> 00:43:04,670 All nice big system. 927 00:43:04,670 --> 00:43:05,370 OK. 928 00:43:05,370 --> 00:43:08,940 So here's diamond, which I was trying to draw over here, four 929 00:43:08,940 --> 00:43:10,390 struts sp3 hybridized. 930 00:43:10,390 --> 00:43:14,560 this is graphite sp2, and these are pi bonds in between. 931 00:43:14,560 --> 00:43:16,010 OK. 932 00:43:16,010 --> 00:43:20,590 Now suppose this fellow asks this girl out, and things get 933 00:43:20,590 --> 00:43:21,940 serious and they get-- 934 00:43:21,940 --> 00:43:24,380 I'm going to use an adverb here-- really serious and he 935 00:43:24,380 --> 00:43:27,540 decides maybe he wants to make a commitment and he wants to 936 00:43:27,540 --> 00:43:31,350 buy her a ring, an engagement ring, and the custom is that 937 00:43:31,350 --> 00:43:34,180 the engagement ring has a stone in it. 938 00:43:34,180 --> 00:43:38,570 Now being an MIT student, he knows that he wants to have 939 00:43:38,570 --> 00:43:40,850 symbolism in that stone. 940 00:43:40,850 --> 00:43:45,540 Now the custom is to get diamond, but it turns out that 941 00:43:45,540 --> 00:43:49,030 if you look carefully, the stable form of carbon at room 942 00:43:49,030 --> 00:43:51,050 temperature and atmospheric pressure is not 943 00:43:51,050 --> 00:43:53,270 diamond, it's graphite. 944 00:43:53,270 --> 00:43:57,270 So if he wants to give his love this stone that 945 00:43:57,270 --> 00:44:02,390 symbolizes eternal commitment, wouldn't he choose the stable 946 00:44:02,390 --> 00:44:05,910 form and not the one that's metastable and doesn't 947 00:44:05,910 --> 00:44:08,380 represent the natural state at room temperature? 948 00:44:11,010 --> 00:44:12,000 That's the truth. 949 00:44:12,000 --> 00:44:13,740 That's the truth. 950 00:44:13,740 --> 00:44:15,120 Graphite is the stable form. 951 00:44:15,120 --> 00:44:18,180 I leave it up to the young man to persuade the young lady 952 00:44:18,180 --> 00:44:23,120 that the gemstone is going to have graphite 953 00:44:23,120 --> 00:44:25,730 in it and not diamond. 954 00:44:25,730 --> 00:44:26,210 OK. 955 00:44:26,210 --> 00:44:29,480 Now something very interesting happened. 956 00:44:29,480 --> 00:44:32,650 What I wanted to play for you as you're leaving today is an 957 00:44:32,650 --> 00:44:35,580 old Beatles song, Lucy in the Sky With Diamonds. 958 00:44:35,580 --> 00:44:36,950 And what happened-- 959 00:44:36,950 --> 00:44:39,810 before I get to it, what happened was-- and this is 960 00:44:39,810 --> 00:44:43,630 even in today's Le Tech if you look on the second page, "New 961 00:44:43,630 --> 00:44:47,840 Fossil Skeleton from africa Predates Lucy." It was just 962 00:44:47,840 --> 00:44:48,570 discovered. 963 00:44:48,570 --> 00:44:49,270 I'm not kidding you. 964 00:44:49,270 --> 00:44:50,500 This isn't a joke. 965 00:44:50,500 --> 00:44:52,340 I heard this on NPR last night. 966 00:44:52,340 --> 00:44:58,240 So it's a female skeleton of a pre-hominid, 4.4 million years 967 00:44:58,240 --> 00:45:00,980 old, which is a million years older than Lucy. 968 00:45:00,980 --> 00:45:05,880 Lucy was discovered in Ethiopia in 1974 and 969 00:45:05,880 --> 00:45:08,820 considered to be 3.18 million years old. 970 00:45:08,820 --> 00:45:12,360 This new one is 4.4 million years old, about four feet 971 00:45:12,360 --> 00:45:18,070 tall, and it just turns the clock even farther back. 972 00:45:18,070 --> 00:45:20,240 The interesting thing is the name. 973 00:45:20,240 --> 00:45:24,290 How did the skeleton discovered in 1974 974 00:45:24,290 --> 00:45:25,920 get the name Lucy? 975 00:45:25,920 --> 00:45:27,050 Interesting story. 976 00:45:27,050 --> 00:45:29,670 There were two American archaeologists that were head 977 00:45:29,670 --> 00:45:34,650 of the team that did the dig, and they were so excited the 978 00:45:34,650 --> 00:45:38,330 night they found this pre-hominid skeleton. 979 00:45:38,330 --> 00:45:41,890 They knew it was a female, and they hadn't dated it yet, but 980 00:45:41,890 --> 00:45:45,440 they knew that it was roughly way, way back, but it was 981 00:45:45,440 --> 00:45:46,630 definitely female. 982 00:45:46,630 --> 00:45:49,740 So they were back at the campfire, kicking back, 983 00:45:49,740 --> 00:45:54,470 drinking, probably heavily, and singing songs, and one of 984 00:45:54,470 --> 00:45:56,950 the songs they were singing was Lucy in the Sky With 985 00:45:56,950 --> 00:46:02,380 Diamonds and that night, they named the skeleton Lucy. 986 00:46:02,380 --> 00:46:04,100 But the story doesn't end there. 987 00:46:04,100 --> 00:46:07,680 Where did this Lucy in the Sky With Diamonds come from? 988 00:46:07,680 --> 00:46:09,800 John Lennon, one of the-- 989 00:46:09,800 --> 00:46:11,800 you may have heard of this group or not, but there was 990 00:46:11,800 --> 00:46:13,130 this group called The Beatles. 991 00:46:13,130 --> 00:46:16,940 So one of the lead composers was John Lennon. 992 00:46:16,940 --> 00:46:20,980 His six-year-old son came home from school one day with a 993 00:46:20,980 --> 00:46:24,290 piece of artwork, and the father asks, what is this? 994 00:46:24,290 --> 00:46:30,600 He says, it's a painting of my classmate Lucy, and it was 995 00:46:30,600 --> 00:46:32,390 Lucy and he had diamonds in the sky. 996 00:46:32,390 --> 00:46:34,510 So it was Lucy in the Sky with Diamonds, and then they wrote 997 00:46:34,510 --> 00:46:36,360 the song about it. 998 00:46:36,360 --> 00:46:38,660 The real Lucy just died this week. 999 00:46:38,660 --> 00:46:39,450 She had Lupus. 1000 00:46:39,450 --> 00:46:41,700 She died at the age of 46. 1001 00:46:41,700 --> 00:46:46,000 And all of these things have come together just because we 1002 00:46:46,000 --> 00:46:51,210 had this lesson today about covalent bonding, which took 1003 00:46:51,210 --> 00:46:54,990 us to this, which then took us to Ethiopia, which then takes 1004 00:46:54,990 --> 00:46:56,390 this up here. 1005 00:46:56,390 --> 00:46:58,910 and so with that, I think we'll go for an early 1006 00:46:58,910 --> 00:47:02,470 dismissal, and I'll wish you a pleasant weekend.