1 00:00:00,030 --> 00:00:02,400 SPEAKER: The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,810 Commons License. 3 00:00:03,810 --> 00:00:06,840 Your support will help MIT OpenCourseWare continue to 4 00:00:06,840 --> 00:00:10,510 offer high quality educational resources for free. 5 00:00:10,510 --> 00:00:13,390 To make a donation or view additional materials from 6 00:00:13,390 --> 00:00:17,340 hundreds of MIT courses visit MIT OpenCourseWare at 7 00:00:17,340 --> 00:00:18,590 ocw.mit.edu. 8 00:00:20,696 --> 00:00:22,670 PROFESSOR: This Wednesday will be the first 9 00:00:22,670 --> 00:00:24,470 celebration of learning. 10 00:00:24,470 --> 00:00:28,830 Test 1 on Wednesday, October 7, you will write during the 11 00:00:28,830 --> 00:00:29,930 normal class time. 12 00:00:29,930 --> 00:00:31,490 So you'll have 50 minutes. 13 00:00:31,490 --> 00:00:37,210 And we want to have a little bit of comfort here, so you 14 00:00:37,210 --> 00:00:38,560 won't be sitting cheek to jowl. 15 00:00:38,560 --> 00:00:42,200 So before long I'll have the room assignment. 16 00:00:42,200 --> 00:00:44,520 So some of you will write in here. 17 00:00:44,520 --> 00:00:48,020 We'll have fewer people than seats so that there'll be 18 00:00:48,020 --> 00:00:50,180 vacancies next to each person. 19 00:00:50,180 --> 00:00:53,270 And then some will write in a few of the other locations, 20 00:00:53,270 --> 00:01:00,120 probably 26-100 and the gym above the Walker Memorial. 21 00:01:00,120 --> 00:01:01,730 And we'll get that out to you. 22 00:01:01,730 --> 00:01:05,600 And next week on the 6th we will have no weekly quiz, 23 00:01:05,600 --> 00:01:08,670 because enough celebrating. 24 00:01:08,670 --> 00:01:11,190 No point in testing you on the 6th and then on the 7th. 25 00:01:11,190 --> 00:01:14,020 There will be, of course, the mini-celebration 26 00:01:14,020 --> 00:01:17,400 tomorrow, quiz 3. 27 00:01:17,400 --> 00:01:19,790 And I'll be available for office hours later today. 28 00:01:23,650 --> 00:01:26,970 Oh, and the coverage, just to remove the mystery, will be 29 00:01:26,970 --> 00:01:30,930 right up to the 7th of October. 30 00:01:30,930 --> 00:01:34,540 I've been doing this for over 30 years and I've learned that 31 00:01:34,540 --> 00:01:38,340 in order to inspire interest on the part of the student it 32 00:01:38,340 --> 00:01:42,420 really pays to have the coverage of the celebration 33 00:01:42,420 --> 00:01:45,710 extend up to the lecture before the celebration. 34 00:01:45,710 --> 00:01:48,120 Now obviously I'm not going to drill deep on something I 35 00:01:48,120 --> 00:01:53,130 taught you on October 5, but it would be a good idea to 36 00:01:53,130 --> 00:01:57,810 stay awake during all of the lectures between now and then. 37 00:01:57,810 --> 00:02:00,200 So last day we talked about ionic bonding. 38 00:02:00,200 --> 00:02:03,950 And ionic bonding occurs with electrostatic attraction 39 00:02:03,950 --> 00:02:07,080 between ions that have formed through electron transfer. 40 00:02:07,080 --> 00:02:10,990 And we saw the energy of the ion pair given by this 41 00:02:10,990 --> 00:02:14,010 formula, where we have Coulomb's law 42 00:02:14,010 --> 00:02:15,770 with the Born exponent. 43 00:02:15,770 --> 00:02:17,540 And then this is plotted. 44 00:02:17,540 --> 00:02:22,570 This is E at r equals r0, and we learned that there were two 45 00:02:22,570 --> 00:02:26,410 terms. The attractive term, which is the Coulombic force 46 00:02:26,410 --> 00:02:28,260 here shown. 47 00:02:28,260 --> 00:02:30,860 And then there's a repulsive term, which results from 48 00:02:30,860 --> 00:02:34,640 electron-electron interaction when the two lines get very, 49 00:02:34,640 --> 00:02:35,950 very close together. 50 00:02:35,950 --> 00:02:37,025 And this is taken from your text. 51 00:02:37,025 --> 00:02:40,130 And I think they did a very nice job here of illustrating 52 00:02:40,130 --> 00:02:43,470 as you go to high values of r they're depicting that you 53 00:02:43,470 --> 00:02:46,590 have the ions separated by considerable distance. 54 00:02:46,590 --> 00:02:48,030 And there's a certain amount of stored 55 00:02:48,030 --> 00:02:49,770 energy, but not a lot. 56 00:02:49,770 --> 00:02:54,470 And then if you go much, much closer than the hard sphere 57 00:02:54,470 --> 00:02:58,600 sum of the ionic radii, I think they're depicting here 58 00:02:58,600 --> 00:03:02,210 that there's some squashing of the electron clouds. 59 00:03:02,210 --> 00:03:04,520 And you can see that the energy has gone way, way up. 60 00:03:04,520 --> 00:03:07,980 So this is an unfavorable situation, meaning that the 61 00:03:07,980 --> 00:03:10,340 energy here is greater than 0. 62 00:03:10,340 --> 00:03:15,880 And there's a sweet spot here at 236 picometers, which 63 00:03:15,880 --> 00:03:19,530 represents the ideal location. 64 00:03:19,530 --> 00:03:25,590 And that is the sum of the radius of the sodium ion and 65 00:03:25,590 --> 00:03:27,100 the radius of the chloride ion. 66 00:03:27,100 --> 00:03:28,780 And so you can see how energy tracks. 67 00:03:28,780 --> 00:03:31,390 And if you go far, far, far away to the point where 68 00:03:31,390 --> 00:03:32,580 they're at infinite separation, 69 00:03:32,580 --> 00:03:33,630 there's no energy stored. 70 00:03:33,630 --> 00:03:35,830 So everything makes sense. 71 00:03:35,830 --> 00:03:38,110 And then we said, well what happens if we keep packing 72 00:03:38,110 --> 00:03:39,510 these things? 73 00:03:39,510 --> 00:03:43,530 We rationalized that they would continue to do so and 74 00:03:43,530 --> 00:03:45,760 ultimately form a 3-dimensional crystal. 75 00:03:45,760 --> 00:03:47,890 And so you can see there's a lot of similarity between 76 00:03:47,890 --> 00:03:49,650 what's above and what's down here. 77 00:03:49,650 --> 00:03:52,260 This has been written for a 1-1 system. 78 00:03:52,260 --> 00:03:56,000 In other words, a cation plus 1 and an anion minus 1. 79 00:03:56,000 --> 00:03:58,670 But it could be mediated by the valences. 80 00:03:58,670 --> 00:04:02,820 And what we have here is the structure factor. 81 00:04:02,820 --> 00:04:10,160 This Madelung constant tells us how much we get decrease in 82 00:04:10,160 --> 00:04:12,400 the energy of the system by going to a 83 00:04:12,400 --> 00:04:13,890 3-dimensional array. 84 00:04:13,890 --> 00:04:16,650 So depending on the atomic arrangement we'll have a 85 00:04:16,650 --> 00:04:18,170 different value of Madelung constant. 86 00:04:18,170 --> 00:04:22,540 We saw that for sodium chloride the value is 1.7476. 87 00:04:22,540 --> 00:04:24,020 And different crystals have different things. 88 00:04:24,020 --> 00:04:27,000 And what determines the crystal structure? 89 00:04:27,000 --> 00:04:29,430 It's a combination of the size of the two 90 00:04:29,430 --> 00:04:32,070 ions and their valence. 91 00:04:32,070 --> 00:04:35,730 So what we saw for sodium chloride, this 92 00:04:35,730 --> 00:04:37,020 is a structure type. 93 00:04:37,020 --> 00:04:41,790 So obviously sodium chloride is sodium chloride crystal 94 00:04:41,790 --> 00:04:47,900 structure, but there is an entire suite of compounds that 95 00:04:47,900 --> 00:04:53,080 have radius ratios and charges that end up with a sodium 96 00:04:53,080 --> 00:04:55,510 chloride type crystal structure. 97 00:04:55,510 --> 00:04:58,660 And then towards the end we started looking at the 98 00:04:58,660 --> 00:05:00,130 Born-Haber Cycle. 99 00:05:00,130 --> 00:05:02,540 And the purpose of the Born-Haber Cycle was to give 100 00:05:02,540 --> 00:05:07,830 us a sense of scale of the various constituents in the 101 00:05:07,830 --> 00:05:09,080 formation of a crystal. 102 00:05:09,080 --> 00:05:11,520 And what we noted, the takeaway message from the 103 00:05:11,520 --> 00:05:15,810 Born-Haber Cycle, is that this enthalpy of crystallization, 104 00:05:15,810 --> 00:05:20,610 which is basically this term here, is huge. 105 00:05:20,610 --> 00:05:21,270 It's huge. 106 00:05:21,270 --> 00:05:24,610 It was the big component of the energy required to form 107 00:05:24,610 --> 00:05:25,330 the crystal. 108 00:05:25,330 --> 00:05:27,960 It's large and it is negative. 109 00:05:27,960 --> 00:05:32,780 So what I want to do today is start by talking about 110 00:05:32,780 --> 00:05:36,880 shortcomings of the business of ionic bonding. 111 00:05:36,880 --> 00:05:38,630 See, how did we get to ionic bonding? 112 00:05:38,630 --> 00:05:42,390 We started with this idea of octet stability. 113 00:05:42,390 --> 00:05:44,890 Octet stability was the driving idea 114 00:05:44,890 --> 00:05:46,230 behind all of this. 115 00:05:46,230 --> 00:05:53,030 Octet stability, and in the case of ionic bonding this was 116 00:05:53,030 --> 00:05:56,910 via electron transfer. 117 00:05:56,910 --> 00:06:02,300 And so that got us a long way, but it has its limitations. 118 00:06:02,300 --> 00:06:04,920 So let's put up some new data. 119 00:06:04,920 --> 00:06:12,340 So suppose I look at compounds like H2, N2, O2. 120 00:06:12,340 --> 00:06:14,930 Do these things form ionic bonds? 121 00:06:14,930 --> 00:06:18,090 How does octet stability play here? 122 00:06:18,090 --> 00:06:21,120 And so let's start by looking at hydrogen. 123 00:06:21,120 --> 00:06:26,130 So if we took hydrogen and started with atomic hydrogen 124 00:06:26,130 --> 00:06:30,590 and added an electron to it, then we would form an anion 125 00:06:30,590 --> 00:06:32,870 known as H minus. 126 00:06:32,870 --> 00:06:36,490 And H minus looks pretty good because it's isoelectronic 127 00:06:36,490 --> 00:06:37,730 with helium. 128 00:06:37,730 --> 00:06:41,180 So maybe this isn't going to be so bad a day. 129 00:06:41,180 --> 00:06:44,150 But if we're going to have a bond then we need 130 00:06:44,150 --> 00:06:45,930 to form an H plus. 131 00:06:45,930 --> 00:06:47,030 So let's do that. 132 00:06:47,030 --> 00:06:53,500 So that would be, then, H goes to H plus, plus an electron. 133 00:06:53,500 --> 00:06:58,700 And that's really nothing more than a proton. 134 00:06:58,700 --> 00:07:00,250 So that doesn't look too appealing. 135 00:07:00,250 --> 00:07:01,820 That's probably a high energy state. 136 00:07:01,820 --> 00:07:06,140 And besides, in the same location at the same time-- 137 00:07:06,140 --> 00:07:09,630 in other words, same temperature, same conditions-- 138 00:07:09,630 --> 00:07:13,480 half of the hydrogens have to acquire electrons and half of 139 00:07:13,480 --> 00:07:15,830 the hydrogens have to lose electrons. 140 00:07:15,830 --> 00:07:17,340 And that's not going to happen. 141 00:07:17,340 --> 00:07:19,840 They're either going to have a propensity for electron gain 142 00:07:19,840 --> 00:07:22,200 or a propensity for electron loss. 143 00:07:22,200 --> 00:07:28,350 So it looks like ionic bonding is not going to help us 144 00:07:28,350 --> 00:07:32,600 explain the formation of molecules such as 145 00:07:32,600 --> 00:07:35,130 H2, N2, and so on. 146 00:07:35,130 --> 00:07:40,110 So who came to the rescue in this case to get 147 00:07:40,110 --> 00:07:41,760 us out of the conundrum? 148 00:07:41,760 --> 00:07:42,200 G.N. 149 00:07:42,200 --> 00:07:43,450 Lewis. 150 00:07:45,340 --> 00:07:45,660 G.N. 151 00:07:45,660 --> 00:07:51,290 Lewis was actually born in Weymouth, Massachusetts and he 152 00:07:51,290 --> 00:07:53,850 finished his PhD at Harvard in 1899. 153 00:07:53,850 --> 00:07:56,300 And then, like so many Americans of the day, went off 154 00:07:56,300 --> 00:07:58,640 to Europe and he postdoc'd in Europe for a while. 155 00:07:58,640 --> 00:08:01,410 And then he came back and got a job at MIT. 156 00:08:01,410 --> 00:08:05,280 And he taught at MIT from 1905 to 1912. 157 00:08:05,280 --> 00:08:09,180 And then in 1912 he was lured to the West Coast where they 158 00:08:09,180 --> 00:08:12,190 were starting to establish the chemistry department, the 159 00:08:12,190 --> 00:08:14,200 University of California at Berkeley, and he went out to 160 00:08:14,200 --> 00:08:17,860 Berkeley and that's where he spent the rest of his career. 161 00:08:17,860 --> 00:08:20,850 And we can speculate why he went. 162 00:08:20,850 --> 00:08:23,920 Maybe he was fed up with the weather here. 163 00:08:23,920 --> 00:08:26,070 Actually, today is one of those few days-- write it 164 00:08:26,070 --> 00:08:30,730 down, because one of the few lovely days in Massachusetts. 165 00:08:30,730 --> 00:08:31,190 So G.N. 166 00:08:31,190 --> 00:08:32,990 Lewis, what did he say? 167 00:08:32,990 --> 00:08:34,760 He said, well I've got an idea here. 168 00:08:34,760 --> 00:08:41,760 He said, what if hydrogen achieved shell filling not by 169 00:08:41,760 --> 00:08:45,120 electron transfer but by electron sharing. 170 00:08:45,120 --> 00:08:51,560 So he posited the idea of shell 171 00:08:51,560 --> 00:08:59,580 filling by electron sharing. 172 00:08:59,580 --> 00:09:01,970 This is in contrast to electron transfer. 173 00:09:04,710 --> 00:09:07,430 So let's see. 174 00:09:07,430 --> 00:09:07,600 Oh. 175 00:09:07,600 --> 00:09:09,100 There's an image of G.N. 176 00:09:09,100 --> 00:09:11,460 Lewis. 177 00:09:11,460 --> 00:09:13,030 He died, actually, on the job. 178 00:09:13,030 --> 00:09:14,640 He came back to his lab one day after 179 00:09:14,640 --> 00:09:17,050 lunch and hit the floor. 180 00:09:17,050 --> 00:09:18,630 So he worked right to the very end. 181 00:09:21,740 --> 00:09:25,450 Here's some data taken from a lab 182 00:09:25,450 --> 00:09:26,950 notebook and memo, actually. 183 00:09:26,950 --> 00:09:30,220 1902. 184 00:09:30,220 --> 00:09:31,630 And what do you see here? 185 00:09:31,630 --> 00:09:36,280 Well, he developed a notation for us, and we still use this 186 00:09:36,280 --> 00:09:38,870 notation to this day: Lewis notation. 187 00:09:38,870 --> 00:09:41,160 So here's lithium and he's got one electron. 188 00:09:41,160 --> 00:09:43,810 But we know lithium has three electrons but 189 00:09:43,810 --> 00:09:46,540 only one valence electron. 190 00:09:46,540 --> 00:09:48,600 And then there's beryllium and magnesium-- 191 00:09:48,600 --> 00:09:49,470 two electrons. 192 00:09:49,470 --> 00:09:51,360 Aluminum with three. 193 00:09:51,360 --> 00:09:55,050 Here's fluorine chlorine, and when they ionize he puts the 194 00:09:55,050 --> 00:09:57,710 eighth electron right here. 195 00:09:57,710 --> 00:09:59,000 And look at this one for silicon. 196 00:09:59,000 --> 00:10:03,530 He's got probably some kernel inside the atom, thus. 197 00:10:03,530 --> 00:10:06,720 So he's even starting to think about concentric shells. 198 00:10:06,720 --> 00:10:07,890 This is 1902. 199 00:10:07,890 --> 00:10:11,690 Remember the Bohr model isn't until 1913. 200 00:10:11,690 --> 00:10:13,170 So you can see people struggling. 201 00:10:13,170 --> 00:10:16,520 And notice that we have eight electrons in a shell-- 202 00:10:16,520 --> 00:10:18,500 that's where we're getting the octet stability-- 203 00:10:18,500 --> 00:10:19,740 and he's using cubes. 204 00:10:19,740 --> 00:10:23,550 Now we know that the cube isn't the shape of the shell, 205 00:10:23,550 --> 00:10:26,160 but it's a pretty good device to help you keep track of 206 00:10:26,160 --> 00:10:27,470 electron number-- 207 00:10:27,470 --> 00:10:29,350 because there's eight corners on a cube. 208 00:10:29,350 --> 00:10:33,180 So it's another example of how that's not what it is but it's 209 00:10:33,180 --> 00:10:36,220 a really good model and it keeps you out of trouble and 210 00:10:36,220 --> 00:10:37,880 allows you to go forward. 211 00:10:37,880 --> 00:10:41,990 So this is going back-- way, way back-- for G.N. 212 00:10:41,990 --> 00:10:42,560 Lewis. 213 00:10:42,560 --> 00:10:47,530 Now let's use this idea and account for the 214 00:10:47,530 --> 00:10:49,930 formation of H2. 215 00:10:49,930 --> 00:10:53,160 So here's hydrogen, and using the Lewis notation we'll put a 216 00:10:53,160 --> 00:10:55,870 dot here for its one electron. 217 00:10:55,870 --> 00:10:58,740 And we'll bring in a second hydrogen and we'll use a 218 00:10:58,740 --> 00:11:02,800 cross, or an x, to indicate the electron 219 00:11:02,800 --> 00:11:04,450 from the second hydrogen. 220 00:11:04,450 --> 00:11:06,465 And now we're going to double count-- 221 00:11:09,550 --> 00:11:11,070 in other words, double attribute. 222 00:11:11,070 --> 00:11:14,660 These are shared electrons so they count for both atoms. 223 00:11:14,660 --> 00:11:19,210 Double count the shared electrons. 224 00:11:19,210 --> 00:11:21,610 And when you do so what do you come up with? 225 00:11:21,610 --> 00:11:28,010 Well, the element on the left has two electrons and, 226 00:11:28,010 --> 00:11:31,550 therefore, is isoelectronic with helium. 227 00:11:31,550 --> 00:11:32,110 OK? 228 00:11:32,110 --> 00:11:33,410 Maybe it was a California thing. 229 00:11:33,410 --> 00:11:34,650 They were sharing. 230 00:11:34,650 --> 00:11:36,350 And then there was sort of another 231 00:11:36,350 --> 00:11:37,820 California concept, like. 232 00:11:37,820 --> 00:11:39,100 So it was like helium. 233 00:11:39,100 --> 00:11:43,660 And then on this side this is also sharing. 234 00:11:43,660 --> 00:11:46,950 And it's kind of like helium. 235 00:11:46,950 --> 00:11:51,810 So now we've achieved the stability of the filled shell 236 00:11:51,810 --> 00:11:53,690 by sharing the electrons. 237 00:11:53,690 --> 00:11:54,060 OK. 238 00:11:54,060 --> 00:11:56,960 And I think I even have another slide of how-- 239 00:11:56,960 --> 00:11:59,590 this is the more modern version of it. 240 00:11:59,590 --> 00:12:00,610 Electron dot. 241 00:12:00,610 --> 00:12:04,260 So the nucleus and the inner-electrons are contained 242 00:12:04,260 --> 00:12:07,290 inside the chemical symbol. 243 00:12:07,290 --> 00:12:08,800 And, actually, this goes all the way to 244 00:12:08,800 --> 00:12:09,970 modern quantum mechanics. 245 00:12:09,970 --> 00:12:13,050 Density functional theory does the same thing: lumps all of 246 00:12:13,050 --> 00:12:17,920 the inner-shell electrons plus the nucleus into one piece, 247 00:12:17,920 --> 00:12:20,780 and then the valence electrons are outside. 248 00:12:20,780 --> 00:12:25,400 And so starting in 1902 with some little dots and crosses 249 00:12:25,400 --> 00:12:28,050 we go all the way to DFT today. 250 00:12:28,050 --> 00:12:29,560 All right. 251 00:12:29,560 --> 00:12:31,280 So let's do another one. 252 00:12:31,280 --> 00:12:32,770 How about nitrogen? 253 00:12:32,770 --> 00:12:33,800 Let's try nitrogen. 254 00:12:33,800 --> 00:12:37,080 So when we going to nitrogen we know the valence 255 00:12:37,080 --> 00:12:39,460 electron's 2s2 2p3. 256 00:12:42,540 --> 00:12:44,070 So put nitrogen here. 257 00:12:44,070 --> 00:12:48,120 One, two, three, four, five. 258 00:12:48,120 --> 00:12:50,250 Now these three electrons here are 259 00:12:50,250 --> 00:12:51,630 according to the Hund rule. 260 00:12:51,630 --> 00:12:56,005 So it's px, py, pz, and this is the 2s2 sitting over here. 261 00:12:56,005 --> 00:13:00,710 And I'll bring in a second nitrogen, and there's its 2s2. 262 00:13:00,710 --> 00:13:03,370 2px, 2py, 2pz. 263 00:13:03,370 --> 00:13:05,070 And now what do I have? 264 00:13:05,070 --> 00:13:06,850 Look at the nitrogen on the left. 265 00:13:06,850 --> 00:13:08,500 Two, four, six, eight. 266 00:13:08,500 --> 00:13:12,360 So the nitrogen on the left feels as though it has access 267 00:13:12,360 --> 00:13:12,990 to eight electrons. 268 00:13:12,990 --> 00:13:15,970 The nitrogen on the right-- two, four, six, eight-- it 269 00:13:15,970 --> 00:13:19,900 feels as though it has access to eight electrons. 270 00:13:19,900 --> 00:13:25,390 So both nitrogens are isoelectronic with neon if we 271 00:13:25,390 --> 00:13:28,010 push on this concept of electron sharing. 272 00:13:28,010 --> 00:13:29,620 Now there's a second thing I want to do. 273 00:13:29,620 --> 00:13:33,230 It's to draw attention to two types of orbitals. 274 00:13:33,230 --> 00:13:37,320 So these three orbitals in the center consist of electrons 275 00:13:37,320 --> 00:13:38,700 that are shared. 276 00:13:38,700 --> 00:13:43,140 So these are going to be called bonding orbitals. 277 00:13:43,140 --> 00:13:46,125 And "bonding" and "blue" both begin with a "b", so I'm going 278 00:13:46,125 --> 00:13:53,900 to denote the bonding orbitals, or bonding domains, 279 00:13:53,900 --> 00:13:59,790 as distinct from the nonbonding domains in red. 280 00:13:59,790 --> 00:14:01,545 Red are nonbonding domains. 281 00:14:04,270 --> 00:14:06,045 Always two electrons per orbital. 282 00:14:06,045 --> 00:14:08,360 They like to live in pairs. 283 00:14:08,360 --> 00:14:09,740 That's the way it works. 284 00:14:09,740 --> 00:14:10,320 OK? 285 00:14:10,320 --> 00:14:13,280 And so each one of these pairs is a bond. 286 00:14:13,280 --> 00:14:18,240 So I can then write nitrogen with three lines through it 287 00:14:18,240 --> 00:14:20,220 indicating I have a triple bond. 288 00:14:20,220 --> 00:14:24,710 Three pairs of electrons, three bonding 289 00:14:24,710 --> 00:14:25,970 domains, triple bond. 290 00:14:25,970 --> 00:14:30,910 This is all in formation according to the concept of 291 00:14:30,910 --> 00:14:32,510 electron sharing. 292 00:14:32,510 --> 00:14:36,210 And Lewis coined a name for the type of bond that is 293 00:14:36,210 --> 00:14:37,720 formed in this way. 294 00:14:37,720 --> 00:14:50,720 He said we get bond formation involves cooperative use-- 295 00:14:50,720 --> 00:14:51,970 sharing, cooperative-- 296 00:14:56,540 --> 00:14:58,210 of valence electrons. 297 00:15:02,710 --> 00:15:07,220 So now we can take the "co" symbol here and the "valence" 298 00:15:07,220 --> 00:15:16,310 here and come up with the term "covalent bond." Covalent 299 00:15:16,310 --> 00:15:18,700 bond, thanks to G.N. 300 00:15:18,700 --> 00:15:18,970 Lewis. 301 00:15:18,970 --> 00:15:22,970 So, again, to make sure we're very clear, ionic bond results 302 00:15:22,970 --> 00:15:27,620 from electron transfer, covalent bond results from 303 00:15:27,620 --> 00:15:30,200 electron sharing. 304 00:15:30,200 --> 00:15:33,950 Now we can do this-- so let's go to heteronuclear molecules. 305 00:15:33,950 --> 00:15:34,980 These are homonuclear. 306 00:15:34,980 --> 00:15:37,230 So let's go to heteronuclear molecules. 307 00:15:37,230 --> 00:15:39,690 And so let's see. 308 00:15:39,690 --> 00:15:42,710 I've got some rules up here, I think. 309 00:15:42,710 --> 00:15:43,450 Yeah. 310 00:15:43,450 --> 00:15:44,470 Drawing Lewis structures. 311 00:15:44,470 --> 00:15:46,940 So let's go to a heteronuclear molecule. 312 00:15:46,940 --> 00:15:50,385 And I'm going to choose as an example sulfuryl chloride. 313 00:15:55,560 --> 00:15:57,100 And I don't expect you to be able to name 314 00:15:57,100 --> 00:15:58,230 these things on site. 315 00:15:58,230 --> 00:15:59,620 I will always give you the name. 316 00:15:59,620 --> 00:16:05,090 I'll say sulfuryl chloride, parenthesis, SO2Cl2, blah, 317 00:16:05,090 --> 00:16:05,980 blah, blah. 318 00:16:05,980 --> 00:16:07,180 OK? 319 00:16:07,180 --> 00:16:09,690 So sulfuryl chloride. 320 00:16:09,690 --> 00:16:11,560 I want to put up the Lewis 321 00:16:11,560 --> 00:16:12,970 structure of sulfuryl chloride. 322 00:16:12,970 --> 00:16:15,910 So center the element with the lowest average valence 323 00:16:15,910 --> 00:16:16,880 electron energy. 324 00:16:16,880 --> 00:16:20,790 So it turns out that the average valence electron 325 00:16:20,790 --> 00:16:22,160 energy stack like this. 326 00:16:22,160 --> 00:16:28,160 Sulfur is the lowest, then chlorine, and then oxygen. 327 00:16:28,160 --> 00:16:30,440 This is this ranking of average 328 00:16:30,440 --> 00:16:31,600 valence electron energies. 329 00:16:31,600 --> 00:16:33,900 And you'd be given those data. 330 00:16:33,900 --> 00:16:36,730 So it says put sulfur in the center. 331 00:16:36,730 --> 00:16:38,640 So I'll put sulfur in the center. 332 00:16:38,640 --> 00:16:40,610 And then what does it say? 333 00:16:40,610 --> 00:16:42,760 We're going to count all the valence electrons. 334 00:16:42,760 --> 00:16:50,010 So sulfur over here is 3s2 3p4. 335 00:16:52,780 --> 00:16:55,200 So that gives me six valence electrons. 336 00:16:55,200 --> 00:16:56,930 And there's two oxygens. 337 00:16:56,930 --> 00:17:01,900 And oxygen lies above sulfur, so that's 2s2 2p4. 338 00:17:01,900 --> 00:17:03,545 So that's 2 times 6. 339 00:17:06,620 --> 00:17:08,526 So that's 12. 340 00:17:08,526 --> 00:17:10,680 All right? 341 00:17:10,680 --> 00:17:12,350 Let's put the 6 over here. 342 00:17:12,350 --> 00:17:15,550 And then there's chlorines in this compound, 343 00:17:15,550 --> 00:17:19,310 so that's 3s2 3p5. 344 00:17:19,310 --> 00:17:23,490 So that's 5 plus 2 is 7, 2 times 7 is 14. 345 00:17:23,490 --> 00:17:25,700 And we add this whole thing up, we get 346 00:17:25,700 --> 00:17:28,910 there's 32 valence electrons. 347 00:17:28,910 --> 00:17:31,710 And draw a single bond from each surrounding atom to the 348 00:17:31,710 --> 00:17:32,995 central atom. 349 00:17:32,995 --> 00:17:35,840 All right. 350 00:17:35,840 --> 00:17:36,950 Again, this is a model. 351 00:17:36,950 --> 00:17:39,270 I'm not saying that this is the shape of the molecule, but 352 00:17:39,270 --> 00:17:40,010 it's a way to count. 353 00:17:40,010 --> 00:17:41,760 All I'm doing is trying to keep track of 354 00:17:41,760 --> 00:17:43,640 bonds and paired electrons. 355 00:17:43,640 --> 00:17:47,900 So I can put chlorine on either side. 356 00:17:47,900 --> 00:17:55,030 And I'll put an oxygen below and an oxygen above. 357 00:17:55,030 --> 00:17:57,150 All right. 358 00:17:57,150 --> 00:17:59,770 So that's already two, four, six, eight. 359 00:17:59,770 --> 00:18:03,350 So I'm losing eight. 360 00:18:03,350 --> 00:18:06,340 So 32 minus 8 is 24. 361 00:18:06,340 --> 00:18:10,990 And so with the 24 that means I've got 12 pairs of 362 00:18:10,990 --> 00:18:14,560 electrons to place. 363 00:18:14,560 --> 00:18:16,410 So let's start putting the Lewis structures up. 364 00:18:16,410 --> 00:18:20,780 So chlorine consists of one, two, three, 365 00:18:20,780 --> 00:18:23,050 four, five, six, seven. 366 00:18:23,050 --> 00:18:24,780 And I'll do the same thing on the other side. 367 00:18:24,780 --> 00:18:27,300 Two, four, six, seven. 368 00:18:27,300 --> 00:18:31,390 And oxygen has six, so that's two, four, six. 369 00:18:31,390 --> 00:18:32,780 And then the lower one, same thing. 370 00:18:32,780 --> 00:18:35,060 Two, four, six. 371 00:18:35,060 --> 00:18:36,340 And sulfur has six. 372 00:18:36,340 --> 00:18:37,810 I'm going to use x's for sulfur. 373 00:18:37,810 --> 00:18:41,160 So I'll put one x with the chlorine, 374 00:18:41,160 --> 00:18:43,280 another x with the chlorine. 375 00:18:43,280 --> 00:18:46,220 Two with the oxygen, two with the oxygen. 376 00:18:46,220 --> 00:18:49,080 And so now we're in pretty good shape, right? 377 00:18:49,080 --> 00:18:51,590 We can identify bonding and nonbonding domains. 378 00:18:51,590 --> 00:18:53,920 Here's the bonding. 379 00:18:53,920 --> 00:18:58,160 One, two, three, four. 380 00:18:58,160 --> 00:19:00,440 And then the nonbonding. 381 00:19:00,440 --> 00:19:02,560 Looks like there's 12. 382 00:19:02,560 --> 00:19:10,435 And sure enough, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. 383 00:19:10,435 --> 00:19:12,070 The 12 nonbonding domains. 384 00:19:12,070 --> 00:19:13,130 4 bonds. 385 00:19:13,130 --> 00:19:14,760 And we have the Lewis structure for 386 00:19:14,760 --> 00:19:16,490 this particular compound. 387 00:19:16,490 --> 00:19:19,660 And one last little piece worth pointing out. 388 00:19:19,660 --> 00:19:23,450 Notice that in the bonds to the chlorine you have two 389 00:19:23,450 --> 00:19:25,390 electrons, as you need. 390 00:19:25,390 --> 00:19:27,740 One electron comes from the chlorine and one electron 391 00:19:27,740 --> 00:19:29,020 comes from sulfur. 392 00:19:29,020 --> 00:19:32,010 But in the bonds between the sulfur and the oxygen the 393 00:19:32,010 --> 00:19:36,490 sulfur's so desperate to form a bond that it actually 394 00:19:36,490 --> 00:19:39,720 donates both electrons to the bond. 395 00:19:39,720 --> 00:19:43,180 And oxygen's happy because it's isoelectronic with neon, 396 00:19:43,180 --> 00:19:45,830 and sulfur's happy because it's going to be isoelectronic 397 00:19:45,830 --> 00:19:46,690 with argon. 398 00:19:46,690 --> 00:19:48,630 But, you know, it has to go to some lengths. 399 00:19:48,630 --> 00:19:53,300 So this is called a dative bond, when both electrons come 400 00:19:53,300 --> 00:19:56,510 from the one element. 401 00:19:56,510 --> 00:19:56,800 OK. 402 00:19:56,800 --> 00:19:58,100 Well, this is great. 403 00:19:58,100 --> 00:19:59,660 I'm going to do one more. 404 00:19:59,660 --> 00:20:03,550 How about methane? 405 00:20:03,550 --> 00:20:05,110 CH4. 406 00:20:05,110 --> 00:20:07,760 So I'm going to start with carbon. 407 00:20:07,760 --> 00:20:09,330 Carbon's going to go with this on there. 408 00:20:09,330 --> 00:20:14,220 And a carbon is 2s2 2p2. 409 00:20:17,600 --> 00:20:23,500 And so I'm going to use the box notation now. 410 00:20:23,500 --> 00:20:28,590 See, this is the Lewis structure, this is chemical 411 00:20:28,590 --> 00:20:31,450 equation, now we're going to a box structure. 412 00:20:31,450 --> 00:20:34,040 We can move fluidly from one model to another. 413 00:20:34,040 --> 00:20:35,670 We had cubes up there. 414 00:20:35,670 --> 00:20:36,620 It's all good. 415 00:20:36,620 --> 00:20:38,480 So this is 2s. 416 00:20:38,480 --> 00:20:44,130 And now this is 2px, 2py, and 2pz. 417 00:20:44,130 --> 00:20:47,420 Now according to this, 2s2, that gives 418 00:20:47,420 --> 00:20:49,450 me an electron pair. 419 00:20:49,450 --> 00:20:52,420 And now I've got 2p2, which according to the Hund rule 420 00:20:52,420 --> 00:20:54,120 goes in like this. 421 00:20:54,120 --> 00:20:55,410 Well I've got a problem here. 422 00:20:55,410 --> 00:20:57,300 How many unpaired electrons? 423 00:20:57,300 --> 00:20:57,725 Two. 424 00:20:57,725 --> 00:21:01,635 Now what's my maximum number of bonds I can form by 425 00:21:01,635 --> 00:21:02,886 electron sharing? 426 00:21:02,886 --> 00:21:05,170 It's two according to this. 427 00:21:05,170 --> 00:21:14,620 So the best I can do, best possible here, is CH2. 428 00:21:14,620 --> 00:21:15,450 And that's no good. 429 00:21:15,450 --> 00:21:19,130 We know from mass measurements it's CH4. 430 00:21:19,130 --> 00:21:21,230 The stoichiometry's CH4. 431 00:21:21,230 --> 00:21:24,530 And besides, what are these orbitals going to look like? 432 00:21:24,530 --> 00:21:28,660 These are the p orbitals, so they're dumbbell-shaped and 433 00:21:28,660 --> 00:21:29,850 they're orthogonal, right? 434 00:21:29,850 --> 00:21:33,200 They're 90 degrees, which means if I formed this thing-- 435 00:21:33,200 --> 00:21:34,650 which is called methylene-- 436 00:21:34,650 --> 00:21:41,230 if I form methylene I'd end up with CH2 looking like this, 437 00:21:41,230 --> 00:21:42,850 which has a dipole moment. 438 00:21:42,850 --> 00:21:46,070 And we know from spectral measurements and electrical 439 00:21:46,070 --> 00:21:48,930 properties measurements that this thing is symmetric. 440 00:21:48,930 --> 00:21:51,630 So this thing-- 441 00:21:51,630 --> 00:21:53,430 electron sharing isn't working. 442 00:21:53,430 --> 00:21:55,060 It's not working. 443 00:21:55,060 --> 00:21:59,790 So we need another patch here, and that patch comes from none 444 00:21:59,790 --> 00:22:02,630 other than Linus Pauling. 445 00:22:02,630 --> 00:22:03,400 Another American. 446 00:22:03,400 --> 00:22:06,360 You see, it's American science today and it's in the 20s. 447 00:22:06,360 --> 00:22:10,230 That's why we have Gershwin playing at the beginning. 448 00:22:10,230 --> 00:22:11,780 Celebration of American science. 449 00:22:11,780 --> 00:22:14,380 So Pauling was born in Portland, Oregon. 450 00:22:14,380 --> 00:22:16,130 He was the son of a pharmacist. And he went to 451 00:22:16,130 --> 00:22:18,990 Caltech, got his PhD in 1925. 452 00:22:18,990 --> 00:22:22,470 So the Rhapsody in Blue came out in 1924 when he was just 453 00:22:22,470 --> 00:22:23,880 hunkering down to his thesis. 454 00:22:23,880 --> 00:22:26,390 Probably listened to it, got some pleasure out of it, as 455 00:22:26,390 --> 00:22:28,110 most people did. 456 00:22:28,110 --> 00:22:32,970 So then he finishes, 1925, at Caltech in Pasadena and he 457 00:22:32,970 --> 00:22:34,230 goes to Europe. 458 00:22:34,230 --> 00:22:35,440 Now he chose wisely. 459 00:22:35,440 --> 00:22:38,830 He chose four postdoctoral positions. 460 00:22:38,830 --> 00:22:40,310 These are people he postdoc'd with. 461 00:22:40,310 --> 00:22:42,990 First with Sommerfeld, then with Bohr, then with 462 00:22:42,990 --> 00:22:45,220 Schrodinger, and then finally with Bragg. 463 00:22:45,220 --> 00:22:46,100 You'll learn about Bragg. 464 00:22:46,100 --> 00:22:48,695 Bragg got the Nobel Prize for X-ray diffraction. 465 00:22:48,695 --> 00:22:54,000 So that's not a bad preparatory start. 466 00:22:54,000 --> 00:22:57,310 So he comes back and teaches at Caltech. 467 00:22:57,310 --> 00:23:00,450 In fact, I have a picture of Linus Pauling. 468 00:23:00,450 --> 00:23:01,070 There he is. 469 00:23:01,070 --> 00:23:05,520 That's a middle-aged Linus Pauling, probably around the 470 00:23:05,520 --> 00:23:09,720 time he got the first of two Nobel Prizes. 471 00:23:09,720 --> 00:23:10,910 So what did Pauling do? 472 00:23:10,910 --> 00:23:14,850 Pauling said, why don't we mix the orbitals? 473 00:23:14,850 --> 00:23:17,510 They're all in the shell n equals 2, and what we're 474 00:23:17,510 --> 00:23:21,050 trying to do is to fill the n equals 2 shell. 475 00:23:21,050 --> 00:23:23,180 So how about mix? 476 00:23:23,180 --> 00:23:33,550 Let's mix 2s and 2p states in order to maximize 477 00:23:33,550 --> 00:23:34,500 the number of bonds. 478 00:23:34,500 --> 00:23:36,740 Remember, when you form a bond you decrease the 479 00:23:36,740 --> 00:23:37,880 energy of the system. 480 00:23:37,880 --> 00:23:40,090 Four bonds is a greater decrease in 481 00:23:40,090 --> 00:23:41,260 energy than two bonds. 482 00:23:41,260 --> 00:23:43,500 So the system, if it could-- 483 00:23:43,500 --> 00:23:45,355 and they're all within the same shell. 484 00:23:45,355 --> 00:23:49,500 You notice he didn't say, gee, if mixing 2s with 2p is good 485 00:23:49,500 --> 00:23:50,910 let's go get some 1s. 486 00:23:50,910 --> 00:23:53,450 Well 1s is down in n equals 1, and there's 487 00:23:53,450 --> 00:23:55,190 no way you can mix. 488 00:23:55,190 --> 00:23:57,460 They had to be in the same shell. 489 00:23:57,460 --> 00:24:07,550 Mix states in order to maximize number of 490 00:24:07,550 --> 00:24:10,235 bonds that can form. 491 00:24:12,800 --> 00:24:14,480 It's all about maximize the number of 492 00:24:14,480 --> 00:24:17,325 bonds that can be formed. 493 00:24:20,630 --> 00:24:23,740 And this, of course, is by electron sharing. 494 00:24:23,740 --> 00:24:26,350 We're talking about covalent bonds here. 495 00:24:26,350 --> 00:24:26,920 OK? 496 00:24:26,920 --> 00:24:35,170 And he termed these mixed orbitals "hybrids." Termed the 497 00:24:35,170 --> 00:24:37,050 mixed orbitals as "hybrid orbitals." They're 498 00:24:37,050 --> 00:24:39,120 cross-breed-- 499 00:24:39,120 --> 00:24:41,720 part s, part p. 500 00:24:41,720 --> 00:24:45,490 So now let's look at the energy level diagram-- 501 00:24:45,490 --> 00:24:46,590 or the box notation. 502 00:24:46,590 --> 00:24:47,330 Forgive me. 503 00:24:47,330 --> 00:24:50,160 So I'm going to mix s and p. 504 00:24:50,160 --> 00:24:53,720 So I've got a single s and I've got three p's, so this is 505 00:24:53,720 --> 00:24:55,260 called sp3. 506 00:24:55,260 --> 00:24:59,695 Each one of these is a mixed sp3 hybrid orbital. 507 00:24:59,695 --> 00:25:01,370 And I've got four of them. 508 00:25:01,370 --> 00:25:03,610 And how many electrons do I have? 509 00:25:03,610 --> 00:25:04,410 Four. 510 00:25:04,410 --> 00:25:07,270 So now I use the Hund rule and in go the electrons. 511 00:25:07,270 --> 00:25:10,090 One, two, three, four. 512 00:25:10,090 --> 00:25:15,260 And now I have the ability to form four bonds. 513 00:25:15,260 --> 00:25:16,460 But it gets better. 514 00:25:16,460 --> 00:25:17,480 Here's the next thing. 515 00:25:17,480 --> 00:25:18,870 These are degenerate. 516 00:25:18,870 --> 00:25:19,900 They're all in the same state. 517 00:25:19,900 --> 00:25:21,770 That's why we're using the Hund rule. 518 00:25:21,770 --> 00:25:26,390 And so degeneracy in energy implies degeneracy in spatial 519 00:25:26,390 --> 00:25:28,050 orientation. 520 00:25:28,050 --> 00:25:28,920 So what does that mean? 521 00:25:28,920 --> 00:25:32,500 It means that if these are four bonds equivalent, then 522 00:25:32,500 --> 00:25:35,560 the way those bonds will arrange themselves in space is 523 00:25:35,560 --> 00:25:36,740 to be equivalent. 524 00:25:36,740 --> 00:25:39,970 So if I've got a central carbon here and I'm going to 525 00:25:39,970 --> 00:25:43,480 put four sticks from the central carbon so as to make 526 00:25:43,480 --> 00:25:47,200 the four sticks symmetrically disposed in space, that 527 00:25:47,200 --> 00:25:50,110 dictates the architecture of the molecule. 528 00:25:50,110 --> 00:25:53,130 And how do I put four sticks off of a central point 529 00:25:53,130 --> 00:25:54,960 symmetrically disposed in space? 530 00:25:54,960 --> 00:25:57,720 One, two, three, and four. 531 00:25:57,720 --> 00:26:00,640 This is meant to be the corners of a tetrahedron. 532 00:26:00,640 --> 00:26:03,520 Each one of these is 109 degrees apart. 533 00:26:03,520 --> 00:26:07,060 And this describes a tetrahedron. 534 00:26:07,060 --> 00:26:10,670 So now I've got carbon in the center, and now I've got the 535 00:26:10,670 --> 00:26:14,900 hydrogens at the four corners of a tetrahedron. 536 00:26:14,900 --> 00:26:17,810 There is the structure of methane. 537 00:26:17,810 --> 00:26:21,080 And each of the hydrogens has a shared electron with the 538 00:26:21,080 --> 00:26:24,040 carbon, making it isoelectronic with helium. 539 00:26:24,040 --> 00:26:28,220 And the carbon has four of its own electrons, four shared 540 00:26:28,220 --> 00:26:30,080 with the four hydrogens to make it 541 00:26:30,080 --> 00:26:31,360 isoelectronic with neon. 542 00:26:31,360 --> 00:26:32,700 So everybody's happy. 543 00:26:32,700 --> 00:26:35,920 Shell filling, and it's all good. 544 00:26:35,920 --> 00:26:39,800 So now it's symmetric and it has no net dipole moment. 545 00:26:39,800 --> 00:26:43,180 Everything squares with the data. 546 00:26:43,180 --> 00:26:45,330 Well, good for Pauling. 547 00:26:45,330 --> 00:26:46,000 But he went further. 548 00:26:46,000 --> 00:26:47,550 He went much further. 549 00:26:47,550 --> 00:26:50,890 What Pauling wanted to do was to make it quantitative. 550 00:26:50,890 --> 00:26:54,020 And so he wanted to have something analogous in 551 00:26:54,020 --> 00:26:58,100 covalent bonding to what we have in ionic bonding. 552 00:26:58,100 --> 00:27:01,740 So what Leslie is now rubbing off the board there is-- 553 00:27:01,740 --> 00:27:02,390 no, keep going. 554 00:27:02,390 --> 00:27:02,670 It's good. 555 00:27:02,670 --> 00:27:03,490 It's OK. 556 00:27:03,490 --> 00:27:06,810 This is a rule of academics: You always erase that which 557 00:27:06,810 --> 00:27:08,160 you will refer back to. 558 00:27:08,160 --> 00:27:10,330 We need more boards in here. 559 00:27:10,330 --> 00:27:12,430 How many boards do I fill in a period? 560 00:27:12,430 --> 00:27:14,120 9, 18? 561 00:27:14,120 --> 00:27:14,580 Maybe what? 562 00:27:14,580 --> 00:27:16,580 We need about 24 boards. 563 00:27:16,580 --> 00:27:18,000 That's a good lecture. 564 00:27:18,000 --> 00:27:18,420 All right. 565 00:27:18,420 --> 00:27:21,760 So here's what Pauling was thinking about. 566 00:27:21,760 --> 00:27:25,170 He was thinking about the analogy, for example, if I 567 00:27:25,170 --> 00:27:32,240 want to get the energy of magnesium oxide I can use the 568 00:27:32,240 --> 00:27:36,060 formula that Leslie has just erased, and 569 00:27:36,060 --> 00:27:37,060 it looks like this. 570 00:27:37,060 --> 00:27:41,320 So if all I need to know is the radius of the anion, the 571 00:27:41,320 --> 00:27:45,910 radius of the cation, it's charge, and the Madelung 572 00:27:45,910 --> 00:27:47,870 constant and then I just plug in, I get the 573 00:27:47,870 --> 00:27:50,030 crystallization energy. 574 00:27:50,030 --> 00:27:53,640 But then suppose instead of magnesium oxide I want to go 575 00:27:53,640 --> 00:27:56,690 to magnesium chloride. 576 00:27:56,690 --> 00:27:58,910 I can use the same formula only I need 577 00:27:58,910 --> 00:28:00,150 the Madelung constant. 578 00:28:00,150 --> 00:28:02,940 This is the Madelung constant for magnesium oxide. 579 00:28:02,940 --> 00:28:04,560 If I have the Madelung constant-- 580 00:28:04,560 --> 00:28:07,150 forgive me, script M-- 581 00:28:07,150 --> 00:28:12,790 for magnesium chloride, and I know the ionic radius of 582 00:28:12,790 --> 00:28:17,540 magnesium cation and chloride anion, away I go again. 583 00:28:17,540 --> 00:28:18,270 I need this. 584 00:28:18,270 --> 00:28:19,760 I need, of course, the Born exponent. 585 00:28:19,760 --> 00:28:22,440 This Born exponent and away we go. 586 00:28:22,440 --> 00:28:24,260 The same formula applies. 587 00:28:24,260 --> 00:28:28,100 So I can build with a library of basic physical data. 588 00:28:28,100 --> 00:28:29,580 So what did Pauling do? 589 00:28:29,580 --> 00:28:32,470 Pauling said, what if we can do the same thing 590 00:28:32,470 --> 00:28:34,340 for covalent bonds? 591 00:28:34,340 --> 00:28:37,250 Is there some kind of an analogy? 592 00:28:37,250 --> 00:28:42,100 So he said, let's take a look at an arbitrary 593 00:28:42,100 --> 00:28:43,310 heteronuclear compound. 594 00:28:43,310 --> 00:28:47,790 So I'm going to do this with HF, hydrogen fluoride. 595 00:28:47,790 --> 00:28:50,680 So, first of all, let's build a hydrogen fluoride molecule. 596 00:28:50,680 --> 00:28:54,090 H with its one electron, and fluorine with its seven. 597 00:28:57,510 --> 00:29:01,900 So now hydrogen sharing an electron with fluorine is 598 00:29:01,900 --> 00:29:03,790 isoelectronic with helium. 599 00:29:03,790 --> 00:29:04,470 It's happy. 600 00:29:04,470 --> 00:29:07,220 And fluorine sharing the electron hydrogen is 601 00:29:07,220 --> 00:29:09,060 isoelectronic with neon. 602 00:29:09,060 --> 00:29:10,010 It's happy. 603 00:29:10,010 --> 00:29:17,840 So again we see shell filling by electron sharing. 604 00:29:17,840 --> 00:29:24,020 So what Pauling wanted to ask is, can I get a measure of the 605 00:29:24,020 --> 00:29:30,890 HF bond energy knowing only the bond 606 00:29:30,890 --> 00:29:35,560 energies of H-H and F-F? 607 00:29:35,560 --> 00:29:38,240 So then if I knew all the homonuclear bond energies and 608 00:29:38,240 --> 00:29:41,580 then I mixed these to make heteronuclear bonds, is there 609 00:29:41,580 --> 00:29:44,050 a path from homonuclear bond energy to 610 00:29:44,050 --> 00:29:46,560 heteronuclear bond energy? 611 00:29:46,560 --> 00:29:51,230 So let's look and see what the numbers are. 612 00:29:51,230 --> 00:29:52,710 So hydrogen. 613 00:29:52,710 --> 00:29:54,370 The hydrogen bond's fairly strong. 614 00:29:54,370 --> 00:29:58,200 It's 435 kilojoules per mole. 615 00:29:58,200 --> 00:29:59,780 That's mole of bonds. 616 00:29:59,780 --> 00:30:00,150 435. 617 00:30:00,150 --> 00:30:03,680 Fluorine-flourine is 160. 618 00:30:03,680 --> 00:30:07,040 And so what do you think the value of the 619 00:30:07,040 --> 00:30:08,900 H-F bond should be? 620 00:30:08,900 --> 00:30:12,260 Well when I first look at this I say, well it's part H and 621 00:30:12,260 --> 00:30:15,970 it's part F, so it's somewhere between 435 and 160. 622 00:30:15,970 --> 00:30:18,140 I don't know if it's the arithmetic mean-- 623 00:30:18,140 --> 00:30:20,470 you know, add these two and divide by two-- or maybe it's 624 00:30:20,470 --> 00:30:21,680 the geometric mean-- 625 00:30:21,680 --> 00:30:23,710 multiply them together and take the square root-- 626 00:30:23,710 --> 00:30:25,630 but it's got to be somewhere in between. 627 00:30:25,630 --> 00:30:27,040 What do the data show? 628 00:30:27,040 --> 00:30:33,460 The number's 569, which is greater than 435. 629 00:30:33,460 --> 00:30:37,090 So I take a bond of 435 and a bond of 160, I put them 630 00:30:37,090 --> 00:30:41,620 together I get 569. 631 00:30:41,620 --> 00:30:44,410 That's very, very strange. 632 00:30:44,410 --> 00:30:45,650 But Pauling was smart. 633 00:30:45,650 --> 00:30:48,250 Pauling said, I have an explanation. 634 00:30:48,250 --> 00:30:55,200 He says, suppose when these electrons are shared in 635 00:30:55,200 --> 00:31:02,260 between the two atoms, suppose they're not shared equally. 636 00:31:02,260 --> 00:31:06,400 Suppose there is a displacement of the electrons. 637 00:31:06,400 --> 00:31:08,620 So instead of putting them dead center, as I've been 638 00:31:08,620 --> 00:31:12,400 doing up until now, suppose the electrons are actually 639 00:31:12,400 --> 00:31:16,680 drawn closer to the fluorine. 640 00:31:16,680 --> 00:31:20,000 So we still have octet stability, or in this case 641 00:31:20,000 --> 00:31:22,880 duet stability, but the sharing of the 642 00:31:22,880 --> 00:31:25,440 electrons is not equal. 643 00:31:25,440 --> 00:31:27,425 So this is charge displacement. 644 00:31:31,160 --> 00:31:33,795 And what does charge displacement constitute? 645 00:31:33,795 --> 00:31:36,605 Well, charge displacement means stored energy. 646 00:31:41,180 --> 00:31:45,930 And Pauling quantified that stored energy. 647 00:31:45,930 --> 00:31:49,820 And so what he did is he said that you increase the bond 648 00:31:49,820 --> 00:31:54,250 strength by thinking of it as a two-step reaction. 649 00:31:54,250 --> 00:31:57,730 So in the heteronuclear bond that is a bond between two 650 00:31:57,730 --> 00:32:07,240 different atoms. So in a heteronuclear bond we form by 651 00:32:07,240 --> 00:32:09,970 what-- and this is my coinage, you don't see this 652 00:32:09,970 --> 00:32:12,310 anywhere in the book-- 653 00:32:12,310 --> 00:32:16,930 two-step, share and then pull. 654 00:32:16,930 --> 00:32:20,160 So share is, as the name implies, we share electrons to 655 00:32:20,160 --> 00:32:21,670 achieve octet stability. 656 00:32:21,670 --> 00:32:26,550 But then because we have unequal atoms we pull towards 657 00:32:26,550 --> 00:32:31,680 one of the atoms. And which one do we pull towards? 658 00:32:31,680 --> 00:32:34,150 Well, we pull towards the one that's got a greater appetite 659 00:32:34,150 --> 00:32:35,370 for electrons. 660 00:32:35,370 --> 00:32:37,720 And we've already gone through this concept. 661 00:32:37,720 --> 00:32:40,720 Which atoms on the periodic table have the highest 662 00:32:40,720 --> 00:32:42,770 appetite for electrons? 663 00:32:42,770 --> 00:32:44,110 The nonmetals. 664 00:32:44,110 --> 00:32:45,730 The weakest appetite is the metals. 665 00:32:45,730 --> 00:32:47,430 The metals are good donors, the 666 00:32:47,430 --> 00:32:49,200 nonmetals are good acceptors. 667 00:32:49,200 --> 00:32:52,320 And fluorine's up in the top right corner, so fluorine has 668 00:32:52,320 --> 00:32:54,830 a very, very high appetite for electrons. 669 00:32:54,830 --> 00:32:57,680 And, indeed, in this bond the electrons are 670 00:32:57,680 --> 00:32:59,660 pulled to the right. 671 00:32:59,660 --> 00:33:04,240 And why Pauling got the Nobel Prize and Lewis didn't-- it's 672 00:33:04,240 --> 00:33:07,170 my theory-- is that Pauling was quantitative. 673 00:33:07,170 --> 00:33:10,020 So he came up with a quantitative measure. 674 00:33:10,020 --> 00:33:22,830 He devised a quantitative measure for the degree of 675 00:33:22,830 --> 00:33:35,000 unequal sharing, thereby allowing us to make these 676 00:33:35,000 --> 00:33:42,090 calculations with some accuracy. 677 00:33:42,090 --> 00:33:50,620 And he called that quantity electronegativity, and it's 678 00:33:50,620 --> 00:33:52,850 denoted by the Greek symbol chi. 679 00:33:56,180 --> 00:33:57,000 OK? 680 00:33:57,000 --> 00:33:58,740 And he devised a whole scale. 681 00:33:58,740 --> 00:34:00,050 How did he get the scale? 682 00:34:00,050 --> 00:34:06,700 He looked at bond energies for all sorts of pairs of elements 683 00:34:06,700 --> 00:34:10,110 across the periodic table and went through an exercise with 684 00:34:10,110 --> 00:34:14,910 pencil and paper that today we would call multivariable 685 00:34:14,910 --> 00:34:16,540 regression analysis. 686 00:34:16,540 --> 00:34:19,075 And came with a set of-- 687 00:34:19,075 --> 00:34:19,420 [SLIDE APPEARING] 688 00:34:19,420 --> 00:34:21,150 PROFESSOR: Oh, I'll come back to this. 689 00:34:21,150 --> 00:34:22,520 This is the structure of methane. 690 00:34:22,520 --> 00:34:23,870 This is the s and p. 691 00:34:23,870 --> 00:34:24,810 Oh, let's take a break. 692 00:34:24,810 --> 00:34:25,950 You can stack. 693 00:34:25,950 --> 00:34:26,370 All right. 694 00:34:26,370 --> 00:34:27,300 So this is what methane looks like. 695 00:34:27,300 --> 00:34:28,960 There's the s, there's the p. 696 00:34:28,960 --> 00:34:31,170 And the sp hybrid looks like this. 697 00:34:31,170 --> 00:34:33,260 It's sort of an asymmetric dumbbell. 698 00:34:33,260 --> 00:34:34,590 And these four things stick out. 699 00:34:34,590 --> 00:34:35,684 And then you bond the hydrogens 700 00:34:35,684 --> 00:34:37,320 and there's the methane. 701 00:34:37,320 --> 00:34:37,960 OK. 702 00:34:37,960 --> 00:34:38,950 So here's what the 703 00:34:38,950 --> 00:34:41,100 electronegativity scale looks like. 704 00:34:41,100 --> 00:34:43,220 It looks a lot like the scale for average 705 00:34:43,220 --> 00:34:44,550 valence electron energies. 706 00:34:44,550 --> 00:34:48,490 The nonmetals have the highest appetite for electrons period, 707 00:34:48,490 --> 00:34:51,460 which means in a bond they're going to hog the electrons. 708 00:34:51,460 --> 00:34:54,200 And the nonmetals have the weakest appetite, and so 709 00:34:54,200 --> 00:34:58,040 they're going to end up having the electrons in a covalent 710 00:34:58,040 --> 00:35:00,480 bond pulled away from them. 711 00:35:00,480 --> 00:35:04,750 So nonmetals have high electronegativity, metals have 712 00:35:04,750 --> 00:35:05,605 low electronegativity. 713 00:35:05,605 --> 00:35:08,230 And now here's taken from the text. 714 00:35:08,230 --> 00:35:12,600 And you see that the electronegativity is periodic. 715 00:35:12,600 --> 00:35:16,090 If you go across a period the metal has the lowest value and 716 00:35:16,090 --> 00:35:18,610 the nonmetal has the highest. And there's fluorine, number 717 00:35:18,610 --> 00:35:21,220 nine, at a value of about 4. 718 00:35:21,220 --> 00:35:25,690 It's got the most intense appetite for electrons. 719 00:35:25,690 --> 00:35:29,010 And then you jump down here to sodium, et cetera, et cetera. 720 00:35:29,010 --> 00:35:32,260 Here we are going across the lanthanides and whatnot. 721 00:35:32,260 --> 00:35:33,590 And this is taken from your text. 722 00:35:33,590 --> 00:35:36,150 There's fluorine, 3.984. 723 00:35:36,150 --> 00:35:37,440 That's the thing. 724 00:35:37,440 --> 00:35:40,990 And down here we have very low values of electronegativity. 725 00:35:40,990 --> 00:35:46,650 So with electronegativity we are now able to make 726 00:35:46,650 --> 00:35:47,900 calculations. 727 00:35:50,140 --> 00:35:54,370 And this is the Pauling formula for calculating the 728 00:35:54,370 --> 00:35:58,580 bond energy in a heteronuclear bond starting from homonuclear 729 00:35:58,580 --> 00:35:59,280 bond energy. 730 00:35:59,280 --> 00:36:01,900 So let's continue with the HF. 731 00:36:01,900 --> 00:36:05,280 So if I want to get the bond energy of HF 732 00:36:05,280 --> 00:36:06,430 I'm going to take-- 733 00:36:06,430 --> 00:36:07,770 and this is the Pauling formula-- 734 00:36:07,770 --> 00:36:08,940 the geometric mean. 735 00:36:08,940 --> 00:36:12,940 So I take the bond energy of hydrogen-hydrogen times the 736 00:36:12,940 --> 00:36:13,960 bond energy of 737 00:36:13,960 --> 00:36:17,700 fluorine-fluorine, and square root. 738 00:36:17,700 --> 00:36:20,290 So that's the geometric mean of the two. 739 00:36:20,290 --> 00:36:24,360 And then comes the Pauling piece that gets 740 00:36:24,360 --> 00:36:26,670 him the Nobel Prize. 741 00:36:26,670 --> 00:36:29,330 You take the difference in the electronegativity between the 742 00:36:29,330 --> 00:36:37,220 two elements squared, and then the factor 96.3 gives us the 743 00:36:37,220 --> 00:36:42,140 unit consistency with kilojoules per mole. 744 00:36:42,140 --> 00:36:45,960 So the greater the difference in electronegativity the 745 00:36:45,960 --> 00:36:51,110 greater the contribution here in terms of the deviation from 746 00:36:51,110 --> 00:36:54,750 just the geometric mean of the two homonuclear bond energies. 747 00:36:54,750 --> 00:36:57,730 Or put another way, if you have a homonuclear atom such 748 00:36:57,730 --> 00:37:02,280 as H2, if it's chi H minus chi H is 0, so this second 749 00:37:02,280 --> 00:37:03,620 term goes to 0. 750 00:37:03,620 --> 00:37:06,090 And obviously when fluorine is one of the members you're 751 00:37:06,090 --> 00:37:07,540 going to get a very, very high number, because 752 00:37:07,540 --> 00:37:08,730 this has the most-- 753 00:37:08,730 --> 00:37:10,640 and it doesn't matter which order you put them in because 754 00:37:10,640 --> 00:37:12,220 you're taking the square, so it's always 755 00:37:12,220 --> 00:37:13,850 going to come out positive. 756 00:37:13,850 --> 00:37:17,220 And I want you to appreciate the sense of scale here, so if 757 00:37:17,220 --> 00:37:18,750 we go in here we'll multiply. 758 00:37:18,750 --> 00:37:23,130 This is going to be 435 times 160. 759 00:37:23,130 --> 00:37:25,590 And I'm going to take the square root of this. 760 00:37:25,590 --> 00:37:29,270 And then this is 96.3. 761 00:37:29,270 --> 00:37:30,830 And you look on your periodic table 762 00:37:30,830 --> 00:37:33,040 this is 2.2 for hydrogen. 763 00:37:33,040 --> 00:37:35,250 Fluorine is 3.98. 764 00:37:35,250 --> 00:37:38,880 And I know there are different tables of electronegativity. 765 00:37:38,880 --> 00:37:39,420 I don't care. 766 00:37:39,420 --> 00:37:41,750 Just whatever you've got on your periodic table. 767 00:37:41,750 --> 00:37:44,580 The one in the book is a little bit different but it 768 00:37:44,580 --> 00:37:47,060 all comes out in the wash. 769 00:37:47,060 --> 00:37:49,770 So you multiply all this out, and we find that the first 770 00:37:49,770 --> 00:37:54,140 term is 264 kilojoules per mole and the 771 00:37:54,140 --> 00:37:56,720 second term is 344. 772 00:37:56,720 --> 00:38:00,080 So this second term is even greater than the first term. 773 00:38:00,080 --> 00:38:03,020 So the amount of energy in that electron displacement is 774 00:38:03,020 --> 00:38:04,290 substantial. 775 00:38:04,290 --> 00:38:08,772 And if you sum the two of these you get 608. 776 00:38:08,772 --> 00:38:10,220 Now you might say, well wait a minute. 777 00:38:10,220 --> 00:38:15,030 The real number is 569. 778 00:38:15,030 --> 00:38:20,700 But 608 takes you in the right direction and accounts for the 779 00:38:20,700 --> 00:38:22,610 contribution of electron displacement. 780 00:38:22,610 --> 00:38:25,080 264 is just plain wrong. 781 00:38:25,080 --> 00:38:31,310 So this was an important start for Pauling. 782 00:38:31,310 --> 00:38:35,050 And he has labels on these two contributions. 783 00:38:35,050 --> 00:38:37,470 This first term, which is just the combination of the 784 00:38:37,470 --> 00:38:44,150 homonuclear bond energies, is called purely covalent. 785 00:38:44,150 --> 00:38:46,033 It's the purely covalent contribution. 786 00:38:49,770 --> 00:38:53,430 And it's what I've been referring to as the sharing. 787 00:38:53,430 --> 00:38:57,640 This is what you get from sharing. 788 00:38:57,640 --> 00:38:58,080 OK? 789 00:38:58,080 --> 00:39:00,650 And then this second term here with the difference in 790 00:39:00,650 --> 00:39:04,000 electronegativity is what you get from what I've been 791 00:39:04,000 --> 00:39:08,050 calling the pull on the electron pair. 792 00:39:08,050 --> 00:39:12,580 And Pauling called this the partial ionic character. 793 00:39:16,392 --> 00:39:19,390 He's not saying that there's electron transfer, but it's a 794 00:39:19,390 --> 00:39:21,320 move in that direction. 795 00:39:21,320 --> 00:39:23,430 Partial electronic character. 796 00:39:23,430 --> 00:39:27,460 So what I've done here is I've decided I'll make a sort of a 797 00:39:27,460 --> 00:39:30,130 panorama of what we've seen up until now. 798 00:39:30,130 --> 00:39:32,510 And so I'm going to make something called the electron 799 00:39:32,510 --> 00:39:36,380 sharing meter. 800 00:39:36,380 --> 00:39:37,090 All right. 801 00:39:37,090 --> 00:39:41,530 So if I look at a homonuclear system like hydrogen. 802 00:39:41,530 --> 00:39:45,240 So my meter reads neutral. 803 00:39:45,240 --> 00:39:47,220 So the arrow's at 12 o'clock. 804 00:39:47,220 --> 00:39:49,920 The electrons are shared equally. 805 00:39:49,920 --> 00:39:55,070 And then if I go to HF what do I have? 806 00:39:55,070 --> 00:39:58,850 Well, I know that the fluorine is pulling the electrons. 807 00:39:58,850 --> 00:40:05,050 And so we can designate that by writing delta minus, delta 808 00:40:05,050 --> 00:40:07,900 plus. delta the physicists use. 809 00:40:07,900 --> 00:40:11,300 The lowercase Greek delta means little bit of. 810 00:40:11,300 --> 00:40:11,970 All right? 811 00:40:11,970 --> 00:40:14,920 So delta minus means it's a little bit negative. 812 00:40:14,920 --> 00:40:18,100 And we've got charge neutrality, so if the fluorine 813 00:40:18,100 --> 00:40:21,170 end is a little bit negative then the hydrogen end has to 814 00:40:21,170 --> 00:40:24,150 be a little bit positive, which means this thing has a 815 00:40:24,150 --> 00:40:27,320 net dipole moment. 816 00:40:27,320 --> 00:40:29,070 It's a dipole. 817 00:40:29,070 --> 00:40:31,430 And the arrow points to the negative end. 818 00:40:31,430 --> 00:40:34,730 One way to think about it is I put a little slash there and 819 00:40:34,730 --> 00:40:36,700 that starts to look a little bit like a plus sign. 820 00:40:36,700 --> 00:40:39,160 You can come up with your own way to remember it. 821 00:40:39,160 --> 00:40:41,590 So it's got a little bit of a dipole moment. 822 00:40:41,590 --> 00:40:45,620 And people depict dipoles usually as ovals, and they'll 823 00:40:45,620 --> 00:40:47,800 put a minus end and a plus end. 824 00:40:47,800 --> 00:40:51,390 So it's net neutral but the charge is not uniformly 825 00:40:51,390 --> 00:40:52,750 distributed. 826 00:40:52,750 --> 00:40:53,980 OK? 827 00:40:53,980 --> 00:40:57,410 So our sharing meter in this case is going to show 828 00:40:57,410 --> 00:41:00,110 something to the right. 829 00:41:00,110 --> 00:41:03,900 We've got electrons that are unequally shared, and that 830 00:41:03,900 --> 00:41:05,580 moves over to the right. 831 00:41:05,580 --> 00:41:10,890 And, you know, the dipoles have interesting properties. 832 00:41:10,890 --> 00:41:12,530 Oh, there's a plot of electronegativity 833 00:41:12,530 --> 00:41:15,620 3-bar in the bar plot. 834 00:41:15,620 --> 00:41:17,590 And actually this is an interesting one. 835 00:41:17,590 --> 00:41:20,310 Just parenthetically, you see hydrogen here? 836 00:41:20,310 --> 00:41:22,340 Hydrogen's weird. 837 00:41:22,340 --> 00:41:25,070 They put it in the periodic table above lithium but it's 838 00:41:25,070 --> 00:41:27,040 not an alkaline metal. 839 00:41:27,040 --> 00:41:29,190 And you can see it just doesn't belong there. 840 00:41:29,190 --> 00:41:31,690 And there's a lot of conversation about putting it 841 00:41:31,690 --> 00:41:36,360 maybe somewhere centered above the p block elements, because 842 00:41:36,360 --> 00:41:39,020 it certainly doesn't belong next to helium. 843 00:41:39,020 --> 00:41:43,470 But it probably doesn't belong above lithium either. 844 00:41:43,470 --> 00:41:45,030 Anyway, I thought that was very interesting. 845 00:41:45,030 --> 00:41:47,910 I can tell from the response of the 846 00:41:47,910 --> 00:41:50,190 class, why does he care? 847 00:41:50,190 --> 00:41:50,490 [LAUGHTER] 848 00:41:50,490 --> 00:41:51,370 PROFESSOR: All right. 849 00:41:51,370 --> 00:41:53,370 Now this is really-- 850 00:41:53,370 --> 00:41:54,360 I'm going to use an adverb here-- 851 00:41:54,360 --> 00:41:55,846 this is really important. 852 00:41:55,846 --> 00:41:56,590 All right? 853 00:41:56,590 --> 00:42:01,410 So here's HCl, is a cousin of HF, and you see in the upper 854 00:42:01,410 --> 00:42:05,430 frame it's just a bunch of HCl molecules just bopping around 855 00:42:05,430 --> 00:42:06,990 any which way. 856 00:42:06,990 --> 00:42:09,850 So there's the delta plus and the delta minus. 857 00:42:09,850 --> 00:42:13,380 Now if you take these dipoles and you put them in an 858 00:42:13,380 --> 00:42:18,340 electric field they will align themselves, and the positive 859 00:42:18,340 --> 00:42:22,275 ends will face the negative plate and the negative ends 860 00:42:22,275 --> 00:42:23,800 will face the positive plate. 861 00:42:23,800 --> 00:42:28,960 And there's energy stored when the random orientation goes 862 00:42:28,960 --> 00:42:31,090 into an ordered orientation. 863 00:42:31,090 --> 00:42:33,880 This is the principle behind a capacitor. 864 00:42:33,880 --> 00:42:35,800 A capacitor is nothing more than a whole 865 00:42:35,800 --> 00:42:37,790 bunch of aligned dipoles. 866 00:42:37,790 --> 00:42:41,460 So if you want to invent a supercapacitor that we can use 867 00:42:41,460 --> 00:42:44,870 on a car to extend the range of the automobile so we can 868 00:42:44,870 --> 00:42:47,960 reduce our dependence on imported petroleum, you're 869 00:42:47,960 --> 00:42:49,650 going to look for molecules that have a 870 00:42:49,650 --> 00:42:51,970 honking big dipole moment. 871 00:42:51,970 --> 00:42:57,770 That way you get more energy per unit electric field. 872 00:42:57,770 --> 00:43:02,460 So, again, a simple idea that tells me how to go and invent. 873 00:43:02,460 --> 00:43:04,830 I can go back to my office and go and invent something right 874 00:43:04,830 --> 00:43:07,320 now just based on this lecture 9. 875 00:43:07,320 --> 00:43:08,670 [LAUGHTER] 876 00:43:08,670 --> 00:43:09,930 PROFESSOR: See, you go and invent. 877 00:43:09,930 --> 00:43:12,050 You start the company, you make the billion. 878 00:43:12,050 --> 00:43:14,670 Remember good old Professor Sadoway at MIT, and 879 00:43:14,670 --> 00:43:17,780 established the fellowship for students, and so. 880 00:43:17,780 --> 00:43:18,260 All right. 881 00:43:18,260 --> 00:43:21,280 But you have to know what a dipole moment is. 882 00:43:21,280 --> 00:43:23,320 Got to know what a dipole moment is. 883 00:43:23,320 --> 00:43:23,640 OK. 884 00:43:23,640 --> 00:43:24,860 So there's the dipole moment. 885 00:43:24,860 --> 00:43:29,100 And then lastly I'm going to put sodium chloride. 886 00:43:29,100 --> 00:43:30,820 So what's sodium chloride look like? 887 00:43:30,820 --> 00:43:33,250 Well it's Na plus and Cl minus. 888 00:43:33,250 --> 00:43:36,330 So the electron has transferred completely. 889 00:43:36,330 --> 00:43:38,670 So this isn't even sharing at all. 890 00:43:38,670 --> 00:43:41,020 So this is really bury the needle. 891 00:43:41,020 --> 00:43:42,270 This is not sharing. 892 00:43:45,230 --> 00:43:49,570 In this instance the sodium doesn't even get visitation 893 00:43:49,570 --> 00:43:50,555 rights to the electron. 894 00:43:50,555 --> 00:43:53,050 The electron's gone. 895 00:43:53,050 --> 00:43:56,990 Whereas here hydrogen gets to see the electron on Saturdays 896 00:43:56,990 --> 00:43:58,190 kind of thing. 897 00:43:58,190 --> 00:44:03,000 Depends what kind of lawyer fluorine had. 898 00:44:03,000 --> 00:44:04,990 That's what it all boils down to. 899 00:44:04,990 --> 00:44:07,390 All right. 900 00:44:07,390 --> 00:44:09,690 This is the same thing that I just showed you. 901 00:44:09,690 --> 00:44:12,380 But you see, the textbook gives you, as the name 902 00:44:12,380 --> 00:44:13,920 implies, dense text. 903 00:44:13,920 --> 00:44:15,320 I gave you the sharing meter. 904 00:44:15,320 --> 00:44:19,380 The sharing meter is far more expositive. 905 00:44:19,380 --> 00:44:19,740 All right. 906 00:44:19,740 --> 00:44:22,990 And then, finally, the percent ionic character is given by 907 00:44:22,990 --> 00:44:24,050 this formula here. 908 00:44:24,050 --> 00:44:26,810 So this is 1 minus the exponential. 909 00:44:26,810 --> 00:44:34,840 So the exp term, this exponential of-- what is it-- 910 00:44:34,840 --> 00:44:38,670 minus 1/4 times the difference in 911 00:44:38,670 --> 00:44:41,220 electronegativities squared. 912 00:44:41,220 --> 00:44:47,380 This notation means e base natural logarithms, minus 1/4, 913 00:44:47,380 --> 00:44:47,960 blah, blah, blah. 914 00:44:47,960 --> 00:44:49,250 That's what this thing is. 915 00:44:49,250 --> 00:44:52,080 So if you plug in, multiply by 100% you get something that 916 00:44:52,080 --> 00:44:53,680 goes from 0 to 100. 917 00:44:53,680 --> 00:44:59,540 So obviously when delta chi is 0 you get 0%. 918 00:44:59,540 --> 00:45:02,150 e to the 0 is 1, 1 minus 1 is 0, and so you 919 00:45:02,150 --> 00:45:04,030 have no ionic character. 920 00:45:04,030 --> 00:45:08,130 And so if you plug in the numbers for HF-- 921 00:45:08,130 --> 00:45:13,620 so you're going to take this difference here, square it-- 922 00:45:13,620 --> 00:45:17,210 it ends up giving you 1.8, which gives you a value of 923 00:45:17,210 --> 00:45:20,040 about 56% ionic character. 924 00:45:20,040 --> 00:45:24,490 So it's as though the electron is sort of half transferred. 925 00:45:24,490 --> 00:45:27,760 But you might also look at it from this perspective. 926 00:45:27,760 --> 00:45:30,110 If you take 344-- 927 00:45:30,110 --> 00:45:37,120 because this is the partial ionic character, which is the 928 00:45:37,120 --> 00:45:40,260 energy of electron displacement over the total 929 00:45:40,260 --> 00:45:42,280 energy in the calculation-- 930 00:45:42,280 --> 00:45:45,000 that turns out to be 57%. 931 00:45:45,000 --> 00:45:46,610 So this stuff makes sense. 932 00:45:46,610 --> 00:45:49,390 There's a sensible metric here at work. 933 00:45:49,390 --> 00:45:53,720 And so this is what Linus Pauling got his Nobel Prize 934 00:45:53,720 --> 00:45:57,190 for, and it's the description of polar covalency. 935 00:46:00,470 --> 00:46:02,850 And polar covalency is operative when you have 936 00:46:02,850 --> 00:46:06,560 heteronuclear bonds, because the two different elements 937 00:46:06,560 --> 00:46:09,920 don't share the electron equally. 938 00:46:09,920 --> 00:46:12,525 And the Pauling formula allows you to calculate that. 939 00:46:12,525 --> 00:46:17,900 And his formative book was written in 1937, called The 940 00:46:17,900 --> 00:46:20,060 Nature of the Chemical Bond. 941 00:46:20,060 --> 00:46:20,870 OK. 942 00:46:20,870 --> 00:46:25,100 So turning to the last five minutes, I want to bring to 943 00:46:25,100 --> 00:46:28,110 your attention some covalent molecules. 944 00:46:28,110 --> 00:46:31,110 Today we're going to talk about Freon. 945 00:46:31,110 --> 00:46:35,270 Freon was an invention, it was a designer chemical, invented 946 00:46:35,270 --> 00:46:36,850 by Thomas Midgley. 947 00:46:36,850 --> 00:46:37,560 This is me. 948 00:46:37,560 --> 00:46:40,700 I named him "sp3." That's his nickname. 949 00:46:40,700 --> 00:46:44,660 Thomas sp3, for the hybridized orbital. 950 00:46:44,660 --> 00:46:47,810 So he was working at the Dayton engineering 951 00:46:47,810 --> 00:46:51,130 laboratories in Dayton, which was owned by General Motors, 952 00:46:51,130 --> 00:46:54,530 and he was working in the 20s at a time when there were no 953 00:46:54,530 --> 00:46:57,160 refrigerators in American kitchens. 954 00:46:57,160 --> 00:47:00,080 The only refrigerants that were used were either toxic or 955 00:47:00,080 --> 00:47:02,160 flammable, things like ammonia, methyl chloride, 956 00:47:02,160 --> 00:47:03,620 sulfur dioxide. 957 00:47:03,620 --> 00:47:05,390 And you read about horrible accidents. 958 00:47:05,390 --> 00:47:09,740 People making ice cream at some plant and the compressor 959 00:47:09,740 --> 00:47:12,170 blows up and two or three people are killed. 960 00:47:12,170 --> 00:47:15,820 So it was deemed unsafe in the American kitchen. 961 00:47:15,820 --> 00:47:19,660 In the 20s Midgley discovered this molecule, which looks 962 00:47:19,660 --> 00:47:23,110 just like methane only we've replace the hydrogens with two 963 00:47:23,110 --> 00:47:25,100 chlorines and two fluorines. 964 00:47:25,100 --> 00:47:28,440 So this is called dichlorodifluoromethane and 965 00:47:28,440 --> 00:47:32,050 it's a chlorofluorocarbon, a CFC. 966 00:47:32,050 --> 00:47:35,480 And this was fantastic stuff. 967 00:47:35,480 --> 00:47:41,170 It it was colorless, odorless, tasteless, non-toxic. 968 00:47:41,170 --> 00:47:44,010 It was not just used as a refrigerant, it was used in 969 00:47:44,010 --> 00:47:44,720 propellant. 970 00:47:44,720 --> 00:47:49,160 When I was your age all of the sprays-- 971 00:47:49,160 --> 00:47:53,400 whether it was hair spray, shaving cream, any aerosol-- 972 00:47:53,400 --> 00:47:56,255 was propelled by Freon-12. 973 00:47:56,255 --> 00:47:58,910 It was fantastic stuff. 974 00:47:58,910 --> 00:48:02,450 Well, it turns out that in the upper atmosphere-- 975 00:48:02,450 --> 00:48:03,710 you know, you go pss pss pss. 976 00:48:03,710 --> 00:48:06,610 You got people all over the world doing this, eventually 977 00:48:06,610 --> 00:48:08,760 this stuff starts floating away. 978 00:48:08,760 --> 00:48:11,130 And what turns out in the upper atmosphere where we 979 00:48:11,130 --> 00:48:14,080 don't have shielding from ultraviolet-- 980 00:48:14,080 --> 00:48:16,190 you know how to do this calculation, because you can 981 00:48:16,190 --> 00:48:18,070 look up the energy. 982 00:48:18,070 --> 00:48:20,040 And, in fact, it's part of your homework, where you look 983 00:48:20,040 --> 00:48:24,070 at the energy differences and the electronegativity 984 00:48:24,070 --> 00:48:26,960 differences, you can compute the wavelength of light that's 985 00:48:26,960 --> 00:48:29,470 capable of breaking the carbon-chlorine bond. 986 00:48:29,470 --> 00:48:31,950 And it turns out to be in the ultraviolet. 987 00:48:31,950 --> 00:48:35,680 Once the chlorine is broken you have a chlorine radical, 988 00:48:35,680 --> 00:48:39,330 and that chlorine radical goes over here and attacks ozone. 989 00:48:39,330 --> 00:48:40,340 [CELL PHONE RINGING] 990 00:48:40,340 --> 00:48:41,280 PROFESSOR: Cell phone-- 991 00:48:41,280 --> 00:48:42,530 out. 992 00:48:45,630 --> 00:48:48,879 Just get up and leave out of courtesy. 993 00:48:48,879 --> 00:48:50,129 [CELL PHONE STILL RINGING] 994 00:48:58,360 --> 00:48:59,610 [LAUGHTER] 995 00:49:04,870 --> 00:49:06,120 PROFESSOR: Hello? 996 00:49:08,490 --> 00:49:13,440 The first year I was teaching 3.091 there was a Nobel Prize 997 00:49:13,440 --> 00:49:17,570 awarded to Mario Molina, who was a faculty member here in 998 00:49:17,570 --> 00:49:21,720 Earth and Planetary Sciences who had worked years earlier 999 00:49:21,720 --> 00:49:26,230 at University of California, Irvine, and had speculated on 1000 00:49:26,230 --> 00:49:31,540 the mechanism by which ozone depletion occurs and linked it 1001 00:49:31,540 --> 00:49:34,790 to rising levels of CFCs. 1002 00:49:34,790 --> 00:49:36,040 Initially-- 1003 00:49:36,040 --> 00:49:38,180 that's why it says a vindication-- 1004 00:49:38,180 --> 00:49:40,410 people pooh poohed it, said it was crazy. 1005 00:49:40,410 --> 00:49:43,100 There wasn't enough of this pss pss to cause any trouble. 1006 00:49:43,100 --> 00:49:45,870 But then later with the NASA program they started taking a 1007 00:49:45,870 --> 00:49:48,780 lot of images and they could track ozone levels in the 1008 00:49:48,780 --> 00:49:51,850 atmosphere and start seeing that not only was ozone 1009 00:49:51,850 --> 00:49:54,240 changing but there were actually pockets where ozone 1010 00:49:54,240 --> 00:49:57,150 was being depleted at an accelerating rate-- because 1011 00:49:57,150 --> 00:50:00,400 obviously the atmosphere isn't constant composition and 1012 00:50:00,400 --> 00:50:01,200 constant temperature. 1013 00:50:01,200 --> 00:50:01,680 Duh. 1014 00:50:01,680 --> 00:50:03,310 So anyways, yeah. 1015 00:50:03,310 --> 00:50:04,270 There he is. 1016 00:50:04,270 --> 00:50:06,320 And this was the paper that was 1017 00:50:06,320 --> 00:50:09,670 published in 1974 in Nature. 1018 00:50:09,670 --> 00:50:13,410 And this was done before computers. 1019 00:50:13,410 --> 00:50:18,560 The PC wasn't invented and commercialized 1020 00:50:18,560 --> 00:50:19,630 until the early 80s. 1021 00:50:19,630 --> 00:50:21,990 So this was typeset, and the person who typeset it 1022 00:50:21,990 --> 00:50:25,080 obviously didn't take 3.091 because instead of "atom" 1023 00:50:25,080 --> 00:50:30,420 hyphen "catalysed" we have "atomc-atalysed." But even 1024 00:50:30,420 --> 00:50:35,130 ignoring the spelling error in a Nobel Prize winning paper-- 1025 00:50:35,130 --> 00:50:36,890 [LAUGHTER] 1026 00:50:36,890 --> 00:50:39,340 PROFESSOR: --the Nobel committee overlooked this. 1027 00:50:39,340 --> 00:50:41,040 Yeah. 1028 00:50:41,040 --> 00:50:42,000 So there it is. 1029 00:50:42,000 --> 00:50:44,900 And then they went to HFCs and so on. 1030 00:50:44,900 --> 00:50:48,300 There's a lot of activity in this. 1031 00:50:48,300 --> 00:50:51,800 And what happened is when we changed from CFCs to HFCs we 1032 00:50:51,800 --> 00:50:53,920 had to change the design of the compressors. 1033 00:50:53,920 --> 00:50:56,050 And what happened was everything got 1034 00:50:56,050 --> 00:50:57,760 much, much more efficient. 1035 00:50:57,760 --> 00:51:01,420 So this was an example of necessity for a change that 1036 00:51:01,420 --> 00:51:03,320 was driven by concern for the environment. 1037 00:51:03,320 --> 00:51:05,820 Instead of putting people out of work and killing an 1038 00:51:05,820 --> 00:51:08,942 industry, gave us much more efficient refrigeration. 1039 00:51:08,942 --> 00:51:10,830 And the last thing I'll show you is this 1040 00:51:10,830 --> 00:51:11,620 to draw your attention. 1041 00:51:11,620 --> 00:51:13,280 This was in your textbook. 1042 00:51:13,280 --> 00:51:16,770 This is the cap at the top of the Washington Monument. 1043 00:51:16,770 --> 00:51:19,080 The Washington Monument was built to celebrate the 1044 00:51:19,080 --> 00:51:21,530 American centennial, 1876. 1045 00:51:21,530 --> 00:51:23,560 They finished it in 1884. 1046 00:51:23,560 --> 00:51:26,780 And this is 100 ounces of aluminum, because aluminum was 1047 00:51:26,780 --> 00:51:27,970 a precious metal. 1048 00:51:27,970 --> 00:51:30,100 It was priced higher than silver. 1049 00:51:30,100 --> 00:51:31,090 1884. 1050 00:51:31,090 --> 00:51:34,490 Two years later Charles Martin Hall and Paul Heroult invent 1051 00:51:34,490 --> 00:51:37,140 an electrochemical process that drives the price of 1052 00:51:37,140 --> 00:51:40,430 aluminum down to the point that we make beer cans-- 1053 00:51:40,430 --> 00:51:42,426 I mean soda cans-- out of it today. 1054 00:51:42,426 --> 00:51:42,890 [LAUGHTER] 1055 00:51:42,890 --> 00:51:47,580 PROFESSOR: And a good example of how chemical innovation can 1056 00:51:47,580 --> 00:51:48,940 lead to superior products. 1057 00:51:48,940 --> 00:51:50,680 I'll see you on Wednesday.