1 00:00:00,030 --> 00:00:02,529 NARRATOR: The following content is provided under a Creative 2 00:00:02,529 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,080 continue to offer high-quality educational resources for free. 5 00:00:10,080 --> 00:00:12,660 To make a donation or to view additional materials 6 00:00:12,660 --> 00:00:15,140 from hundreds of MIT courses, visit 7 00:00:15,140 --> 00:00:17,250 MITOpenCourseWare@ocw.mit.edu. 8 00:00:26,021 --> 00:00:30,490 CATHERINE DRENNAN: So now we go on to the breakdown 9 00:00:30,490 --> 00:00:32,880 of the octet rule. 10 00:00:32,880 --> 00:00:34,720 So we've established the octet rule 11 00:00:34,720 --> 00:00:40,940 and now we're going to shake it all up and see its flaws. 12 00:00:40,940 --> 00:00:44,490 So case 1, we're going to consider what happens-- 13 00:00:44,490 --> 00:00:46,580 and this again is today's handout-- 14 00:00:46,580 --> 00:00:50,780 consider what happens when you have an odd number 15 00:00:50,780 --> 00:00:52,990 of valence electrons. 16 00:00:52,990 --> 00:00:58,990 Case 1, all right, so for molecules 17 00:00:58,990 --> 00:01:02,490 with an odd number of electrons, odd number 18 00:01:02,490 --> 00:01:06,180 of valence electrons, it's not possible for every atom 19 00:01:06,180 --> 00:01:08,270 to have a complete octet. 20 00:01:08,270 --> 00:01:10,400 That involves pairing, that involves 21 00:01:10,400 --> 00:01:13,600 even numbers of electrons. 22 00:01:13,600 --> 00:01:15,830 So let's look at an example of a place 23 00:01:15,830 --> 00:01:18,800 where we have an odd number of valence electrons. 24 00:01:18,800 --> 00:01:21,780 And this is our friend, CH3, our methyl group 25 00:01:21,780 --> 00:01:23,790 that we saw a few minutes ago. 26 00:01:23,790 --> 00:01:26,520 So we can also write it like this. 27 00:01:26,520 --> 00:01:32,750 And we can come up with a Lewis structure for our methyl group. 28 00:01:32,750 --> 00:01:36,380 So hydrogen, remember, it only brings one valence electron 29 00:01:36,380 --> 00:01:37,620 to the table. 30 00:01:37,620 --> 00:01:39,390 There are three hydrogens. 31 00:01:39,390 --> 00:01:42,180 Carbon brings four valence electrons. 32 00:01:42,180 --> 00:01:47,920 So in sum, we have seven, an odd number of electrons. 33 00:01:47,920 --> 00:01:50,260 To complete our octets-- remember, 34 00:01:50,260 --> 00:01:53,760 hydrogen is an exception, it only needs two, 35 00:01:53,760 --> 00:01:59,730 it just has that 1s orbital-- so we need two electrons for it 36 00:01:59,730 --> 00:02:00,660 for it to be happy. 37 00:02:00,660 --> 00:02:02,130 There are three hydrogens. 38 00:02:02,130 --> 00:02:03,650 That's six. 39 00:02:03,650 --> 00:02:05,100 Carbon needs eight. 40 00:02:05,100 --> 00:02:11,780 So total you need 14 to make carbon and hydrogens happy. 41 00:02:11,780 --> 00:02:15,090 So if you consider then how many bonding electrons you have, 42 00:02:15,090 --> 00:02:19,780 that's the number of electrons needed for your octet, 43 00:02:19,780 --> 00:02:22,890 minus the number of valence electrons you have, 44 00:02:22,890 --> 00:02:25,580 so we get seven bonding electrons. 45 00:02:25,580 --> 00:02:28,030 How do you have seven bonding electrons? 46 00:02:28,030 --> 00:02:30,870 A bond has two electrons in it. 47 00:02:30,870 --> 00:02:32,840 So let's look at that, what do we do. 48 00:02:32,840 --> 00:02:37,510 Well, we can put six-- six is good-- 49 00:02:37,510 --> 00:02:40,390 assign two electrons for the bond between this carbon 50 00:02:40,390 --> 00:02:44,130 and this hydrogen, two between this carbon and this hydrogen, 51 00:02:44,130 --> 00:02:48,090 two between this carbon and this hydrogen, six is good. 52 00:02:48,090 --> 00:02:50,430 But now what do we do with the extra one? 53 00:02:50,430 --> 00:02:51,690 We have seven. 54 00:02:51,690 --> 00:02:56,440 So we're just going to stick that right there on the carbon. 55 00:02:56,440 --> 00:02:59,010 So what is this molecule, then? 56 00:02:59,010 --> 00:03:02,630 Well, what this molecule is is a radical species. 57 00:03:02,630 --> 00:03:05,250 So a radical species is a molecule 58 00:03:05,250 --> 00:03:07,920 with an unpaired electron. 59 00:03:07,920 --> 00:03:11,440 And usually radical species are very reactive. 60 00:03:11,440 --> 00:03:14,230 Bonding electrons don't want to be unpaired, 61 00:03:14,230 --> 00:03:16,460 they want to be in bonds. 62 00:03:16,460 --> 00:03:19,230 So they're highly reactive, they're sort of searching out 63 00:03:19,230 --> 00:03:20,900 to either get rid of their electron, 64 00:03:20,900 --> 00:03:22,180 to get another electron. 65 00:03:22,180 --> 00:03:26,270 They're very, very reactive species. 66 00:03:26,270 --> 00:03:31,120 So radicals turn out to be very important in human health. 67 00:03:31,120 --> 00:03:36,590 And free radicals in biology are both good and bad. 68 00:03:36,590 --> 00:03:40,500 So in the bad category, we have the fact 69 00:03:40,500 --> 00:03:44,810 that free radicals damage DNA, that leads to mutations 70 00:03:44,810 --> 00:03:46,160 in cancer and death. 71 00:03:46,160 --> 00:03:47,940 Not good. 72 00:03:47,940 --> 00:03:50,340 And there are a lot of radical species, many of which 73 00:03:50,340 --> 00:03:54,410 involve oxygen, oxygen radical species that 74 00:03:54,410 --> 00:03:57,750 can be byproducts of metabolism, common cigarette smoke. 75 00:03:57,750 --> 00:04:00,380 So there are natural causes and also 76 00:04:00,380 --> 00:04:02,890 man-made causes of radical species. 77 00:04:02,890 --> 00:04:04,840 And this is very harmful. 78 00:04:04,840 --> 00:04:09,070 But radical species can also be really good. 79 00:04:09,070 --> 00:04:12,480 So they're also essential for life. 80 00:04:12,480 --> 00:04:14,740 They're signaling agents in the body. 81 00:04:14,740 --> 00:04:17,160 Radicals are short-lived, they're highly reactive, 82 00:04:17,160 --> 00:04:18,750 so they're not around very long. 83 00:04:18,750 --> 00:04:20,329 That's a great signaling agent. 84 00:04:20,329 --> 00:04:22,320 You want the signal to turn itself off, 85 00:04:22,320 --> 00:04:25,580 so you want to have something short-lived. 86 00:04:25,580 --> 00:04:28,110 They're also essential for enzyme reactions. 87 00:04:28,110 --> 00:04:32,070 We have no cell division, no growth, no multicellular 88 00:04:32,070 --> 00:04:35,740 organisms if it weren't for radical enzymes. 89 00:04:35,740 --> 00:04:39,430 And we use radicals in our body when 90 00:04:39,430 --> 00:04:41,090 we're fighting off illness. 91 00:04:41,090 --> 00:04:44,310 We generate radical species, or the white blood cells 92 00:04:44,310 --> 00:04:46,420 do, to kill invading microbes. 93 00:04:46,420 --> 00:04:49,770 So we're using radical defense in our bodies 94 00:04:49,770 --> 00:04:51,350 to fight off illness. 95 00:04:51,350 --> 00:04:53,750 So radicals are both good and bad. 96 00:04:53,750 --> 00:04:57,630 So we talked about oxygen radicals a little bit. 97 00:04:57,630 --> 00:05:01,740 One other radical species is nitric oxide. 98 00:05:01,740 --> 00:05:06,460 And now why don't you identify for me which of these molecules 99 00:05:06,460 --> 00:05:10,008 is nitric oxide, knowing that it's a radical? 100 00:05:21,610 --> 00:05:23,605 All right, just 10 more seconds. 101 00:05:41,120 --> 00:05:42,930 OK, so some of you might have just known 102 00:05:42,930 --> 00:05:45,390 that that molecule is nitric oxide. 103 00:05:45,390 --> 00:05:49,970 But if you had done the calculation, 104 00:05:49,970 --> 00:05:51,960 you would have realized that NO is 105 00:05:51,960 --> 00:05:54,490 the only one where you get an odd number of bonding 106 00:05:54,490 --> 00:05:55,350 electrons. 107 00:05:55,350 --> 00:05:58,290 For these other molecules, you have an even number 108 00:05:58,290 --> 00:06:00,970 and they can't be radical species with an even number. 109 00:06:00,970 --> 00:06:03,500 You need an odd number of bonding electrons 110 00:06:03,500 --> 00:06:05,160 to have a radical species. 111 00:06:05,160 --> 00:06:09,216 So this is nitric oxide, NO. 112 00:06:09,216 --> 00:06:10,610 And what does NO do? 113 00:06:10,610 --> 00:06:14,130 So NO is an important signaling molecule in the body. 114 00:06:14,130 --> 00:06:17,740 So it activates something called guanylyl cyclase, which 115 00:06:17,740 --> 00:06:22,100 is an enzyme, which then starts a second signaling system that 116 00:06:22,100 --> 00:06:28,210 involves GTP forming another signaling agent, cyclic GMP. 117 00:06:28,210 --> 00:06:31,680 And this causes vasodilaiton. 118 00:06:31,680 --> 00:06:35,540 So scientists were very interested in this pathway, 119 00:06:35,540 --> 00:06:39,390 and they noted that cyclic GMP can 120 00:06:39,390 --> 00:06:43,280 be degraded by an enzyme turning off this signal. 121 00:06:43,280 --> 00:06:46,960 And they thought, if we inhibit this enzyme, 122 00:06:46,960 --> 00:06:51,670 then we can have a more continued effect 123 00:06:51,670 --> 00:06:53,090 of vasodilaiton. 124 00:06:53,090 --> 00:06:54,630 This could be good for people who 125 00:06:54,630 --> 00:06:59,110 are suffering from heart disease, 126 00:06:59,110 --> 00:07:01,830 they need to get more oxygen to their lungs, 127 00:07:01,830 --> 00:07:03,480 they might have emphysema. 128 00:07:03,480 --> 00:07:06,440 This could be a very good thing for these people. 129 00:07:06,440 --> 00:07:08,900 So they designed an inhibitor for this, 130 00:07:08,900 --> 00:07:12,270 but it turned out it didn't work exactly as they expected. 131 00:07:12,270 --> 00:07:14,860 It did increase vasodilaiton, but only 132 00:07:14,860 --> 00:07:20,100 in one part of the body and only for one gender of individual. 133 00:07:20,100 --> 00:07:25,360 So this led to the bestselling pharmaceutical molecule 134 00:07:25,360 --> 00:07:28,360 of all time. 135 00:07:28,360 --> 00:07:32,140 So a good example of how science sometimes doesn't take you 136 00:07:32,140 --> 00:07:34,610 exactly where you wanted to go but nonetheless 137 00:07:34,610 --> 00:07:36,300 was highly profitable. 138 00:07:36,300 --> 00:07:40,290 All right, so nitric oxide-- and I 139 00:07:40,290 --> 00:07:41,860 could tell you that actually a lot 140 00:07:41,860 --> 00:07:45,640 of the early work on nitric oxide was done at MIT. 141 00:07:45,640 --> 00:07:48,180 And for people who came from MIT, 142 00:07:48,180 --> 00:07:51,860 there's a whole just-say-no club here at MIT. 143 00:07:51,860 --> 00:07:55,810 And if you see this just-say-no, it's about nitric oxide. 144 00:07:55,810 --> 00:07:58,410 So just be aware of that. 145 00:07:58,410 --> 00:08:02,000 OK, so now let's do a Lewis structure for this 146 00:08:02,000 --> 00:08:06,450 and see this radical species. 147 00:08:06,450 --> 00:08:10,031 So again, we need an odd number of valence electrons, 148 00:08:10,031 --> 00:08:12,280 if it's going to be a radical, and that's what we get. 149 00:08:12,280 --> 00:08:16,650 So nitrogen has five valence electrons and oxygen has six. 150 00:08:16,650 --> 00:08:18,910 So overall, we have 11. 151 00:08:18,910 --> 00:08:22,130 To have complete octets, nitrogen needs eight, 152 00:08:22,130 --> 00:08:27,150 oxygen needs eight, so we need 16, which gives us five bonding 153 00:08:27,150 --> 00:08:30,320 electrons, 16 minus 11. 154 00:08:30,320 --> 00:08:32,710 That makes it a radical species. 155 00:08:32,710 --> 00:08:38,780 And now we have six remaining and we can put everything on. 156 00:08:38,780 --> 00:08:42,480 So we can put four bonding electrons in between. 157 00:08:42,480 --> 00:08:45,090 We have five, we can only put four 158 00:08:45,090 --> 00:08:47,790 because you can't have a bond with just one electron. 159 00:08:47,790 --> 00:08:51,610 So the fifth one is why it's a radical species. 160 00:08:51,610 --> 00:08:55,650 But we have also six remaining valence electrons. 161 00:08:55,650 --> 00:08:58,980 And we can distribute those around one, two, three, four, 162 00:08:58,980 --> 00:09:00,270 five, six. 163 00:09:00,270 --> 00:09:03,330 And that would be our molecule. 164 00:09:03,330 --> 00:09:06,520 All right, so this is a highly reactive radical species. 165 00:09:06,520 --> 00:09:07,162 Yeah. 166 00:09:07,162 --> 00:09:09,994 AUDIENCE: How do you know that the radical one goes 167 00:09:09,994 --> 00:09:11,410 on the end instead of [INAUDIBLE]? 168 00:09:11,410 --> 00:09:14,080 CATHERINE DRENNAN: Yeah, I actually 169 00:09:14,080 --> 00:09:17,970 was going to calculate whether we could use this, 170 00:09:17,970 --> 00:09:20,780 but that is in fact where that one is going. 171 00:09:20,780 --> 00:09:23,030 So that can be a clicker question maybe for next time. 172 00:09:23,030 --> 00:09:24,363 I didn't actually get to try it. 173 00:09:24,363 --> 00:09:26,640 I was thinking, we should calculate the formal charges 174 00:09:26,640 --> 00:09:28,830 and think about where things are going to go, 175 00:09:28,830 --> 00:09:30,700 but I didn't do that. 176 00:09:30,700 --> 00:09:32,780 But that's a good question. 177 00:09:32,780 --> 00:09:39,620 All right, so let's just look at another example. 178 00:09:39,620 --> 00:09:44,350 Let's look at molecular oxygen for a minute and do this one. 179 00:09:44,350 --> 00:09:47,320 So we're getting lots of practice with Lewis structures 180 00:09:47,320 --> 00:09:47,820 today. 181 00:09:51,250 --> 00:09:54,330 So we need to think about valence electrons. 182 00:09:54,330 --> 00:09:57,570 So we have six for oxygen plus six, 183 00:09:57,570 --> 00:10:02,080 so we have 12 valence electrons. 184 00:10:02,080 --> 00:10:10,540 And we need eight plus eight, we need 16. 185 00:10:10,540 --> 00:10:13,660 Then we need to figure out how many bonding electrons. 186 00:10:13,660 --> 00:10:18,150 That's going to be 16 minus 12, or four. 187 00:10:18,150 --> 00:10:19,390 And now we can assign. 188 00:10:22,680 --> 00:10:32,310 And so we'll put first two, and we have two more. 189 00:10:32,310 --> 00:10:34,430 So we're going to assign them. 190 00:10:34,430 --> 00:10:42,210 We have two more, and we'll put those in. 191 00:10:42,210 --> 00:10:44,220 So we have a double bond. 192 00:10:44,220 --> 00:10:48,100 And then we can think about whether we have any left. 193 00:10:48,100 --> 00:10:51,080 And we had 12 valence electrons. 194 00:10:51,080 --> 00:10:56,090 Now we've used four, so we have eight more 195 00:10:56,090 --> 00:10:57,430 that we can distribute. 196 00:11:00,540 --> 00:11:03,080 So we're going to make some lone pairs here. 197 00:11:03,080 --> 00:11:07,940 So we can put two up here, a lone pair up here, 198 00:11:07,940 --> 00:11:09,300 a lone pair down here. 199 00:11:09,300 --> 00:11:15,470 One, two, three, four-- our eight extra electrons. 200 00:11:15,470 --> 00:11:16,750 And that looks lovely. 201 00:11:16,750 --> 00:11:18,630 Everything is happy. 202 00:11:18,630 --> 00:11:21,450 That seems like a great structure, 203 00:11:21,450 --> 00:11:24,660 except for the fact that it's wrong. 204 00:11:24,660 --> 00:11:26,740 Lewis structures have failed us. 205 00:11:30,140 --> 00:11:36,150 So it turns out that it's a biradical. 206 00:11:36,150 --> 00:11:41,990 And this is really why we live life as we know it. 207 00:11:41,990 --> 00:11:45,520 Oxygen-- there was a time before oxygen, 208 00:11:45,520 --> 00:11:47,330 life was very different. 209 00:11:47,330 --> 00:11:51,880 Oxygen came around, changed life as we know it, 210 00:11:51,880 --> 00:11:53,740 partly because it's a biradical. 211 00:11:53,740 --> 00:11:57,350 It has amazing properties, it can do amazing chemistry. 212 00:11:57,350 --> 00:12:01,070 It allows us to break down sugars and make energy. 213 00:12:01,070 --> 00:12:04,470 And so in reality, oxygen is a biradical. 214 00:12:04,470 --> 00:12:08,910 We know that it is not a double bond, as drawn there. 215 00:12:08,910 --> 00:12:10,590 This is the structure. 216 00:12:10,590 --> 00:12:15,500 We have one bond, not a double bond, lone pairs. 217 00:12:15,500 --> 00:12:18,330 But then we have a single dot over here, 218 00:12:18,330 --> 00:12:19,630 a single dot over here. 219 00:12:19,630 --> 00:12:21,210 That makes it a biradical. 220 00:12:21,210 --> 00:12:24,920 One radical on this oxygen, one radical on that oxygen. 221 00:12:24,920 --> 00:12:28,630 And we can't explain this with Lewis structures. 222 00:12:28,630 --> 00:12:31,556 We need molecular orbital theory. 223 00:12:31,556 --> 00:12:32,930 But you're in luck because that's 224 00:12:32,930 --> 00:12:35,160 what we're going to talk about on Monday. 225 00:12:35,160 --> 00:12:39,750 So Monday, I'll explain to you why this is a biradical, 226 00:12:39,750 --> 00:12:41,690 but you're going to have to wait for then. 227 00:12:41,690 --> 00:12:45,360 Because we have more problems to address first, 228 00:12:45,360 --> 00:12:48,172 we have octet deficient molecules. 229 00:12:51,920 --> 00:12:55,540 Some of these atoms, they're just messing everything up 230 00:12:55,540 --> 00:12:56,540 for us. 231 00:12:56,540 --> 00:13:01,610 OK, but the good news is, octet deficient molecules-- 232 00:13:01,610 --> 00:13:03,530 incomplete octet. 233 00:13:03,530 --> 00:13:08,370 There's only two of them you need to know-- boron, aluminum. 234 00:13:08,370 --> 00:13:11,840 There they are, just those two. 235 00:13:11,840 --> 00:13:13,890 So let's look at boron. 236 00:13:13,890 --> 00:13:17,440 So we have boron and three fluorines. 237 00:13:17,440 --> 00:13:21,677 So we put boron in the middle, three fluorines around it. 238 00:13:21,677 --> 00:13:23,760 How come I'm not putting a fluorine in the middle? 239 00:13:26,860 --> 00:13:28,830 Yeah, fluorine is always going to be terminal, 240 00:13:28,830 --> 00:13:31,260 it does not want to be anywhere else. 241 00:13:31,260 --> 00:13:34,880 OK, so now we can look at part of the periodic table. 242 00:13:34,880 --> 00:13:37,800 We see here is boron. 243 00:13:37,800 --> 00:13:39,290 Right below it, aluminum. 244 00:13:39,290 --> 00:13:42,220 So that helps us remember those exceptions, 245 00:13:42,220 --> 00:13:44,030 they're right there. 246 00:13:44,030 --> 00:13:46,900 And now let's just calculate this Lewis structure 247 00:13:46,900 --> 00:13:53,330 and see about the octet deficiency. 248 00:13:53,330 --> 00:13:58,540 So normally we would say boron has three valence electrons. 249 00:13:58,540 --> 00:14:00,680 And fluorine-- there are three fluorines, 250 00:14:00,680 --> 00:14:02,430 it has seven, then 24. 251 00:14:02,430 --> 00:14:04,500 OK, that actually would always be true. 252 00:14:04,500 --> 00:14:07,310 But now we're saying boron should want eight-- 253 00:14:07,310 --> 00:14:09,630 and we'll come back to that assumption-- 254 00:14:09,630 --> 00:14:10,850 to have complete octet. 255 00:14:10,850 --> 00:14:13,391 That's definitely how much it wants to have a complete octet, 256 00:14:13,391 --> 00:14:15,370 but doesn't really need a complete octet. 257 00:14:15,370 --> 00:14:17,370 Fluorines also want eight. 258 00:14:17,370 --> 00:14:20,030 That's a total of 32. 259 00:14:20,030 --> 00:14:25,970 And now, if we subtract, we have eight bonding electrons. 260 00:14:25,970 --> 00:14:29,070 So we can assign two per bond first. 261 00:14:29,070 --> 00:14:32,800 Now we've used six, and we have two more. 262 00:14:32,800 --> 00:14:35,520 So we have two extras that we can put in, 263 00:14:35,520 --> 00:14:37,460 that gives us a double bond. 264 00:14:37,460 --> 00:14:40,830 We can look at our extra electrons. 265 00:14:40,830 --> 00:14:46,020 We had 24, we used eight now to do bonding, so we have 16 left. 266 00:14:46,020 --> 00:14:48,090 And so we can put two on this fluorine 267 00:14:48,090 --> 00:14:49,490 that has the double bond. 268 00:14:49,490 --> 00:14:52,020 We can put six on the fluorine with the single bond, 269 00:14:52,020 --> 00:14:54,160 and another six on this fluorine. 270 00:14:54,160 --> 00:14:57,160 And that adds up, that's great. 271 00:14:57,160 --> 00:14:59,360 And now we can assign formal charges 272 00:14:59,360 --> 00:15:03,880 and think about how good this structure is. 273 00:15:03,880 --> 00:15:09,820 So if we do the formal charges, let's look at boron first. 274 00:15:09,820 --> 00:15:12,550 So it had three valence electrons, 275 00:15:12,550 --> 00:15:16,770 it has no lone pairs, and it has half of eight bonding 276 00:15:16,770 --> 00:15:18,370 electrons, minus 1. 277 00:15:18,370 --> 00:15:20,980 Not bad for a formal charge. 278 00:15:20,980 --> 00:15:24,490 What about this fluorine with the double bond? 279 00:15:24,490 --> 00:15:26,280 And that's a clicker question, I told you 280 00:15:26,280 --> 00:15:27,863 you'd be really good at formal charges 281 00:15:27,863 --> 00:15:29,006 by the end of class today. 282 00:15:44,990 --> 00:15:48,030 OK, 10 more seconds. 283 00:15:48,030 --> 00:15:49,596 I think we can get in the 90's. 284 00:15:58,200 --> 00:15:59,512 Ah, it's killing me. 285 00:16:03,182 --> 00:16:04,890 All right, we have to keep working on it. 286 00:16:04,890 --> 00:16:07,745 There's going to be more clicker competition Friday. 287 00:16:07,745 --> 00:16:11,600 There will be a formal charge question on it. 288 00:16:11,600 --> 00:16:13,680 And by the way, it's OK to sit in groups. 289 00:16:13,680 --> 00:16:17,950 We've had back-to-back wins of one recitation. 290 00:16:17,950 --> 00:16:21,420 I have noted that some of them sit in a group. 291 00:16:21,420 --> 00:16:23,580 Just saying that is allowed. 292 00:16:23,580 --> 00:16:31,960 OK, so here, we have seven valence electrons for fluorine, 293 00:16:31,960 --> 00:16:36,170 and there were four lone pair electrons. 294 00:16:36,170 --> 00:16:43,950 Half of four bonding electrons, so we 295 00:16:43,950 --> 00:16:47,910 have a plus 1 charge there. 296 00:16:47,910 --> 00:16:50,200 And then let's finish up. 297 00:16:50,200 --> 00:16:56,650 So our single-bonded fluorines, seven minus six lone pairs 298 00:16:56,650 --> 00:16:57,740 for those. 299 00:16:57,740 --> 00:16:59,620 And they have one bond, so they have 300 00:16:59,620 --> 00:17:03,030 half of two bonding electrons, and then there's zero. 301 00:17:03,030 --> 00:17:05,430 Overall, this is a neutral molecule. 302 00:17:05,430 --> 00:17:10,250 So we have plus 1, minus 1, that's a neutral molecule. 303 00:17:10,250 --> 00:17:19,760 This all seems good, except that someone did an experiment 304 00:17:19,760 --> 00:17:23,000 and found that all three boron fluorine 305 00:17:23,000 --> 00:17:25,970 bonds were single bonds. 306 00:17:25,970 --> 00:17:29,230 So again, Lewis structures failed us. 307 00:17:29,230 --> 00:17:32,680 Experiment tells us it's a single bond. 308 00:17:32,680 --> 00:17:36,180 And this is in fact the correct structure here. 309 00:17:36,180 --> 00:17:40,230 Poor boron has an incomplete octet. 310 00:17:40,230 --> 00:17:43,780 So it does not have all the electrons, 311 00:17:43,780 --> 00:17:45,240 but it's happy with that. 312 00:17:45,240 --> 00:17:48,850 It's OK with that because it's one of the exceptions. 313 00:17:48,850 --> 00:17:51,940 And if we look at the formal charges for it, 314 00:17:51,940 --> 00:17:55,630 now we have boron, it had three valence electrons, 315 00:17:55,630 --> 00:18:00,560 no lone pairs, half of six bonding electrons is three. 316 00:18:00,560 --> 00:18:03,260 So now it has a formal charge of zero. 317 00:18:03,260 --> 00:18:07,050 And all three fluorines, we have seven minus six, 318 00:18:07,050 --> 00:18:10,320 they have one bond, so half of two bonding electrons, 319 00:18:10,320 --> 00:18:13,220 they also have a formal charge of zero. 320 00:18:13,220 --> 00:18:16,560 So most elements would not be OK with this. 321 00:18:16,560 --> 00:18:19,220 But boron, with its three valence electrons, 322 00:18:19,220 --> 00:18:20,960 thinks that this is just fine. 323 00:18:20,960 --> 00:18:22,667 So it's one of the exceptions. 324 00:18:22,667 --> 00:18:24,250 And again, we wouldn't have known this 325 00:18:24,250 --> 00:18:26,880 except for experimental data. 326 00:18:26,880 --> 00:18:29,890 Lewis structures work 90% of the time, 327 00:18:29,890 --> 00:18:32,580 but there are some where experiment tells us 328 00:18:32,580 --> 00:18:34,730 that the structure we normally would've 329 00:18:34,730 --> 00:18:39,480 done with our rules of complete octets don't hold. 330 00:18:39,480 --> 00:18:42,830 OK, and we can rationalize this by the fact 331 00:18:42,830 --> 00:18:45,907 the formal charges are more favorable on this molecule. 332 00:18:48,520 --> 00:18:55,670 One more exception-- valence shell expansion. 333 00:18:55,670 --> 00:19:01,270 This again, is not a property of just any atom. 334 00:19:01,270 --> 00:19:04,154 It needs to be elements that have been an n. 335 00:19:04,154 --> 00:19:04,820 What is n again? 336 00:19:08,240 --> 00:19:09,520 n stands for? 337 00:19:09,520 --> 00:19:11,370 AUDIENCE: Principle quantum number. 338 00:19:11,370 --> 00:19:14,850 CATHERINE DRENNAN: Principle quantum number, yes. 339 00:19:14,850 --> 00:19:17,390 You can't forget anything, we're coming back to it, 340 00:19:17,390 --> 00:19:19,430 we're coming back to all of it. 341 00:19:19,430 --> 00:19:21,390 Have a principle quantum number that's 342 00:19:21,390 --> 00:19:25,098 equal or greater than three. 343 00:19:25,098 --> 00:19:28,260 And what do we know about things, what kind of orbitals 344 00:19:28,260 --> 00:19:31,780 do we start talking about when we have principle quantum 345 00:19:31,780 --> 00:19:34,870 numbers of n that are equal to or greater 346 00:19:34,870 --> 00:19:36,492 than three, what happens? 347 00:19:36,492 --> 00:19:37,562 AUDIENCE: d-orbitals. 348 00:19:37,562 --> 00:19:41,750 CATHERINE DRENNAN: We get some d-orbitals, exactly. 349 00:19:41,750 --> 00:19:44,542 d-orbitals-- I love d-orbitals. 350 00:19:44,542 --> 00:19:46,000 Closer to Thanksgiving, we're going 351 00:19:46,000 --> 00:19:50,750 to have a whole unit where we're talking about d-orbitals. 352 00:19:50,750 --> 00:19:53,190 We save that for a really special occasion. 353 00:19:53,190 --> 00:19:55,580 I love d-orbitals. d-orbitals allow 354 00:19:55,580 --> 00:19:58,050 for all sorts of craziness to happen, 355 00:19:58,050 --> 00:20:00,650 including valence shell expansion. 356 00:20:03,780 --> 00:20:09,360 So this is most common when the central atom is large 357 00:20:09,360 --> 00:20:13,970 and it has a bunch of little small, electronegative atoms 358 00:20:13,970 --> 00:20:15,320 around it. 359 00:20:15,320 --> 00:20:18,950 And the transition metals, with their d-orbitals, 360 00:20:18,950 --> 00:20:20,010 they like this. 361 00:20:20,010 --> 00:20:24,270 And other things with d-orbitals are cool with this as well. 362 00:20:24,270 --> 00:20:26,450 So the first example we're going to look at 363 00:20:26,450 --> 00:20:30,740 is phosphorus with five chlorines around it. 364 00:20:30,740 --> 00:20:33,460 Phosphorus is pretty large, and it 365 00:20:33,460 --> 00:20:37,950 has chlorine, which is a fairly small electronegative 366 00:20:37,950 --> 00:20:38,800 atom around it. 367 00:20:38,800 --> 00:20:40,900 And it has five of them around it. 368 00:20:40,900 --> 00:20:43,270 So we'll put our phosphorus in the middle 369 00:20:43,270 --> 00:20:46,990 and we'll put our five chlorines around it. 370 00:20:46,990 --> 00:20:49,640 Now we need to count valence electrons because we 371 00:20:49,640 --> 00:20:53,210 need to draw Lewis structures. 372 00:20:53,210 --> 00:20:58,300 So phosphorous brings five valence electrons to the gain. 373 00:20:58,300 --> 00:21:01,400 The chlorines each have seven, and there are five of them. 374 00:21:01,400 --> 00:21:05,530 So we have 40 altogether. 375 00:21:05,530 --> 00:21:07,870 Phosphorus, we think, wants eight. 376 00:21:07,870 --> 00:21:10,230 It would, for a complete octet. 377 00:21:10,230 --> 00:21:13,250 Also, chlorine wants eight, and there are five of those. 378 00:21:13,250 --> 00:21:18,710 So total, if we have complete octets, we need 48 electrons. 379 00:21:18,710 --> 00:21:23,160 48 minus 40, the number needed for the complete octet 380 00:21:23,160 --> 00:21:24,755 minus the number of valence electrons 381 00:21:24,755 --> 00:21:27,870 gives you eight bonding electrons. 382 00:21:27,870 --> 00:21:32,610 So we're going to assign two per bond, up to eight, 383 00:21:32,610 --> 00:21:33,900 and there we go. 384 00:21:33,900 --> 00:21:36,360 Now does that look like a good Lewis structure to you? 385 00:21:36,360 --> 00:21:37,200 AUDIENCE: No. 386 00:21:37,200 --> 00:21:39,033 CATHERINE DRENNAN: Yeah, it's pretty obvious 387 00:21:39,033 --> 00:21:41,230 that there's something wrong here. 388 00:21:41,230 --> 00:21:44,390 So this is not going to work out. 389 00:21:44,390 --> 00:21:46,790 We're not going to add chlorine to another chlorine 390 00:21:46,790 --> 00:21:49,050 because they don't like to do that. 391 00:21:49,050 --> 00:21:51,270 Halogens really like to be terminal. 392 00:21:51,270 --> 00:21:52,800 So that's not going to work. 393 00:21:52,800 --> 00:21:57,470 So we've got to use some extra electrons and make a new bond. 394 00:21:57,470 --> 00:22:00,070 So we need to have five bonds, so that 395 00:22:00,070 --> 00:22:01,740 means we need to use 10. 396 00:22:01,740 --> 00:22:04,410 We need to have 10 bonding electrons. 397 00:22:04,410 --> 00:22:09,660 And when we borrow another two, we can make another bond. 398 00:22:09,660 --> 00:22:12,430 We had 40, we've now used 10. 399 00:22:12,430 --> 00:22:16,730 We had no choice, we had to use 10, so we have 30 lone pairs. 400 00:22:16,730 --> 00:22:23,050 And we have five chlorines that all want six lone pair 401 00:22:23,050 --> 00:22:27,060 electrons to complete their octet, and six times five 402 00:22:27,060 --> 00:22:30,680 is 30, so this works out beautifully. 403 00:22:30,680 --> 00:22:33,330 So this is a very happy structure now. 404 00:22:33,330 --> 00:22:36,380 Phosphorus is OK, it's large. 405 00:22:36,380 --> 00:22:38,040 It has an n equal 3. 406 00:22:38,040 --> 00:22:41,520 It's OK being expanded, and chlorine is very happy. 407 00:22:41,520 --> 00:22:45,100 So the bottom line is there are certain kinds of elements 408 00:22:45,100 --> 00:22:46,230 that you can expand. 409 00:22:46,230 --> 00:22:49,220 They can have more than an octet. 410 00:22:49,220 --> 00:22:50,660 They can have five bonds. 411 00:22:50,660 --> 00:22:54,420 Others, carbon-- no, carbon is not going to have five bonds. 412 00:22:54,420 --> 00:22:56,040 But phosphorus is OK. 413 00:22:56,040 --> 00:23:00,490 So some things can be expanded, some things can't be expanded. 414 00:23:00,490 --> 00:23:04,410 So you have to think about what the expansion would do 415 00:23:04,410 --> 00:23:08,250 and whether that would be allowed. 416 00:23:08,250 --> 00:23:10,275 Let's do one more example. 417 00:23:13,650 --> 00:23:19,450 Chromium with four oxygens, minus 2 charge. 418 00:23:19,450 --> 00:23:21,360 We see chromium over here. 419 00:23:21,360 --> 00:23:24,540 It's got d electrons. 420 00:23:24,540 --> 00:23:27,170 Let's see what happens. 421 00:23:27,170 --> 00:23:30,700 So here we have our skeleton structure. 422 00:23:30,700 --> 00:23:35,350 We put chromium in the middle and our four oxygens around it. 423 00:23:35,350 --> 00:23:38,050 Now we have a bracket around it with a little minus 2. 424 00:23:38,050 --> 00:23:41,040 Note the minus 2 because it has a charge on it. 425 00:23:41,040 --> 00:23:43,430 Don't forget about the charge. 426 00:23:43,430 --> 00:23:47,720 And now we need to see how many valence electrons. 427 00:23:47,720 --> 00:23:50,030 So chromium is going to bring six. 428 00:23:50,030 --> 00:23:52,710 So we can look at where it is in the periodic table-- one, two, 429 00:23:52,710 --> 00:23:55,170 three, four, five, six. 430 00:23:55,170 --> 00:23:56,820 Our oxygen is up here. 431 00:23:56,820 --> 00:23:59,850 Also, have six valence electrons. 432 00:23:59,850 --> 00:24:04,010 And don't forget about the charge on the molecule. 433 00:24:04,010 --> 00:24:07,160 So when it's minus 2, you have to add two electrons. 434 00:24:07,160 --> 00:24:10,150 It has extra two electrons, so add those in, 435 00:24:10,150 --> 00:24:13,910 which gives you a total of 32. 436 00:24:13,910 --> 00:24:16,640 What do you need for your octet? 437 00:24:16,640 --> 00:24:20,520 So eight for the chromium, four times eight 438 00:24:20,520 --> 00:24:22,240 for all the four oxygens. 439 00:24:22,240 --> 00:24:27,040 We need 40, then we can take our 40, minus 32, 440 00:24:27,040 --> 00:24:29,900 and we have eight bonding electrons. 441 00:24:29,900 --> 00:24:32,460 That seems good, two per bond. 442 00:24:32,460 --> 00:24:34,670 We'll put those on. 443 00:24:34,670 --> 00:24:38,430 Then we'll see if we have any lone pairs, and we do. 444 00:24:38,430 --> 00:24:42,160 32 minus eight gives us 24. 445 00:24:42,160 --> 00:24:44,960 The oxygens are going to want six, 446 00:24:44,960 --> 00:24:50,490 and there are four of them, so that just looks awesome. 447 00:24:50,490 --> 00:24:53,280 So maybe this doesn't need to expand. 448 00:24:53,280 --> 00:24:54,960 Let's see. 449 00:24:54,960 --> 00:24:58,710 What do we do to test this? 450 00:24:58,710 --> 00:25:02,400 Experiments, or, if we don't have a lab, we do-- 451 00:25:02,400 --> 00:25:03,460 AUDIENCE: Formal charge. 452 00:25:03,460 --> 00:25:04,910 CATHERINE DRENNAN: Formal charge. 453 00:25:04,910 --> 00:25:06,850 Yes, that's easier to do in the classroom. 454 00:25:06,850 --> 00:25:09,980 All right, so we'll calculate a formal charge. 455 00:25:09,980 --> 00:25:12,420 Chromium brought six valence electrons. 456 00:25:12,420 --> 00:25:16,740 It has no lone pairs, it has half of eight bonding 457 00:25:16,740 --> 00:25:20,960 electrons, or four bonds, plus two oxygens, 458 00:25:20,960 --> 00:25:25,250 have six valence electrons, six loan pairs, one bond, so half 459 00:25:25,250 --> 00:25:27,480 of two, or minus 1. 460 00:25:27,480 --> 00:25:30,220 So the total charge here, because you have four 461 00:25:30,220 --> 00:25:33,400 of those, plus 2, is minus 2. 462 00:25:33,400 --> 00:25:35,380 So that's OK. 463 00:25:35,380 --> 00:25:38,310 But this has a pretty big charge distribution. 464 00:25:38,310 --> 00:25:40,850 Everything has a formal charge on it. 465 00:25:40,850 --> 00:25:43,700 And that doesn't make molecules very happy, 466 00:25:43,700 --> 00:25:47,110 they want formal charges of zero, maybe plus or minus 1. 467 00:25:47,110 --> 00:25:49,890 But this is a lot of formal charge. 468 00:25:49,890 --> 00:25:52,270 So maybe we can do better. 469 00:25:52,270 --> 00:25:54,950 And as someone said, someone else 470 00:25:54,950 --> 00:26:00,970 did an experiment and-- yup-- they're not single bonds. 471 00:26:00,970 --> 00:26:03,990 So the experiment showed that it's somewhere 472 00:26:03,990 --> 00:26:07,310 between a single and double bond. 473 00:26:07,310 --> 00:26:09,890 So this is not the right Lewis structure. 474 00:26:09,890 --> 00:26:12,630 And one of the tip-offs could be, whenever you see plus 2, 475 00:26:12,630 --> 00:26:18,310 make sure there's not another possible structure, or minus 2. 476 00:26:18,310 --> 00:26:21,350 So you can make double bonds here. 477 00:26:21,350 --> 00:26:25,770 And if you do that and calculate the formal charge then 478 00:26:25,770 --> 00:26:29,800 on chromium, it had six, no lone pairs, 479 00:26:29,800 --> 00:26:34,640 but now it has six bonds, so it has a formal charge of zero. 480 00:26:34,640 --> 00:26:40,620 The double-bonded oxygens had brought six valence electrons, 481 00:26:40,620 --> 00:26:45,930 they have four lone pairs, and they have half of four bonding 482 00:26:45,930 --> 00:26:48,270 electrons, now they're zero. 483 00:26:48,270 --> 00:26:52,050 And the two single oxygens here are still minus 1. 484 00:26:52,050 --> 00:26:54,980 There are two of them, so that equals the minus 2. 485 00:26:54,980 --> 00:26:57,480 But this has much less charge separation. 486 00:26:57,480 --> 00:26:59,010 You have lots of zeros. 487 00:26:59,010 --> 00:27:02,980 You don't have any more things with charges of 2. 488 00:27:02,980 --> 00:27:06,760 But this is not done. 489 00:27:06,760 --> 00:27:08,197 What else do we need here? 490 00:27:08,197 --> 00:27:09,630 AUDIENCE: Resonance structures. 491 00:27:09,630 --> 00:27:11,629 CATHERINE DRENNAN: We need resonance structures, 492 00:27:11,629 --> 00:27:15,450 that's right, because I just put the double bonds here. 493 00:27:15,450 --> 00:27:19,870 I could have put them there, or maybe somewhere else. 494 00:27:19,870 --> 00:27:22,940 And you can tell me how many other structures 495 00:27:22,940 --> 00:27:24,780 you need to complete this. 496 00:27:37,055 --> 00:27:38,715 All right, 10 more seconds. 497 00:27:55,760 --> 00:27:58,630 Yup, this is a good one, I think, 498 00:27:58,630 --> 00:28:00,930 that helps when you do it in a team. 499 00:28:00,930 --> 00:28:02,950 Everyone sees different ones. 500 00:28:02,950 --> 00:28:05,050 So there actually are four other ones. 501 00:28:05,050 --> 00:28:07,910 You can try to write that out and prove it to yourself. 502 00:28:07,910 --> 00:28:10,720 There are four other structures you can do. 503 00:28:10,720 --> 00:28:14,920 And you need all of those to get the correct structure. 504 00:28:14,920 --> 00:28:18,790 So these structures are correct, it is a resonance structure. 505 00:28:18,790 --> 00:28:21,600 There's less formal charge separation. 506 00:28:21,600 --> 00:28:24,200 OK, so that's the end of Lewis structures. 507 00:28:24,200 --> 00:28:27,590 And we're going to talk about shapes and molecules on Friday. 508 00:28:27,590 --> 00:28:30,080 And problem set is due on Friday, 509 00:28:30,080 --> 00:28:32,390 and it has lots of Lewis structures 510 00:28:32,390 --> 00:28:35,940 in it, so don't leave it to the last minute.