1 00:00:16,166 --> 00:00:20,571 OK now, exam one. 2 00:00:20,571 --> 00:00:22,272 I mentioned this before. 3 00:00:22,272 --> 00:00:25,476 I thought this would be helpful to everyone. 4 00:00:25,476 --> 00:00:31,348 What I did is put together what I call a concept map. 5 00:00:31,348 --> 00:00:33,584 And I will post this, here it is, oh, there 6 00:00:33,584 --> 00:00:37,354 is a periodic table, isn't that lovely? 7 00:00:37,354 --> 00:00:40,257 This is what I put together for you guys. 8 00:00:40,257 --> 00:00:42,192 And I call it exam one concept map. 9 00:00:42,192 --> 00:00:44,094 Now, I'm not going to go through it now. 10 00:00:44,094 --> 00:00:47,931 But I wanted you guys to have the big picture of what 11 00:00:47,931 --> 00:00:49,933 we've done. 12 00:00:49,933 --> 00:00:53,270 This really is a celebration, especially when 13 00:00:53,270 --> 00:00:55,706 you look at all of the things that we've learned 14 00:00:55,706 --> 00:00:57,474 and how they're all connected together. 15 00:00:57,474 --> 00:00:59,710 And that's what I wanted you guys to see. 16 00:00:59,710 --> 00:01:00,811 OK so, look at this. 17 00:01:00,811 --> 00:01:03,347 So here are on the left here are-- 18 00:01:03,347 --> 00:01:05,783 this is all not graded stuff, here's what's been graded. 19 00:01:05,783 --> 00:01:08,252 Oh, and this is what's graded on Monday, 20 00:01:08,252 --> 00:01:10,487 exam one problem topics. 21 00:01:10,487 --> 00:01:11,555 Look at all those things. 22 00:01:11,555 --> 00:01:13,323 Now over here, we have the three goody bags 23 00:01:13,323 --> 00:01:14,558 that you guys have had. 24 00:01:14,558 --> 00:01:16,794 And there have been problems in there. 25 00:01:16,794 --> 00:01:19,129 And here we have the problems that you've 26 00:01:19,129 --> 00:01:21,965 had from the textbooks and the topics there. 27 00:01:21,965 --> 00:01:23,767 Here we've had your seven recitations. 28 00:01:23,767 --> 00:01:26,637 Here you've had-- it's even color coded. 29 00:01:26,637 --> 00:01:28,472 It's even color coded. 30 00:01:28,472 --> 00:01:29,940 So here you've got the lectures. 31 00:01:29,940 --> 00:01:32,176 So these the nine lectures and all the topics 32 00:01:32,176 --> 00:01:33,577 that we've talked about. 33 00:01:33,577 --> 00:01:34,878 Today is number 10. 34 00:01:34,878 --> 00:01:37,581 And as I said, I will tell you something today 35 00:01:37,581 --> 00:01:42,586 about Lewis structures that will not be on the exam, 36 00:01:42,586 --> 00:01:44,221 and that is resonant Lewis structures. 37 00:01:44,221 --> 00:01:46,590 So we'll teach that. 38 00:01:46,590 --> 00:01:48,125 And then those are the three quizzes. 39 00:01:48,125 --> 00:01:49,626 Now you notice, there's some things, 40 00:01:49,626 --> 00:01:53,864 like here, OK, we've covered this stuff, like Lewis dots, 41 00:01:53,864 --> 00:01:56,600 and electronegativity, Lewis structures, 42 00:01:56,600 --> 00:01:59,169 stuff like that we've covered that has not been on a quiz. 43 00:01:59,169 --> 00:02:01,805 But it could be on the exam. 44 00:02:01,805 --> 00:02:03,707 So this is a concept map that I hope 45 00:02:03,707 --> 00:02:07,744 gives you a sense of how this all connects together. 46 00:02:07,744 --> 00:02:11,415 And how this connects together to give you 47 00:02:11,415 --> 00:02:14,618 the knowledge that you need to answer questions 48 00:02:14,618 --> 00:02:17,588 about these topics. 49 00:02:17,588 --> 00:02:20,190 Now, the exam is not-- 50 00:02:20,190 --> 00:02:24,228 these exams walk the balance. 51 00:02:24,228 --> 00:02:29,299 We want you to learn how to solve these problems. 52 00:02:29,299 --> 00:02:32,668 What I want you to do on Monday is run out of knowledge, 53 00:02:32,668 --> 00:02:33,670 not run out of time. 54 00:02:37,274 --> 00:02:37,941 Did I get a-- 55 00:02:37,941 --> 00:02:39,343 thank you. 56 00:02:39,343 --> 00:02:42,779 Now, you might run out of both, but listen-- 57 00:02:42,779 --> 00:02:45,949 [LAUGHTER] 58 00:02:45,949 --> 00:02:48,986 But these are not meant to break you in a speed test. 59 00:02:48,986 --> 00:02:51,555 These are meant to see if you have learned the concepts. 60 00:02:51,555 --> 00:02:55,392 So what I ask you to do on this exam is show us what you know. 61 00:02:58,795 --> 00:03:01,999 As you may know already, I am a big fan of partial credit. 62 00:03:01,999 --> 00:03:03,100 Lots of partial credit. 63 00:03:03,100 --> 00:03:07,070 If you show me you know something, you will get credit. 64 00:03:07,070 --> 00:03:09,306 And so that gets into, also, exam strategies, 65 00:03:09,306 --> 00:03:11,275 but a lot of you already know about this stuff. 66 00:03:11,275 --> 00:03:15,779 But please remember, don't just go in order, 67 00:03:15,779 --> 00:03:17,214 and then run out of time, and not 68 00:03:17,214 --> 00:03:19,283 even see the last two questions. 69 00:03:19,283 --> 00:03:20,083 Take a look. 70 00:03:20,083 --> 00:03:21,151 Tell me what you know. 71 00:03:21,151 --> 00:03:22,052 Show us what you know. 72 00:03:24,688 --> 00:03:27,958 We're not about the last decimal on some calculation. 73 00:03:27,958 --> 00:03:30,227 I'm much more about do you understand the concept? 74 00:03:30,227 --> 00:03:31,828 Do you know how to solve this problem? 75 00:03:31,828 --> 00:03:34,798 So show us that, please, on the exam. 76 00:03:34,798 --> 00:03:39,236 OK, now was that another loud snap in favor. 77 00:03:39,236 --> 00:03:42,072 I think that means it's like positive reinforcement. 78 00:03:46,243 --> 00:03:52,316 Topics, OK so, the other thing I want to say about the exam 79 00:03:52,316 --> 00:03:56,420 is we're going to connect concepts together sometimes. 80 00:03:56,420 --> 00:03:58,088 That's the point. 81 00:03:58,088 --> 00:04:01,325 Exams aren't just going to be another three quizzes plus one 82 00:04:01,325 --> 00:04:03,927 more question on Lewis. 83 00:04:03,927 --> 00:04:07,197 No, we're going to see if you can also weave together 84 00:04:07,197 --> 00:04:09,399 the knowledge. 85 00:04:09,399 --> 00:04:12,502 So an example would be, if I were to ask you, OK, I'm 86 00:04:12,502 --> 00:04:13,503 going to give you a PES. 87 00:04:16,206 --> 00:04:20,444 So let's see, here would be like a question about a PES 88 00:04:20,444 --> 00:04:20,944 question. 89 00:04:20,944 --> 00:04:23,447 So here's a PES diagram. 90 00:04:23,447 --> 00:04:25,749 And here I'm going to draw, I'm going to say, OK, 91 00:04:25,749 --> 00:04:28,018 I give you this PES. 92 00:04:28,018 --> 00:04:33,357 Here we go, let's label it, three, four, five, six. 93 00:04:33,357 --> 00:04:35,192 We don't need more than that. 94 00:04:35,192 --> 00:04:38,228 OK so, we're going to go, oh, and a break in the axis. 95 00:04:38,228 --> 00:04:39,429 Now you know all about that. 96 00:04:39,429 --> 00:04:42,099 And we're going to go all the way up to six here. 97 00:04:42,099 --> 00:04:44,668 And we'll do another break. 98 00:04:44,668 --> 00:04:46,470 We'll go up to two there. 99 00:04:46,470 --> 00:04:50,841 And now this is going to go up to here. 100 00:04:50,841 --> 00:04:52,542 And you can read off of here. 101 00:04:52,542 --> 00:04:56,813 Remember, with the PES, that's a relative electron count. 102 00:04:56,813 --> 00:05:02,119 I'm blasting this atom with enough photon energy 103 00:05:02,119 --> 00:05:03,353 to get all the electrons out. 104 00:05:03,353 --> 00:05:05,756 And then I'm counting their relative-- 105 00:05:05,756 --> 00:05:08,125 I measure the kinetic energy, I can get the energy. 106 00:05:08,125 --> 00:05:13,163 Oh by the way, this would be like decreasing, 107 00:05:13,163 --> 00:05:14,498 the way I plotted it before. 108 00:05:14,498 --> 00:05:19,036 And the way we usually plot PES diagrams is the x-axis 109 00:05:19,036 --> 00:05:20,904 is going down in energy. 110 00:05:20,904 --> 00:05:25,242 Because those are the 1S electrons there. 111 00:05:25,242 --> 00:05:27,277 These would be the 2S. 112 00:05:27,277 --> 00:05:30,147 And these would be the 2P. 113 00:05:30,147 --> 00:05:31,715 And I can keep on going. 114 00:05:31,715 --> 00:05:33,383 So I've got 3S. 115 00:05:33,383 --> 00:05:34,651 Now what do we got here? 116 00:05:34,651 --> 00:05:38,021 We've got 3P, How many? 117 00:05:38,021 --> 00:05:38,522 Four. 118 00:05:38,522 --> 00:05:40,457 OK, good. 119 00:05:40,457 --> 00:05:44,895 OK, a question you might be asked, what atom is that? 120 00:05:44,895 --> 00:05:45,762 Sulfur. 121 00:05:45,762 --> 00:05:47,364 OK, good. 122 00:05:47,364 --> 00:05:48,765 OK that's sulfur. 123 00:05:48,765 --> 00:05:50,801 So I can answer that kind of question. 124 00:05:50,801 --> 00:05:54,805 But now, let's take another concept and weave it in here. 125 00:05:54,805 --> 00:05:59,543 OK, if that's sulfur and I say, well, 126 00:05:59,543 --> 00:06:02,245 what if I went up to chlorine. 127 00:06:02,245 --> 00:06:04,381 Right so, sulfur has 16. 128 00:06:04,381 --> 00:06:06,116 Sulfur has 16. 129 00:06:06,116 --> 00:06:07,617 And chlorine is 17. 130 00:06:10,454 --> 00:06:13,190 What would this look like for chlorine? 131 00:06:13,190 --> 00:06:14,257 I gave you this. 132 00:06:14,257 --> 00:06:16,727 Well, you might say, well, I know, 133 00:06:16,727 --> 00:06:18,195 it's going to just be like this. 134 00:06:20,731 --> 00:06:22,466 But let's go to the right height. 135 00:06:22,466 --> 00:06:25,802 And now I filled that with the one more electron. 136 00:06:25,802 --> 00:06:27,437 And so it must be chlorine. 137 00:06:27,437 --> 00:06:31,575 But that's not taking everything you know into account. 138 00:06:31,575 --> 00:06:34,444 Because you know that if I go up to chlorine, 139 00:06:34,444 --> 00:06:36,046 I've added a proton. 140 00:06:36,046 --> 00:06:38,281 But I haven't really done much more 141 00:06:38,281 --> 00:06:42,185 to shield those outer electrons. 142 00:06:42,185 --> 00:06:44,254 And so everything wants to be closer 143 00:06:44,254 --> 00:06:45,655 to that more positive charge. 144 00:06:45,655 --> 00:06:48,759 So you've got a reflect that in the PES. 145 00:06:48,759 --> 00:06:52,496 So if I were really going to say, well now, you know. 146 00:06:52,496 --> 00:06:54,331 You might not even know quantitatively. 147 00:06:54,331 --> 00:06:56,032 But you know qualitatively, if I'm 148 00:06:56,032 --> 00:06:57,601 going to draw this peak for chlorine, 149 00:06:57,601 --> 00:07:01,805 it better be to the left of that peak. 150 00:07:01,805 --> 00:07:04,741 It's got to move in just like the orbitals and all 151 00:07:04,741 --> 00:07:06,076 those things we've talked about. 152 00:07:06,076 --> 00:07:08,111 That's the kind of thing that I mean. 153 00:07:08,111 --> 00:07:10,981 I want you to be able to synthesize concepts as we 154 00:07:10,981 --> 00:07:13,683 think about exam questions. 155 00:07:13,683 --> 00:07:16,052 Everything is going to shift over, if you go to chlorine. 156 00:07:20,657 --> 00:07:29,032 Now, on Wednesday, we talked about Lewis structures. 157 00:07:29,032 --> 00:07:32,936 So let's talk about Lewis structures more. 158 00:07:32,936 --> 00:07:36,473 Let's suppose I've got acetaldehyde, 159 00:07:36,473 --> 00:07:42,245 a wonderful toxic compound, H4O. 160 00:07:42,245 --> 00:07:48,552 OK, here is a possible Lewis structure for this. 161 00:07:48,552 --> 00:07:52,189 All right, let's see, I'm going to draw the two carbons here. 162 00:07:52,189 --> 00:07:56,226 Here's my oxygen. It's got some lone pairs, put 163 00:07:56,226 --> 00:07:58,295 another one there, why not. 164 00:07:58,295 --> 00:08:03,200 Hydrogen here, and we got one more hydrogen there. 165 00:08:03,200 --> 00:08:04,534 What is wrong with this? 166 00:08:07,604 --> 00:08:12,008 Double bond on the hydrogen. Question 167 00:08:12,008 --> 00:08:16,346 really is, what is not wrong with this? 168 00:08:16,346 --> 00:08:19,082 I can count three things, right now, that are wrong with this. 169 00:08:19,082 --> 00:08:19,749 What are they? 170 00:08:19,749 --> 00:08:25,121 Double bond-- this has to have, remember, these things 171 00:08:25,121 --> 00:08:29,993 need to be happy and follow their octet. 172 00:08:29,993 --> 00:08:34,331 Octet means for hydrogen, two. 173 00:08:34,331 --> 00:08:37,534 But otherwise, so that's good. 174 00:08:37,534 --> 00:08:38,034 That's good. 175 00:08:38,034 --> 00:08:39,269 That's not good. 176 00:08:39,269 --> 00:08:40,437 OK so, we're not-- 177 00:08:40,437 --> 00:08:41,905 is this good? 178 00:08:41,905 --> 00:08:45,008 No, no. 179 00:08:45,008 --> 00:08:46,009 Is that good? 180 00:08:48,778 --> 00:08:52,148 Now, hold on, because OK, but there's something else that's 181 00:08:52,148 --> 00:08:55,352 fundamentally wrong with this. 182 00:08:55,352 --> 00:08:57,287 Anybody notice? 183 00:08:57,287 --> 00:08:58,922 How many electrons? 184 00:08:58,922 --> 00:09:01,024 This is how you start a Lewis problem. 185 00:09:01,024 --> 00:09:04,394 How many electrons do I have? 186 00:09:04,394 --> 00:09:05,629 How many electrons do I have? 187 00:09:05,629 --> 00:09:08,131 Remember Lewis is all about the valence. 188 00:09:08,131 --> 00:09:12,402 So I've got 4 times 2, that's 8. 189 00:09:12,402 --> 00:09:15,972 Eight electrons, four electrons here, and six electrons here, 190 00:09:15,972 --> 00:09:18,275 how many do I have? 191 00:09:18,275 --> 00:09:21,545 So I've got 18 electrons total. 192 00:09:21,545 --> 00:09:23,847 Now what did I put into this diagram? 193 00:09:23,847 --> 00:09:26,182 So each of these bonds, remember, a line 194 00:09:26,182 --> 00:09:27,717 is two electrons. 195 00:09:27,717 --> 00:09:31,221 It's a bonding pair. 196 00:09:31,221 --> 00:09:39,262 So I've got 2, 4, 6, 8, 10, 12, 14, 16, 18, 20. 197 00:09:39,262 --> 00:09:43,233 I didn't even get the electron count right. 198 00:09:43,233 --> 00:09:47,871 I didn't say it's acetaldehyde two minus. 199 00:09:47,871 --> 00:09:52,142 Wrong number of electrons. 200 00:09:54,744 --> 00:10:00,083 And octets not formed. 201 00:10:00,083 --> 00:10:01,051 There's something else. 202 00:10:01,051 --> 00:10:02,485 There's another way you can see how 203 00:10:02,485 --> 00:10:04,688 this Lewis structure is wrong. 204 00:10:04,688 --> 00:10:06,523 How is that? 205 00:10:06,523 --> 00:10:08,458 What else did we learn on Wednesday? 206 00:10:08,458 --> 00:10:11,161 Really important concept. 207 00:10:11,161 --> 00:10:13,096 Formal charge. 208 00:10:13,096 --> 00:10:17,767 Formal charges are all over the place. 209 00:10:17,767 --> 00:10:26,176 Lowest not lowest formal charge. 210 00:10:26,176 --> 00:10:31,147 Now, when I say lowest, I mean closest to zero, 211 00:10:31,147 --> 00:10:35,485 because that is what gives you the most stable structure, 212 00:10:35,485 --> 00:10:36,820 as we talked about on Wednesday. 213 00:10:36,820 --> 00:10:40,557 You can see this, formal charges here are all over the place. 214 00:10:40,557 --> 00:10:44,394 So the formal charge on this carbon is-- 215 00:10:44,394 --> 00:10:46,363 OK so, you go the number of valence electrons 216 00:10:46,363 --> 00:10:52,569 of the free atom minus dots minus 6. 217 00:10:52,569 --> 00:10:58,875 So 4 minus 1, 2, 3, so that's plus 1. 218 00:10:58,875 --> 00:11:03,580 Oh, 6 minus 1, 2, 3, 4, 5, 6, 7. 219 00:11:03,580 --> 00:11:05,649 So that would be minus 1. 220 00:11:05,649 --> 00:11:08,351 So this is not a good Lewis structure 221 00:11:08,351 --> 00:11:09,786 for all of these reasons. 222 00:11:09,786 --> 00:11:14,090 The right Lewis structure would look something like this. 223 00:11:14,090 --> 00:11:15,125 Very easy fix. 224 00:11:17,761 --> 00:11:21,665 And C, and here you go. 225 00:11:21,665 --> 00:11:25,802 And now, you can check all of these things 226 00:11:25,802 --> 00:11:26,870 that we just talked about. 227 00:11:26,870 --> 00:11:28,204 You can check the formal charge. 228 00:11:28,204 --> 00:11:31,041 You can check the total charge. 229 00:11:31,041 --> 00:11:33,176 I could've taken two electrons off of here. 230 00:11:33,176 --> 00:11:34,644 And it still would have been wrong. 231 00:11:37,313 --> 00:11:38,915 But this is the right Lewis structure, 232 00:11:38,915 --> 00:11:41,951 because we satisfy all the things that we talked about 233 00:11:41,951 --> 00:11:43,286 on Wednesday. 234 00:11:43,286 --> 00:11:49,225 OK, good, now, as I mentioned, there is one other concept oh-- 235 00:11:51,194 --> 00:11:53,863 The last concept that will not be on the exam, 236 00:11:53,863 --> 00:11:58,334 oh, but it could be on exam two, I don't know, 237 00:11:58,334 --> 00:12:00,403 is the last concept about Lewis structures 238 00:12:00,403 --> 00:12:02,806 that's very important. 239 00:12:02,806 --> 00:12:04,374 And it's what happens when you have 240 00:12:04,374 --> 00:12:07,010 structures that look similar. 241 00:12:10,113 --> 00:12:11,748 If I look at all these things, number 242 00:12:11,748 --> 00:12:16,720 of electrons, octets forming, formal charges, 243 00:12:16,720 --> 00:12:18,321 these look good. 244 00:12:18,321 --> 00:12:21,124 These both look good. 245 00:12:21,124 --> 00:12:25,228 And so the question is, which one of them is right? 246 00:12:25,228 --> 00:12:29,232 And this gets into this concept that I 247 00:12:29,232 --> 00:12:32,769 want to teach you, which is called resonance. 248 00:12:35,405 --> 00:12:41,945 And just to define resonance, I'll put that on the board. 249 00:12:41,945 --> 00:12:44,214 These are called resonance structures. 250 00:12:44,214 --> 00:12:53,990 OK now, resonance, when referring to Lewis structures, 251 00:12:53,990 --> 00:12:55,492 the way I want you to think about it 252 00:12:55,492 --> 00:13:00,563 is that it describes delocalization. 253 00:13:00,563 --> 00:13:07,270 I'm going all caps, delocalization, why not? 254 00:13:10,306 --> 00:13:11,241 I'm not shouting it. 255 00:13:11,241 --> 00:13:13,810 I'm just saying it's an important concept. 256 00:13:13,810 --> 00:13:22,085 Describes delocalization of electrons in molecules. 257 00:13:26,823 --> 00:13:29,392 So in the case of ozone, which is something 258 00:13:29,392 --> 00:13:34,597 we broke apart and talked about, we talked about ozone already, 259 00:13:34,597 --> 00:13:37,967 in the case of ozone, now, what's happening? 260 00:13:37,967 --> 00:13:43,807 Well if I draw the ozone molecule the way-- 261 00:13:43,807 --> 00:13:45,375 if I draw it the way one of those 262 00:13:45,375 --> 00:13:48,878 is, so I'll draw it the way the left hand picture is, 263 00:13:48,878 --> 00:13:51,181 like this. 264 00:13:51,181 --> 00:13:54,184 And O and, 265 00:13:54,184 --> 00:13:56,085 I'm not talking about shape. 266 00:13:56,085 --> 00:13:58,021 No shape yet. 267 00:13:58,021 --> 00:14:00,123 [INAUDIBLE] I don't know. 268 00:14:00,123 --> 00:14:02,058 Next week, shape. 269 00:14:02,058 --> 00:14:05,428 This week, no shape. 270 00:14:05,428 --> 00:14:06,796 OK, hold on. 271 00:14:06,796 --> 00:14:10,733 Now, if I look at this molecule, this is a double bond. 272 00:14:10,733 --> 00:14:14,737 There's more electrons on this bond than here. 273 00:14:14,737 --> 00:14:16,773 And what we know about that is that that's going 274 00:14:16,773 --> 00:14:18,374 to make that bond stronger. 275 00:14:18,374 --> 00:14:19,976 And those atoms are going to be closer. 276 00:14:19,976 --> 00:14:26,182 So this length would actually be around 1.2 angstroms. 277 00:14:26,182 --> 00:14:28,585 And this one would be around 1.5ish. 278 00:14:33,590 --> 00:14:36,693 But see, and now, you say, well, maybe it's the other one. 279 00:14:36,693 --> 00:14:39,495 1.2 over here 1.5 over there. 280 00:14:39,495 --> 00:14:41,898 But now, it take ozone, and I can measure it. 281 00:14:41,898 --> 00:14:44,500 I can measure those bond lengths. 282 00:14:44,500 --> 00:14:45,735 And I do that. 283 00:14:45,735 --> 00:14:50,106 And when I do that, what I find is that it's neither of these. 284 00:14:52,642 --> 00:15:06,522 But in experiment, the bond length is 1.3. 285 00:15:06,522 --> 00:15:08,892 And it's the same. 286 00:15:08,892 --> 00:15:11,160 It's not different. 287 00:15:11,160 --> 00:15:12,028 So what is happening? 288 00:15:12,028 --> 00:15:16,966 Physically, what is happening is delocalization. 289 00:15:16,966 --> 00:15:22,372 And remember, electrons do things to be happy. 290 00:15:22,372 --> 00:15:27,110 Happiness means lowering the energy of the whole system. 291 00:15:27,110 --> 00:15:30,914 And so what these electrons realize, 292 00:15:30,914 --> 00:15:33,283 what these extra two electrons here realize is well, OK, 293 00:15:33,283 --> 00:15:35,418 I couldn't make this bond short and that bond long. 294 00:15:35,418 --> 00:15:39,422 But maybe if I delocalize across the whole molecule, 295 00:15:39,422 --> 00:15:42,859 maybe if I do that, I can be happier. 296 00:15:42,859 --> 00:15:44,694 Meaning I can be in a lower energy state. 297 00:15:44,694 --> 00:15:46,829 And that is what resonance talks about. 298 00:15:46,829 --> 00:15:48,398 That is what it tells us about. 299 00:15:48,398 --> 00:15:51,200 It tells us when I have these different states that 300 00:15:51,200 --> 00:15:54,370 look equivalent, that the molecule 301 00:15:54,370 --> 00:15:56,205 essentially, pictorially, is going 302 00:15:56,205 --> 00:15:59,575 back and forth between them so fast that it's an average. 303 00:15:59,575 --> 00:16:02,245 That it's a delocalization. 304 00:16:02,245 --> 00:16:07,550 Now, the way that we think about this in terms of our Lewis 305 00:16:07,550 --> 00:16:14,824 structures is by having one resonant form 306 00:16:14,824 --> 00:16:17,527 and another resonant form. 307 00:16:17,527 --> 00:16:19,295 OK now, hold on. 308 00:16:19,295 --> 00:16:28,004 So, here OK, let's do this, and this, and double 309 00:16:28,004 --> 00:16:29,305 bond on this side, good. 310 00:16:35,278 --> 00:16:40,717 Chemists are very particular about how you use arrows. 311 00:16:40,717 --> 00:16:43,753 And you have to be very careful in chemistry. 312 00:16:43,753 --> 00:16:46,723 But here's a case where they really like the arrow. 313 00:16:46,723 --> 00:16:49,325 And they really like curvy arrows in particular. 314 00:16:49,325 --> 00:16:52,128 And what the curvy arrow shows you 315 00:16:52,128 --> 00:16:55,698 is what happened to the electrons. 316 00:16:55,698 --> 00:16:58,334 So it's really just a representation 317 00:16:58,334 --> 00:17:02,238 of where the electrons went to get from one resonance 318 00:17:02,238 --> 00:17:04,540 structure to another, how they changed. 319 00:17:04,540 --> 00:17:07,242 But it can be, actually, helpful to understand 320 00:17:07,242 --> 00:17:09,612 whether you really have a resonance structure 321 00:17:09,612 --> 00:17:10,445 and what's happened. 322 00:17:10,445 --> 00:17:12,415 So for example, in this case, these 323 00:17:12,415 --> 00:17:16,185 are the two resonance structures for ozone. 324 00:17:16,185 --> 00:17:17,420 OK so, what happened? 325 00:17:17,420 --> 00:17:21,457 Well, to get from this structure to this structure, you can see, 326 00:17:21,457 --> 00:17:27,230 what's happened is that you can imagine this lone pair has 327 00:17:27,230 --> 00:17:29,132 gone onto that bond. 328 00:17:32,735 --> 00:17:37,807 And this bond, where am I? 329 00:17:41,544 --> 00:17:44,881 This lone pair has gone onto this bond. 330 00:17:44,881 --> 00:17:49,085 OK, so I've made a double bond to get to there. 331 00:17:49,085 --> 00:17:51,454 And I've made the double bond here to get to there. 332 00:17:51,454 --> 00:17:54,057 Now what's missing? 333 00:17:54,057 --> 00:17:57,360 Well, I've got to make the other lone pair appear here 334 00:17:57,360 --> 00:17:58,694 from this bond. 335 00:17:58,694 --> 00:18:00,763 So you can look at your resonance structures 336 00:18:00,763 --> 00:18:04,333 and realize that that also happened. 337 00:18:04,333 --> 00:18:07,036 And you can look over here, and I've got three lone pairs 338 00:18:07,036 --> 00:18:07,537 on that one. 339 00:18:07,537 --> 00:18:09,372 So the only way to get that would have been, 340 00:18:09,372 --> 00:18:13,776 let's try draw these over here to make room, if that happened. 341 00:18:13,776 --> 00:18:18,347 Those curvy arrows help us just see 342 00:18:18,347 --> 00:18:21,751 how the changes in the electrons happened in these resonance 343 00:18:21,751 --> 00:18:23,786 structures. 344 00:18:23,786 --> 00:18:25,621 But the way to really think about this, 345 00:18:25,621 --> 00:18:31,627 and this is why I wanted to capitalize this word, 346 00:18:31,627 --> 00:18:36,999 is that the actual structure is a combination of these. 347 00:18:36,999 --> 00:18:40,970 So let's see, I'll do this. 348 00:18:40,970 --> 00:18:43,339 And the way you would write the actual structure 349 00:18:43,339 --> 00:18:44,607 would be like this. 350 00:18:44,607 --> 00:18:58,187 OK, this, and this, and this, and dashed lines, oh. 351 00:18:58,187 --> 00:19:01,557 And this is called the resonant hybrid structure, 352 00:19:01,557 --> 00:19:06,295 Resonant hybrid. 353 00:19:06,295 --> 00:19:09,298 It's a mixture of the structures. 354 00:19:09,298 --> 00:19:12,368 And it shows, with the dashed lines, the delocalization. 355 00:19:12,368 --> 00:19:16,272 And if you want to be complete about this resonance structure 356 00:19:16,272 --> 00:19:18,641 concept, if you want to be complete, 357 00:19:18,641 --> 00:19:22,411 then you would also count formal charges. 358 00:19:22,411 --> 00:19:25,047 And so you would see that the formal charge-- let 359 00:19:25,047 --> 00:19:27,049 me make sure I get this right. 360 00:19:27,049 --> 00:19:29,018 So over here, we've got what? 361 00:19:29,018 --> 00:19:32,488 6 minus 1, 2, 3, 4, 5, 6, 7. 362 00:19:32,488 --> 00:19:36,125 So the formal charge here was minus 1. 363 00:19:36,125 --> 00:19:40,229 The formal charge here, 6 minus 1, 2, 3, 4, 5. 364 00:19:40,229 --> 00:19:45,601 So that's plus one in the middle. 365 00:19:45,601 --> 00:19:50,439 And the formal charge here is 6 minus 1, 2, 3, 4, 5, 6. 366 00:19:50,439 --> 00:19:53,743 So that's zero. 367 00:19:53,743 --> 00:19:56,512 And over here, I'm not going to do the counting again, 368 00:19:56,512 --> 00:19:57,213 it's the same. 369 00:19:57,213 --> 00:20:03,152 But the zeros and the minus ones have switched, as you can see. 370 00:20:03,152 --> 00:20:07,089 Now, in the resonant hybrid structure, what's happened-- 371 00:20:07,089 --> 00:20:08,925 notice these are the two Lewis structures. 372 00:20:08,925 --> 00:20:13,196 So you do have formal charge. 373 00:20:13,196 --> 00:20:15,965 Formal charges are not zero everywhere, but that's OK. 374 00:20:15,965 --> 00:20:18,334 This is the best you can do. 375 00:20:18,334 --> 00:20:20,136 But you've got the two that are equivalent. 376 00:20:20,136 --> 00:20:22,305 And they go back and forth. 377 00:20:22,305 --> 00:20:24,273 And in this case, the way you would 378 00:20:24,273 --> 00:20:28,911 draw that is that you would have the plus is still there, 379 00:20:28,911 --> 00:20:34,817 but now you're sort of sharing the formal charge on the ends. 380 00:20:34,817 --> 00:20:36,219 So if you want to think about this 381 00:20:36,219 --> 00:20:37,486 in terms of formal charge-- 382 00:20:37,486 --> 00:20:40,223 notice something about formal charge. 383 00:20:40,223 --> 00:20:43,993 I did not add or take away charge, in this case. 384 00:20:43,993 --> 00:20:46,596 I got it added up. 385 00:20:46,596 --> 00:20:48,998 The addition of all formal charges, 386 00:20:48,998 --> 00:20:51,434 by the definition of formal charge, 387 00:20:51,434 --> 00:20:57,607 must be equal to the charge on the molecule, must be. 388 00:20:57,607 --> 00:21:01,043 So if the molecule is neutral, then the formal charge 389 00:21:01,043 --> 00:21:06,215 must add up to zero, which it does, does, and it does. 390 00:21:06,215 --> 00:21:09,752 Good way to check your work. 391 00:21:09,752 --> 00:21:11,020 OK so, we can go further. 392 00:21:11,020 --> 00:21:12,555 So that's the concept of resonance. 393 00:21:12,555 --> 00:21:15,424 And I did it, as I like to do, kind of slowly. 394 00:21:15,424 --> 00:21:17,093 And now I will give you another example. 395 00:21:17,093 --> 00:21:19,595 But I'm not going to go through it as slowly. 396 00:21:19,595 --> 00:21:24,400 And but oh, let's ask this question, so if I had-- 397 00:21:24,400 --> 00:21:26,569 OK so, here's two other cases. 398 00:21:26,569 --> 00:21:28,604 Now, this one we're going to get through quickly. 399 00:21:28,604 --> 00:21:31,574 Is this a resonance structure? 400 00:21:31,574 --> 00:21:33,909 And I'm going to give you the answer, the answer is no. 401 00:21:33,909 --> 00:21:36,812 And the reason is that resonance structures only 402 00:21:36,812 --> 00:21:40,850 involve the curvy arrow motion of electrons. 403 00:21:43,552 --> 00:21:46,789 The curvy arrows showing us how electrons might change 404 00:21:46,789 --> 00:21:48,891 in this, but not of atoms. 405 00:21:48,891 --> 00:21:53,629 Notice I cannot get from here to here without moving an atom. 406 00:21:53,629 --> 00:21:55,865 That's not a resonance structure. 407 00:21:55,865 --> 00:21:57,667 That is not a residence structure. 408 00:21:57,667 --> 00:22:00,069 So we're done with that. 409 00:22:00,069 --> 00:22:05,541 And we can put, well, let's put a little thing here. 410 00:22:05,541 --> 00:22:25,995 Only change location of electrons, not atoms. 411 00:22:25,995 --> 00:22:27,296 Let's keep this separate. 412 00:22:29,832 --> 00:22:30,966 Not atoms. 413 00:22:30,966 --> 00:22:32,535 But now, we get to another one, where 414 00:22:32,535 --> 00:22:33,803 we didn't change the location of atoms, 415 00:22:33,803 --> 00:22:35,738 but we did change the location of electrons. 416 00:22:38,541 --> 00:22:40,276 And you can see the structure up there. 417 00:22:40,276 --> 00:22:43,612 And I won't do this, because I just did it for ozone, 418 00:22:43,612 --> 00:22:46,949 but you can go through this yourself and show, 419 00:22:46,949 --> 00:22:48,384 with curvy arrows, what's happening 420 00:22:48,384 --> 00:22:49,985 between these structures. 421 00:22:49,985 --> 00:22:53,823 These are three resonance structures. 422 00:22:53,823 --> 00:22:57,259 These are three resonance structures 423 00:22:57,259 --> 00:22:58,794 And you can see how they go. 424 00:22:58,794 --> 00:23:01,831 So you can see, like here, I had that extra lone pair 425 00:23:01,831 --> 00:23:04,834 on the oxygen, that might have a curvy arrow to here to give you 426 00:23:04,834 --> 00:23:06,702 the double bond there. 427 00:23:06,702 --> 00:23:09,004 But as I did that, in order for carbon 428 00:23:09,004 --> 00:23:13,342 to keep its formal charge, to keep its formal charge, 429 00:23:13,342 --> 00:23:17,413 I had to do something with this double bond. 430 00:23:17,413 --> 00:23:20,349 And so this is how you think about resonance structures. 431 00:23:20,349 --> 00:23:23,953 And just to give you the final for that one, 432 00:23:23,953 --> 00:23:25,621 just show you what it would look like. 433 00:23:25,621 --> 00:23:29,325 Once you've got the hybrid structure, 434 00:23:29,325 --> 00:23:30,760 it would look something like this. 435 00:23:34,196 --> 00:23:41,871 So there, there, and oxygen with its two lone pairs, 436 00:23:41,871 --> 00:23:47,877 and oxygen with its two lone pairs, and the delocalized 437 00:23:47,877 --> 00:23:48,511 electrons. 438 00:23:48,511 --> 00:23:50,246 And then you've got your charge. 439 00:23:50,246 --> 00:23:59,054 And this would be minus 2/3, minus 2/3, minus 2/3. 440 00:23:59,054 --> 00:24:01,357 Notice on these three oxygen atoms, 441 00:24:01,357 --> 00:24:06,195 notice that, again, the formal charge adds up 442 00:24:06,195 --> 00:24:07,430 to the charge on the molecule. 443 00:24:07,430 --> 00:24:08,197 There it is. 444 00:24:08,197 --> 00:24:11,934 Charge on the molecule, that's a 2 minus. 445 00:24:11,934 --> 00:24:15,404 This is CO3 2 minus. 446 00:24:18,741 --> 00:24:21,844 Formal charge adds up to the charge on the molecule, good. 447 00:24:21,844 --> 00:24:24,914 OK, one more example, because now we're 448 00:24:24,914 --> 00:24:29,585 going to combine this and learn one other really important 449 00:24:29,585 --> 00:24:32,588 concept about Lewis structures. 450 00:24:32,588 --> 00:24:36,725 Let's see if this, OK. 451 00:24:36,725 --> 00:24:39,195 So here's an example-- 452 00:24:39,195 --> 00:24:40,429 do I have this up here? 453 00:24:40,429 --> 00:24:40,930 No. 454 00:24:44,600 --> 00:24:48,904 Here's an example, what if I had the thiocyanate ion? 455 00:24:48,904 --> 00:24:50,873 This is a favorite. 456 00:24:50,873 --> 00:24:56,745 So this is CNS minus. 457 00:24:56,745 --> 00:25:01,183 Might as well write the name here, thiocyanate. 458 00:25:01,183 --> 00:25:04,553 This is a favorite for demonstrating concepts 459 00:25:04,553 --> 00:25:05,054 about Lewis. 460 00:25:05,054 --> 00:25:07,957 And you'll see why in a minute. 461 00:25:07,957 --> 00:25:11,727 Because first I'm going to draw a Lewis structure. 462 00:25:11,727 --> 00:25:13,696 This is a minus, so it's an ion. 463 00:25:13,696 --> 00:25:15,631 First I'm going to draw a Lewis structure that 464 00:25:15,631 --> 00:25:16,465 seems to make sense. 465 00:25:16,465 --> 00:25:20,336 But then as you'll see, there are resonant Lewis structures 466 00:25:20,336 --> 00:25:21,637 that also seem to make sense. 467 00:25:21,637 --> 00:25:23,606 So I need to use some of the concepts we've 468 00:25:23,606 --> 00:25:25,774 learned to figure it out. 469 00:25:25,774 --> 00:25:28,944 So if I draw this, let's see, I'm going to draw three. 470 00:25:28,944 --> 00:25:37,653 So N, triple bond, C, sulfur, lone pairs, lone pair. 471 00:25:37,653 --> 00:25:45,694 Or I could have N double bond C, double bond sulfur, lone pair, 472 00:25:45,694 --> 00:25:46,662 lone pair. 473 00:25:46,662 --> 00:25:52,835 Or try to squeeze this in underneath, so that you can, 474 00:25:52,835 --> 00:25:53,903 put it here. 475 00:25:53,903 --> 00:26:02,378 N, three lone pairs, single bond C, triple bond sulfur. 476 00:26:02,378 --> 00:26:04,013 Now, these are resonance structures. 477 00:26:04,013 --> 00:26:06,849 I can get them by drawing my curvy arrows 478 00:26:06,849 --> 00:26:08,183 and moving electrons around. 479 00:26:11,654 --> 00:26:13,589 And so you might say, well, is this not just 480 00:26:13,589 --> 00:26:15,190 going to be an average. 481 00:26:15,190 --> 00:26:18,093 Are these electrons delocalized according 482 00:26:18,093 --> 00:26:19,395 to their resonance structures. 483 00:26:19,395 --> 00:26:23,532 But see, once we start thinking about formal charge, 484 00:26:23,532 --> 00:26:25,200 we get more insight. 485 00:26:25,200 --> 00:26:28,971 So the formal charge here is minus 2. 486 00:26:28,971 --> 00:26:35,110 5 minus 1, 2, 3, 4, 5, 6, 7 minus 2. 487 00:26:35,110 --> 00:26:36,979 Here it's zero. 488 00:26:36,979 --> 00:26:40,416 And here it's plus 1. 489 00:26:40,416 --> 00:26:44,987 Sulfur 6 minus 1, 2, 3, 4, 5. 490 00:26:44,987 --> 00:26:48,557 So that looks pretty bad, a large formal charge minus two. 491 00:26:48,557 --> 00:26:52,027 OK over here, oh, I didn't draw the lone pairs, 492 00:26:52,027 --> 00:26:53,095 sorry about that. 493 00:26:53,095 --> 00:26:56,332 Here's two lone pairs here. 494 00:26:56,332 --> 00:27:01,337 We will not mess up the electron count again today. 495 00:27:01,337 --> 00:27:03,906 And so here, the formal charge, let's see, 496 00:27:03,906 --> 00:27:07,209 this one would be minus one. 497 00:27:07,209 --> 00:27:10,346 And in here, it's zero. 498 00:27:10,346 --> 00:27:11,480 And over here, it's zero. 499 00:27:11,480 --> 00:27:12,948 That looks better. 500 00:27:12,948 --> 00:27:15,017 According to my rules on Wednesday, 501 00:27:15,017 --> 00:27:18,454 this looks like a more stable structure than that. 502 00:27:18,454 --> 00:27:20,422 So even though it's resonant, I'm 503 00:27:20,422 --> 00:27:21,991 going to say that's probably not going 504 00:27:21,991 --> 00:27:25,628 to be part of the most stable ground state structure. 505 00:27:25,628 --> 00:27:27,162 But let's look at this last one. 506 00:27:27,162 --> 00:27:31,033 Zero, and over here, I've got zero, 507 00:27:31,033 --> 00:27:32,868 and over here, I've got minus one. 508 00:27:36,772 --> 00:27:38,140 What do we do? 509 00:27:38,140 --> 00:27:41,377 There's one last point that we need to learn. 510 00:27:41,377 --> 00:27:43,545 And it has to do with the formal charges. 511 00:27:43,545 --> 00:27:47,650 And it's a general rule in writing Lewis structures. 512 00:27:47,650 --> 00:27:51,954 And that is that, let's see if I can fit it here, 513 00:27:51,954 --> 00:27:54,089 because it's very relevant to here, 514 00:27:54,089 --> 00:28:11,674 that atoms with the negative formal charge should be-- 515 00:28:11,674 --> 00:28:14,209 I'm not going to fit it here-- should 516 00:28:14,209 --> 00:28:30,926 be on the more electronegative ion, electroneg atom, not 517 00:28:30,926 --> 00:28:32,961 ion, atom. 518 00:28:32,961 --> 00:28:34,863 So let's take a look and see what this means. 519 00:28:34,863 --> 00:28:36,065 Let's understand it, first. 520 00:28:36,065 --> 00:28:37,232 I just wrote something down. 521 00:28:37,232 --> 00:28:39,134 Let's make sure we understand it. 522 00:28:39,134 --> 00:28:45,407 So atoms with the negative formal charge 523 00:28:45,407 --> 00:28:49,178 should be on more electronegative atoms. 524 00:28:49,178 --> 00:28:50,512 That kind of makes sense, right? 525 00:28:50,512 --> 00:28:53,015 Because remember what our definition of electronegativity 526 00:28:53,015 --> 00:28:53,816 is. 527 00:28:53,816 --> 00:28:59,054 Electronegativity is the desire of an atom 528 00:28:59,054 --> 00:29:01,056 to bring bonded pairs towards itself. 529 00:29:01,056 --> 00:29:04,993 Remember, and it makes a molecule polar, polar covalent, 530 00:29:04,993 --> 00:29:06,562 for example, maybe ionic. 531 00:29:06,562 --> 00:29:08,764 But anyway, it's that desire to bring 532 00:29:08,764 --> 00:29:10,599 bonded pairs towards itself. 533 00:29:10,599 --> 00:29:15,237 So if an atom has a negative formal charge, 534 00:29:15,237 --> 00:29:17,372 then you think, well, that should 535 00:29:17,372 --> 00:29:19,308 be on the more electronegative atom. 536 00:29:19,308 --> 00:29:20,976 Because the negative formal charge means 537 00:29:20,976 --> 00:29:22,544 it's got a little bit extra charge 538 00:29:22,544 --> 00:29:24,513 compared to when it was free. 539 00:29:27,082 --> 00:29:29,685 And so if you look at this, oh, now you 540 00:29:29,685 --> 00:29:31,720 need to know electronegativities, 541 00:29:31,720 --> 00:29:34,790 something that we know. 542 00:29:34,790 --> 00:29:41,130 The electronegativity of nitrogen, so chi, remember, 543 00:29:41,130 --> 00:29:44,967 of nitrogen is around 3.0. 544 00:29:44,967 --> 00:29:49,605 And chi for sulfur is around 2.5. 545 00:29:49,605 --> 00:29:57,713 And that tells us that this is the ground state structure. 546 00:29:57,713 --> 00:30:00,215 All from the concepts we already know. 547 00:30:00,215 --> 00:30:01,784 All from the concepts we already know. 548 00:30:01,784 --> 00:30:08,824 Now, at the same time, the atoms that are more electropositive 549 00:30:08,824 --> 00:30:12,494 would want to have positive formal charges, in general. 550 00:30:12,494 --> 00:30:14,096 So the same rule applies the other way. 551 00:30:16,632 --> 00:30:18,267 OK now. 552 00:30:18,267 --> 00:30:23,472 So that's the last part of Lewis structures 553 00:30:23,472 --> 00:30:25,908 that I want you to know about. 554 00:30:25,908 --> 00:30:28,210 And it all ties together. 555 00:30:28,210 --> 00:30:30,412 You can see resonance, this is why 556 00:30:30,412 --> 00:30:33,582 this is such a great example, you can think about resonance. 557 00:30:33,582 --> 00:30:35,117 And could you move electrons around 558 00:30:35,117 --> 00:30:36,318 and get different structures? 559 00:30:36,318 --> 00:30:39,188 Yes, but then those aren't actually equivalent, 560 00:30:39,188 --> 00:30:41,924 because they have very different formal charges. 561 00:30:41,924 --> 00:30:44,693 And then two of them had kind of similarish formal charges, 562 00:30:44,693 --> 00:30:50,098 but one is more stable because of that. 563 00:30:50,098 --> 00:30:51,233 Now why does this matter? 564 00:30:51,233 --> 00:30:56,839 And I'm going to tell you an example of why this matters. 565 00:30:56,839 --> 00:31:03,278 I'm talking about delocalization to stabilize. 566 00:31:03,278 --> 00:31:07,983 And I'm talking about how the actual structure of a molecule 567 00:31:07,983 --> 00:31:13,255 isn't this rigid two bonds here, one bond there, 568 00:31:13,255 --> 00:31:16,859 that's not what that says, but it was in ozone. 569 00:31:16,859 --> 00:31:20,329 But it's this shared equivalent bonds 570 00:31:20,329 --> 00:31:22,898 where the electron is shared and the whole system is lowered. 571 00:31:22,898 --> 00:31:27,169 Now, there is one molecule where this is very much important 572 00:31:27,169 --> 00:31:29,171 and dictates all of its chemical behavior. 573 00:31:29,171 --> 00:31:31,006 And that's benzene. 574 00:31:31,006 --> 00:31:36,545 And back in the day, in the 1930s, oh, by the way, 575 00:31:36,545 --> 00:31:38,714 Pauling, electronegativity scale. we 576 00:31:38,714 --> 00:31:40,582 talked about him on Wednesday. 577 00:31:40,582 --> 00:31:42,985 But back in the day, they didn't know what benzene was. 578 00:31:42,985 --> 00:31:44,620 They could kind of measure some things. 579 00:31:44,620 --> 00:31:46,622 But none of it worked. 580 00:31:46,622 --> 00:31:49,625 And Pauling wrote a number of structural formulae 581 00:31:49,625 --> 00:31:52,494 have been proposed for benzene, but none of them 582 00:31:52,494 --> 00:31:56,632 is free from very serious objections. 583 00:31:56,632 --> 00:31:58,500 The way they wrote stuff. 584 00:31:58,500 --> 00:32:00,369 Look at these shapes for benzene. 585 00:32:00,369 --> 00:32:04,940 If we don't know the right shape and why you have the right 586 00:32:04,940 --> 00:32:09,544 shape for benzene, then we cannot understand all 587 00:32:09,544 --> 00:32:13,382 of the mega amounts of organic chemistry that we do with 588 00:32:13,382 --> 00:32:14,483 benzene. 589 00:32:14,483 --> 00:32:16,685 So this is what's happening with benzene. 590 00:32:16,685 --> 00:32:20,622 You have these two structures that benzene actually averages 591 00:32:20,622 --> 00:32:22,524 over. 592 00:32:22,524 --> 00:32:24,293 These are resonance structures. 593 00:32:24,293 --> 00:32:27,029 Back in the day, they called them Kekulé structures 594 00:32:27,029 --> 00:32:30,999 because Kekulé, as you can see, was kind of close and thought 595 00:32:30,999 --> 00:32:32,034 about this as well. 596 00:32:32,034 --> 00:32:33,335 These are resonance structures. 597 00:32:33,335 --> 00:32:35,304 It totally dictates the chemistry 598 00:32:35,304 --> 00:32:39,942 of benzene, which then leads to massive amounts of things 599 00:32:39,942 --> 00:32:41,777 that we do with benzene. 600 00:32:41,777 --> 00:32:43,445 Because with benzene, and this is just, 601 00:32:43,445 --> 00:32:45,113 we don't need to look at this in detail, 602 00:32:45,113 --> 00:32:49,985 this is an example of a few out of almost unlimited variations 603 00:32:49,985 --> 00:32:51,320 of benzene that we can make. 604 00:32:51,320 --> 00:32:53,288 Because we can take one of those hydrogen atoms 605 00:32:53,288 --> 00:32:56,558 and put something on it, or two places. 606 00:32:56,558 --> 00:32:59,962 And each time we do that, we get a different material, 607 00:32:59,962 --> 00:33:01,697 a different molecule that gives us 608 00:33:01,697 --> 00:33:03,632 different properties and different uses. 609 00:33:06,268 --> 00:33:09,938 But all of this, even though we draw it, still 610 00:33:09,938 --> 00:33:15,177 with those lines, we know that inside, it's delocalized. 611 00:33:15,177 --> 00:33:15,911 It's delocalized. 612 00:33:15,911 --> 00:33:17,846 Well, it might change once you add stuff to it. 613 00:33:17,846 --> 00:33:21,483 But by itself here, by itself here, it's delocalized. 614 00:33:21,483 --> 00:33:24,186 And that sets up the properties of benzene. 615 00:33:24,186 --> 00:33:28,156 Now OK, let's go a little farther. 616 00:33:28,156 --> 00:33:31,093 So I like these last two here. 617 00:33:31,093 --> 00:33:35,364 Naphthalene, now, you can keep on going, 618 00:33:35,364 --> 00:33:37,366 and keep adding benzenes, and we're 619 00:33:37,366 --> 00:33:39,101 going to get somewhere that I really love. 620 00:33:41,737 --> 00:33:43,972 This is getting me excited. 621 00:33:43,972 --> 00:33:46,241 Oh, look I'm going to add one there. 622 00:33:46,241 --> 00:33:49,044 I'm just going to keep adding benzene. 623 00:33:49,044 --> 00:33:55,283 Now I could stay here, oh, don't get benzene wrong. 624 00:33:55,283 --> 00:33:57,486 I can keep going. 625 00:33:57,486 --> 00:34:01,223 And if I kept going, I might get sheets of benzene 626 00:34:01,223 --> 00:34:03,525 that are now called graphene. 627 00:34:03,525 --> 00:34:06,061 And if I stack those up, I've got graphite. 628 00:34:06,061 --> 00:34:07,029 I've got graphite. 629 00:34:07,029 --> 00:34:08,530 That's what graphite is. 630 00:34:08,530 --> 00:34:10,431 It's these very large sheets of benzene. 631 00:34:10,431 --> 00:34:14,636 Now, there are two very well-known phases 632 00:34:14,636 --> 00:34:16,271 of pure carbon. 633 00:34:16,271 --> 00:34:18,139 One is diamond. 634 00:34:18,139 --> 00:34:19,908 And the other is graphite. 635 00:34:19,908 --> 00:34:21,710 Does anybody know which one is more stable? 636 00:34:24,612 --> 00:34:26,915 How many people think graphite? 637 00:34:26,915 --> 00:34:28,784 How many people think diamonds are forever? 638 00:34:32,987 --> 00:34:38,159 I'm giving you guys some advice, especially like, you're out, 639 00:34:38,159 --> 00:34:40,395 and maybe you're out on a date. 640 00:34:40,395 --> 00:34:42,597 And it's the weekend. 641 00:34:42,597 --> 00:34:44,733 And there's a candle. 642 00:34:44,733 --> 00:34:46,967 And you've talked about combustion. 643 00:34:46,967 --> 00:34:48,170 And you've written that down. 644 00:34:48,170 --> 00:34:51,172 And you talked about how long you have in that room, 645 00:34:51,172 --> 00:34:54,376 because you know about oxygen and the limiting reagent 646 00:34:54,376 --> 00:34:55,844 that it's either you or the candle. 647 00:34:55,844 --> 00:34:56,710 We talked about that. 648 00:34:56,710 --> 00:34:58,947 We also talked about how you need your periodic table 649 00:34:58,947 --> 00:34:59,781 on that date. 650 00:34:59,781 --> 00:35:02,751 And how you may need your spectroscope, 651 00:35:02,751 --> 00:35:06,755 because they might give you an LED candle instead. 652 00:35:06,755 --> 00:35:08,757 You need to talk about LEDs. 653 00:35:08,757 --> 00:35:11,793 And I'm not saying that all of these conversations involving 654 00:35:11,793 --> 00:35:13,228 knowledge you gained in this class 655 00:35:13,228 --> 00:35:14,996 will lead to this place of seriousness, 656 00:35:14,996 --> 00:35:16,865 but it could lead to engagement. 657 00:35:16,865 --> 00:35:18,366 It could. 658 00:35:18,366 --> 00:35:21,236 And now, some people when they get engaged, 659 00:35:21,236 --> 00:35:24,773 one person buys the other a ring. 660 00:35:24,773 --> 00:35:28,477 And sometimes that involves this material. 661 00:35:28,477 --> 00:35:33,682 My point to you right now is when you go to the store, 662 00:35:33,682 --> 00:35:36,351 maybe it's Tiffany's, maybe it's somewhere else, 663 00:35:36,351 --> 00:35:40,222 and you buy that ring, ask for a warranty. 664 00:35:40,222 --> 00:35:46,394 And make sure it's about 100,000 years, because after that time, 665 00:35:46,394 --> 00:35:48,396 that ring is going to turn into graphite, 666 00:35:48,396 --> 00:35:52,267 because graphite is the lowest most energetically 667 00:35:52,267 --> 00:35:54,536 stable form of carbon. 668 00:35:54,536 --> 00:35:56,371 So I'm just, again, always trying to help. 669 00:35:56,371 --> 00:36:01,910 But this is the most stable form of carbon. 670 00:36:01,910 --> 00:36:05,313 And it's a bunch of benzene, but repeated. 671 00:36:05,313 --> 00:36:06,815 Not benzene, the hydrogens are gone. 672 00:36:06,815 --> 00:36:09,017 It's just pure graphene, but repeated 673 00:36:09,017 --> 00:36:12,454 in these beautiful rings. 674 00:36:12,454 --> 00:36:16,691 And when we come back to hybridisation 675 00:36:16,691 --> 00:36:19,694 and molecular orbitals, which we'll start after exam one, 676 00:36:19,694 --> 00:36:21,997 you'll see other beautiful properties 677 00:36:21,997 --> 00:36:23,965 of structures like this. 678 00:36:23,965 --> 00:36:29,704 By the way, speaking of seeing, you can see this material. 679 00:36:29,704 --> 00:36:32,741 What are you throwing at this material to see it like that? 680 00:36:32,741 --> 00:36:38,680 Not light, not photons, you're throwing electrons. 681 00:36:38,680 --> 00:36:41,283 I just came across this the other day in Wired, 682 00:36:41,283 --> 00:36:44,286 New microscope shows the quantum world in crazy detail. 683 00:36:44,286 --> 00:36:46,721 They took electrons and they shine it 684 00:36:46,721 --> 00:36:49,124 on these little particles of platinum and iron. 685 00:36:49,124 --> 00:36:51,793 And they're able to literally see every single atom as they 686 00:36:51,793 --> 00:36:53,995 break apart the particle. 687 00:36:53,995 --> 00:36:55,430 By just throwing electrons at it, 688 00:36:55,430 --> 00:36:57,799 they say, the transmission electron microscope 689 00:36:57,799 --> 00:37:00,302 was designed to break records. 690 00:37:00,302 --> 00:37:01,937 Using its beam of electrons, scientists 691 00:37:01,937 --> 00:37:04,372 have glimpsed many types of viruses, et cetera, et cetera. 692 00:37:04,372 --> 00:37:08,910 They got down to 0.4 angstrom resolution in that work. 693 00:37:08,910 --> 00:37:10,045 It's a beautiful thing. 694 00:37:10,045 --> 00:37:12,013 And being able to see these materials 695 00:37:12,013 --> 00:37:15,750 has revolutionized what we can do with them. 696 00:37:15,750 --> 00:37:19,788 Diamonds and graphite makes me, when I talk about graphite, 697 00:37:19,788 --> 00:37:21,790 I get excited. 698 00:37:21,790 --> 00:37:22,657 I get excited. 699 00:37:22,657 --> 00:37:25,026 Now I'm working on my arm here. 700 00:37:25,026 --> 00:37:28,330 So I got to go up that way. 701 00:37:28,330 --> 00:37:29,764 And I got to go this way. 702 00:37:35,570 --> 00:37:40,242 All right, hold on, hold on, let's go that way. 703 00:37:40,242 --> 00:37:41,710 Now, wait a second. 704 00:37:41,710 --> 00:37:44,346 I'm very excited. 705 00:37:44,346 --> 00:37:46,081 Oh wait, there was a lot of noise. 706 00:37:51,019 --> 00:37:51,920 I tried. 707 00:37:51,920 --> 00:37:53,088 I got to work on my arm. 708 00:37:57,926 --> 00:38:01,029 It's such a beautiful material. 709 00:38:01,029 --> 00:38:02,430 Too excited. 710 00:38:02,430 --> 00:38:03,098 We'll have more. 711 00:38:08,503 --> 00:38:17,479 Now I don't mean to bring us down after why 712 00:38:17,479 --> 00:38:18,980 this matters and excitement. 713 00:38:18,980 --> 00:38:22,017 But here's the thing, if there's one thing 714 00:38:22,017 --> 00:38:23,585 you know about chemistry by now, it's 715 00:38:23,585 --> 00:38:26,488 that chemistry lives in the fast lane. 716 00:38:26,488 --> 00:38:28,723 And what I mean is, they know about the rules. 717 00:38:28,723 --> 00:38:31,760 And they also know they're meant to be broken. 718 00:38:31,760 --> 00:38:36,031 And Lewis and octets are no exception. 719 00:38:36,031 --> 00:38:42,037 And so there are exceptions to Lewis's octet rule. 720 00:38:42,037 --> 00:38:44,739 And I'll give you a couple of examples. 721 00:38:44,739 --> 00:38:46,274 And it has to do with the concepts 722 00:38:46,274 --> 00:38:51,880 that we've been talking about, but in particular, like boron. 723 00:38:51,880 --> 00:38:57,285 Boron trifluoride, well, that one, you could draw it. 724 00:38:57,285 --> 00:39:01,890 If you wanted to, you could draw it like this. 725 00:39:01,890 --> 00:39:10,498 And let's see, here, F, and you can do this, F. 726 00:39:10,498 --> 00:39:11,966 But the problem with this is then, 727 00:39:11,966 --> 00:39:14,336 when you look at the formal charges, you've got a minus 1 728 00:39:14,336 --> 00:39:16,971 there and a minus 1 there. 729 00:39:16,971 --> 00:39:20,975 And so actually, what this structure does, 730 00:39:20,975 --> 00:39:24,846 is boron is like, you know what, I'm flexible. 731 00:39:24,846 --> 00:39:27,649 I don't need my eight. 732 00:39:27,649 --> 00:39:29,984 I can be OK with six. 733 00:39:29,984 --> 00:39:32,620 As long as you keep sharing, fluorine atoms, 734 00:39:32,620 --> 00:39:36,024 as long as you keep sharing those electrons 735 00:39:36,024 --> 00:39:40,862 so we can make these three covalent bonds all equivalent, 736 00:39:40,862 --> 00:39:43,498 I'm OK. 737 00:39:43,498 --> 00:39:46,134 And so this is actually-- and you can see, 738 00:39:46,134 --> 00:39:49,304 the formal charges on all the atoms here is zero. 739 00:39:49,304 --> 00:39:53,174 This is actually the stable structure, 740 00:39:53,174 --> 00:39:55,410 even though boron does not, this does not 741 00:39:55,410 --> 00:39:58,313 satisfy the octet rule. 742 00:39:58,313 --> 00:40:02,083 Chemistry, breaking the rules. 743 00:40:02,083 --> 00:40:04,753 So boron is said to be happy. 744 00:40:04,753 --> 00:40:11,693 Boron can be, what is called in this world, electron deficient. 745 00:40:15,797 --> 00:40:19,033 And by the same token, you can go the other way. 746 00:40:19,033 --> 00:40:23,671 And sulfur likes to go the other way, because sometimes sulfur, 747 00:40:23,671 --> 00:40:25,273 so there we have sulfur tetrafluoride. 748 00:40:25,273 --> 00:40:27,475 So I'm using the same external atom, 749 00:40:27,475 --> 00:40:35,750 but sulfur also prefers to look like this 750 00:40:35,750 --> 00:40:40,088 and to have one extra lone pair there. 751 00:40:40,088 --> 00:40:47,028 And now we can go like this, and like that, and like that. 752 00:40:47,028 --> 00:40:52,801 And so what sulfur will do, oh boy, lots of dots, 753 00:40:52,801 --> 00:40:56,037 and so what sulfur will do is it will 754 00:40:56,037 --> 00:41:05,213 form what's called an expanded octet, expanded octet. 755 00:41:05,213 --> 00:41:08,416 And this will become also, a little bit-- 756 00:41:08,416 --> 00:41:10,585 we'll sort of understand this also a little bit more 757 00:41:10,585 --> 00:41:11,986 once we look at shapes. 758 00:41:11,986 --> 00:41:13,721 Shapes are going to play an important role 759 00:41:13,721 --> 00:41:16,558 in molecular stability. 760 00:41:16,558 --> 00:41:18,626 But for now, I just wanted you to know that there 761 00:41:18,626 --> 00:41:21,963 are exceptions to the rules. 762 00:41:21,963 --> 00:41:24,132 And in particular, as you go beyond the first couple 763 00:41:24,132 --> 00:41:28,403 of rows, so you go down in the period table, 764 00:41:28,403 --> 00:41:31,339 you do find atoms more willing, like sulfur, 765 00:41:31,339 --> 00:41:35,743 to have an expanded octet in order to reach the lowest 766 00:41:35,743 --> 00:41:37,278 energy state. 767 00:41:37,278 --> 00:41:41,149 Now, in the last two minutes, I come back to this. 768 00:41:41,149 --> 00:41:43,284 Let's just bring it all back home 769 00:41:43,284 --> 00:41:46,988 around Lewis, and around electronegativity, 770 00:41:46,988 --> 00:41:52,060 and around all these covalent bonding concepts. 771 00:41:52,060 --> 00:41:53,428 And what I want to leave you with 772 00:41:53,428 --> 00:41:56,130 is what was kind of the point, part 773 00:41:56,130 --> 00:41:58,433 of the point of your goody bag. 774 00:41:58,433 --> 00:42:01,035 Which is, I wanted you to get your hands on these materials 775 00:42:01,035 --> 00:42:04,606 so you could touch and feel the differences 776 00:42:04,606 --> 00:42:10,178 and see that actually it's the properties at the end that 777 00:42:10,178 --> 00:42:13,982 are also very important for determining what a material is. 778 00:42:13,982 --> 00:42:19,420 So you can make rules like electronegativity difference 779 00:42:19,420 --> 00:42:24,025 less than 1.6 is going to be polar covalent 780 00:42:24,025 --> 00:42:25,894 or non-polar covalent, or greater than two, 781 00:42:25,894 --> 00:42:27,395 it's probably ionic. 782 00:42:27,395 --> 00:42:29,764 But at the end of the day, you've 783 00:42:29,764 --> 00:42:31,332 got to come back to properties. 784 00:42:37,605 --> 00:42:39,774 I'll put up a slide showing this, 785 00:42:39,774 --> 00:42:41,442 but ionic solids are hard, brittle, 786 00:42:41,442 --> 00:42:43,244 and solid at room temperature. 787 00:42:43,244 --> 00:42:47,815 Covalent solids can be gases, liquids, or solids. 788 00:42:47,815 --> 00:42:50,018 Covalent solids have a low melting point. 789 00:42:50,018 --> 00:42:52,854 Ionic solids are typically high. 790 00:42:52,854 --> 00:42:54,923 Covalent solids are poor conductors. 791 00:42:54,923 --> 00:42:59,060 Ionic ones are usually insulators as a solid. 792 00:42:59,060 --> 00:43:00,962 This is what you need to also do. 793 00:43:00,962 --> 00:43:04,132 You can't just go by one metric. 794 00:43:04,132 --> 00:43:05,833 You got to look at the properties. 795 00:43:05,833 --> 00:43:07,168 It's what Mendeleev did. 796 00:43:07,168 --> 00:43:10,204 And it's what chemists have always done to make progress. 797 00:43:10,204 --> 00:43:13,575 OK so, on that note, have a great weekend. 798 00:43:13,575 --> 00:43:16,177 And see you at the exam. 799 00:43:16,177 --> 00:43:17,412 [CROWD NOISE]