1 00:00:00,090 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,850 Commons license. 3 00:00:03,850 --> 00:00:06,060 Your support will help MIT OpenCourseWare 4 00:00:06,060 --> 00:00:10,150 continue to offer high quality educational resources for free. 5 00:00:10,150 --> 00:00:12,690 To make a donation or to view additional materials 6 00:00:12,690 --> 00:00:16,650 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,650 --> 00:00:17,450 at ocw.mit.edu. 8 00:00:25,962 --> 00:00:27,462 CATHERINE DRENNAN: Lewis structures. 9 00:00:30,180 --> 00:00:34,080 So I tell students who have sort of no background in chemistry 10 00:00:34,080 --> 00:00:35,610 before they come into this class, 11 00:00:35,610 --> 00:00:40,050 and there always are some, that there are some topics in 5.111 12 00:00:40,050 --> 00:00:42,960 that having no experience with the topic 13 00:00:42,960 --> 00:00:45,210 is actually a good thing, and Lewis structures is 14 00:00:45,210 --> 00:00:46,459 one of these things. 15 00:00:46,459 --> 00:00:48,750 I think if you've seen it before, you're like, oh yeah, 16 00:00:48,750 --> 00:00:49,530 that's easy. 17 00:00:49,530 --> 00:00:50,640 I know how to do that. 18 00:00:50,640 --> 00:00:52,800 You don't practice and you get on an exam 19 00:00:52,800 --> 00:00:54,910 and you're like, oh wait a minute. 20 00:00:54,910 --> 00:00:57,090 I forgot how to do this. 21 00:00:57,090 --> 00:00:58,980 And the students who haven't seen it before 22 00:00:58,980 --> 00:01:02,490 know all the rules and just get brilliant, perfect scores 23 00:01:02,490 --> 00:01:03,120 on the exam. 24 00:01:03,120 --> 00:01:05,640 And the other students are like, man. 25 00:01:05,640 --> 00:01:08,530 I forgot how to do my Lewis structures. 26 00:01:08,530 --> 00:01:11,190 So here are Lewis structures. 27 00:01:11,190 --> 00:01:12,990 We're going to go over them. 28 00:01:12,990 --> 00:01:15,300 So I really like Lewis structures 29 00:01:15,300 --> 00:01:19,440 because they're relatively simple, and they work. 30 00:01:19,440 --> 00:01:23,520 Like 90% of the time, they are the correct structure. 31 00:01:23,520 --> 00:01:27,540 And I'm a big fan of simple things working. 32 00:01:27,540 --> 00:01:30,810 If you wanted to get from 90% to 100%, 33 00:01:30,810 --> 00:01:33,480 you'd have to use Schrodinger's equation, 34 00:01:33,480 --> 00:01:37,380 but you can get 90% just with the simple Lewis structures 35 00:01:37,380 --> 00:01:38,620 I'm a big fan of that. 36 00:01:38,620 --> 00:01:41,590 I love it when simple things really work pretty well. 37 00:01:41,590 --> 00:01:42,230 OK. 38 00:01:42,230 --> 00:01:45,000 So when Lewis structures the key to this 39 00:01:45,000 --> 00:01:49,950 is thinking about the electrons being shared so that you 40 00:01:49,950 --> 00:01:53,790 get a full valence shell. 41 00:01:53,790 --> 00:01:58,620 And having the electrons distributed in such a way 42 00:01:58,620 --> 00:02:02,880 that all of the atoms have the number of electrons 43 00:02:02,880 --> 00:02:05,250 that make them happy, which is usually 44 00:02:05,250 --> 00:02:08,970 eight electrons, which is an octet, that noble gas 45 00:02:08,970 --> 00:02:11,400 configuration. 46 00:02:11,400 --> 00:02:13,860 So every dot in a Lewis structure 47 00:02:13,860 --> 00:02:17,220 represents a valence electron. 48 00:02:17,220 --> 00:02:23,010 And we can then look at some atoms 49 00:02:23,010 --> 00:02:26,220 and put dots around them to indicate the number 50 00:02:26,220 --> 00:02:27,810 of valence electrons. 51 00:02:27,810 --> 00:02:32,220 So we also have to know how many valence electrons atoms have. 52 00:02:32,220 --> 00:02:33,720 And so why didn't you just practice 53 00:02:33,720 --> 00:02:35,106 with a clicker question. 54 00:02:38,270 --> 00:02:43,620 And here's part of the periodic table up here if you need it. 55 00:02:56,070 --> 00:02:58,610 All right. 56 00:02:58,610 --> 00:02:59,970 I'm told 10 seconds. 57 00:02:59,970 --> 00:03:01,610 Everyone was crazy fast. 58 00:03:13,110 --> 00:03:14,820 Yes. 59 00:03:14,820 --> 00:03:17,760 So seven is the correct answer. 60 00:03:17,760 --> 00:03:19,800 You could look at the periodic table 61 00:03:19,800 --> 00:03:23,790 and sometimes with these it's a counting thing. 62 00:03:23,790 --> 00:03:28,680 So this is one where you want to always double check 63 00:03:28,680 --> 00:03:30,480 if things don't make sense. 64 00:03:30,480 --> 00:03:31,290 All right. 65 00:03:31,290 --> 00:03:35,550 So we can put seven electrons around fluorine, 66 00:03:35,550 --> 00:03:37,530 and we'll have two fluorines here. 67 00:03:37,530 --> 00:03:41,649 They'll both have seven electrons around them. 68 00:03:41,649 --> 00:03:43,440 And now I'm going to jump to another slide, 69 00:03:43,440 --> 00:03:45,870 but I'm going to show you the seven again in case 70 00:03:45,870 --> 00:03:47,207 you haven't written them down. 71 00:03:47,207 --> 00:03:49,290 If you don't want to, they're not in your handout, 72 00:03:49,290 --> 00:03:52,860 but that's probably OK. 73 00:03:52,860 --> 00:03:54,390 So when you bring them together, you 74 00:03:54,390 --> 00:03:58,290 can bring them together in such a way that they can all share. 75 00:03:58,290 --> 00:04:02,940 And so if we put in green, then, one of the fluorine's seven. 76 00:04:02,940 --> 00:04:06,450 And then we put in blue the other fluorine's seven, 77 00:04:06,450 --> 00:04:08,880 you can see that they can share two in the middle 78 00:04:08,880 --> 00:04:11,680 and both are very, very happy. 79 00:04:11,680 --> 00:04:15,870 So just thinking about this really simple idea, 80 00:04:15,870 --> 00:04:21,579 how many electrons will give you an octet, will give you eight? 81 00:04:21,579 --> 00:04:23,340 And how can you put things together 82 00:04:23,340 --> 00:04:27,370 in such a way that allows for that to happen? 83 00:04:27,370 --> 00:04:32,010 Now there are a few elements that 84 00:04:32,010 --> 00:04:36,930 do not want eight in their valence shell, 85 00:04:36,930 --> 00:04:38,280 and hydrogen is one of them. 86 00:04:38,280 --> 00:04:40,800 It just has that one S. So it only 87 00:04:40,800 --> 00:04:43,230 wants two, that's all it can handle. 88 00:04:43,230 --> 00:04:46,050 So this is an exception, hydrogen 89 00:04:46,050 --> 00:04:47,940 is going to want two electrons. 90 00:04:47,940 --> 00:04:50,070 Hydrogen loves to interact with things, though. 91 00:04:50,070 --> 00:04:52,320 It interacts with lots and lots of things. 92 00:04:52,320 --> 00:04:55,950 And here hydrogen with its one valence electron 93 00:04:55,950 --> 00:04:59,220 is interacting with chlorine with its seven 94 00:04:59,220 --> 00:05:03,720 valence electrons, and they are sharing two electrons forming 95 00:05:03,720 --> 00:05:06,410 a bond together. 96 00:05:06,410 --> 00:05:08,690 So when we're talking about Lewis structures, 97 00:05:08,690 --> 00:05:12,590 we're talking about different kinds of electrons. 98 00:05:12,590 --> 00:05:16,820 So we're talking about bonding electrons, the electrons that 99 00:05:16,820 --> 00:05:20,640 are involved in the bond, and also lone pair electrons. 100 00:05:20,640 --> 00:05:26,240 So chloride in HCl is going to have two bonding electrons, 101 00:05:26,240 --> 00:05:30,650 one was its, and one came from hydrogen. 102 00:05:30,650 --> 00:05:35,450 And it's also going to have six lone pair electrons, 103 00:05:35,450 --> 00:05:38,490 or we could say three lone pairs. 104 00:05:38,490 --> 00:05:43,670 So when we say a lone pair, that indicates two electrons there. 105 00:05:43,670 --> 00:05:49,160 So it has one, two, three, four, five, six or one pair, 106 00:05:49,160 --> 00:05:52,980 two pairs, three pairs. 107 00:05:52,980 --> 00:05:55,850 Now there are rules to Lewis structures, 108 00:05:55,850 --> 00:05:57,830 and here is the complete rule. 109 00:05:57,830 --> 00:06:00,200 In your handout, this wouldn't fit on one page. 110 00:06:00,200 --> 00:06:03,050 It's on two pages. 111 00:06:03,050 --> 00:06:06,140 And these rules, if you do work these problems, 112 00:06:06,140 --> 00:06:09,140 you will remember these rules, and they become pretty easy. 113 00:06:09,140 --> 00:06:12,050 But it's important to work Lewis structure problems 114 00:06:12,050 --> 00:06:16,070 so that the rules become really familiar to you. 115 00:06:16,070 --> 00:06:18,660 And it takes time to work Lewis structure problems, 116 00:06:18,660 --> 00:06:22,110 so don't wait to the last minute to start this problem set. 117 00:06:22,110 --> 00:06:24,124 There's a lot of Lewis structure problems on it, 118 00:06:24,124 --> 00:06:25,790 which means it's not difficult, but it's 119 00:06:25,790 --> 00:06:27,421 going to take some time. 120 00:06:27,421 --> 00:06:27,920 All right. 121 00:06:27,920 --> 00:06:31,070 So let's briefly go over these rules. 122 00:06:31,070 --> 00:06:34,790 First what you want to do is draw in the skeleton structure. 123 00:06:34,790 --> 00:06:37,880 Just put the atoms down. 124 00:06:37,880 --> 00:06:42,200 Hydrogen and fluorine are always going to be terminal atoms. 125 00:06:42,200 --> 00:06:44,210 Don't put them in the middle of a molecule. 126 00:06:44,210 --> 00:06:46,880 That gets chemistry professors really upset 127 00:06:46,880 --> 00:06:49,130 to see hydrogen in the middle with lots of bonds 128 00:06:49,130 --> 00:06:51,950 to things, so don't do it. 129 00:06:51,950 --> 00:06:55,580 And typically the element with the lowest ionization energy 130 00:06:55,580 --> 00:06:58,070 goes in the middle, and there are some exceptions 131 00:06:58,070 --> 00:06:59,720 and we'll see some of those exceptions. 132 00:06:59,720 --> 00:07:03,440 But that should be your first guess. 133 00:07:03,440 --> 00:07:06,180 You want to count the number of valence electrons. 134 00:07:06,180 --> 00:07:09,200 If there's a negative charge, you need to count that in 135 00:07:09,200 --> 00:07:11,900 or if there's a positive charge you need to subtract that 136 00:07:11,900 --> 00:07:13,246 from the total. 137 00:07:13,246 --> 00:07:15,620 Then you want to figure out the total number of electrons 138 00:07:15,620 --> 00:07:19,070 needed, so everyone has their full valence shell. 139 00:07:19,070 --> 00:07:23,570 You need to subtract these two to get the number of bonding 140 00:07:23,570 --> 00:07:24,350 electrons. 141 00:07:24,350 --> 00:07:26,210 And here are some of the things that it's 142 00:07:26,210 --> 00:07:29,360 really easy to make math mistakes here, 143 00:07:29,360 --> 00:07:31,550 so if your structure makes zero sense at the end, 144 00:07:31,550 --> 00:07:33,670 go back and check your math. 145 00:07:33,670 --> 00:07:37,100 Assign to bonding electrons to each bond. 146 00:07:37,100 --> 00:07:39,350 If any remain, you want to think about 147 00:07:39,350 --> 00:07:42,320 whether you have double or triple bonds. 148 00:07:42,320 --> 00:07:44,900 And there's only certain kinds of atoms that can 149 00:07:44,900 --> 00:07:46,530 have double and triple bonds. 150 00:07:46,530 --> 00:07:48,110 So be careful where you're putting 151 00:07:48,110 --> 00:07:50,600 your double and triple bonds. 152 00:07:50,600 --> 00:07:54,020 If any valence electrons remain, those are loan pairs. 153 00:07:54,020 --> 00:07:56,750 And then lastly, you want to figure out the formal charge 154 00:07:56,750 --> 00:07:59,862 on all of the atoms in your structure to make sure 155 00:07:59,862 --> 00:08:01,820 that this is a valid structure, and we're going 156 00:08:01,820 --> 00:08:03,900 to talk about formal charge. 157 00:08:03,900 --> 00:08:06,260 So first, let's just try an example. 158 00:08:06,260 --> 00:08:10,700 And your sheet has two examples on the same page. 159 00:08:10,700 --> 00:08:14,060 We have HCN and we also have CN minus. 160 00:08:14,060 --> 00:08:17,949 So we're going to do HCN first, so don't fill your entire page 161 00:08:17,949 --> 00:08:19,490 because you're going to have to write 162 00:08:19,490 --> 00:08:21,920 things for CN minus as well. 163 00:08:21,920 --> 00:08:25,520 But before we start, we need to figure out which atom is likely 164 00:08:25,520 --> 00:08:27,660 going to be in the middle. 165 00:08:27,660 --> 00:08:29,960 And so why don't you tell me what you 166 00:08:29,960 --> 00:08:31,240 think on the clicker question. 167 00:09:07,680 --> 00:09:08,813 OK, 10 more seconds. 168 00:09:24,520 --> 00:09:25,060 Yup. 169 00:09:25,060 --> 00:09:29,080 So here again we want to have one that has a lower ionization 170 00:09:29,080 --> 00:09:31,780 energy, and you also want to consider other things 171 00:09:31,780 --> 00:09:35,610 like hydrogen can't be in the middle. 172 00:09:35,610 --> 00:09:36,160 OK. 173 00:09:36,160 --> 00:09:38,243 And it was written that way, but sometimes they're 174 00:09:38,243 --> 00:09:42,290 written in a way that is not as straightforward. 175 00:09:42,290 --> 00:09:42,790 OK. 176 00:09:45,680 --> 00:09:47,225 So I'll put that up. 177 00:09:47,225 --> 00:09:48,850 All right so we'll go through the rules 178 00:09:48,850 --> 00:09:50,630 and we'll try to work this out. 179 00:09:50,630 --> 00:09:53,522 So first I can write-- I'm going to start-- 180 00:09:53,522 --> 00:09:55,930 I guess I'll write over here. 181 00:09:55,930 --> 00:10:00,790 So number one, we're just going to write HCN with C 182 00:10:00,790 --> 00:10:02,740 in the middle. 183 00:10:02,740 --> 00:10:04,780 So that's the first thing we're going to do. 184 00:10:04,780 --> 00:10:07,630 Next we're going to consider the valence electrons. 185 00:10:07,630 --> 00:10:10,060 And you can just help me out by yelling things out. 186 00:10:10,060 --> 00:10:12,370 How many valence electrons does hydrogen have? 187 00:10:12,370 --> 00:10:13,780 AUDIENCE: One. 188 00:10:13,780 --> 00:10:15,820 CATHERINE DRENNAN: What about carbon? 189 00:10:15,820 --> 00:10:17,000 AUDIENCE: Four. 190 00:10:17,000 --> 00:10:18,166 CATHERINE DRENNAN: Nitrogen? 191 00:10:18,166 --> 00:10:18,850 AUDIENCE: Five. 192 00:10:18,850 --> 00:10:20,950 CATHERINE DRENNAN: And you can always check me on my math. 193 00:10:20,950 --> 00:10:22,166 How much does that equal? 194 00:10:22,166 --> 00:10:23,040 AUDIENCE: 10. 195 00:10:23,040 --> 00:10:24,248 CATHERINE DRENNAN: Excellent. 196 00:10:26,590 --> 00:10:28,690 There's nothing like adding simple numbers 197 00:10:28,690 --> 00:10:33,310 in front of 350 people to really put the stress in one's day. 198 00:10:33,310 --> 00:10:37,420 OK, so to have a complete full valence shell, what 199 00:10:37,420 --> 00:10:38,447 do I need for hydrogen? 200 00:10:38,447 --> 00:10:39,149 AUDIENCE: Two. 201 00:10:39,149 --> 00:10:40,690 CATHERINE DRENNAN: What about carbon? 202 00:10:40,690 --> 00:10:42,480 AUDIENCE: Four. 203 00:10:42,480 --> 00:10:44,630 CATHERINE DRENNAN: Eight to be complete. 204 00:10:44,630 --> 00:10:45,170 Nitrogen? 205 00:10:45,170 --> 00:10:45,940 AUDIENCE: Eight. 206 00:10:45,940 --> 00:10:47,572 CATHERINE DRENNAN: Eight. 207 00:10:47,572 --> 00:10:50,000 And I think we add this up, we should get 18. 208 00:10:50,000 --> 00:10:51,360 How's that? 209 00:10:51,360 --> 00:10:52,180 All right. 210 00:10:52,180 --> 00:10:57,250 So for four, now we're going to subtract these numbers 211 00:10:57,250 --> 00:11:01,060 from each other to tell us how many bonding electrons we have. 212 00:11:01,060 --> 00:11:04,090 So we have 18 minus 10. 213 00:11:04,090 --> 00:11:07,780 And we should have eight to bonding electrons. 214 00:11:07,780 --> 00:11:13,820 For five I'm going to now assign two per bond. 215 00:11:13,820 --> 00:11:16,030 So I'm going to put one here. 216 00:11:16,030 --> 00:11:17,560 Another here. 217 00:11:17,560 --> 00:11:18,220 Another here. 218 00:11:18,220 --> 00:11:19,030 Another here. 219 00:11:19,030 --> 00:11:21,790 So I've assigned two per bond. 220 00:11:21,790 --> 00:11:25,210 And now I see if I have any left. 221 00:11:25,210 --> 00:11:26,245 Do I have some left? 222 00:11:28,930 --> 00:11:31,060 I've used four, I had eight. 223 00:11:31,060 --> 00:11:35,500 So yes, I have four more. 224 00:11:35,500 --> 00:11:38,170 And if you have more bonding electrons, 225 00:11:38,170 --> 00:11:43,120 then you are supposed to assign those bonding electrons. 226 00:11:43,120 --> 00:11:44,620 And think about whether it's allowed 227 00:11:44,620 --> 00:11:48,040 to have double or triple bonds. 228 00:11:48,040 --> 00:11:50,830 Can hydrogen be involved in the double bond? 229 00:11:50,830 --> 00:11:51,760 No. 230 00:11:51,760 --> 00:11:52,480 Carbon nitrogen? 231 00:11:52,480 --> 00:11:53,182 AUDIENCE: Yes. 232 00:11:53,182 --> 00:11:54,890 CATHERINE DRENNAN: Yes, or a triple bond. 233 00:11:54,890 --> 00:11:56,650 So I have four more, so I'm going 234 00:11:56,650 --> 00:12:00,670 to put one, two, three, four. 235 00:12:00,670 --> 00:12:02,650 So I'm going to have a triple bond 236 00:12:02,650 --> 00:12:06,970 between my carbon and my nitrogen. And now I'm good. 237 00:12:06,970 --> 00:12:12,010 So now I want to see do I have any extra electrons. 238 00:12:12,010 --> 00:12:19,210 So for this, I had 10, I used eight, so I have two left. 239 00:12:19,210 --> 00:12:23,950 So I'm going to assign those two as a lone pair. 240 00:12:23,950 --> 00:12:26,050 Should I put them on hydrogen? 241 00:12:26,050 --> 00:12:27,591 What about carbon? 242 00:12:27,591 --> 00:12:28,090 No. 243 00:12:28,090 --> 00:12:32,500 Carbon already has its eight, because it has four bonds. 244 00:12:32,500 --> 00:12:38,350 So then I'm going to put it on nitrogen. 245 00:12:38,350 --> 00:12:42,040 And then the only thing left is formal charge, which 246 00:12:42,040 --> 00:12:44,210 we haven't talked about yet. 247 00:12:44,210 --> 00:12:46,270 So before we do formal charge, I just 248 00:12:46,270 --> 00:12:49,460 want to do the same thing with CN minus. 249 00:12:55,250 --> 00:12:56,960 And I will say if you want to draw 250 00:12:56,960 --> 00:12:58,700 triple bonds, that's fine too. 251 00:12:58,700 --> 00:13:02,780 You don't have to indicate the dots as bonds. 252 00:13:02,780 --> 00:13:06,120 It's perfectly fine to write out a triple bond. 253 00:13:06,120 --> 00:13:07,977 So I could have written this as well. 254 00:13:13,490 --> 00:13:16,550 OK, so let's look at CN minus. 255 00:13:16,550 --> 00:13:23,540 So how many valence electrons does carbon have again? 256 00:13:23,540 --> 00:13:24,200 Four. 257 00:13:24,200 --> 00:13:25,930 Nitrogen? 258 00:13:25,930 --> 00:13:26,640 Five. 259 00:13:26,640 --> 00:13:27,680 Am I done? 260 00:13:27,680 --> 00:13:28,400 AUDIENCE: No. 261 00:13:28,400 --> 00:13:29,316 CATHERINE DRENNAN: No. 262 00:13:29,316 --> 00:13:30,860 I need to add one because there's 263 00:13:30,860 --> 00:13:33,950 a charge on this molecule of minus one. 264 00:13:33,950 --> 00:13:42,130 So now so I have 10 again. 265 00:13:42,130 --> 00:13:45,050 Three I'm going to figure out how many 266 00:13:45,050 --> 00:13:48,350 electrons I need to complete my valence shell. 267 00:13:48,350 --> 00:13:50,768 How many from carbon? 268 00:13:50,768 --> 00:13:51,700 AUDIENCE: Eight. 269 00:13:51,700 --> 00:13:52,741 CATHERINE DRENNAN: Eight. 270 00:13:52,741 --> 00:13:54,140 Nitrogen? 271 00:13:54,140 --> 00:13:56,730 Eight so now I have 16. 272 00:13:59,510 --> 00:14:00,720 I will subtract. 273 00:14:00,720 --> 00:14:08,480 Now 10 from 16 and get six bonding electrons. 274 00:14:08,480 --> 00:14:11,410 And I'm going to assign first just two of them. 275 00:14:11,410 --> 00:14:15,020 So I'll assign one, two here. 276 00:14:15,020 --> 00:14:17,990 And then we-- this is to assign. 277 00:14:17,990 --> 00:14:20,960 Six said, do you have any left over? 278 00:14:20,960 --> 00:14:28,160 I had six, and I only used two, so the answer is yes. 279 00:14:28,160 --> 00:14:30,770 I have four more. 280 00:14:30,770 --> 00:14:34,820 So I can put those in one, two, three, four. 281 00:14:34,820 --> 00:14:38,030 Again, we're going to have a triple bond. 282 00:14:38,030 --> 00:14:41,510 And then we ask are there any electrons left? 283 00:14:41,510 --> 00:14:46,770 So we had 10, we used six of them. 284 00:14:46,770 --> 00:14:52,230 And so we're going to have now four more here. 285 00:14:52,230 --> 00:14:55,460 So now I can assign-- this only has-- this 286 00:14:55,460 --> 00:14:57,480 is not complete for carbon. 287 00:14:57,480 --> 00:14:59,690 So I can put a lone pair on carbon, 288 00:14:59,690 --> 00:15:02,060 and I can put a lone pair on nitrogen. 289 00:15:02,060 --> 00:15:05,630 And now they have their complete octet. 290 00:15:05,630 --> 00:15:08,880 And we get to assign formal charge. 291 00:15:08,880 --> 00:15:14,360 So I could write it this way, or I could have written it 292 00:15:14,360 --> 00:15:16,786 with a triple bond here. 293 00:15:16,786 --> 00:15:20,850 And don't forget to put the charge on the end. 294 00:15:20,850 --> 00:15:21,560 All right. 295 00:15:21,560 --> 00:15:26,590 So now let's consider formal charge, 296 00:15:26,590 --> 00:15:28,900 because we're never done with our Lewis structures 297 00:15:28,900 --> 00:15:31,910 until we've considered formal charge. 298 00:15:31,910 --> 00:15:35,560 So formal charge is a measure of the extent 299 00:15:35,560 --> 00:15:38,380 to which the atom has really lost or gained 300 00:15:38,380 --> 00:15:42,670 an electron in the process of forming this covalent bond. 301 00:15:42,670 --> 00:15:45,610 So as we'll talk about, it's not the same as oxidation number 302 00:15:45,610 --> 00:15:47,830 where you have like sodium plus one, 303 00:15:47,830 --> 00:15:51,520 but again, there's some differences 304 00:15:51,520 --> 00:15:53,190 in how many are brought to the table 305 00:15:53,190 --> 00:15:55,750 and what it ends up with in the end. 306 00:15:55,750 --> 00:15:57,996 So there's an equation which you'll have to learn, 307 00:15:57,996 --> 00:15:59,620 but if you do enough of these problems, 308 00:15:59,620 --> 00:16:02,080 it'll be stuck in your brain and you can't purge it 309 00:16:02,080 --> 00:16:04,490 even if you want to. 310 00:16:04,490 --> 00:16:08,710 And so formal charge, FC, is equal to the number 311 00:16:08,710 --> 00:16:13,090 of valence electrons, symbol V, here, 312 00:16:13,090 --> 00:16:17,800 minus the number of lone pair electrons 313 00:16:17,800 --> 00:16:22,480 minus half of the number of bonding electrons. 314 00:16:22,480 --> 00:16:24,790 So at least this equation makes sense. 315 00:16:24,790 --> 00:16:27,610 If you forget what they mean, you can probably think about it 316 00:16:27,610 --> 00:16:30,820 and it'll come back to you. 317 00:16:30,820 --> 00:16:34,630 So in doing these formal charges, 318 00:16:34,630 --> 00:16:39,070 you want the formal charges to add up 319 00:16:39,070 --> 00:16:41,780 to the charge on the molecule. 320 00:16:41,780 --> 00:16:45,760 So if we had HCN, that's a neutral molecule 321 00:16:45,760 --> 00:16:49,360 so the sum of all of the formal charges must be zero 322 00:16:49,360 --> 00:16:52,090 or you did something wrong. 323 00:16:52,090 --> 00:16:56,200 If it's CN minus, the sum of the formal charges 324 00:16:56,200 --> 00:17:00,380 has to add up to minus one or you did something wrong. 325 00:17:00,380 --> 00:17:03,010 So this is a good way of checking your math. 326 00:17:03,010 --> 00:17:06,280 So always remember that the sum needs 327 00:17:06,280 --> 00:17:09,190 to add up to the total charge on the molecule. 328 00:17:09,190 --> 00:17:12,609 If you remember that, that's a really good check to make sure 329 00:17:12,609 --> 00:17:15,010 you didn't make some kind of weird math mistake 330 00:17:15,010 --> 00:17:18,220 and add things wrong and have an appropriate number of loan 331 00:17:18,220 --> 00:17:20,690 pairs or something going on. 332 00:17:20,690 --> 00:17:21,190 OK. 333 00:17:21,190 --> 00:17:24,220 So let's calculate formal charge now 334 00:17:24,220 --> 00:17:28,390 on our CN minus molecule up here. 335 00:17:28,390 --> 00:17:32,540 So the formal charge now on carbon here. 336 00:17:32,540 --> 00:17:35,110 So how many valence electrons do carbon have? 337 00:17:35,110 --> 00:17:37,270 AUDIENCE: [INAUDIBLE] 338 00:17:37,270 --> 00:17:38,320 CATHERINE DRENNAN: Four. 339 00:17:38,320 --> 00:17:40,106 How many lone pairs does it have? 340 00:17:40,106 --> 00:17:41,860 AUDIENCE: [INAUDIBLE] 341 00:17:41,860 --> 00:17:44,110 CATHERINE DRENNAN: It has one lone pair, two lone pair 342 00:17:44,110 --> 00:17:44,710 electrons. 343 00:17:44,710 --> 00:17:48,310 This is the total number of lone pair electrons here, 344 00:17:48,310 --> 00:17:51,550 and then half the number of bonding electrons. 345 00:17:51,550 --> 00:17:55,930 So it is expanding electrons and half of that is three. 346 00:17:55,930 --> 00:17:59,079 And so that should add up to a charge of minus one. 347 00:17:59,079 --> 00:18:01,120 You can also, instead of thinking half the number 348 00:18:01,120 --> 00:18:02,680 of bonding electrons, you can also 349 00:18:02,680 --> 00:18:08,110 just think about number of bonds if you want to to do this. 350 00:18:08,110 --> 00:18:10,900 All right so to see if you have the hang of it, 351 00:18:10,900 --> 00:18:12,270 let's do a clicker question. 352 00:18:53,810 --> 00:18:55,670 OK, 10 more seconds. 353 00:19:12,759 --> 00:19:13,800 Most people got it right. 354 00:19:13,800 --> 00:19:16,980 I can't actually read the number, but that was very good. 355 00:19:16,980 --> 00:19:19,320 It always seems wrong to put zero as the answer, 356 00:19:19,320 --> 00:19:22,510 but that is, in fact, the answer here. 357 00:19:22,510 --> 00:19:29,400 So if we look at this again, we have five valence electrons 358 00:19:29,400 --> 00:19:34,050 on nitrogen. We have two lone pair electrons, 359 00:19:34,050 --> 00:19:36,960 and we have six bonding electrons. 360 00:19:36,960 --> 00:19:39,750 Half of six is three, and so that's zero. 361 00:19:39,750 --> 00:19:42,570 Or you could have said, well, if this is minus one, 362 00:19:42,570 --> 00:19:44,010 and the charge is minus one, then 363 00:19:44,010 --> 00:19:47,490 that had to have been zero, otherwise Professor Drennan 364 00:19:47,490 --> 00:19:51,310 did something wrong, and that's just not possible. 365 00:19:51,310 --> 00:19:53,730 So the answer there would be zero. 366 00:19:53,730 --> 00:19:58,500 And you can see that the total formal charge is minus one, 367 00:19:58,500 --> 00:20:02,160 and the total charge of the molecules also minus one. 368 00:20:02,160 --> 00:20:06,290 So again formal charge does not equal oxidation number, 369 00:20:06,290 --> 00:20:07,800 it's something special. 370 00:20:07,800 --> 00:20:11,340 It tells you kind of in this arrangement of atoms 371 00:20:11,340 --> 00:20:13,920 in the molecule, did this atom kind 372 00:20:13,920 --> 00:20:15,990 of come up with a little bit more at the end 373 00:20:15,990 --> 00:20:19,020 or a little bit less, depending on where it started from 374 00:20:19,020 --> 00:20:20,320 and where it is. 375 00:20:20,320 --> 00:20:22,440 And where you want in these structures 376 00:20:22,440 --> 00:20:25,450 for the formal charge to be low. 377 00:20:25,450 --> 00:20:31,560 So we can use a formal charge to decide between Lewis structures 378 00:20:31,560 --> 00:20:33,330 so that the structures with the lowest 379 00:20:33,330 --> 00:20:36,090 absolute values of this formal charge 380 00:20:36,090 --> 00:20:38,020 are more stable structures. 381 00:20:38,020 --> 00:20:40,710 So if you have really high formal charges, 382 00:20:40,710 --> 00:20:43,590 that means that molecule isn't really very stable 383 00:20:43,590 --> 00:20:45,090 because you want low charges. 384 00:20:45,090 --> 00:20:47,570 You want lower energy. 385 00:20:47,570 --> 00:20:52,890 You want things to be in a more neutral and happy state. 386 00:20:52,890 --> 00:20:55,770 So we want to figure out which ones 387 00:20:55,770 --> 00:20:58,240 are going to have low charges. 388 00:20:58,240 --> 00:21:02,460 So let's look at another example, Thiocynate ion, 389 00:21:02,460 --> 00:21:07,380 and it has a carbon, sulfur, and nitrogen in it, 390 00:21:07,380 --> 00:21:10,010 and it has a charge of minus one. 391 00:21:10,010 --> 00:21:14,790 So I might tell you the ionization energies for carbon, 392 00:21:14,790 --> 00:21:17,960 sulfur, and nitrogen, and then ask you 393 00:21:17,960 --> 00:21:21,360 which atom do you think is going to be 394 00:21:21,360 --> 00:21:23,335 in the center of the molecule? 395 00:21:23,335 --> 00:21:24,210 So what do you think? 396 00:21:24,210 --> 00:21:26,280 What's in the center of this molecule based 397 00:21:26,280 --> 00:21:28,590 on those numbers? 398 00:21:28,590 --> 00:21:29,545 Just yell it out. 399 00:21:29,545 --> 00:21:30,420 AUDIENCE: [INAUDIBLE] 400 00:21:30,420 --> 00:21:32,480 CATHERINE DRENNAN: Sulfur. 401 00:21:32,480 --> 00:21:35,640 So sulfur has the lowest ionization energy, 402 00:21:35,640 --> 00:21:38,850 and I told you that's usually the thing in the center. 403 00:21:38,850 --> 00:21:42,030 But you can start with that. 404 00:21:42,030 --> 00:21:43,890 It's always good to start with that, 405 00:21:43,890 --> 00:21:45,980 but then you want to check the structure 406 00:21:45,980 --> 00:21:48,840 and make sure that a structure with sulfur in the middle 407 00:21:48,840 --> 00:21:51,430 has the lowest formal charge. 408 00:21:51,430 --> 00:21:53,450 So let's take a look at that. 409 00:21:53,450 --> 00:21:57,630 So we can draw structure A with sulfur in the middle, 410 00:21:57,630 --> 00:22:02,210 and then calculate the formal charges on that. 411 00:22:02,210 --> 00:22:06,360 And if we do that, we see for the nitrogen here, 412 00:22:06,360 --> 00:22:09,020 nitrogen has five valence electrons, 413 00:22:09,020 --> 00:22:14,550 it has four lone pair electrons, and it has half of four bonding 414 00:22:14,550 --> 00:22:19,290 electrons, so it would have a formal charge a minus one 415 00:22:19,290 --> 00:22:22,830 in this particular structure that has sulfur in the middle. 416 00:22:22,830 --> 00:22:24,010 We can look at carbon. 417 00:22:24,010 --> 00:22:27,540 Carbon has four valence electrons, Four lone pair 418 00:22:27,540 --> 00:22:31,410 electrons, and half of four bonding electrons, 419 00:22:31,410 --> 00:22:34,920 so it has a charge of minus two. 420 00:22:34,920 --> 00:22:37,370 Then we can look at sulfur. 421 00:22:37,370 --> 00:22:41,340 Sulfur has six valence electrons, zero loan pairs, 422 00:22:41,340 --> 00:22:44,310 and half of eight bonding electrons, 423 00:22:44,310 --> 00:22:47,250 so it has a formal charge of plus two. 424 00:22:47,250 --> 00:22:50,550 So overall, this does equal the minus one. 425 00:22:50,550 --> 00:22:52,650 So it's a valid structure. 426 00:22:52,650 --> 00:22:56,370 But is that the lowest energy one? 427 00:22:56,370 --> 00:23:00,120 We also could put carbon in the middle or nitrogen 428 00:23:00,120 --> 00:23:01,270 in the middle. 429 00:23:01,270 --> 00:23:04,770 So let's look at what this does for us. 430 00:23:04,770 --> 00:23:08,960 So with the formal charge on nitrogen now, 431 00:23:08,960 --> 00:23:14,190 we have five minus four lone pair electrons, minus half 432 00:23:14,190 --> 00:23:16,770 of four bonding electrons, minus one. 433 00:23:16,770 --> 00:23:19,140 So that's the same. 434 00:23:19,140 --> 00:23:21,180 Now we can look at carbon. 435 00:23:21,180 --> 00:23:24,500 We have now just no lone pair electrons. 436 00:23:24,500 --> 00:23:26,803 It has four minus zero minus four, 437 00:23:26,803 --> 00:23:31,520 half of eight bonding electrons, or zero. 438 00:23:31,520 --> 00:23:35,550 And for sulfur, six minus four lone pair electrons, 439 00:23:35,550 --> 00:23:38,245 and half of four bonding electrons, or zero. 440 00:23:41,640 --> 00:23:47,260 Next, structure C, five minus zero lone pairs minus half 441 00:23:47,260 --> 00:23:49,690 of eight, plus one. 442 00:23:49,690 --> 00:23:52,240 So that one's different. 443 00:23:52,240 --> 00:23:55,630 Carbon, we have four valence electrons 444 00:23:55,630 --> 00:24:01,550 minus four lone pairs minus half of four, bonding minus two. 445 00:24:01,550 --> 00:24:06,070 And for the sulfur, six minus four lone pairs 446 00:24:06,070 --> 00:24:08,880 electrons, half of three or zero. 447 00:24:08,880 --> 00:24:13,160 So now with the clicker, tell me which is most stable. 448 00:24:39,500 --> 00:24:40,488 All right, 10 seconds. 449 00:24:45,190 --> 00:24:46,506 I think this can be 98%. 450 00:24:49,150 --> 00:24:50,300 That's what I'm thinking. 451 00:24:50,300 --> 00:24:51,254 I'm feeling good. 452 00:24:55,180 --> 00:24:57,540 Well, close. 453 00:24:57,540 --> 00:24:59,700 Yeah. 454 00:24:59,700 --> 00:25:00,200 What? 455 00:25:00,200 --> 00:25:01,056 No, no. 456 00:25:01,056 --> 00:25:01,555 Sorry. 457 00:25:05,360 --> 00:25:13,210 It should be B. Yeah, it should be B. Yeah. 458 00:25:13,210 --> 00:25:15,830 Sorry I actually-- Yay. 459 00:25:15,830 --> 00:25:16,790 There we go. 460 00:25:16,790 --> 00:25:19,320 Yeah, B is correct. 461 00:25:19,320 --> 00:25:20,990 So if we just look at it over here, 462 00:25:20,990 --> 00:25:23,300 it has the lowest number of formal charges, 463 00:25:23,300 --> 00:25:51,600 so the answer is B. OK. 464 00:25:51,600 --> 00:25:55,590 Let's start with this simply. 465 00:25:55,590 --> 00:25:59,310 Who wants to tell me why one, how they could look at this 466 00:25:59,310 --> 00:26:01,665 and realize one was not the correct answer? 467 00:26:07,230 --> 00:26:08,060 I think this is on. 468 00:26:08,060 --> 00:26:10,230 Give it a try. 469 00:26:10,230 --> 00:26:13,470 AUDIENCE: One's not correct because if you look back 470 00:26:13,470 --> 00:26:16,230 at your atomic radius chart, this is pretty much doing 471 00:26:16,230 --> 00:26:18,330 the exact opposite of that 472 00:26:18,330 --> 00:26:20,940 CATHERINE DRENNAN: Yeah, so helium definitely 473 00:26:20,940 --> 00:26:24,260 not the biggest atom there is. 474 00:26:24,260 --> 00:26:31,880 OK so six got a lot of attention, and so did two. 475 00:26:31,880 --> 00:26:36,840 And ionization energy, electron affinity, and electronegativity 476 00:26:36,840 --> 00:26:40,110 are definitely connected to each other, 477 00:26:40,110 --> 00:26:46,680 but there is a clue that electron affinity would not 478 00:26:46,680 --> 00:26:47,510 be the correct. 479 00:26:47,510 --> 00:26:49,945 Does someone want to say what you might have noticed? 480 00:26:54,240 --> 00:26:57,420 AUDIENCE: For electron affinity, it increases and then 481 00:26:57,420 --> 00:27:00,480 stops at the noble gases because noble gases do not 482 00:27:00,480 --> 00:27:02,080 want electrons. 483 00:27:02,080 --> 00:27:05,120 So in this particular chart, all the noble gases 484 00:27:05,120 --> 00:27:08,040 are like the highest-- are the highest ones 485 00:27:08,040 --> 00:27:09,570 in the relative area, which would 486 00:27:09,570 --> 00:27:11,822 mean that electron affinity would be incorrect. 487 00:27:11,822 --> 00:27:12,780 CATHERINE DRENNAN: Yep. 488 00:27:12,780 --> 00:27:15,920 That's right so the noble gases were the clue. 489 00:27:15,920 --> 00:27:18,130 So [INAUDIBLE]. 490 00:27:18,130 --> 00:27:18,860 Yep. 491 00:27:18,860 --> 00:27:24,090 And so if electron affinity also is not high at the noble gases, 492 00:27:24,090 --> 00:27:25,620 they're also not electronegative. 493 00:27:25,620 --> 00:27:27,384 Noble gases just don't want extra light. 494 00:27:27,384 --> 00:27:28,800 They don't want to lose electrons, 495 00:27:28,800 --> 00:27:30,216 they don't want to gain electrons, 496 00:27:30,216 --> 00:27:33,030 they just want to be left alone. 497 00:27:33,030 --> 00:27:36,810 So this trend is for ionization energy. 498 00:27:36,810 --> 00:27:39,920 And because noble gases want to be left alone, 499 00:27:39,920 --> 00:27:44,980 they don't want to lose any of their electrons. 500 00:27:44,980 --> 00:27:45,480 Great. 501 00:27:45,480 --> 00:27:48,120 So this is good to be thinking about, 502 00:27:48,120 --> 00:27:53,900 because we're finishing up now for the handout from last time. 503 00:27:53,900 --> 00:27:57,170 And we're going to be talking about electronegativity again. 504 00:27:57,170 --> 00:27:59,690 We never move very far away from a lot of these topics. 505 00:27:59,690 --> 00:28:01,300 They just keep coming back. 506 00:28:01,300 --> 00:28:02,905 So we just keep reviewing them. 507 00:28:02,905 --> 00:28:04,530 All right, I don't need to microphones, 508 00:28:04,530 --> 00:28:07,260 although, I don't know, I kind of like having this one. 509 00:28:07,260 --> 00:28:09,210 Anyway. 510 00:28:09,210 --> 00:28:12,270 So let's take out the handout from last time, 511 00:28:12,270 --> 00:28:13,430 and let's finish it up. 512 00:28:13,430 --> 00:28:16,800 We were talking about formal charge, 513 00:28:16,800 --> 00:28:20,900 and we had looked at examples where we calculated 514 00:28:20,900 --> 00:28:23,580 formal charge, and then we looked 515 00:28:23,580 --> 00:28:25,800 at which structure would be lowest in energy, 516 00:28:25,800 --> 00:28:28,710 and that was the structure where you had the least 517 00:28:28,710 --> 00:28:32,360 separation between the charges on them, 518 00:28:32,360 --> 00:28:35,250 so the smallest absolute numbers. 519 00:28:35,250 --> 00:28:38,850 If you have formal charges of zero, that's fantastic. 520 00:28:38,850 --> 00:28:41,250 That's what the molecule wants. 521 00:28:41,250 --> 00:28:43,830 Minus one, plus one, if you must, 522 00:28:43,830 --> 00:28:47,180 but when you start having plus two minus two, that's 523 00:28:47,180 --> 00:28:50,690 a lot of charge separation, so that's less favorable. 524 00:28:50,690 --> 00:28:52,560 So we're going to have more examples of that 525 00:28:52,560 --> 00:28:54,160 as we go along. 526 00:28:54,160 --> 00:29:00,710 But now, what-- if you had calculated formal charge 527 00:29:00,710 --> 00:29:02,370 and they're all the same, how do you 528 00:29:02,370 --> 00:29:04,810 know which structure is correct? 529 00:29:04,810 --> 00:29:08,930 So what if you have-- and this is-- just some people who 530 00:29:08,930 --> 00:29:09,790 are having trouble. 531 00:29:09,790 --> 00:29:13,400 This is the top of page four from the handout. 532 00:29:13,400 --> 00:29:17,190 You have two valid Lewis structures 533 00:29:17,190 --> 00:29:20,040 that have the same formal charge, 534 00:29:20,040 --> 00:29:21,650 how do you know where it goes? 535 00:29:21,650 --> 00:29:25,000 And the answer is that the negative formal charge 536 00:29:25,000 --> 00:29:28,910 should go on the most electronegative atom. 537 00:29:28,910 --> 00:29:31,770 And so that's why we are sort of talking about electronegativity 538 00:29:31,770 --> 00:29:32,610 again. 539 00:29:32,610 --> 00:29:35,940 And so electronegativity-- remember electronegativity 540 00:29:35,940 --> 00:29:39,090 is high when the electron affinity is high, 541 00:29:39,090 --> 00:29:41,900 meaning that the atom wants to get an electron, 542 00:29:41,900 --> 00:29:43,670 has a high affinity for electrons, 543 00:29:43,670 --> 00:29:45,780 and also a high ionization energy, which means it 544 00:29:45,780 --> 00:29:47,610 doesn't want to give up its electrons. 545 00:29:47,610 --> 00:29:50,340 So that's something-- it likes to have electrons, 546 00:29:50,340 --> 00:29:52,800 and so you want to put a negative value, which 547 00:29:52,800 --> 00:29:56,100 indicates there's more electrons on something 548 00:29:56,100 --> 00:29:57,890 that's electronegative. 549 00:29:57,890 --> 00:30:00,757 So negative on negative over here. 550 00:30:00,757 --> 00:30:02,340 And so that's what you're looking for. 551 00:30:02,340 --> 00:30:05,140 That's how you're going to make a decision. 552 00:30:05,140 --> 00:30:07,210 So let's look at an example. 553 00:30:07,210 --> 00:30:09,480 So here is a molecule, and we're going 554 00:30:09,480 --> 00:30:13,670 to look at two possible Lewis structures of this 555 00:30:13,670 --> 00:30:16,960 with similar formal charges, and decide which 556 00:30:16,960 --> 00:30:18,690 has the correct structure. 557 00:30:18,690 --> 00:30:21,540 So first let me give you a couple hints that can be useful 558 00:30:21,540 --> 00:30:23,790 in problem sets and in exams. 559 00:30:23,790 --> 00:30:28,320 When you see CH3, that's a methyl group, 560 00:30:28,320 --> 00:30:31,320 and that's going to be terminal so you're 561 00:30:31,320 --> 00:30:34,190 going to have these three hydrogens associated 562 00:30:34,190 --> 00:30:36,600 with that carbon, and that's going to be 563 00:30:36,600 --> 00:30:38,860 at an end of the molecule. 564 00:30:38,860 --> 00:30:41,580 So it could look like one of these two things. 565 00:30:41,580 --> 00:30:43,920 So we have the carbon, we have it attached 566 00:30:43,920 --> 00:30:48,060 to three hydrogens, a carbon attached to three hydrogens, 567 00:30:48,060 --> 00:30:53,350 and then attached to something else, this nitrogen here. 568 00:30:53,350 --> 00:30:56,080 And this structure where it's just kind of written out 569 00:30:56,080 --> 00:31:00,760 in a line or a chain of atoms, what we 570 00:31:00,760 --> 00:31:03,450 call chain molecules sometimes. 571 00:31:03,450 --> 00:31:05,560 Often the atoms are actually written 572 00:31:05,560 --> 00:31:08,080 in the order in which they're attached, 573 00:31:08,080 --> 00:31:09,900 so that's definitely true here. 574 00:31:09,900 --> 00:31:13,850 CH3, three hydrogens attached to the carbon, 575 00:31:13,850 --> 00:31:18,100 so they're attached to the atom that came before in the chain. 576 00:31:18,100 --> 00:31:21,990 The nitrogen is also going to be attached to the carbon. 577 00:31:21,990 --> 00:31:23,640 Even though it follows the hydrogen, 578 00:31:23,640 --> 00:31:25,140 you're not going to have hydrogen 579 00:31:25,140 --> 00:31:26,306 in the middle of a bond. 580 00:31:26,306 --> 00:31:28,055 It's not going to be bonded to two things. 581 00:31:28,055 --> 00:31:31,620 Hydrogen is always terminal, so even though nitrogen is here, 582 00:31:31,620 --> 00:31:33,660 it's got to be attached to the carbon. 583 00:31:33,660 --> 00:31:35,500 So we have three hydrogens and then 584 00:31:35,500 --> 00:31:39,490 a bond between the carbon and the nitrogen. 585 00:31:39,490 --> 00:31:41,860 Now we have a hydrogen after the nitrogen, 586 00:31:41,860 --> 00:31:44,260 and by this rule it should go on the nitrogen. 587 00:31:44,260 --> 00:31:46,860 But you might want to double check that that's true. 588 00:31:46,860 --> 00:31:48,520 And then we have an oxygen. Again, 589 00:31:48,520 --> 00:31:51,690 the oxygen is going to have a bond with the nitrogen. 590 00:31:51,690 --> 00:31:54,520 You're not going to have a bond with hydrogen in the middle. 591 00:31:54,520 --> 00:31:56,350 Hydrogen's always terminal. 592 00:31:56,350 --> 00:31:58,170 So the only real choice we have here 593 00:31:58,170 --> 00:32:00,390 is we can put the hydrogen on nitrogen 594 00:32:00,390 --> 00:32:03,730 or we can put the hydrogen on the oxygen here. 595 00:32:03,730 --> 00:32:09,120 And so we can use this rule about electronegativity and 596 00:32:09,120 --> 00:32:11,830 formal charges to figure out which of these structures 597 00:32:11,830 --> 00:32:13,410 is right. 598 00:32:13,410 --> 00:32:17,160 So in this particular case, all of the formal charges 599 00:32:17,160 --> 00:32:21,100 are zero on all of the atoms, except there's 600 00:32:21,100 --> 00:32:23,290 one minus one charge. 601 00:32:23,290 --> 00:32:25,360 And in this structure, the minus one charge 602 00:32:25,360 --> 00:32:28,380 would be on the oxygen, and in this structure 603 00:32:28,380 --> 00:32:31,330 the minus one charge would be on the nitrogen. 604 00:32:31,330 --> 00:32:34,030 And if everything else is zero then you 605 00:32:34,030 --> 00:32:36,630 have the sum of your formal charges, 606 00:32:36,630 --> 00:32:39,630 minus one, equal to the charge on the molecule, 607 00:32:39,630 --> 00:32:40,750 which is minus one. 608 00:32:40,750 --> 00:32:43,300 So both of these are valid structures. 609 00:32:43,300 --> 00:32:47,400 Both have low values of formal charge, which is right. 610 00:32:47,400 --> 00:32:49,750 So it's going to be the structure that 611 00:32:49,750 --> 00:32:54,450 has the negative charge on the most electronegative atom is 612 00:32:54,450 --> 00:32:55,950 the right structure. 613 00:32:55,950 --> 00:32:58,780 And so here you need to remember some of your rules 614 00:32:58,780 --> 00:33:00,820 about electronegativity. 615 00:33:00,820 --> 00:33:03,630 And in terms of electronegativity, 616 00:33:03,630 --> 00:33:07,750 we see that oxygen has greater electronegativity 617 00:33:07,750 --> 00:33:11,460 than the nitrogen. And so that's where 618 00:33:11,460 --> 00:33:14,190 we would want to put our negative charge-- 619 00:33:14,190 --> 00:33:16,330 on the negative charge goes on the atom that's 620 00:33:16,330 --> 00:33:17,860 the most electronegative. 621 00:33:17,860 --> 00:33:21,740 And that would generate the lower energy structure. 622 00:33:21,740 --> 00:33:25,380 So if you're given a table of electronegativities, which 623 00:33:25,380 --> 00:33:27,250 you often are, you can look it up 624 00:33:27,250 --> 00:33:30,100 and validate that that's going to be the correct place. 625 00:33:30,100 --> 00:33:32,700 And in fact, experimentally we know that that's 626 00:33:32,700 --> 00:33:34,920 the right structure. 627 00:33:34,920 --> 00:33:36,080 So that works. 628 00:33:36,080 --> 00:33:40,920 So if you have two structures, identical formal charges, 629 00:33:40,920 --> 00:33:43,510 valid structures, then the last step 630 00:33:43,510 --> 00:33:46,210 is to think about where that negative charge should 631 00:33:46,210 --> 00:33:49,216 go, and pick the atom that is the most electronegative. 632 00:33:52,170 --> 00:33:54,720 All right we have one more thing we 633 00:33:54,720 --> 00:33:57,070 need to talk about in Lewis structures 634 00:33:57,070 --> 00:33:59,740 before we start violating various rules 635 00:33:59,740 --> 00:34:02,560 that we've learned, and that is that we 636 00:34:02,560 --> 00:34:05,050 need to talk about resonance structures. 637 00:34:05,050 --> 00:34:08,190 And so to explain to you what a resonance structure is, 638 00:34:08,190 --> 00:34:10,679 it's really helpful to start with an example, 639 00:34:10,679 --> 00:34:12,250 so that's what we're going to do. 640 00:34:12,250 --> 00:34:14,400 And we're going to consider ozone, 641 00:34:14,400 --> 00:34:20,190 which is three atoms of oxygen. And we have the ozone layer, 642 00:34:20,190 --> 00:34:24,659 which protects us from UV damage and is very valuable, 643 00:34:24,659 --> 00:34:27,010 and we should not destroy it with chemicals being 644 00:34:27,010 --> 00:34:28,889 released into the environment. 645 00:34:28,889 --> 00:34:32,429 And because you don't have complete say over that, 646 00:34:32,429 --> 00:34:33,870 always wear sunscreen. 647 00:34:33,870 --> 00:34:35,130 OK. 648 00:34:35,130 --> 00:34:38,050 So let's build up these Lewis structures, 649 00:34:38,050 --> 00:34:42,199 and then consider what's meant by a resonance structure. 650 00:34:42,199 --> 00:34:44,350 So here we have part of the Periodic Table 651 00:34:44,350 --> 00:34:47,110 that you're going to need, and we 652 00:34:47,110 --> 00:34:49,920 need to think first about the valence electrons. 653 00:34:49,920 --> 00:34:52,929 So oxygen has six valence electrons 654 00:34:52,929 --> 00:34:57,030 and there are three oxygens, so that's 18. 655 00:34:57,030 --> 00:35:01,300 To get a full octet for each of the oxygens, three oxygens, 656 00:35:01,300 --> 00:35:06,250 an octet is eight valence electrons, so that would be 24. 657 00:35:06,250 --> 00:35:08,350 To figure out the number of bonding electrons, 658 00:35:08,350 --> 00:35:11,940 we're going to be subtracting our octet electrons 659 00:35:11,940 --> 00:35:13,860 from our valence electrons. 660 00:35:13,860 --> 00:35:18,600 So 24 minus 18 is six. 661 00:35:18,600 --> 00:35:21,790 And then our next step is to assign those bonding 662 00:35:21,790 --> 00:35:25,290 electrons two at a time, two per bond. 663 00:35:25,290 --> 00:35:28,080 So let's take a look at that. 664 00:35:28,080 --> 00:35:32,250 We can put one bond here between these two oxygens, 665 00:35:32,250 --> 00:35:34,750 one bond here between these two oxygens, 666 00:35:34,750 --> 00:35:37,440 and then ask do we have any more? 667 00:35:37,440 --> 00:35:38,500 And we do. 668 00:35:38,500 --> 00:35:41,770 Because we have six bonding electrons, we used four, 669 00:35:41,770 --> 00:35:44,230 so we have two more bonding electrons. 670 00:35:44,230 --> 00:35:46,350 So we need to make a double bond. 671 00:35:46,350 --> 00:35:51,100 But now we have the question of where to make that double bond. 672 00:35:51,100 --> 00:35:53,920 Am I going to put it between these two oxygens 673 00:35:53,920 --> 00:35:56,770 or am I going to put it between those two oxygens? 674 00:35:56,770 --> 00:36:02,150 And so I could say put it there, but I could also, 675 00:36:02,150 --> 00:36:05,670 in structure two, put the double bond over here. 676 00:36:05,670 --> 00:36:06,170 All right. 677 00:36:06,170 --> 00:36:08,480 We'll come back to that question in a minute. 678 00:36:08,480 --> 00:36:11,570 Let's first figure out if we have any remaining valence 679 00:36:11,570 --> 00:36:14,300 electrons, and we do. 680 00:36:14,300 --> 00:36:20,060 So we had 18 and we've only used six, so we have 12 left. 681 00:36:20,060 --> 00:36:23,510 So we're going to put those in as lone pairs. 682 00:36:23,510 --> 00:36:25,640 And so I can put them in over here. 683 00:36:25,640 --> 00:36:30,200 One set here, one, two, three, four, five, six. 684 00:36:30,200 --> 00:36:32,210 And I can also do that over here. 685 00:36:32,210 --> 00:36:36,140 One, two, three, four, five, six. 686 00:36:36,140 --> 00:36:38,570 And now we have two structures, so we 687 00:36:38,570 --> 00:36:40,340 need to think about formal charges 688 00:36:40,340 --> 00:36:43,550 to see if that can help differentiate structure 689 00:36:43,550 --> 00:36:47,390 one from structure two. 690 00:36:47,390 --> 00:36:48,590 OK. 691 00:36:48,590 --> 00:36:50,360 And let's look at that. 692 00:36:50,360 --> 00:36:52,670 Be sure everyone's ready. 693 00:36:52,670 --> 00:36:57,440 And that is a clicker question. 694 00:36:57,440 --> 00:36:59,050 So I'll put that back up. 695 00:37:49,141 --> 00:37:49,640 All right. 696 00:37:49,640 --> 00:37:52,340 Let's just take 10 more seconds, and we'll 697 00:37:52,340 --> 00:37:56,120 talk about what the right answer is, and a little bit of a trick 698 00:37:56,120 --> 00:37:58,055 for doing these, perhaps, a bit faster. 699 00:38:10,490 --> 00:38:11,600 OK. 700 00:38:11,600 --> 00:38:12,600 So that was pretty good. 701 00:38:12,600 --> 00:38:15,660 So let's look at why that's the right answer. 702 00:38:15,660 --> 00:38:18,870 And we'll take a look at that over here. 703 00:38:18,870 --> 00:38:22,470 So let's do the calculations. 704 00:38:22,470 --> 00:38:25,120 So you have to remember the equation for formal charge 705 00:38:25,120 --> 00:38:27,070 for sure and once you do enough problems 706 00:38:27,070 --> 00:38:30,160 it should stick in your head pretty easily. 707 00:38:30,160 --> 00:38:32,400 So if we look over here-- so this 708 00:38:32,400 --> 00:38:36,060 is the formal charge on oxygen A. 709 00:38:36,060 --> 00:38:37,830 There were six valence electrons, 710 00:38:37,830 --> 00:38:40,900 there are four lone pair electrons, minus half 711 00:38:40,900 --> 00:38:42,640 of the bonding electrons. 712 00:38:42,640 --> 00:38:45,300 There are eight bonding electrons, so that's two. 713 00:38:45,300 --> 00:38:48,870 So that's a formal charge of zero. 714 00:38:48,870 --> 00:38:54,510 For oxygen B over here, we again have six valence electrons, 715 00:38:54,510 --> 00:38:57,690 and we have two lone pair electrons. 716 00:38:57,690 --> 00:39:01,260 We have six bonding electrons, so half of six 717 00:39:01,260 --> 00:39:04,620 is three, which is plus one. 718 00:39:04,620 --> 00:39:10,290 And on oxygen C over here, six minus six 719 00:39:10,290 --> 00:39:14,600 lone parallel electrons, half of one bond, so half of two 720 00:39:14,600 --> 00:39:19,110 is one, minus one, overall the charge here is neutral. 721 00:39:19,110 --> 00:39:22,080 And that's good because it's a neutral molecule. 722 00:39:22,080 --> 00:39:24,930 So to do this structure faster, you 723 00:39:24,930 --> 00:39:28,500 have to realize that oxygen A over here 724 00:39:28,500 --> 00:39:31,330 is the same as C over there. 725 00:39:31,330 --> 00:39:34,050 So you can just copy down what you 726 00:39:34,050 --> 00:39:40,890 had for C is now A. B is exactly the same in the two structures, 727 00:39:40,890 --> 00:39:43,630 so it's the same as what you calculated. 728 00:39:43,630 --> 00:39:46,710 And now this C was the same as this A, 729 00:39:46,710 --> 00:39:49,260 so we can put that same value that we 730 00:39:49,260 --> 00:39:56,496 calculated for A into C. So they're both the same. 731 00:39:56,496 --> 00:40:01,980 Same formal charges, which structure is correct? 732 00:40:01,980 --> 00:40:04,950 And the answer is both of them. 733 00:40:04,950 --> 00:40:07,200 And in fact, you need both of them, 734 00:40:07,200 --> 00:40:11,640 so there's data-- chemists love data-- 735 00:40:11,640 --> 00:40:13,830 and the data is that the bonds are actually 736 00:40:13,830 --> 00:40:15,550 equivalent in the molecule. 737 00:40:15,550 --> 00:40:17,940 So it isn't that there's one double bond and one 738 00:40:17,940 --> 00:40:24,030 single bond, there's just one kind of bond in ozone, 739 00:40:24,030 --> 00:40:28,160 and it's between a single and a double bond. 740 00:40:28,160 --> 00:40:32,850 And so this is how one would draw that kind of thing. 741 00:40:32,850 --> 00:40:36,420 You would have structure one here and structure two here. 742 00:40:36,420 --> 00:40:38,610 You would put them both in brackets, 743 00:40:38,610 --> 00:40:40,620 and you would put this special double headed 744 00:40:40,620 --> 00:40:43,680 arrow between them and that indicates 745 00:40:43,680 --> 00:40:47,100 that both of these structures are needed to describe 746 00:40:47,100 --> 00:40:49,290 the properties of the molecule. 747 00:40:49,290 --> 00:40:54,880 You do not have a stationary double bond and a single bond. 748 00:40:54,880 --> 00:40:58,170 You have kind of a mixture between these two, 749 00:40:58,170 --> 00:41:01,290 and that's what a resonance structure is. 750 00:41:01,290 --> 00:41:04,560 So let's take a little bit more of a look at this. 751 00:41:04,560 --> 00:41:07,980 So this is just what I had before, experimental evidence 752 00:41:07,980 --> 00:41:09,870 is that the bonds are equivalent. 753 00:41:09,870 --> 00:41:11,930 There isn't a double and a single, 754 00:41:11,930 --> 00:41:15,750 there's something in between, a one and a half. 755 00:41:15,750 --> 00:41:18,360 And this is called a resonance hybrid, 756 00:41:18,360 --> 00:41:20,580 and it's of length of these two structures. 757 00:41:20,580 --> 00:41:24,600 So this structure is blended with that structure. 758 00:41:24,600 --> 00:41:27,870 So some of you are aware of hybrids from biology, 759 00:41:27,870 --> 00:41:29,810 and now with cars. 760 00:41:29,810 --> 00:41:32,190 So a mule would be an example. 761 00:41:32,190 --> 00:41:34,230 And if you're thinking about what a mule is, 762 00:41:34,230 --> 00:41:36,990 you don't walk out into your barnyard one day 763 00:41:36,990 --> 00:41:39,520 and see a donkey one day and you go out the next day 764 00:41:39,520 --> 00:41:40,950 and you see a horse. 765 00:41:40,950 --> 00:41:44,490 A mule is a hybrid between a donkey and a horse. 766 00:41:44,490 --> 00:41:47,090 So if you were a chemist, you would do this. 767 00:41:47,090 --> 00:41:48,900 You would put the donkey in brackets, 768 00:41:48,900 --> 00:41:52,150 and the horse in brackets, and put your double headed arrow. 769 00:41:52,150 --> 00:41:55,620 And if you see this, you'd say, oh yes, a mule. 770 00:41:55,620 --> 00:41:58,230 So that is what this is. 771 00:41:58,230 --> 00:42:01,830 Both structures are needed to describe ozone. 772 00:42:01,830 --> 00:42:04,030 One structure isn't enough. 773 00:42:04,030 --> 00:42:05,100 You need both of them. 774 00:42:05,100 --> 00:42:07,860 They're in resonance with each other. 775 00:42:07,860 --> 00:42:12,210 And so what's true about the electrons in ozone 776 00:42:12,210 --> 00:42:18,210 is that they're delocalized across all of these bonds, 777 00:42:18,210 --> 00:42:21,030 so there isn't like a double single. 778 00:42:21,030 --> 00:42:23,760 All of those electrons are delocalized. 779 00:42:23,760 --> 00:42:27,387 They're shared over this set of atoms. 780 00:42:27,387 --> 00:42:29,970 And you can have two resonance structures, you can have three, 781 00:42:29,970 --> 00:42:30,810 you can have four. 782 00:42:30,810 --> 00:42:33,180 It depends on the molecule in question. 783 00:42:33,180 --> 00:42:38,040 And in all those cases, those electrons would be delocalized. 784 00:42:38,040 --> 00:42:42,150 So just to sum that up, resonance structures, 785 00:42:42,150 --> 00:42:47,050 two or more, same arrangement of atoms-- and that's important. 786 00:42:47,050 --> 00:42:48,947 It's the electrons that are different. 787 00:42:48,947 --> 00:42:51,030 And this isn't in your handout, out but just think 788 00:42:51,030 --> 00:42:53,310 about this for a second-- because it was in your hand 789 00:42:53,310 --> 00:42:54,510 out a minute ago. 790 00:42:54,510 --> 00:42:57,180 Are these resonance structures? 791 00:42:57,180 --> 00:42:58,060 No. 792 00:42:58,060 --> 00:43:00,540 They're not resonance structures. 793 00:43:00,540 --> 00:43:02,760 The atoms are in different positions. 794 00:43:02,760 --> 00:43:05,490 So one of these structures is right one of these structures 795 00:43:05,490 --> 00:43:06,450 is wrong. 796 00:43:06,450 --> 00:43:09,420 With resonance structures, they're both correct 797 00:43:09,420 --> 00:43:12,600 and both needed to define the structure. 798 00:43:12,600 --> 00:43:13,590 So pay attention. 799 00:43:13,590 --> 00:43:15,060 This is a common mistake. 800 00:43:15,060 --> 00:43:15,740 Pay attention. 801 00:43:15,740 --> 00:43:17,950 Ask yourself, are the atoms in a different position? 802 00:43:17,950 --> 00:43:19,200 That's not resonance. 803 00:43:19,200 --> 00:43:22,020 You're just looking-- atoms are the same, formal charge 804 00:43:22,020 --> 00:43:23,670 are the same, you're just looking 805 00:43:23,670 --> 00:43:27,890 at whether you have different arrangements of electrons.