WEBVTT 00:00:00.090 --> 00:00:02.430 The following content is provided under a Creative 00:00:02.430 --> 00:00:03.850 Commons license. 00:00:03.850 --> 00:00:06.060 Your support will help MIT OpenCourseWare 00:00:06.060 --> 00:00:10.150 continue to offer high quality educational resources for free. 00:00:10.150 --> 00:00:12.690 To make a donation or to view additional materials 00:00:12.690 --> 00:00:16.650 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.650 --> 00:00:17.450 at ocw.mit.edu. 00:00:25.962 --> 00:00:27.462 CATHERINE DRENNAN: Lewis structures. 00:00:30.180 --> 00:00:34.080 So I tell students who have sort of no background in chemistry 00:00:34.080 --> 00:00:35.610 before they come into this class, 00:00:35.610 --> 00:00:40.050 and there always are some, that there are some topics in 5.111 00:00:40.050 --> 00:00:42.960 that having no experience with the topic 00:00:42.960 --> 00:00:45.210 is actually a good thing, and Lewis structures is 00:00:45.210 --> 00:00:46.459 one of these things. 00:00:46.459 --> 00:00:48.750 I think if you've seen it before, you're like, oh yeah, 00:00:48.750 --> 00:00:49.530 that's easy. 00:00:49.530 --> 00:00:50.640 I know how to do that. 00:00:50.640 --> 00:00:52.800 You don't practice and you get on an exam 00:00:52.800 --> 00:00:54.910 and you're like, oh wait a minute. 00:00:54.910 --> 00:00:57.090 I forgot how to do this. 00:00:57.090 --> 00:00:58.980 And the students who haven't seen it before 00:00:58.980 --> 00:01:02.490 know all the rules and just get brilliant, perfect scores 00:01:02.490 --> 00:01:03.120 on the exam. 00:01:03.120 --> 00:01:05.640 And the other students are like, man. 00:01:05.640 --> 00:01:08.530 I forgot how to do my Lewis structures. 00:01:08.530 --> 00:01:11.190 So here are Lewis structures. 00:01:11.190 --> 00:01:12.990 We're going to go over them. 00:01:12.990 --> 00:01:15.300 So I really like Lewis structures 00:01:15.300 --> 00:01:19.440 because they're relatively simple, and they work. 00:01:19.440 --> 00:01:23.520 Like 90% of the time, they are the correct structure. 00:01:23.520 --> 00:01:27.540 And I'm a big fan of simple things working. 00:01:27.540 --> 00:01:30.810 If you wanted to get from 90% to 100%, 00:01:30.810 --> 00:01:33.480 you'd have to use Schrodinger's equation, 00:01:33.480 --> 00:01:37.380 but you can get 90% just with the simple Lewis structures 00:01:37.380 --> 00:01:38.620 I'm a big fan of that. 00:01:38.620 --> 00:01:41.590 I love it when simple things really work pretty well. 00:01:41.590 --> 00:01:42.230 OK. 00:01:42.230 --> 00:01:45.000 So when Lewis structures the key to this 00:01:45.000 --> 00:01:49.950 is thinking about the electrons being shared so that you 00:01:49.950 --> 00:01:53.790 get a full valence shell. 00:01:53.790 --> 00:01:58.620 And having the electrons distributed in such a way 00:01:58.620 --> 00:02:02.880 that all of the atoms have the number of electrons 00:02:02.880 --> 00:02:05.250 that make them happy, which is usually 00:02:05.250 --> 00:02:08.970 eight electrons, which is an octet, that noble gas 00:02:08.970 --> 00:02:11.400 configuration. 00:02:11.400 --> 00:02:13.860 So every dot in a Lewis structure 00:02:13.860 --> 00:02:17.220 represents a valence electron. 00:02:17.220 --> 00:02:23.010 And we can then look at some atoms 00:02:23.010 --> 00:02:26.220 and put dots around them to indicate the number 00:02:26.220 --> 00:02:27.810 of valence electrons. 00:02:27.810 --> 00:02:32.220 So we also have to know how many valence electrons atoms have. 00:02:32.220 --> 00:02:33.720 And so why didn't you just practice 00:02:33.720 --> 00:02:35.106 with a clicker question. 00:02:38.270 --> 00:02:43.620 And here's part of the periodic table up here if you need it. 00:02:56.070 --> 00:02:58.610 All right. 00:02:58.610 --> 00:02:59.970 I'm told 10 seconds. 00:02:59.970 --> 00:03:01.610 Everyone was crazy fast. 00:03:13.110 --> 00:03:14.820 Yes. 00:03:14.820 --> 00:03:17.760 So seven is the correct answer. 00:03:17.760 --> 00:03:19.800 You could look at the periodic table 00:03:19.800 --> 00:03:23.790 and sometimes with these it's a counting thing. 00:03:23.790 --> 00:03:28.680 So this is one where you want to always double check 00:03:28.680 --> 00:03:30.480 if things don't make sense. 00:03:30.480 --> 00:03:31.290 All right. 00:03:31.290 --> 00:03:35.550 So we can put seven electrons around fluorine, 00:03:35.550 --> 00:03:37.530 and we'll have two fluorines here. 00:03:37.530 --> 00:03:41.649 They'll both have seven electrons around them. 00:03:41.649 --> 00:03:43.440 And now I'm going to jump to another slide, 00:03:43.440 --> 00:03:45.870 but I'm going to show you the seven again in case 00:03:45.870 --> 00:03:47.207 you haven't written them down. 00:03:47.207 --> 00:03:49.290 If you don't want to, they're not in your handout, 00:03:49.290 --> 00:03:52.860 but that's probably OK. 00:03:52.860 --> 00:03:54.390 So when you bring them together, you 00:03:54.390 --> 00:03:58.290 can bring them together in such a way that they can all share. 00:03:58.290 --> 00:04:02.940 And so if we put in green, then, one of the fluorine's seven. 00:04:02.940 --> 00:04:06.450 And then we put in blue the other fluorine's seven, 00:04:06.450 --> 00:04:08.880 you can see that they can share two in the middle 00:04:08.880 --> 00:04:11.680 and both are very, very happy. 00:04:11.680 --> 00:04:15.870 So just thinking about this really simple idea, 00:04:15.870 --> 00:04:21.579 how many electrons will give you an octet, will give you eight? 00:04:21.579 --> 00:04:23.340 And how can you put things together 00:04:23.340 --> 00:04:27.370 in such a way that allows for that to happen? 00:04:27.370 --> 00:04:32.010 Now there are a few elements that 00:04:32.010 --> 00:04:36.930 do not want eight in their valence shell, 00:04:36.930 --> 00:04:38.280 and hydrogen is one of them. 00:04:38.280 --> 00:04:40.800 It just has that one S. So it only 00:04:40.800 --> 00:04:43.230 wants two, that's all it can handle. 00:04:43.230 --> 00:04:46.050 So this is an exception, hydrogen 00:04:46.050 --> 00:04:47.940 is going to want two electrons. 00:04:47.940 --> 00:04:50.070 Hydrogen loves to interact with things, though. 00:04:50.070 --> 00:04:52.320 It interacts with lots and lots of things. 00:04:52.320 --> 00:04:55.950 And here hydrogen with its one valence electron 00:04:55.950 --> 00:04:59.220 is interacting with chlorine with its seven 00:04:59.220 --> 00:05:03.720 valence electrons, and they are sharing two electrons forming 00:05:03.720 --> 00:05:06.410 a bond together. 00:05:06.410 --> 00:05:08.690 So when we're talking about Lewis structures, 00:05:08.690 --> 00:05:12.590 we're talking about different kinds of electrons. 00:05:12.590 --> 00:05:16.820 So we're talking about bonding electrons, the electrons that 00:05:16.820 --> 00:05:20.640 are involved in the bond, and also lone pair electrons. 00:05:20.640 --> 00:05:26.240 So chloride in HCl is going to have two bonding electrons, 00:05:26.240 --> 00:05:30.650 one was its, and one came from hydrogen. 00:05:30.650 --> 00:05:35.450 And it's also going to have six lone pair electrons, 00:05:35.450 --> 00:05:38.490 or we could say three lone pairs. 00:05:38.490 --> 00:05:43.670 So when we say a lone pair, that indicates two electrons there. 00:05:43.670 --> 00:05:49.160 So it has one, two, three, four, five, six or one pair, 00:05:49.160 --> 00:05:52.980 two pairs, three pairs. 00:05:52.980 --> 00:05:55.850 Now there are rules to Lewis structures, 00:05:55.850 --> 00:05:57.830 and here is the complete rule. 00:05:57.830 --> 00:06:00.200 In your handout, this wouldn't fit on one page. 00:06:00.200 --> 00:06:03.050 It's on two pages. 00:06:03.050 --> 00:06:06.140 And these rules, if you do work these problems, 00:06:06.140 --> 00:06:09.140 you will remember these rules, and they become pretty easy. 00:06:09.140 --> 00:06:12.050 But it's important to work Lewis structure problems 00:06:12.050 --> 00:06:16.070 so that the rules become really familiar to you. 00:06:16.070 --> 00:06:18.660 And it takes time to work Lewis structure problems, 00:06:18.660 --> 00:06:22.110 so don't wait to the last minute to start this problem set. 00:06:22.110 --> 00:06:24.124 There's a lot of Lewis structure problems on it, 00:06:24.124 --> 00:06:25.790 which means it's not difficult, but it's 00:06:25.790 --> 00:06:27.421 going to take some time. 00:06:27.421 --> 00:06:27.920 All right. 00:06:27.920 --> 00:06:31.070 So let's briefly go over these rules. 00:06:31.070 --> 00:06:34.790 First what you want to do is draw in the skeleton structure. 00:06:34.790 --> 00:06:37.880 Just put the atoms down. 00:06:37.880 --> 00:06:42.200 Hydrogen and fluorine are always going to be terminal atoms. 00:06:42.200 --> 00:06:44.210 Don't put them in the middle of a molecule. 00:06:44.210 --> 00:06:46.880 That gets chemistry professors really upset 00:06:46.880 --> 00:06:49.130 to see hydrogen in the middle with lots of bonds 00:06:49.130 --> 00:06:51.950 to things, so don't do it. 00:06:51.950 --> 00:06:55.580 And typically the element with the lowest ionization energy 00:06:55.580 --> 00:06:58.070 goes in the middle, and there are some exceptions 00:06:58.070 --> 00:06:59.720 and we'll see some of those exceptions. 00:06:59.720 --> 00:07:03.440 But that should be your first guess. 00:07:03.440 --> 00:07:06.180 You want to count the number of valence electrons. 00:07:06.180 --> 00:07:09.200 If there's a negative charge, you need to count that in 00:07:09.200 --> 00:07:11.900 or if there's a positive charge you need to subtract that 00:07:11.900 --> 00:07:13.246 from the total. 00:07:13.246 --> 00:07:15.620 Then you want to figure out the total number of electrons 00:07:15.620 --> 00:07:19.070 needed, so everyone has their full valence shell. 00:07:19.070 --> 00:07:23.570 You need to subtract these two to get the number of bonding 00:07:23.570 --> 00:07:24.350 electrons. 00:07:24.350 --> 00:07:26.210 And here are some of the things that it's 00:07:26.210 --> 00:07:29.360 really easy to make math mistakes here, 00:07:29.360 --> 00:07:31.550 so if your structure makes zero sense at the end, 00:07:31.550 --> 00:07:33.670 go back and check your math. 00:07:33.670 --> 00:07:37.100 Assign to bonding electrons to each bond. 00:07:37.100 --> 00:07:39.350 If any remain, you want to think about 00:07:39.350 --> 00:07:42.320 whether you have double or triple bonds. 00:07:42.320 --> 00:07:44.900 And there's only certain kinds of atoms that can 00:07:44.900 --> 00:07:46.530 have double and triple bonds. 00:07:46.530 --> 00:07:48.110 So be careful where you're putting 00:07:48.110 --> 00:07:50.600 your double and triple bonds. 00:07:50.600 --> 00:07:54.020 If any valence electrons remain, those are loan pairs. 00:07:54.020 --> 00:07:56.750 And then lastly, you want to figure out the formal charge 00:07:56.750 --> 00:07:59.862 on all of the atoms in your structure to make sure 00:07:59.862 --> 00:08:01.820 that this is a valid structure, and we're going 00:08:01.820 --> 00:08:03.900 to talk about formal charge. 00:08:03.900 --> 00:08:06.260 So first, let's just try an example. 00:08:06.260 --> 00:08:10.700 And your sheet has two examples on the same page. 00:08:10.700 --> 00:08:14.060 We have HCN and we also have CN minus. 00:08:14.060 --> 00:08:17.949 So we're going to do HCN first, so don't fill your entire page 00:08:17.949 --> 00:08:19.490 because you're going to have to write 00:08:19.490 --> 00:08:21.920 things for CN minus as well. 00:08:21.920 --> 00:08:25.520 But before we start, we need to figure out which atom is likely 00:08:25.520 --> 00:08:27.660 going to be in the middle. 00:08:27.660 --> 00:08:29.960 And so why don't you tell me what you 00:08:29.960 --> 00:08:31.240 think on the clicker question. 00:09:07.680 --> 00:09:08.813 OK, 10 more seconds. 00:09:24.520 --> 00:09:25.060 Yup. 00:09:25.060 --> 00:09:29.080 So here again we want to have one that has a lower ionization 00:09:29.080 --> 00:09:31.780 energy, and you also want to consider other things 00:09:31.780 --> 00:09:35.610 like hydrogen can't be in the middle. 00:09:35.610 --> 00:09:36.160 OK. 00:09:36.160 --> 00:09:38.243 And it was written that way, but sometimes they're 00:09:38.243 --> 00:09:42.290 written in a way that is not as straightforward. 00:09:42.290 --> 00:09:42.790 OK. 00:09:45.680 --> 00:09:47.225 So I'll put that up. 00:09:47.225 --> 00:09:48.850 All right so we'll go through the rules 00:09:48.850 --> 00:09:50.630 and we'll try to work this out. 00:09:50.630 --> 00:09:53.522 So first I can write-- I'm going to start-- 00:09:53.522 --> 00:09:55.930 I guess I'll write over here. 00:09:55.930 --> 00:10:00.790 So number one, we're just going to write HCN with C 00:10:00.790 --> 00:10:02.740 in the middle. 00:10:02.740 --> 00:10:04.780 So that's the first thing we're going to do. 00:10:04.780 --> 00:10:07.630 Next we're going to consider the valence electrons. 00:10:07.630 --> 00:10:10.060 And you can just help me out by yelling things out. 00:10:10.060 --> 00:10:12.370 How many valence electrons does hydrogen have? 00:10:12.370 --> 00:10:13.780 AUDIENCE: One. 00:10:13.780 --> 00:10:15.820 CATHERINE DRENNAN: What about carbon? 00:10:15.820 --> 00:10:17.000 AUDIENCE: Four. 00:10:17.000 --> 00:10:18.166 CATHERINE DRENNAN: Nitrogen? 00:10:18.166 --> 00:10:18.850 AUDIENCE: Five. 00:10:18.850 --> 00:10:20.950 CATHERINE DRENNAN: And you can always check me on my math. 00:10:20.950 --> 00:10:22.166 How much does that equal? 00:10:22.166 --> 00:10:23.040 AUDIENCE: 10. 00:10:23.040 --> 00:10:24.248 CATHERINE DRENNAN: Excellent. 00:10:26.590 --> 00:10:28.690 There's nothing like adding simple numbers 00:10:28.690 --> 00:10:33.310 in front of 350 people to really put the stress in one's day. 00:10:33.310 --> 00:10:37.420 OK, so to have a complete full valence shell, what 00:10:37.420 --> 00:10:38.447 do I need for hydrogen? 00:10:38.447 --> 00:10:39.149 AUDIENCE: Two. 00:10:39.149 --> 00:10:40.690 CATHERINE DRENNAN: What about carbon? 00:10:40.690 --> 00:10:42.480 AUDIENCE: Four. 00:10:42.480 --> 00:10:44.630 CATHERINE DRENNAN: Eight to be complete. 00:10:44.630 --> 00:10:45.170 Nitrogen? 00:10:45.170 --> 00:10:45.940 AUDIENCE: Eight. 00:10:45.940 --> 00:10:47.572 CATHERINE DRENNAN: Eight. 00:10:47.572 --> 00:10:50.000 And I think we add this up, we should get 18. 00:10:50.000 --> 00:10:51.360 How's that? 00:10:51.360 --> 00:10:52.180 All right. 00:10:52.180 --> 00:10:57.250 So for four, now we're going to subtract these numbers 00:10:57.250 --> 00:11:01.060 from each other to tell us how many bonding electrons we have. 00:11:01.060 --> 00:11:04.090 So we have 18 minus 10. 00:11:04.090 --> 00:11:07.780 And we should have eight to bonding electrons. 00:11:07.780 --> 00:11:13.820 For five I'm going to now assign two per bond. 00:11:13.820 --> 00:11:16.030 So I'm going to put one here. 00:11:16.030 --> 00:11:17.560 Another here. 00:11:17.560 --> 00:11:18.220 Another here. 00:11:18.220 --> 00:11:19.030 Another here. 00:11:19.030 --> 00:11:21.790 So I've assigned two per bond. 00:11:21.790 --> 00:11:25.210 And now I see if I have any left. 00:11:25.210 --> 00:11:26.245 Do I have some left? 00:11:28.930 --> 00:11:31.060 I've used four, I had eight. 00:11:31.060 --> 00:11:35.500 So yes, I have four more. 00:11:35.500 --> 00:11:38.170 And if you have more bonding electrons, 00:11:38.170 --> 00:11:43.120 then you are supposed to assign those bonding electrons. 00:11:43.120 --> 00:11:44.620 And think about whether it's allowed 00:11:44.620 --> 00:11:48.040 to have double or triple bonds. 00:11:48.040 --> 00:11:50.830 Can hydrogen be involved in the double bond? 00:11:50.830 --> 00:11:51.760 No. 00:11:51.760 --> 00:11:52.480 Carbon nitrogen? 00:11:52.480 --> 00:11:53.182 AUDIENCE: Yes. 00:11:53.182 --> 00:11:54.890 CATHERINE DRENNAN: Yes, or a triple bond. 00:11:54.890 --> 00:11:56.650 So I have four more, so I'm going 00:11:56.650 --> 00:12:00.670 to put one, two, three, four. 00:12:00.670 --> 00:12:02.650 So I'm going to have a triple bond 00:12:02.650 --> 00:12:06.970 between my carbon and my nitrogen. And now I'm good. 00:12:06.970 --> 00:12:12.010 So now I want to see do I have any extra electrons. 00:12:12.010 --> 00:12:19.210 So for this, I had 10, I used eight, so I have two left. 00:12:19.210 --> 00:12:23.950 So I'm going to assign those two as a lone pair. 00:12:23.950 --> 00:12:26.050 Should I put them on hydrogen? 00:12:26.050 --> 00:12:27.591 What about carbon? 00:12:27.591 --> 00:12:28.090 No. 00:12:28.090 --> 00:12:32.500 Carbon already has its eight, because it has four bonds. 00:12:32.500 --> 00:12:38.350 So then I'm going to put it on nitrogen. 00:12:38.350 --> 00:12:42.040 And then the only thing left is formal charge, which 00:12:42.040 --> 00:12:44.210 we haven't talked about yet. 00:12:44.210 --> 00:12:46.270 So before we do formal charge, I just 00:12:46.270 --> 00:12:49.460 want to do the same thing with CN minus. 00:12:55.250 --> 00:12:56.960 And I will say if you want to draw 00:12:56.960 --> 00:12:58.700 triple bonds, that's fine too. 00:12:58.700 --> 00:13:02.780 You don't have to indicate the dots as bonds. 00:13:02.780 --> 00:13:06.120 It's perfectly fine to write out a triple bond. 00:13:06.120 --> 00:13:07.977 So I could have written this as well. 00:13:13.490 --> 00:13:16.550 OK, so let's look at CN minus. 00:13:16.550 --> 00:13:23.540 So how many valence electrons does carbon have again? 00:13:23.540 --> 00:13:24.200 Four. 00:13:24.200 --> 00:13:25.930 Nitrogen? 00:13:25.930 --> 00:13:26.640 Five. 00:13:26.640 --> 00:13:27.680 Am I done? 00:13:27.680 --> 00:13:28.400 AUDIENCE: No. 00:13:28.400 --> 00:13:29.316 CATHERINE DRENNAN: No. 00:13:29.316 --> 00:13:30.860 I need to add one because there's 00:13:30.860 --> 00:13:33.950 a charge on this molecule of minus one. 00:13:33.950 --> 00:13:42.130 So now so I have 10 again. 00:13:42.130 --> 00:13:45.050 Three I'm going to figure out how many 00:13:45.050 --> 00:13:48.350 electrons I need to complete my valence shell. 00:13:48.350 --> 00:13:50.768 How many from carbon? 00:13:50.768 --> 00:13:51.700 AUDIENCE: Eight. 00:13:51.700 --> 00:13:52.741 CATHERINE DRENNAN: Eight. 00:13:52.741 --> 00:13:54.140 Nitrogen? 00:13:54.140 --> 00:13:56.730 Eight so now I have 16. 00:13:59.510 --> 00:14:00.720 I will subtract. 00:14:00.720 --> 00:14:08.480 Now 10 from 16 and get six bonding electrons. 00:14:08.480 --> 00:14:11.410 And I'm going to assign first just two of them. 00:14:11.410 --> 00:14:15.020 So I'll assign one, two here. 00:14:15.020 --> 00:14:17.990 And then we-- this is to assign. 00:14:17.990 --> 00:14:20.960 Six said, do you have any left over? 00:14:20.960 --> 00:14:28.160 I had six, and I only used two, so the answer is yes. 00:14:28.160 --> 00:14:30.770 I have four more. 00:14:30.770 --> 00:14:34.820 So I can put those in one, two, three, four. 00:14:34.820 --> 00:14:38.030 Again, we're going to have a triple bond. 00:14:38.030 --> 00:14:41.510 And then we ask are there any electrons left? 00:14:41.510 --> 00:14:46.770 So we had 10, we used six of them. 00:14:46.770 --> 00:14:52.230 And so we're going to have now four more here. 00:14:52.230 --> 00:14:55.460 So now I can assign-- this only has-- this 00:14:55.460 --> 00:14:57.480 is not complete for carbon. 00:14:57.480 --> 00:14:59.690 So I can put a lone pair on carbon, 00:14:59.690 --> 00:15:02.060 and I can put a lone pair on nitrogen. 00:15:02.060 --> 00:15:05.630 And now they have their complete octet. 00:15:05.630 --> 00:15:08.880 And we get to assign formal charge. 00:15:08.880 --> 00:15:14.360 So I could write it this way, or I could have written it 00:15:14.360 --> 00:15:16.786 with a triple bond here. 00:15:16.786 --> 00:15:20.850 And don't forget to put the charge on the end. 00:15:20.850 --> 00:15:21.560 All right. 00:15:21.560 --> 00:15:26.590 So now let's consider formal charge, 00:15:26.590 --> 00:15:28.900 because we're never done with our Lewis structures 00:15:28.900 --> 00:15:31.910 until we've considered formal charge. 00:15:31.910 --> 00:15:35.560 So formal charge is a measure of the extent 00:15:35.560 --> 00:15:38.380 to which the atom has really lost or gained 00:15:38.380 --> 00:15:42.670 an electron in the process of forming this covalent bond. 00:15:42.670 --> 00:15:45.610 So as we'll talk about, it's not the same as oxidation number 00:15:45.610 --> 00:15:47.830 where you have like sodium plus one, 00:15:47.830 --> 00:15:51.520 but again, there's some differences 00:15:51.520 --> 00:15:53.190 in how many are brought to the table 00:15:53.190 --> 00:15:55.750 and what it ends up with in the end. 00:15:55.750 --> 00:15:57.996 So there's an equation which you'll have to learn, 00:15:57.996 --> 00:15:59.620 but if you do enough of these problems, 00:15:59.620 --> 00:16:02.080 it'll be stuck in your brain and you can't purge it 00:16:02.080 --> 00:16:04.490 even if you want to. 00:16:04.490 --> 00:16:08.710 And so formal charge, FC, is equal to the number 00:16:08.710 --> 00:16:13.090 of valence electrons, symbol V, here, 00:16:13.090 --> 00:16:17.800 minus the number of lone pair electrons 00:16:17.800 --> 00:16:22.480 minus half of the number of bonding electrons. 00:16:22.480 --> 00:16:24.790 So at least this equation makes sense. 00:16:24.790 --> 00:16:27.610 If you forget what they mean, you can probably think about it 00:16:27.610 --> 00:16:30.820 and it'll come back to you. 00:16:30.820 --> 00:16:34.630 So in doing these formal charges, 00:16:34.630 --> 00:16:39.070 you want the formal charges to add up 00:16:39.070 --> 00:16:41.780 to the charge on the molecule. 00:16:41.780 --> 00:16:45.760 So if we had HCN, that's a neutral molecule 00:16:45.760 --> 00:16:49.360 so the sum of all of the formal charges must be zero 00:16:49.360 --> 00:16:52.090 or you did something wrong. 00:16:52.090 --> 00:16:56.200 If it's CN minus, the sum of the formal charges 00:16:56.200 --> 00:17:00.380 has to add up to minus one or you did something wrong. 00:17:00.380 --> 00:17:03.010 So this is a good way of checking your math. 00:17:03.010 --> 00:17:06.280 So always remember that the sum needs 00:17:06.280 --> 00:17:09.190 to add up to the total charge on the molecule. 00:17:09.190 --> 00:17:12.609 If you remember that, that's a really good check to make sure 00:17:12.609 --> 00:17:15.010 you didn't make some kind of weird math mistake 00:17:15.010 --> 00:17:18.220 and add things wrong and have an appropriate number of loan 00:17:18.220 --> 00:17:20.690 pairs or something going on. 00:17:20.690 --> 00:17:21.190 OK. 00:17:21.190 --> 00:17:24.220 So let's calculate formal charge now 00:17:24.220 --> 00:17:28.390 on our CN minus molecule up here. 00:17:28.390 --> 00:17:32.540 So the formal charge now on carbon here. 00:17:32.540 --> 00:17:35.110 So how many valence electrons do carbon have? 00:17:35.110 --> 00:17:37.270 AUDIENCE: [INAUDIBLE] 00:17:37.270 --> 00:17:38.320 CATHERINE DRENNAN: Four. 00:17:38.320 --> 00:17:40.106 How many lone pairs does it have? 00:17:40.106 --> 00:17:41.860 AUDIENCE: [INAUDIBLE] 00:17:41.860 --> 00:17:44.110 CATHERINE DRENNAN: It has one lone pair, two lone pair 00:17:44.110 --> 00:17:44.710 electrons. 00:17:44.710 --> 00:17:48.310 This is the total number of lone pair electrons here, 00:17:48.310 --> 00:17:51.550 and then half the number of bonding electrons. 00:17:51.550 --> 00:17:55.930 So it is expanding electrons and half of that is three. 00:17:55.930 --> 00:17:59.079 And so that should add up to a charge of minus one. 00:17:59.079 --> 00:18:01.120 You can also, instead of thinking half the number 00:18:01.120 --> 00:18:02.680 of bonding electrons, you can also 00:18:02.680 --> 00:18:08.110 just think about number of bonds if you want to to do this. 00:18:08.110 --> 00:18:10.900 All right so to see if you have the hang of it, 00:18:10.900 --> 00:18:12.270 let's do a clicker question. 00:18:53.810 --> 00:18:55.670 OK, 10 more seconds. 00:19:12.759 --> 00:19:13.800 Most people got it right. 00:19:13.800 --> 00:19:16.980 I can't actually read the number, but that was very good. 00:19:16.980 --> 00:19:19.320 It always seems wrong to put zero as the answer, 00:19:19.320 --> 00:19:22.510 but that is, in fact, the answer here. 00:19:22.510 --> 00:19:29.400 So if we look at this again, we have five valence electrons 00:19:29.400 --> 00:19:34.050 on nitrogen. We have two lone pair electrons, 00:19:34.050 --> 00:19:36.960 and we have six bonding electrons. 00:19:36.960 --> 00:19:39.750 Half of six is three, and so that's zero. 00:19:39.750 --> 00:19:42.570 Or you could have said, well, if this is minus one, 00:19:42.570 --> 00:19:44.010 and the charge is minus one, then 00:19:44.010 --> 00:19:47.490 that had to have been zero, otherwise Professor Drennan 00:19:47.490 --> 00:19:51.310 did something wrong, and that's just not possible. 00:19:51.310 --> 00:19:53.730 So the answer there would be zero. 00:19:53.730 --> 00:19:58.500 And you can see that the total formal charge is minus one, 00:19:58.500 --> 00:20:02.160 and the total charge of the molecules also minus one. 00:20:02.160 --> 00:20:06.290 So again formal charge does not equal oxidation number, 00:20:06.290 --> 00:20:07.800 it's something special. 00:20:07.800 --> 00:20:11.340 It tells you kind of in this arrangement of atoms 00:20:11.340 --> 00:20:13.920 in the molecule, did this atom kind 00:20:13.920 --> 00:20:15.990 of come up with a little bit more at the end 00:20:15.990 --> 00:20:19.020 or a little bit less, depending on where it started from 00:20:19.020 --> 00:20:20.320 and where it is. 00:20:20.320 --> 00:20:22.440 And where you want in these structures 00:20:22.440 --> 00:20:25.450 for the formal charge to be low. 00:20:25.450 --> 00:20:31.560 So we can use a formal charge to decide between Lewis structures 00:20:31.560 --> 00:20:33.330 so that the structures with the lowest 00:20:33.330 --> 00:20:36.090 absolute values of this formal charge 00:20:36.090 --> 00:20:38.020 are more stable structures. 00:20:38.020 --> 00:20:40.710 So if you have really high formal charges, 00:20:40.710 --> 00:20:43.590 that means that molecule isn't really very stable 00:20:43.590 --> 00:20:45.090 because you want low charges. 00:20:45.090 --> 00:20:47.570 You want lower energy. 00:20:47.570 --> 00:20:52.890 You want things to be in a more neutral and happy state. 00:20:52.890 --> 00:20:55.770 So we want to figure out which ones 00:20:55.770 --> 00:20:58.240 are going to have low charges. 00:20:58.240 --> 00:21:02.460 So let's look at another example, Thiocynate ion, 00:21:02.460 --> 00:21:07.380 and it has a carbon, sulfur, and nitrogen in it, 00:21:07.380 --> 00:21:10.010 and it has a charge of minus one. 00:21:10.010 --> 00:21:14.790 So I might tell you the ionization energies for carbon, 00:21:14.790 --> 00:21:17.960 sulfur, and nitrogen, and then ask you 00:21:17.960 --> 00:21:21.360 which atom do you think is going to be 00:21:21.360 --> 00:21:23.335 in the center of the molecule? 00:21:23.335 --> 00:21:24.210 So what do you think? 00:21:24.210 --> 00:21:26.280 What's in the center of this molecule based 00:21:26.280 --> 00:21:28.590 on those numbers? 00:21:28.590 --> 00:21:29.545 Just yell it out. 00:21:29.545 --> 00:21:30.420 AUDIENCE: [INAUDIBLE] 00:21:30.420 --> 00:21:32.480 CATHERINE DRENNAN: Sulfur. 00:21:32.480 --> 00:21:35.640 So sulfur has the lowest ionization energy, 00:21:35.640 --> 00:21:38.850 and I told you that's usually the thing in the center. 00:21:38.850 --> 00:21:42.030 But you can start with that. 00:21:42.030 --> 00:21:43.890 It's always good to start with that, 00:21:43.890 --> 00:21:45.980 but then you want to check the structure 00:21:45.980 --> 00:21:48.840 and make sure that a structure with sulfur in the middle 00:21:48.840 --> 00:21:51.430 has the lowest formal charge. 00:21:51.430 --> 00:21:53.450 So let's take a look at that. 00:21:53.450 --> 00:21:57.630 So we can draw structure A with sulfur in the middle, 00:21:57.630 --> 00:22:02.210 and then calculate the formal charges on that. 00:22:02.210 --> 00:22:06.360 And if we do that, we see for the nitrogen here, 00:22:06.360 --> 00:22:09.020 nitrogen has five valence electrons, 00:22:09.020 --> 00:22:14.550 it has four lone pair electrons, and it has half of four bonding 00:22:14.550 --> 00:22:19.290 electrons, so it would have a formal charge a minus one 00:22:19.290 --> 00:22:22.830 in this particular structure that has sulfur in the middle. 00:22:22.830 --> 00:22:24.010 We can look at carbon. 00:22:24.010 --> 00:22:27.540 Carbon has four valence electrons, Four lone pair 00:22:27.540 --> 00:22:31.410 electrons, and half of four bonding electrons, 00:22:31.410 --> 00:22:34.920 so it has a charge of minus two. 00:22:34.920 --> 00:22:37.370 Then we can look at sulfur. 00:22:37.370 --> 00:22:41.340 Sulfur has six valence electrons, zero loan pairs, 00:22:41.340 --> 00:22:44.310 and half of eight bonding electrons, 00:22:44.310 --> 00:22:47.250 so it has a formal charge of plus two. 00:22:47.250 --> 00:22:50.550 So overall, this does equal the minus one. 00:22:50.550 --> 00:22:52.650 So it's a valid structure. 00:22:52.650 --> 00:22:56.370 But is that the lowest energy one? 00:22:56.370 --> 00:23:00.120 We also could put carbon in the middle or nitrogen 00:23:00.120 --> 00:23:01.270 in the middle. 00:23:01.270 --> 00:23:04.770 So let's look at what this does for us. 00:23:04.770 --> 00:23:08.960 So with the formal charge on nitrogen now, 00:23:08.960 --> 00:23:14.190 we have five minus four lone pair electrons, minus half 00:23:14.190 --> 00:23:16.770 of four bonding electrons, minus one. 00:23:16.770 --> 00:23:19.140 So that's the same. 00:23:19.140 --> 00:23:21.180 Now we can look at carbon. 00:23:21.180 --> 00:23:24.500 We have now just no lone pair electrons. 00:23:24.500 --> 00:23:26.803 It has four minus zero minus four, 00:23:26.803 --> 00:23:31.520 half of eight bonding electrons, or zero. 00:23:31.520 --> 00:23:35.550 And for sulfur, six minus four lone pair electrons, 00:23:35.550 --> 00:23:38.245 and half of four bonding electrons, or zero. 00:23:41.640 --> 00:23:47.260 Next, structure C, five minus zero lone pairs minus half 00:23:47.260 --> 00:23:49.690 of eight, plus one. 00:23:49.690 --> 00:23:52.240 So that one's different. 00:23:52.240 --> 00:23:55.630 Carbon, we have four valence electrons 00:23:55.630 --> 00:24:01.550 minus four lone pairs minus half of four, bonding minus two. 00:24:01.550 --> 00:24:06.070 And for the sulfur, six minus four lone pairs 00:24:06.070 --> 00:24:08.880 electrons, half of three or zero. 00:24:08.880 --> 00:24:13.160 So now with the clicker, tell me which is most stable. 00:24:39.500 --> 00:24:40.488 All right, 10 seconds. 00:24:45.190 --> 00:24:46.506 I think this can be 98%. 00:24:49.150 --> 00:24:50.300 That's what I'm thinking. 00:24:50.300 --> 00:24:51.254 I'm feeling good. 00:24:55.180 --> 00:24:57.540 Well, close. 00:24:57.540 --> 00:24:59.700 Yeah. 00:24:59.700 --> 00:25:00.200 What? 00:25:00.200 --> 00:25:01.056 No, no. 00:25:01.056 --> 00:25:01.555 Sorry. 00:25:05.360 --> 00:25:13.210 It should be B. Yeah, it should be B. Yeah. 00:25:13.210 --> 00:25:15.830 Sorry I actually-- Yay. 00:25:15.830 --> 00:25:16.790 There we go. 00:25:16.790 --> 00:25:19.320 Yeah, B is correct. 00:25:19.320 --> 00:25:20.990 So if we just look at it over here, 00:25:20.990 --> 00:25:23.300 it has the lowest number of formal charges, 00:25:23.300 --> 00:25:51.600 so the answer is B. OK. 00:25:51.600 --> 00:25:55.590 Let's start with this simply. 00:25:55.590 --> 00:25:59.310 Who wants to tell me why one, how they could look at this 00:25:59.310 --> 00:26:01.665 and realize one was not the correct answer? 00:26:07.230 --> 00:26:08.060 I think this is on. 00:26:08.060 --> 00:26:10.230 Give it a try. 00:26:10.230 --> 00:26:13.470 AUDIENCE: One's not correct because if you look back 00:26:13.470 --> 00:26:16.230 at your atomic radius chart, this is pretty much doing 00:26:16.230 --> 00:26:18.330 the exact opposite of that 00:26:18.330 --> 00:26:20.940 CATHERINE DRENNAN: Yeah, so helium definitely 00:26:20.940 --> 00:26:24.260 not the biggest atom there is. 00:26:24.260 --> 00:26:31.880 OK so six got a lot of attention, and so did two. 00:26:31.880 --> 00:26:36.840 And ionization energy, electron affinity, and electronegativity 00:26:36.840 --> 00:26:40.110 are definitely connected to each other, 00:26:40.110 --> 00:26:46.680 but there is a clue that electron affinity would not 00:26:46.680 --> 00:26:47.510 be the correct. 00:26:47.510 --> 00:26:49.945 Does someone want to say what you might have noticed? 00:26:54.240 --> 00:26:57.420 AUDIENCE: For electron affinity, it increases and then 00:26:57.420 --> 00:27:00.480 stops at the noble gases because noble gases do not 00:27:00.480 --> 00:27:02.080 want electrons. 00:27:02.080 --> 00:27:05.120 So in this particular chart, all the noble gases 00:27:05.120 --> 00:27:08.040 are like the highest-- are the highest ones 00:27:08.040 --> 00:27:09.570 in the relative area, which would 00:27:09.570 --> 00:27:11.822 mean that electron affinity would be incorrect. 00:27:11.822 --> 00:27:12.780 CATHERINE DRENNAN: Yep. 00:27:12.780 --> 00:27:15.920 That's right so the noble gases were the clue. 00:27:15.920 --> 00:27:18.130 So [INAUDIBLE]. 00:27:18.130 --> 00:27:18.860 Yep. 00:27:18.860 --> 00:27:24.090 And so if electron affinity also is not high at the noble gases, 00:27:24.090 --> 00:27:25.620 they're also not electronegative. 00:27:25.620 --> 00:27:27.384 Noble gases just don't want extra light. 00:27:27.384 --> 00:27:28.800 They don't want to lose electrons, 00:27:28.800 --> 00:27:30.216 they don't want to gain electrons, 00:27:30.216 --> 00:27:33.030 they just want to be left alone. 00:27:33.030 --> 00:27:36.810 So this trend is for ionization energy. 00:27:36.810 --> 00:27:39.920 And because noble gases want to be left alone, 00:27:39.920 --> 00:27:44.980 they don't want to lose any of their electrons. 00:27:44.980 --> 00:27:45.480 Great. 00:27:45.480 --> 00:27:48.120 So this is good to be thinking about, 00:27:48.120 --> 00:27:53.900 because we're finishing up now for the handout from last time. 00:27:53.900 --> 00:27:57.170 And we're going to be talking about electronegativity again. 00:27:57.170 --> 00:27:59.690 We never move very far away from a lot of these topics. 00:27:59.690 --> 00:28:01.300 They just keep coming back. 00:28:01.300 --> 00:28:02.905 So we just keep reviewing them. 00:28:02.905 --> 00:28:04.530 All right, I don't need to microphones, 00:28:04.530 --> 00:28:07.260 although, I don't know, I kind of like having this one. 00:28:07.260 --> 00:28:09.210 Anyway. 00:28:09.210 --> 00:28:12.270 So let's take out the handout from last time, 00:28:12.270 --> 00:28:13.430 and let's finish it up. 00:28:13.430 --> 00:28:16.800 We were talking about formal charge, 00:28:16.800 --> 00:28:20.900 and we had looked at examples where we calculated 00:28:20.900 --> 00:28:23.580 formal charge, and then we looked 00:28:23.580 --> 00:28:25.800 at which structure would be lowest in energy, 00:28:25.800 --> 00:28:28.710 and that was the structure where you had the least 00:28:28.710 --> 00:28:32.360 separation between the charges on them, 00:28:32.360 --> 00:28:35.250 so the smallest absolute numbers. 00:28:35.250 --> 00:28:38.850 If you have formal charges of zero, that's fantastic. 00:28:38.850 --> 00:28:41.250 That's what the molecule wants. 00:28:41.250 --> 00:28:43.830 Minus one, plus one, if you must, 00:28:43.830 --> 00:28:47.180 but when you start having plus two minus two, that's 00:28:47.180 --> 00:28:50.690 a lot of charge separation, so that's less favorable. 00:28:50.690 --> 00:28:52.560 So we're going to have more examples of that 00:28:52.560 --> 00:28:54.160 as we go along. 00:28:54.160 --> 00:29:00.710 But now, what-- if you had calculated formal charge 00:29:00.710 --> 00:29:02.370 and they're all the same, how do you 00:29:02.370 --> 00:29:04.810 know which structure is correct? 00:29:04.810 --> 00:29:08.930 So what if you have-- and this is-- just some people who 00:29:08.930 --> 00:29:09.790 are having trouble. 00:29:09.790 --> 00:29:13.400 This is the top of page four from the handout. 00:29:13.400 --> 00:29:17.190 You have two valid Lewis structures 00:29:17.190 --> 00:29:20.040 that have the same formal charge, 00:29:20.040 --> 00:29:21.650 how do you know where it goes? 00:29:21.650 --> 00:29:25.000 And the answer is that the negative formal charge 00:29:25.000 --> 00:29:28.910 should go on the most electronegative atom. 00:29:28.910 --> 00:29:31.770 And so that's why we are sort of talking about electronegativity 00:29:31.770 --> 00:29:32.610 again. 00:29:32.610 --> 00:29:35.940 And so electronegativity-- remember electronegativity 00:29:35.940 --> 00:29:39.090 is high when the electron affinity is high, 00:29:39.090 --> 00:29:41.900 meaning that the atom wants to get an electron, 00:29:41.900 --> 00:29:43.670 has a high affinity for electrons, 00:29:43.670 --> 00:29:45.780 and also a high ionization energy, which means it 00:29:45.780 --> 00:29:47.610 doesn't want to give up its electrons. 00:29:47.610 --> 00:29:50.340 So that's something-- it likes to have electrons, 00:29:50.340 --> 00:29:52.800 and so you want to put a negative value, which 00:29:52.800 --> 00:29:56.100 indicates there's more electrons on something 00:29:56.100 --> 00:29:57.890 that's electronegative. 00:29:57.890 --> 00:30:00.757 So negative on negative over here. 00:30:00.757 --> 00:30:02.340 And so that's what you're looking for. 00:30:02.340 --> 00:30:05.140 That's how you're going to make a decision. 00:30:05.140 --> 00:30:07.210 So let's look at an example. 00:30:07.210 --> 00:30:09.480 So here is a molecule, and we're going 00:30:09.480 --> 00:30:13.670 to look at two possible Lewis structures of this 00:30:13.670 --> 00:30:16.960 with similar formal charges, and decide which 00:30:16.960 --> 00:30:18.690 has the correct structure. 00:30:18.690 --> 00:30:21.540 So first let me give you a couple hints that can be useful 00:30:21.540 --> 00:30:23.790 in problem sets and in exams. 00:30:23.790 --> 00:30:28.320 When you see CH3, that's a methyl group, 00:30:28.320 --> 00:30:31.320 and that's going to be terminal so you're 00:30:31.320 --> 00:30:34.190 going to have these three hydrogens associated 00:30:34.190 --> 00:30:36.600 with that carbon, and that's going to be 00:30:36.600 --> 00:30:38.860 at an end of the molecule. 00:30:38.860 --> 00:30:41.580 So it could look like one of these two things. 00:30:41.580 --> 00:30:43.920 So we have the carbon, we have it attached 00:30:43.920 --> 00:30:48.060 to three hydrogens, a carbon attached to three hydrogens, 00:30:48.060 --> 00:30:53.350 and then attached to something else, this nitrogen here. 00:30:53.350 --> 00:30:56.080 And this structure where it's just kind of written out 00:30:56.080 --> 00:31:00.760 in a line or a chain of atoms, what we 00:31:00.760 --> 00:31:03.450 call chain molecules sometimes. 00:31:03.450 --> 00:31:05.560 Often the atoms are actually written 00:31:05.560 --> 00:31:08.080 in the order in which they're attached, 00:31:08.080 --> 00:31:09.900 so that's definitely true here. 00:31:09.900 --> 00:31:13.850 CH3, three hydrogens attached to the carbon, 00:31:13.850 --> 00:31:18.100 so they're attached to the atom that came before in the chain. 00:31:18.100 --> 00:31:21.990 The nitrogen is also going to be attached to the carbon. 00:31:21.990 --> 00:31:23.640 Even though it follows the hydrogen, 00:31:23.640 --> 00:31:25.140 you're not going to have hydrogen 00:31:25.140 --> 00:31:26.306 in the middle of a bond. 00:31:26.306 --> 00:31:28.055 It's not going to be bonded to two things. 00:31:28.055 --> 00:31:31.620 Hydrogen is always terminal, so even though nitrogen is here, 00:31:31.620 --> 00:31:33.660 it's got to be attached to the carbon. 00:31:33.660 --> 00:31:35.500 So we have three hydrogens and then 00:31:35.500 --> 00:31:39.490 a bond between the carbon and the nitrogen. 00:31:39.490 --> 00:31:41.860 Now we have a hydrogen after the nitrogen, 00:31:41.860 --> 00:31:44.260 and by this rule it should go on the nitrogen. 00:31:44.260 --> 00:31:46.860 But you might want to double check that that's true. 00:31:46.860 --> 00:31:48.520 And then we have an oxygen. Again, 00:31:48.520 --> 00:31:51.690 the oxygen is going to have a bond with the nitrogen. 00:31:51.690 --> 00:31:54.520 You're not going to have a bond with hydrogen in the middle. 00:31:54.520 --> 00:31:56.350 Hydrogen's always terminal. 00:31:56.350 --> 00:31:58.170 So the only real choice we have here 00:31:58.170 --> 00:32:00.390 is we can put the hydrogen on nitrogen 00:32:00.390 --> 00:32:03.730 or we can put the hydrogen on the oxygen here. 00:32:03.730 --> 00:32:09.120 And so we can use this rule about electronegativity and 00:32:09.120 --> 00:32:11.830 formal charges to figure out which of these structures 00:32:11.830 --> 00:32:13.410 is right. 00:32:13.410 --> 00:32:17.160 So in this particular case, all of the formal charges 00:32:17.160 --> 00:32:21.100 are zero on all of the atoms, except there's 00:32:21.100 --> 00:32:23.290 one minus one charge. 00:32:23.290 --> 00:32:25.360 And in this structure, the minus one charge 00:32:25.360 --> 00:32:28.380 would be on the oxygen, and in this structure 00:32:28.380 --> 00:32:31.330 the minus one charge would be on the nitrogen. 00:32:31.330 --> 00:32:34.030 And if everything else is zero then you 00:32:34.030 --> 00:32:36.630 have the sum of your formal charges, 00:32:36.630 --> 00:32:39.630 minus one, equal to the charge on the molecule, 00:32:39.630 --> 00:32:40.750 which is minus one. 00:32:40.750 --> 00:32:43.300 So both of these are valid structures. 00:32:43.300 --> 00:32:47.400 Both have low values of formal charge, which is right. 00:32:47.400 --> 00:32:49.750 So it's going to be the structure that 00:32:49.750 --> 00:32:54.450 has the negative charge on the most electronegative atom is 00:32:54.450 --> 00:32:55.950 the right structure. 00:32:55.950 --> 00:32:58.780 And so here you need to remember some of your rules 00:32:58.780 --> 00:33:00.820 about electronegativity. 00:33:00.820 --> 00:33:03.630 And in terms of electronegativity, 00:33:03.630 --> 00:33:07.750 we see that oxygen has greater electronegativity 00:33:07.750 --> 00:33:11.460 than the nitrogen. And so that's where 00:33:11.460 --> 00:33:14.190 we would want to put our negative charge-- 00:33:14.190 --> 00:33:16.330 on the negative charge goes on the atom that's 00:33:16.330 --> 00:33:17.860 the most electronegative. 00:33:17.860 --> 00:33:21.740 And that would generate the lower energy structure. 00:33:21.740 --> 00:33:25.380 So if you're given a table of electronegativities, which 00:33:25.380 --> 00:33:27.250 you often are, you can look it up 00:33:27.250 --> 00:33:30.100 and validate that that's going to be the correct place. 00:33:30.100 --> 00:33:32.700 And in fact, experimentally we know that that's 00:33:32.700 --> 00:33:34.920 the right structure. 00:33:34.920 --> 00:33:36.080 So that works. 00:33:36.080 --> 00:33:40.920 So if you have two structures, identical formal charges, 00:33:40.920 --> 00:33:43.510 valid structures, then the last step 00:33:43.510 --> 00:33:46.210 is to think about where that negative charge should 00:33:46.210 --> 00:33:49.216 go, and pick the atom that is the most electronegative. 00:33:52.170 --> 00:33:54.720 All right we have one more thing we 00:33:54.720 --> 00:33:57.070 need to talk about in Lewis structures 00:33:57.070 --> 00:33:59.740 before we start violating various rules 00:33:59.740 --> 00:34:02.560 that we've learned, and that is that we 00:34:02.560 --> 00:34:05.050 need to talk about resonance structures. 00:34:05.050 --> 00:34:08.190 And so to explain to you what a resonance structure is, 00:34:08.190 --> 00:34:10.679 it's really helpful to start with an example, 00:34:10.679 --> 00:34:12.250 so that's what we're going to do. 00:34:12.250 --> 00:34:14.400 And we're going to consider ozone, 00:34:14.400 --> 00:34:20.190 which is three atoms of oxygen. And we have the ozone layer, 00:34:20.190 --> 00:34:24.659 which protects us from UV damage and is very valuable, 00:34:24.659 --> 00:34:27.010 and we should not destroy it with chemicals being 00:34:27.010 --> 00:34:28.889 released into the environment. 00:34:28.889 --> 00:34:32.429 And because you don't have complete say over that, 00:34:32.429 --> 00:34:33.870 always wear sunscreen. 00:34:33.870 --> 00:34:35.130 OK. 00:34:35.130 --> 00:34:38.050 So let's build up these Lewis structures, 00:34:38.050 --> 00:34:42.199 and then consider what's meant by a resonance structure. 00:34:42.199 --> 00:34:44.350 So here we have part of the Periodic Table 00:34:44.350 --> 00:34:47.110 that you're going to need, and we 00:34:47.110 --> 00:34:49.920 need to think first about the valence electrons. 00:34:49.920 --> 00:34:52.929 So oxygen has six valence electrons 00:34:52.929 --> 00:34:57.030 and there are three oxygens, so that's 18. 00:34:57.030 --> 00:35:01.300 To get a full octet for each of the oxygens, three oxygens, 00:35:01.300 --> 00:35:06.250 an octet is eight valence electrons, so that would be 24. 00:35:06.250 --> 00:35:08.350 To figure out the number of bonding electrons, 00:35:08.350 --> 00:35:11.940 we're going to be subtracting our octet electrons 00:35:11.940 --> 00:35:13.860 from our valence electrons. 00:35:13.860 --> 00:35:18.600 So 24 minus 18 is six. 00:35:18.600 --> 00:35:21.790 And then our next step is to assign those bonding 00:35:21.790 --> 00:35:25.290 electrons two at a time, two per bond. 00:35:25.290 --> 00:35:28.080 So let's take a look at that. 00:35:28.080 --> 00:35:32.250 We can put one bond here between these two oxygens, 00:35:32.250 --> 00:35:34.750 one bond here between these two oxygens, 00:35:34.750 --> 00:35:37.440 and then ask do we have any more? 00:35:37.440 --> 00:35:38.500 And we do. 00:35:38.500 --> 00:35:41.770 Because we have six bonding electrons, we used four, 00:35:41.770 --> 00:35:44.230 so we have two more bonding electrons. 00:35:44.230 --> 00:35:46.350 So we need to make a double bond. 00:35:46.350 --> 00:35:51.100 But now we have the question of where to make that double bond. 00:35:51.100 --> 00:35:53.920 Am I going to put it between these two oxygens 00:35:53.920 --> 00:35:56.770 or am I going to put it between those two oxygens? 00:35:56.770 --> 00:36:02.150 And so I could say put it there, but I could also, 00:36:02.150 --> 00:36:05.670 in structure two, put the double bond over here. 00:36:05.670 --> 00:36:06.170 All right. 00:36:06.170 --> 00:36:08.480 We'll come back to that question in a minute. 00:36:08.480 --> 00:36:11.570 Let's first figure out if we have any remaining valence 00:36:11.570 --> 00:36:14.300 electrons, and we do. 00:36:14.300 --> 00:36:20.060 So we had 18 and we've only used six, so we have 12 left. 00:36:20.060 --> 00:36:23.510 So we're going to put those in as lone pairs. 00:36:23.510 --> 00:36:25.640 And so I can put them in over here. 00:36:25.640 --> 00:36:30.200 One set here, one, two, three, four, five, six. 00:36:30.200 --> 00:36:32.210 And I can also do that over here. 00:36:32.210 --> 00:36:36.140 One, two, three, four, five, six. 00:36:36.140 --> 00:36:38.570 And now we have two structures, so we 00:36:38.570 --> 00:36:40.340 need to think about formal charges 00:36:40.340 --> 00:36:43.550 to see if that can help differentiate structure 00:36:43.550 --> 00:36:47.390 one from structure two. 00:36:47.390 --> 00:36:48.590 OK. 00:36:48.590 --> 00:36:50.360 And let's look at that. 00:36:50.360 --> 00:36:52.670 Be sure everyone's ready. 00:36:52.670 --> 00:36:57.440 And that is a clicker question. 00:36:57.440 --> 00:36:59.050 So I'll put that back up. 00:37:49.141 --> 00:37:49.640 All right. 00:37:49.640 --> 00:37:52.340 Let's just take 10 more seconds, and we'll 00:37:52.340 --> 00:37:56.120 talk about what the right answer is, and a little bit of a trick 00:37:56.120 --> 00:37:58.055 for doing these, perhaps, a bit faster. 00:38:10.490 --> 00:38:11.600 OK. 00:38:11.600 --> 00:38:12.600 So that was pretty good. 00:38:12.600 --> 00:38:15.660 So let's look at why that's the right answer. 00:38:15.660 --> 00:38:18.870 And we'll take a look at that over here. 00:38:18.870 --> 00:38:22.470 So let's do the calculations. 00:38:22.470 --> 00:38:25.120 So you have to remember the equation for formal charge 00:38:25.120 --> 00:38:27.070 for sure and once you do enough problems 00:38:27.070 --> 00:38:30.160 it should stick in your head pretty easily. 00:38:30.160 --> 00:38:32.400 So if we look over here-- so this 00:38:32.400 --> 00:38:36.060 is the formal charge on oxygen A. 00:38:36.060 --> 00:38:37.830 There were six valence electrons, 00:38:37.830 --> 00:38:40.900 there are four lone pair electrons, minus half 00:38:40.900 --> 00:38:42.640 of the bonding electrons. 00:38:42.640 --> 00:38:45.300 There are eight bonding electrons, so that's two. 00:38:45.300 --> 00:38:48.870 So that's a formal charge of zero. 00:38:48.870 --> 00:38:54.510 For oxygen B over here, we again have six valence electrons, 00:38:54.510 --> 00:38:57.690 and we have two lone pair electrons. 00:38:57.690 --> 00:39:01.260 We have six bonding electrons, so half of six 00:39:01.260 --> 00:39:04.620 is three, which is plus one. 00:39:04.620 --> 00:39:10.290 And on oxygen C over here, six minus six 00:39:10.290 --> 00:39:14.600 lone parallel electrons, half of one bond, so half of two 00:39:14.600 --> 00:39:19.110 is one, minus one, overall the charge here is neutral. 00:39:19.110 --> 00:39:22.080 And that's good because it's a neutral molecule. 00:39:22.080 --> 00:39:24.930 So to do this structure faster, you 00:39:24.930 --> 00:39:28.500 have to realize that oxygen A over here 00:39:28.500 --> 00:39:31.330 is the same as C over there. 00:39:31.330 --> 00:39:34.050 So you can just copy down what you 00:39:34.050 --> 00:39:40.890 had for C is now A. B is exactly the same in the two structures, 00:39:40.890 --> 00:39:43.630 so it's the same as what you calculated. 00:39:43.630 --> 00:39:46.710 And now this C was the same as this A, 00:39:46.710 --> 00:39:49.260 so we can put that same value that we 00:39:49.260 --> 00:39:56.496 calculated for A into C. So they're both the same. 00:39:56.496 --> 00:40:01.980 Same formal charges, which structure is correct? 00:40:01.980 --> 00:40:04.950 And the answer is both of them. 00:40:04.950 --> 00:40:07.200 And in fact, you need both of them, 00:40:07.200 --> 00:40:11.640 so there's data-- chemists love data-- 00:40:11.640 --> 00:40:13.830 and the data is that the bonds are actually 00:40:13.830 --> 00:40:15.550 equivalent in the molecule. 00:40:15.550 --> 00:40:17.940 So it isn't that there's one double bond and one 00:40:17.940 --> 00:40:24.030 single bond, there's just one kind of bond in ozone, 00:40:24.030 --> 00:40:28.160 and it's between a single and a double bond. 00:40:28.160 --> 00:40:32.850 And so this is how one would draw that kind of thing. 00:40:32.850 --> 00:40:36.420 You would have structure one here and structure two here. 00:40:36.420 --> 00:40:38.610 You would put them both in brackets, 00:40:38.610 --> 00:40:40.620 and you would put this special double headed 00:40:40.620 --> 00:40:43.680 arrow between them and that indicates 00:40:43.680 --> 00:40:47.100 that both of these structures are needed to describe 00:40:47.100 --> 00:40:49.290 the properties of the molecule. 00:40:49.290 --> 00:40:54.880 You do not have a stationary double bond and a single bond. 00:40:54.880 --> 00:40:58.170 You have kind of a mixture between these two, 00:40:58.170 --> 00:41:01.290 and that's what a resonance structure is. 00:41:01.290 --> 00:41:04.560 So let's take a little bit more of a look at this. 00:41:04.560 --> 00:41:07.980 So this is just what I had before, experimental evidence 00:41:07.980 --> 00:41:09.870 is that the bonds are equivalent. 00:41:09.870 --> 00:41:11.930 There isn't a double and a single, 00:41:11.930 --> 00:41:15.750 there's something in between, a one and a half. 00:41:15.750 --> 00:41:18.360 And this is called a resonance hybrid, 00:41:18.360 --> 00:41:20.580 and it's of length of these two structures. 00:41:20.580 --> 00:41:24.600 So this structure is blended with that structure. 00:41:24.600 --> 00:41:27.870 So some of you are aware of hybrids from biology, 00:41:27.870 --> 00:41:29.810 and now with cars. 00:41:29.810 --> 00:41:32.190 So a mule would be an example. 00:41:32.190 --> 00:41:34.230 And if you're thinking about what a mule is, 00:41:34.230 --> 00:41:36.990 you don't walk out into your barnyard one day 00:41:36.990 --> 00:41:39.520 and see a donkey one day and you go out the next day 00:41:39.520 --> 00:41:40.950 and you see a horse. 00:41:40.950 --> 00:41:44.490 A mule is a hybrid between a donkey and a horse. 00:41:44.490 --> 00:41:47.090 So if you were a chemist, you would do this. 00:41:47.090 --> 00:41:48.900 You would put the donkey in brackets, 00:41:48.900 --> 00:41:52.150 and the horse in brackets, and put your double headed arrow. 00:41:52.150 --> 00:41:55.620 And if you see this, you'd say, oh yes, a mule. 00:41:55.620 --> 00:41:58.230 So that is what this is. 00:41:58.230 --> 00:42:01.830 Both structures are needed to describe ozone. 00:42:01.830 --> 00:42:04.030 One structure isn't enough. 00:42:04.030 --> 00:42:05.100 You need both of them. 00:42:05.100 --> 00:42:07.860 They're in resonance with each other. 00:42:07.860 --> 00:42:12.210 And so what's true about the electrons in ozone 00:42:12.210 --> 00:42:18.210 is that they're delocalized across all of these bonds, 00:42:18.210 --> 00:42:21.030 so there isn't like a double single. 00:42:21.030 --> 00:42:23.760 All of those electrons are delocalized. 00:42:23.760 --> 00:42:27.387 They're shared over this set of atoms. 00:42:27.387 --> 00:42:29.970 And you can have two resonance structures, you can have three, 00:42:29.970 --> 00:42:30.810 you can have four. 00:42:30.810 --> 00:42:33.180 It depends on the molecule in question. 00:42:33.180 --> 00:42:38.040 And in all those cases, those electrons would be delocalized. 00:42:38.040 --> 00:42:42.150 So just to sum that up, resonance structures, 00:42:42.150 --> 00:42:47.050 two or more, same arrangement of atoms-- and that's important. 00:42:47.050 --> 00:42:48.947 It's the electrons that are different. 00:42:48.947 --> 00:42:51.030 And this isn't in your handout, out but just think 00:42:51.030 --> 00:42:53.310 about this for a second-- because it was in your hand 00:42:53.310 --> 00:42:54.510 out a minute ago. 00:42:54.510 --> 00:42:57.180 Are these resonance structures? 00:42:57.180 --> 00:42:58.060 No. 00:42:58.060 --> 00:43:00.540 They're not resonance structures. 00:43:00.540 --> 00:43:02.760 The atoms are in different positions. 00:43:02.760 --> 00:43:05.490 So one of these structures is right one of these structures 00:43:05.490 --> 00:43:06.450 is wrong. 00:43:06.450 --> 00:43:09.420 With resonance structures, they're both correct 00:43:09.420 --> 00:43:12.600 and both needed to define the structure. 00:43:12.600 --> 00:43:13.590 So pay attention. 00:43:13.590 --> 00:43:15.060 This is a common mistake. 00:43:15.060 --> 00:43:15.740 Pay attention. 00:43:15.740 --> 00:43:17.950 Ask yourself, are the atoms in a different position? 00:43:17.950 --> 00:43:19.200 That's not resonance. 00:43:19.200 --> 00:43:22.020 You're just looking-- atoms are the same, formal charge 00:43:22.020 --> 00:43:23.670 are the same, you're just looking 00:43:23.670 --> 00:43:27.890 at whether you have different arrangements of electrons.