WEBVTT 00:00:00.000 --> 00:00:00.016 The following content is provided under a Creative 00:00:00.016 --> 00:00:00.022 Commons license. 00:00:00.022 --> 00:00:00.038 Your support will help MIT OpenCourseWare continue to 00:00:00.038 --> 00:00:00.054 offer high quality educational resources for free. 00:00:00.054 --> 00:00:00.072 To make a donation or view additional materials from 00:00:00.072 --> 00:00:00.088 hundreds of MIT courses, visit MIT OpenCourseWare at 00:00:00.088 --> 00:00:00.110 ocw.mit.edu. 00:00:00.110 --> 00:00:23.660 PROFESSOR: OK, let's get started here. 00:00:23.660 --> 00:00:26.350 Go ahead and take 10 more seconds on the clicker 00:00:26.350 --> 00:00:29.100 question, which probably looks all too familiar at this 00:00:29.100 --> 00:00:35.770 point, if you went to recitation yesterday. 00:00:35.770 --> 00:00:38.370 All right, and let's see how we do here. 00:00:38.370 --> 00:00:38.900 OK. 00:00:38.900 --> 00:00:41.540 So, let's talk about this for one second. 00:00:41.540 --> 00:00:44.830 So what we're asking here, if we can settle down and listen 00:00:44.830 --> 00:00:47.440 up, is which equations can be used if we're talking about 00:00:47.440 --> 00:00:50.590 converting wavelength to energy for an electron. 00:00:50.590 --> 00:00:53.260 Remember, the key word here is electron. 00:00:53.260 --> 00:00:56.060 This might look familiar to the first part of problem one 00:00:56.060 --> 00:00:59.230 on the exam, and problem one on the exam is what tended to 00:00:59.230 --> 00:01:02.290 be the huge problem on the exam. 00:01:02.290 --> 00:01:06.310 I think over 2/3 of you decided on the exam to use 00:01:06.310 --> 00:01:09.530 this first equation, e equals h c over wavelength. 00:01:09.530 --> 00:01:13.250 So I just want to reiterate one more time, why can we not 00:01:13.250 --> 00:01:17.720 use this equation if we're talking about an electron? 00:01:17.720 --> 00:01:19.500 C. OK, good, good, I'm hearing it. 00:01:19.500 --> 00:01:20.990 So the answer is c. 00:01:20.990 --> 00:01:23.430 What you need to do is you need to ask yourself if you're 00:01:23.430 --> 00:01:25.810 trying to convert from wavelength to energy for an 00:01:25.810 --> 00:01:29.820 electron, and you are tempted, because we are all tempted to 00:01:29.820 --> 00:01:33.800 use this equation, and if you were tempted, say, does an 00:01:33.800 --> 00:01:35.280 electron travel at the speed of light? 00:01:35.280 --> 00:01:38.150 And if the answer is no, an electron does not travel at 00:01:38.150 --> 00:01:40.570 the speed of light, light travels at the speed of light, 00:01:40.570 --> 00:01:43.290 then you want to stay away from using this equation. 00:01:43.290 --> 00:01:46.440 And I know how tempting it is to do that, but we have other 00:01:46.440 --> 00:01:49.320 equations we can use -- the DeBroglie wavelength, and this 00:01:49.320 --> 00:01:52.560 is just a combination of energy equals 1/2 m v squared, 00:01:52.560 --> 00:01:54.740 and the definition of momentum, so we can combine 00:01:54.740 --> 00:01:56.540 those things to get it. 00:01:56.540 --> 00:01:59.110 You might be wondering why I'm telling you this now, you've 00:01:59.110 --> 00:02:01.500 already -- if you've lost points on that, lost the 00:02:01.500 --> 00:02:04.200 points on it, and what I'm saying to you is if there are 00:02:04.200 --> 00:02:07.790 parts of exam 1 that you did not do well on, you will have 00:02:07.790 --> 00:02:10.900 a chance to show us again that you now understand that 00:02:10.900 --> 00:02:12.360 material on the final. 00:02:12.360 --> 00:02:15.940 One quarter of the final is going to be exam 1 material, 00:02:15.940 --> 00:02:18.290 and what that means is when we look at your grade at the end 00:02:18.290 --> 00:02:21.090 of the semester, and we take a look at what you got on exam 00:02:21.090 --> 00:02:24.810 1, and you're right at that borderline, and we say well, 00:02:24.810 --> 00:02:27.630 what happened, did they understand more at the end of 00:02:27.630 --> 00:02:30.090 the semester, did the concepts kind of 00:02:30.090 --> 00:02:31.710 solidify over the semester? 00:02:31.710 --> 00:02:34.210 And if they did and if you showed us that they did, then 00:02:34.210 --> 00:02:36.840 you're going to get bumped up into that next grade category. 00:02:36.840 --> 00:02:39.730 So keep that in mind as you're reviewing the exam, sometimes 00:02:39.730 --> 00:02:42.150 if things don't go as well as you want them to, the 00:02:42.150 --> 00:02:45.180 temptation is just to put that exam away forever and ever. 00:02:45.180 --> 00:02:47.720 But the reality is that new material builds on that 00:02:47.720 --> 00:02:54.610 material, and specifically exam 1 a, question 1 a that 00:02:54.610 --> 00:02:56.740 deals with converting wavelength to 00:02:56.740 --> 00:02:58.410 energy for an electron. 00:02:58.410 --> 00:03:01.610 I really want you guys know this and to understand it, so 00:03:01.610 --> 00:03:04.800 I can guarantee you that you will see this on the final. 00:03:04.800 --> 00:03:08.640 Specifically, question 1, part a. 00:03:08.640 --> 00:03:10.260 You will see something very, very similar 00:03:10.260 --> 00:03:11.750 to this on the final. 00:03:11.750 --> 00:03:14.930 If you are thinking about 1 thing to go back and study on 00:03:14.930 --> 00:03:20.420 exam 1, 1 a is a really good choice for that. 00:03:20.420 --> 00:03:22.820 This is important to me, so you're going to 00:03:22.820 --> 00:03:24.110 see it on the final. 00:03:24.110 --> 00:03:27.960 So if you have friends that aren't here, you might want to 00:03:27.960 --> 00:03:30.270 mention it to them, or maybe not, maybe this is your reward 00:03:30.270 --> 00:03:33.480 for coming to class, which is fine with me as well. 00:03:33.480 --> 00:03:33.840 All right. 00:03:33.840 --> 00:03:36.070 So I want to talk a little bit about exam 1. 00:03:36.070 --> 00:03:39.320 I know most you picked up your examine in recitation. 00:03:39.320 --> 00:03:43.020 If you didn't, any extra exams can be picked up in the 00:03:43.020 --> 00:03:47.410 Chemistry Education office, that's room 2204. 00:03:47.410 --> 00:03:51.780 So, the class average for the exam was a 68%, which is 00:03:51.780 --> 00:03:54.890 actually a strong, solid average for an exam 1 grade in 00:03:54.890 --> 00:03:57.610 the fall semester of 511-1. 00:03:57.610 --> 00:04:01.280 What we typically see is something right in this range, 00:04:01.280 --> 00:04:05.060 either ranging from the 50's for an exam 1 average to 00:04:05.060 --> 00:04:08.110 occasionally getting into the 70's, but most commonly what 00:04:08.110 --> 00:04:13.220 we've seen for exam 1 averages is 60, 61 -- those low 60's. 00:04:13.220 --> 00:04:18.120 So in many ways, seeing this 68 here, this is a great sign 00:04:18.120 --> 00:04:21.740 that we are off to a good start for this semester. 00:04:21.740 --> 00:04:24.060 And I do want to address, because I know many of you, 00:04:24.060 --> 00:04:27.130 this is only your second exam at MIT, and perhaps you've 00:04:27.130 --> 00:04:30.750 never gotten an exam back that didn't start with a 90 or 00:04:30.750 --> 00:04:33.570 start with an 80 in terms of the grades. 00:04:33.570 --> 00:04:36.910 So one thing you need to keep in mind is don't just look at 00:04:36.910 --> 00:04:37.870 the number grade. 00:04:37.870 --> 00:04:40.730 The reason that we give you these letters grade categories 00:04:40.730 --> 00:04:44.190 is that you can understand what it actually means, what 00:04:44.190 --> 00:04:47.150 your exam score actually says in terms of how we perceive 00:04:47.150 --> 00:04:49.360 you as understanding the material. 00:04:49.360 --> 00:04:53.030 So, for example, and this is the same categories that were 00:04:53.030 --> 00:04:56.900 shared in recitation, so I apologize for repeating, but I 00:04:56.900 --> 00:04:59.770 know sometimes when you get an exam back, no more information 00:04:59.770 --> 00:05:02.180 comes into your head except obsessing over the exam, so 00:05:02.180 --> 00:05:05.130 I'm just going to say it one more time, and that is between 00:05:05.130 --> 00:05:08.540 88 and 100, so that's 20% of you got an A. This is just 00:05:08.540 --> 00:05:11.810 absolutely fantastic, you really nailed this very hard 00:05:11.810 --> 00:05:14.140 material and these hard questions on the exam where 00:05:14.140 --> 00:05:17.850 you had to both use equations and solve problems, but also 00:05:17.850 --> 00:05:20.280 understand the concept in order to get yourself started 00:05:20.280 --> 00:05:21.910 on solving the problem. 00:05:21.910 --> 00:05:26.690 The same with the B, the B range was between 69 and 87 -- 00:05:26.690 --> 00:05:30.280 anywhere in between those ranges, you've got a B, some 00:05:30.280 --> 00:05:32.100 sort of B on the exam. 00:05:32.100 --> 00:05:35.360 So again, if you're in the A or the B category here, this 00:05:35.360 --> 00:05:38.330 is really something to be proud of, you really earned 00:05:38.330 --> 00:05:39.130 these grades. 00:05:39.130 --> 00:05:43.650 You know these exams, our 511-1 exams, we're not giving 00:05:43.650 --> 00:05:45.900 you points here, there are no give me, easy points, you 00:05:45.900 --> 00:05:47.870 earned every single one of these points. 00:05:47.870 --> 00:05:51.100 So, A and B here, these are refrigerator-worthy grades, 00:05:51.100 --> 00:05:52.690 hang those up in your dorm. 00:05:52.690 --> 00:05:54.560 This is something to feel good about. 00:05:54.560 --> 00:05:55.090 All right. 00:05:55.090 --> 00:05:59.890 So, for those of you that got between a 51 and a 68, this is 00:05:59.890 --> 00:06:01.690 somewhere in the C range. 00:06:01.690 --> 00:06:04.240 For some people, they feel comfortable being in the C 00:06:04.240 --> 00:06:08.280 range, other people really do not like being in this range. 00:06:08.280 --> 00:06:10.950 We understand that, there is plenty of room up there with 00:06:10.950 --> 00:06:11.940 the A's and the B's. 00:06:11.940 --> 00:06:15.930 You are welcome to come up to these higher ranges starting 00:06:15.930 --> 00:06:17.360 with the next exam. 00:06:17.360 --> 00:06:20.010 And what I want to tell you if you are in the C range, and 00:06:20.010 --> 00:06:22.430 this is not a place that you want to be in, anyone that's 00:06:22.430 --> 00:06:26.910 got below the class average, so below a 68 -- or a 68 or 00:06:26.910 --> 00:06:29.850 below, is eligible for free tutoring, and I put the 00:06:29.850 --> 00:06:32.250 website on the front page of your notes. 00:06:32.250 --> 00:06:34.700 This means you get a one-on-one tutor paid for by 00:06:34.700 --> 00:06:36.950 the Chemistry Department to help you if it's concepts 00:06:36.950 --> 00:06:40.315 you're not quite up on, if it's exam strategy that you 00:06:40.315 --> 00:06:42.020 need to work on more. 00:06:42.020 --> 00:06:44.520 Whatever it is that you need to work on, we want to help 00:06:44.520 --> 00:06:45.350 you get there. 00:06:45.350 --> 00:06:47.680 So, if you have a grade that you're not happy with, that 00:06:47.680 --> 00:06:52.040 you're feeling upset or discouraged about, please, I'm 00:06:52.040 --> 00:06:54.050 happy to talk to all of you about your grades 00:06:54.050 --> 00:06:54.940 individually. 00:06:54.940 --> 00:06:58.070 You can come talk to me, bring your exam, and we'll go over 00:06:58.070 --> 00:06:59.810 what the strategy should be in terms of you 00:06:59.810 --> 00:07:01.070 succeeding on the next exam. 00:07:01.070 --> 00:07:03.300 You can do the same thing with all of your TAs are more than 00:07:03.300 --> 00:07:05.370 happy to meet with each and every one of you. 00:07:05.370 --> 00:07:07.640 And then in addition to that, we can set you up with a tutor 00:07:07.640 --> 00:07:11.010 if you are in the C range or below, in terms 00:07:11.010 --> 00:07:13.220 of this first exam. 00:07:13.220 --> 00:07:13.570 All right. 00:07:13.570 --> 00:07:16.720 So 44 to 50, this is going to be in the D range. 00:07:16.720 --> 00:07:19.390 And then anything below a 44 is going to be 00:07:19.390 --> 00:07:20.880 failing on this exam. 00:07:20.880 --> 00:07:22.860 And also keep in mind, for those of you that are 00:07:22.860 --> 00:07:25.470 freshman, you need at least a C to pass the class. 00:07:25.470 --> 00:07:29.270 So, if you did get a D or an F on the first exam, you are 00:07:29.270 --> 00:07:32.980 going to need to really evaluate why that happened and 00:07:32.980 --> 00:07:35.100 make some changes, and we're absolutely here 00:07:35.100 --> 00:07:36.530 to help you do that. 00:07:36.530 --> 00:07:40.030 So the real key is identifying where the problem is -- is it 00:07:40.030 --> 00:07:42.240 with understanding the concepts, are you in a study 00:07:42.240 --> 00:07:44.190 group that's dragging you along but you're not 00:07:44.190 --> 00:07:45.340 understanding? 00:07:45.340 --> 00:07:47.300 Do you kind of panic when you get in the exam? 00:07:47.300 --> 00:07:49.270 There are all sorts of scenarios we can talk about 00:07:49.270 --> 00:07:51.270 and we want to talk about them with you. 00:07:51.270 --> 00:07:54.610 Seriously, even if we had a huge range in this exam from 00:07:54.610 --> 00:07:58.420 17 to 100, if you're sitting there and you're the 17, and 00:07:58.420 --> 00:08:01.505 actually there's more than 1 so don't feel alone, if you're 00:08:01.505 --> 00:08:05.210 a 17 or you're a 20, it's not time to give up, it's not time 00:08:05.210 --> 00:08:06.740 to drop the class and say I'm no good at 00:08:06.740 --> 00:08:08.070 chemistry, I can't do this. 00:08:08.070 --> 00:08:11.840 You still can, this is your first couple of exams, 00:08:11.840 --> 00:08:14.400 certainly your first in this class, potentially one of your 00:08:14.400 --> 00:08:17.690 first at MIT, so there's tons of room to improve 00:08:17.690 --> 00:08:18.450 from here on out. 00:08:18.450 --> 00:08:21.340 This is only 100 points out of 750. 00:08:21.340 --> 00:08:23.610 So, the same thing goes if you did really well, you still 00:08:23.610 --> 00:08:26.180 have 650 other points that you need to deal with. 00:08:26.180 --> 00:08:28.930 So, make sure you don't just rest on your high score from 00:08:28.930 --> 00:08:31.270 this first exam. 00:08:31.270 --> 00:08:34.150 So, OK, so that's pretty much what I wanted to say about the 00:08:34.150 --> 00:08:38.280 exam, and in terms of there's tons of resources if things 00:08:38.280 --> 00:08:39.610 didn't work out quite as you wanted. 00:08:39.610 --> 00:08:42.320 If you feel upset in any way, please come and talk to me. 00:08:42.320 --> 00:08:44.370 We want you to love chemistry and feel good about your 00:08:44.370 --> 00:08:45.480 ability to do it. 00:08:45.480 --> 00:08:48.380 Nobody get into MIT by mistake, so you all deserve to 00:08:48.380 --> 00:08:50.650 be sitting here, and you all can pass this class and do 00:08:50.650 --> 00:08:53.650 well in it, so we can help you get there no matter what. 00:08:53.650 --> 00:08:56.080 You all absolutely can do this. 00:08:56.080 --> 00:08:58.590 And then one more time, to reiterate, in case anyone 00:08:58.590 --> 00:09:01.990 missed it, 1 a, make sure you understand that, I feel like 00:09:01.990 --> 00:09:02.700 that's important. 00:09:02.700 --> 00:09:03.920 And actually all of 1 -- 00:09:03.920 --> 00:09:06.200 I really feel like the photoelectric effect is 00:09:06.200 --> 00:09:08.710 important for understanding all of these energy concepts. 00:09:08.710 --> 00:09:12.380 So, as you go on in this class, make sure you don't go 00:09:12.380 --> 00:09:14.030 on before you go back and make sure you 00:09:14.030 --> 00:09:16.380 understand that problem. 00:09:16.380 --> 00:09:22.260 All right, so let's move on to material for exam 2 now, and 00:09:22.260 --> 00:09:25.550 we're already three lectures into exam 2 material. 00:09:25.550 --> 00:09:29.370 And I do want to say that in terms of 511-1, what tends to 00:09:29.370 --> 00:09:33.140 happen is the exam scores go up and up and up, in terms of 00:09:33.140 --> 00:09:36.250 as we go from exam 1, to exam 2, to exam 3. 00:09:36.250 --> 00:09:38.470 One of these reasons is we are building on material, the 00:09:38.470 --> 00:09:40.510 other reason is you'll be shocked at how much better you 00:09:40.510 --> 00:09:43.830 are at taking an exam just a few weeks from now. 00:09:43.830 --> 00:09:47.660 So this will be on, starting with the Lewis structures, so 00:09:47.660 --> 00:09:50.300 go back in your notes -- if this doesn't sound familiar, 00:09:50.300 --> 00:09:54.030 if you spent too much time -- or not too much time, spent a 00:09:54.030 --> 00:09:56.520 lot of time studying exam 1 and didn't move on here. 00:09:56.520 --> 00:09:58.400 Today we're going to talk about the breakdown of the 00:09:58.400 --> 00:09:59.550 octet rule. 00:09:59.550 --> 00:10:02.310 Cases where we don't have eight electrons around our 00:10:02.310 --> 00:10:04.600 Lewis structures, then we'll move on to 00:10:04.600 --> 00:10:06.260 talking about ionic bonds. 00:10:06.260 --> 00:10:08.590 We had already talked about covalent bonds, and then we 00:10:08.590 --> 00:10:11.060 talked about Lewis structures, which describe the electron 00:10:11.060 --> 00:10:13.060 configuration in covalent bonds. 00:10:13.060 --> 00:10:16.160 So now let's think about the other extreme of ionic bonds, 00:10:16.160 --> 00:10:19.390 and then we'll talk about polar covalent bonds to end, 00:10:19.390 --> 00:10:23.670 if we get there or will start with that in class on Monday. 00:10:23.670 --> 00:10:26.330 Also, public service announcement for all of you, 00:10:26.330 --> 00:10:28.620 voter registration in Massachusetts, which is where 00:10:28.620 --> 00:10:32.620 we are, is on Monday, the deadline if you want to 00:10:32.620 --> 00:10:33.800 register to vote. 00:10:33.800 --> 00:10:36.750 There's some websites up there that can guide you through 00:10:36.750 --> 00:10:38.850 registering and also can guide you through, if you need an 00:10:38.850 --> 00:10:41.480 absentee ballot for your home state. 00:10:41.480 --> 00:10:44.500 And I actually saw, and I saw a 5.111 student manning, 00:10:44.500 --> 00:10:47.230 there's some booths around MIT that will register you or get 00:10:47.230 --> 00:10:48.790 you an absentee ballot. 00:10:48.790 --> 00:10:52.120 So, the deadline's coming soon, so patriotic duty, I 00:10:52.120 --> 00:10:54.580 need to remind you of that as your chemistry teacher -- 00:10:54.580 --> 00:10:56.940 chemistry issues are important in politics as well. 00:10:56.940 --> 00:11:00.540 So make sure you get registered to vote. 00:11:00.540 --> 00:11:02.330 I just remembered one more announcement, too, that I did 00:11:02.330 --> 00:11:05.770 want to mention, some of you may have friends in 511-2 and 00:11:05.770 --> 00:11:08.610 have heard their class average for exam 1. 00:11:08.610 --> 00:11:11.100 And I want to tell you, this happens every year, their 00:11:11.100 --> 00:11:14.040 average was 15 points higher than our average. 00:11:14.040 --> 00:11:16.090 Last year, their average was 15 points 00:11:16.090 --> 00:11:17.140 higher than our average. 00:11:17.140 --> 00:11:18.690 This is for exam 1. 00:11:18.690 --> 00:11:23.660 This is what tends to happen to 511-2 grades as 00:11:23.660 --> 00:11:24.430 the exam goes on. 00:11:24.430 --> 00:11:25.800 This is what happens to 511-1. 00:11:25.800 --> 00:11:27.600 You guys are in a good spot. 00:11:27.600 --> 00:11:30.160 Also, I want to point out that what's not important is just 00:11:30.160 --> 00:11:32.610 that number grade, but also the letter that goes with it. 00:11:32.610 --> 00:11:36.990 So, for example, if you got a 69 in this class on this exam, 00:11:36.990 --> 00:11:38.260 that's a B minus. 00:11:38.260 --> 00:11:41.870 If you got a 69 on your exam in 511-2, that's a D, you 00:11:41.870 --> 00:11:42.920 didn't pass the exam. 00:11:42.920 --> 00:11:45.970 So keep that in mind when your friend might have gotten a 00:11:45.970 --> 00:11:49.670 higher number grade than you and you know you understand 00:11:49.670 --> 00:11:51.490 the similar material just as well. 00:11:51.490 --> 00:11:54.600 Similarly, an 80 in this class on the exam was a B plus, a 00:11:54.600 --> 00:11:58.790 very high B. An 80 in that class is going to be a C. So, 00:11:58.790 --> 00:12:01.890 just don't worry so much about exactly where that average 00:12:01.890 --> 00:12:03.330 lies, you really want to think about what the 00:12:03.330 --> 00:12:04.510 letter grade means. 00:12:04.510 --> 00:12:05.290 OK, I've said enough. 00:12:05.290 --> 00:12:05.650 I just -- 00:12:05.650 --> 00:12:07.550 I hate to see people discouraged, and I know that a 00:12:07.550 --> 00:12:10.030 few people have been feeling discouraged, so that's my 00:12:10.030 --> 00:12:14.030 long-winded explanation of exam 1 grades. 00:12:14.030 --> 00:12:14.390 All right. 00:12:14.390 --> 00:12:17.890 So, let's move on with life though, so talking about the 00:12:17.890 --> 00:12:20.070 breakdown of the octet rule. 00:12:20.070 --> 00:12:22.400 The first example where we're going to see a breakdown is 00:12:22.400 --> 00:12:25.470 any time we have an odd number of valence electrons. 00:12:25.470 --> 00:12:28.900 This is probably the easiest to explain and to think about, 00:12:28.900 --> 00:12:31.740 because if we have an odd number that means that we 00:12:31.740 --> 00:12:34.440 can't have our octet rule, because our octet rule works 00:12:34.440 --> 00:12:36.140 by pairing electrons. 00:12:36.140 --> 00:12:40.020 And if we have an odd number, we automatically have an odd 00:12:40.020 --> 00:12:41.440 electron out. 00:12:41.440 --> 00:12:44.560 So, if we look at an example, the methyl radical, we can 00:12:44.560 --> 00:12:47.120 first think about how we draw the Lewis structure -- we draw 00:12:47.120 --> 00:12:49.150 the skeletal structure here. 00:12:49.150 --> 00:12:50.900 And then what we're going to do is add up our valence 00:12:50.900 --> 00:12:55.580 electrons -- we have 3 times 1 for the hydrogen atoms, carbon 00:12:55.580 --> 00:12:58.730 has 4 valence electrons, so we have a total of 7. 00:12:58.730 --> 00:13:02.000 If we want to fill all of our valence shells in each of 00:13:02.000 --> 00:13:06.010 these atoms, we're going to need a total of 14 electrons. 00:13:06.010 --> 00:13:07.770 So, what we see we're left with is that we 00:13:07.770 --> 00:13:10.340 have 7 bonding electrons. 00:13:10.340 --> 00:13:14.020 So we can fill in 6 of those straightforward here, because 00:13:14.020 --> 00:13:16.370 we know that we need to make 3 different bonds. 00:13:16.370 --> 00:13:18.670 And now we're left over with 1 electron, we 00:13:18.670 --> 00:13:19.890 can't make a bond. 00:13:19.890 --> 00:13:24.190 So, what we'll do is carbon does not have an octet yet. 00:13:24.190 --> 00:13:27.195 We can't get it one, but we can do the best we can and 00:13:27.195 --> 00:13:30.890 help it out with adding that extra electron onto the carbon 00:13:30.890 --> 00:13:33.420 atom, so that at least we're getting as close as possible 00:13:33.420 --> 00:13:36.380 to filling our octets. 00:13:36.380 --> 00:13:40.280 This is what we call a radical species or a free radical. 00:13:40.280 --> 00:13:43.090 Free radical or radical species is essentially any 00:13:43.090 --> 00:13:46.580 type of a molecule that has this unpaired electron on one 00:13:46.580 --> 00:13:49.890 of the atoms. This might look really strange, we're used to 00:13:49.890 --> 00:13:51.180 seeing octets. 00:13:51.180 --> 00:13:54.630 But you'll realize, if you calculate the formal charge on 00:13:54.630 --> 00:13:56.470 this molecule, that it's not the worst 00:13:56.470 --> 00:13:58.190 situation ever for carbon. 00:13:58.190 --> 00:14:01.680 At least it's formal charge is zero, even if it doesn't have 00:14:01.680 --> 00:14:03.830 -- it would rather have an extra bond 00:14:03.830 --> 00:14:05.420 and have a full octet. 00:14:05.420 --> 00:14:08.120 But it's not the worst scenario that we can imagine. 00:14:08.120 --> 00:14:11.170 But still, radicals tend to be incredibly reactive because 00:14:11.170 --> 00:14:13.030 they do want to fill that octet. 00:14:13.030 --> 00:14:16.000 So, what happens when you have a radical is it tends to react 00:14:16.000 --> 00:14:18.640 with the first thing that it runs into, especially highly 00:14:18.640 --> 00:14:21.450 reactive radicals that are not stabilized in some other way, 00:14:21.450 --> 00:14:24.030 which you'll tend to talk about it organic chemistry -- 00:14:24.030 --> 00:14:26.510 how you can stabilize radicals. 00:14:26.510 --> 00:14:29.330 So the term free radical should sound familiar to you, 00:14:29.330 --> 00:14:32.390 whether you've heard it in chemistry before, or you 00:14:32.390 --> 00:14:35.330 haven't heard it in chemistry, but maybe have heard it, I 00:14:35.330 --> 00:14:37.710 don't know, commercials for facial 00:14:37.710 --> 00:14:40.150 products or other things. 00:14:40.150 --> 00:14:44.350 People like to talk about free radicals, and they're sort of 00:14:44.350 --> 00:14:46.680 the hero that gets rid of free radicals, which are 00:14:46.680 --> 00:14:47.320 antioxidants. 00:14:47.320 --> 00:14:51.450 So you hear in a lot of different creams or products 00:14:51.450 --> 00:14:54.120 or vitamins that they have antioxidants in them, which 00:14:54.120 --> 00:14:55.840 get rid of free radicals. 00:14:55.840 --> 00:14:58.010 The reason you would want to get rid of free radicals is 00:14:58.010 --> 00:15:00.630 that free radicals can damage DNA, so 00:15:00.630 --> 00:15:01.810 they're incredibly reactive. 00:15:01.810 --> 00:15:04.120 It makes sense that if they hit a strand of DNA, they're 00:15:04.120 --> 00:15:06.250 going to react with the DNA, you end up breaking the 00:15:06.250 --> 00:15:09.320 strands of DNA and causing DNA damage. 00:15:09.320 --> 00:15:12.580 So, this is actually what happens in aging because we 00:15:12.580 --> 00:15:14.950 have a lot of free radicals in our body. 00:15:14.950 --> 00:15:17.940 We can introduce them artificially, for example, 00:15:17.940 --> 00:15:20.760 cigarette smoke has a lot of really dangerous free radicals 00:15:20.760 --> 00:15:24.000 that get into the cells in your lungs, which damage your 00:15:24.000 --> 00:15:26.280 lung DNA, which can cause lung cancer. 00:15:26.280 --> 00:15:29.290 But also, all of us are living and breathing, which means 00:15:29.290 --> 00:15:33.250 we're having metabolism go on in our body, which means that 00:15:33.250 --> 00:15:37.550 as we use oxygen and as we metabolize our food, we are 00:15:37.550 --> 00:15:40.850 actually producing free radicals as well. 00:15:40.850 --> 00:15:44.510 So it's kind of a paradox because we need them because 00:15:44.510 --> 00:15:47.780 they are a natural by-product of these important processes, 00:15:47.780 --> 00:15:50.070 but then they can go on and damage cells, which is what 00:15:50.070 --> 00:15:54.160 kind of is causing aging and can lead to cancer. 00:15:54.160 --> 00:15:58.090 We have enzymes in our body that repair damage that is 00:15:58.090 --> 00:16:00.730 done by free radicals, that will put the strands of DNA 00:16:00.730 --> 00:16:01.710 back together. 00:16:01.710 --> 00:16:04.370 And we also have antioxidants in our body. 00:16:04.370 --> 00:16:08.920 So, you might know that, for example, very brightly colored 00:16:08.920 --> 00:16:12.830 fruit is full of antioxidants, they're full of chemicals that 00:16:12.830 --> 00:16:14.880 will neutralize free radicals. 00:16:14.880 --> 00:16:17.850 Lots of vitamins are also antioxidants, so we have 00:16:17.850 --> 00:16:20.870 vitamin A on the top there and vitamin E. 00:16:20.870 --> 00:16:22.980 So, the most common thing we think of when we think of free 00:16:22.980 --> 00:16:25.110 radicals is very reactive, bad for your 00:16:25.110 --> 00:16:28.250 body, causes DNA damage. 00:16:28.250 --> 00:16:30.530 But the reality is that free radicals are also 00:16:30.530 --> 00:16:31.990 essential for life. 00:16:31.990 --> 00:16:34.530 So this is kind of interesting to think about. 00:16:34.530 --> 00:16:38.080 And, for example, certain enzymes or proteins actually 00:16:38.080 --> 00:16:41.950 use free radicals in order to carry out the reactions that 00:16:41.950 --> 00:16:43.540 they carry out in your body. 00:16:43.540 --> 00:16:46.280 So, for example, this is a picture or a snapshot of a 00:16:46.280 --> 00:16:49.350 protein, this is a crystal structure of ribonucleotide 00:16:49.350 --> 00:16:51.830 reductase is what it's called. 00:16:51.830 --> 00:16:56.100 It's an enzyme that catalyzes the reaction of an essential 00:16:56.100 --> 00:16:59.980 step in both DNA synthesis and also DNA repair, and it 00:16:59.980 --> 00:17:04.090 requires having radicals within its active site in 00:17:04.090 --> 00:17:05.830 order to carry out the chemistry. 00:17:05.830 --> 00:17:08.910 So, this is kind of a neat paradox, because radicals 00:17:08.910 --> 00:17:13.350 damage DNA, but in order to repair your DNA, you need 00:17:13.350 --> 00:17:15.970 certain enzymes, and those enzymes require different 00:17:15.970 --> 00:17:17.610 types of free radicals. 00:17:17.610 --> 00:17:20.090 So, free radicals are definitely very interesting, 00:17:20.090 --> 00:17:22.720 and once we get -- or hopefully you will get into 00:17:22.720 --> 00:17:24.680 organic chemistry at some point and get to really think 00:17:24.680 --> 00:17:28.530 about what they do in terms of a radical mechanism. 00:17:28.530 --> 00:17:31.120 We can think about radicals that are also more stable, so 00:17:31.120 --> 00:17:35.690 let's do another example with the molecule nitric acid. 00:17:35.690 --> 00:17:40.220 So we can again, draw the skeleton here, and just by 00:17:40.220 --> 00:17:42.510 looking at it we might not know it's a radical, but as we 00:17:42.510 --> 00:17:45.190 start to count valence electrons, we should be able 00:17:45.190 --> 00:17:48.790 to figure it out very quickly, because what we have is 11 00:17:48.790 --> 00:17:50.350 valence electrons. 00:17:50.350 --> 00:17:53.630 We need 16 electrons to have full octets. 00:17:53.630 --> 00:17:56.270 So, we're left with 5 bonding electrons. 00:17:56.270 --> 00:17:59.760 We put a double bond in between our nitrogen and our 00:17:59.760 --> 00:18:03.840 oxygen, so what we're left over with is this single 00:18:03.840 --> 00:18:06.830 bonding electron, and we'll put that on the nitrogen here. 00:18:06.830 --> 00:18:09.140 And I'll explain why we put it on the nitrogen and not the 00:18:09.140 --> 00:18:11.080 oxygen in just a minute. 00:18:11.080 --> 00:18:16.350 But what we find is then once we fill in the rest of the 00:18:16.350 --> 00:18:19.480 valence electrons in terms of lone pairs, this is the 00:18:19.480 --> 00:18:21.460 structure that we get. 00:18:21.460 --> 00:18:24.340 And if you add up all of the formal charges on the nitrogen 00:18:24.340 --> 00:18:27.450 and on the oxygen, what you'll see is they're both 0. 00:18:27.450 --> 00:18:30.600 So if you happen to try drawing this structure and you 00:18:30.600 --> 00:18:33.710 put the lone pair on oxygen and then you figured out the 00:18:33.710 --> 00:18:36.900 formal charge and saw that you had a split charge, a plus 1 00:18:36.900 --> 00:18:39.440 and a minus 1, the first thing you might want to try is 00:18:39.440 --> 00:18:41.430 putting it on the other atom, and once you did that you'd 00:18:41.430 --> 00:18:42.990 see that you had a better structure 00:18:42.990 --> 00:18:46.500 with no formal charge. 00:18:46.500 --> 00:18:49.300 I have to mention what nitric oxide does, because it's a 00:18:49.300 --> 00:18:50.850 very interesting molecule. 00:18:50.850 --> 00:18:54.670 Don't get it confused with nitrous oxide, which is happy 00:18:54.670 --> 00:18:56.880 gas, that's n o 2. 00:18:56.880 --> 00:19:00.880 This is nitric oxide, and it's actually much more interesting 00:19:00.880 --> 00:19:02.430 than nitrous oxide. 00:19:02.430 --> 00:19:04.690 It's a signaling molecule in your body, it's one of the 00:19:04.690 --> 00:19:08.050 very few signaling molecules that is a gas, and obviously, 00:19:08.050 --> 00:19:09.440 it's also a radical. 00:19:09.440 --> 00:19:12.350 What happens with n o is that it's produced in the 00:19:12.350 --> 00:19:15.210 endothelium of your blood vessels, so the inner lining 00:19:15.210 --> 00:19:20.280 of your blood vessels, and it signals for smooth muscle that 00:19:20.280 --> 00:19:23.180 line your blood vessels to relax, which causes 00:19:23.180 --> 00:19:25.900 vasodilation , and by vasodilation, I just mean a 00:19:25.900 --> 00:19:27.850 widening of the blood vessels. 00:19:27.850 --> 00:19:31.320 So, n o signals for your blood vessels to get wider and allow 00:19:31.320 --> 00:19:33.070 more blood to flow through. 00:19:33.070 --> 00:19:35.860 And if you think about what consequences this could have, 00:19:35.860 --> 00:19:39.320 in terms of places where they have high altitude, so they 00:19:39.320 --> 00:19:42.080 have lower oxygen levels, do you think that they produce 00:19:42.080 --> 00:19:47.440 more or less and n o their body? 00:19:47.440 --> 00:19:48.030 More? 00:19:48.030 --> 00:19:49.750 Yeah, it turns out they do produce more. 00:19:49.750 --> 00:19:52.810 The reason they produce more is that they want to have more 00:19:52.810 --> 00:19:55.340 blood flowing through their veins so that they can get 00:19:55.340 --> 00:19:59.060 more oxygenated blood into different parts of their body. 00:19:59.060 --> 00:20:02.770 N o is also a target in the pharmaceutical industry. 00:20:02.770 --> 00:20:06.260 A very famous one that became famous I guess over 10 years 00:20:06.260 --> 00:20:10.700 ago now, and this is from a drug that actually targets one 00:20:10.700 --> 00:20:14.440 of n o's receptors, and this drug has the net effect of 00:20:14.440 --> 00:20:18.190 vasodilation or widening of blood vessels in a certain 00:20:18.190 --> 00:20:19.650 area in the body. 00:20:19.650 --> 00:20:22.880 So this is viagra, some of you may be familiar, I think 00:20:22.880 --> 00:20:24.880 everyone's heard of viagra. 00:20:24.880 --> 00:20:27.370 Now you know how viagra works. 00:20:27.370 --> 00:20:31.770 Viagra breaks down, or it inhibits the breakdown of n 00:20:31.770 --> 00:20:35.010 o's binding partner in just certain areas, not everywhere 00:20:35.010 --> 00:20:36.910 in your body. 00:20:36.910 --> 00:20:40.060 So, in those areas, what happens is you get more n o 00:20:40.060 --> 00:20:42.330 signaling, you get more vasodilation, you get 00:20:42.330 --> 00:20:44.220 increased blood flow. 00:20:44.220 --> 00:20:45.770 So that's a little bit of pharmacology 00:20:45.770 --> 00:20:47.750 for you here today. 00:20:47.750 --> 00:20:50.480 All right, so let's talk about one more example in terms of 00:20:50.480 --> 00:20:52.990 the breakdown of the octet rule with radicals. 00:20:52.990 --> 00:20:57.230 Let's think about molecular oxygen. 00:20:57.230 --> 00:21:00.970 So let's go ahead and quickly draw this Lewis structure. 00:21:00.970 --> 00:21:02.840 We have o 2. 00:21:02.840 --> 00:21:04.130 The second thing we need to do is 00:21:04.130 --> 00:21:06.900 figure out valence electrons. 00:21:06.900 --> 00:21:11.060 6 plus 6, so we would expect to see 12. 00:21:11.060 --> 00:21:17.850 For a complete octet we would need 8 electrons each, so 16. 00:21:17.850 --> 00:21:25.250 So in terms of bonding electrons, what we have is 4 00:21:25.250 --> 00:21:26.190 bonding electrons. 00:21:26.190 --> 00:21:29.890 So, we can go ahead and fill those in as a double bond 00:21:29.890 --> 00:21:32.200 between the two oxygens. 00:21:32.200 --> 00:21:36.140 So, what we end up having left, and this would be step 00:21:36.140 --> 00:21:40.820 six then because five was just filling in that, is 12 minus 00:21:40.820 --> 00:21:45.890 4, so we have 8 lone pair electrons left. 00:21:45.890 --> 00:21:51.290 So we can just fill it in to our oxygens like this. 00:21:51.290 --> 00:21:53.970 All right, so using everything we've learned about Lewis 00:21:53.970 --> 00:21:58.730 structures, we here have the structure of molecular oxygen. 00:21:58.730 --> 00:22:01.100 And I just want to point out for anyone that gets confused, 00:22:01.100 --> 00:22:05.500 when we talk about oxygen as an atom, that's o, but 00:22:05.500 --> 00:22:08.240 molecular oxygen is actually o 2, the same for molecular 00:22:08.240 --> 00:22:09.960 hydrogen, for example. 00:22:09.960 --> 00:22:12.430 All right, so let's look at what the actual Lewis 00:22:12.430 --> 00:22:15.780 structure is for molecular oxygen, and it turns out that 00:22:15.780 --> 00:22:19.030 actually we don't have a double bond, we have a single 00:22:19.030 --> 00:22:21.030 bond, and we have two radicals. 00:22:21.030 --> 00:22:23.820 And any time we have two radicals, we talk about what's 00:22:23.820 --> 00:22:26.220 called a biradical. 00:22:26.220 --> 00:22:30.480 And while using this exception to the Lewis structure rule, 00:22:30.480 --> 00:22:33.390 to the octet rule for odd numbers of valence electrons 00:22:33.390 --> 00:22:37.020 can clue us into the fact that we have a radical, there's 00:22:37.020 --> 00:22:40.220 really no way for us to use Lewis structures to predict 00:22:40.220 --> 00:22:42.970 when we have a biradical, right, because we would just 00:22:42.970 --> 00:22:45.660 predict that we would get this Lewis structure here. 00:22:45.660 --> 00:22:48.270 So, when I first introduced Lewis structures, I said these 00:22:48.270 --> 00:22:51.790 are great, they're really easy to use and they work about 90% 00:22:51.790 --> 00:22:53.080 of the time. 00:22:53.080 --> 00:22:55.750 This falls into that 10% that Lewis structures 00:22:55.750 --> 00:22:56.930 don't work for us. 00:22:56.930 --> 00:23:00.260 It turns out, in order to understand that this is the 00:23:00.260 --> 00:23:03.950 electron configuration for o 2, we need to use something 00:23:03.950 --> 00:23:07.870 called molecular orbital theory, and just wait till 00:23:07.870 --> 00:23:10.190 next Wednesday and we will tell you what that is, and we 00:23:10.190 --> 00:23:12.970 will, in fact, use it for oxygen. 00:23:12.970 --> 00:23:15.650 But until that point, I'll just tell you that molecular 00:23:15.650 --> 00:23:19.150 orbital theory takes into account quantum mechanics, 00:23:19.150 --> 00:23:21.060 which Lewis theory does not. 00:23:21.060 --> 00:23:23.950 So that's why, in fact, there are those 10% of cases that 00:23:23.950 --> 00:23:26.960 Lewis structures don't work for. 00:23:26.960 --> 00:23:29.850 All right, the second case of exceptions to the octet rule 00:23:29.850 --> 00:23:32.530 are when we have octet deficient molecules. 00:23:32.530 --> 00:23:34.570 So basically, this means we're going to have a molecule 00:23:34.570 --> 00:23:38.530 that's stable, even though it doesn't have a complete octet. 00:23:38.530 --> 00:23:41.450 And these tend to happen in group 13 molecules, and 00:23:41.450 --> 00:23:44.790 actually happen almost exclusively in group 13 00:23:44.790 --> 00:23:48.210 molecules, specifically with boron and aluminum. 00:23:48.210 --> 00:23:51.010 So, any time you see a Lewis structure with boron or 00:23:51.010 --> 00:23:53.760 aluminum, you want to just remember that I should look 00:23:53.760 --> 00:23:57.080 out to make sure that these might have an incomplete 00:23:57.080 --> 00:24:01.070 octet, so look out for that when you see those atoms. 00:24:01.070 --> 00:24:05.910 So, let's look at b f 3 as our example here. 00:24:05.910 --> 00:24:09.290 And what we see for b f 3 is the number of valence 00:24:09.290 --> 00:24:13.420 electrons that we have are 24, because the valence number of 00:24:13.420 --> 00:24:16.330 electrons for boron is 3, and then 3 00:24:16.330 --> 00:24:19.910 times 7 for each fluorine. 00:24:19.910 --> 00:24:24.070 For total filled octets we need 32, so that means we need 00:24:24.070 --> 00:24:25.420 8 bonding electrons. 00:24:25.420 --> 00:24:28.920 So, let's assign two to each bond here, and then we're 00:24:28.920 --> 00:24:31.160 going to have two extra bonding electrons, so let's 00:24:31.160 --> 00:24:33.350 just arbitrarily pick a fluorine to give 00:24:33.350 --> 00:24:35.850 a double bond to. 00:24:35.850 --> 00:24:38.340 And then we can fill in the lone pair electrons, we have 00:24:38.340 --> 00:24:40.250 16 left over. 00:24:40.250 --> 00:24:42.660 So thinking about what the formal charge is, if we want 00:24:42.660 --> 00:24:45.470 to figure out the formal charge for the boron here, 00:24:45.470 --> 00:24:48.590 what we're talking about is the valence number for boron, 00:24:48.590 --> 00:24:52.430 which is 3, minus 0 because there are no lone pairs, minus 00:24:52.430 --> 00:24:55.450 1/2 of 8 because there are eight shared electrons. 00:24:55.450 --> 00:24:58.520 We get a formal charge of minus 1. 00:24:58.520 --> 00:25:01.380 What is our formal charge since we learned this on 00:25:01.380 --> 00:25:05.250 Monday for thinking about the double 00:25:05.250 --> 00:25:07.770 bonded fluorine in boron? 00:25:07.770 --> 00:25:10.280 So, look at your notes and look at the fluorine that has 00:25:10.280 --> 00:25:12.940 a double bond with it, and I want you to go ahead and tell 00:25:12.940 --> 00:25:32.030 me what that formal charge should be. 00:25:32.030 --> 00:25:46.650 All right, let's take 10 more seconds on that. 00:25:46.650 --> 00:25:50.400 OK, so 49%. 00:25:50.400 --> 00:25:54.800 So, let's go look back at the notes, we'll talk about why 00:25:54.800 --> 00:25:58.610 about 50% of you are right, and 50% need to review, which 00:25:58.610 --> 00:26:00.880 I totally understand you haven't had time to do yet, 00:26:00.880 --> 00:26:04.190 your formal charge rules from Monday's class, there were 00:26:04.190 --> 00:26:05.520 other things going on. 00:26:05.520 --> 00:26:08.490 But let's talk about how we figure out formal charge. 00:26:08.490 --> 00:26:12.390 Formal charge is just the number of valence electrons 00:26:12.390 --> 00:26:14.720 you have. So fluorine has 7. 00:26:14.720 --> 00:26:17.140 You should be able to look at a periodic table and see that 00:26:17.140 --> 00:26:18.550 fluorine has seven. 00:26:18.550 --> 00:26:21.760 What we subtract from that is the number of lone pair 00:26:21.760 --> 00:26:25.280 electrons, and there are four lone pair electrons on this 00:26:25.280 --> 00:26:28.770 double bonded fluorine, so it's minus 4. 00:26:28.770 --> 00:26:32.150 Then we subtract 1/2 of the shared electrons. 00:26:32.150 --> 00:26:34.870 Well we have a double bond with boron here, so we have a 00:26:34.870 --> 00:26:37.050 total of 4 shared electrons. 00:26:37.050 --> 00:26:41.350 And when we do the subtraction here, what we end up with is a 00:26:41.350 --> 00:26:46.180 formal charge plus 1 on the double bonded fluorine. 00:26:46.180 --> 00:26:48.580 Without even doing a calculation, what do you think 00:26:48.580 --> 00:26:50.540 that the formal charge should be on you 00:26:50.540 --> 00:26:52.940 single bonded fluorines? 00:26:52.940 --> 00:26:53.400 Good. 00:26:53.400 --> 00:26:56.700 OK, it should be 0 and it is 0. 00:26:56.700 --> 00:27:01.420 The reason it's zero in terms of calculating it is 7 minus 6 00:27:01.420 --> 00:27:04.150 lone pair electrons minus 1/2 half of 2 shared 00:27:04.150 --> 00:27:05.840 electrons is 0. 00:27:05.840 --> 00:27:09.140 The reason that you all told me, I think, and I hope, is 00:27:09.140 --> 00:27:12.890 that you know that the formal charge on individual atoms has 00:27:12.890 --> 00:27:15.120 to equal the total charge on the molecule. 00:27:15.120 --> 00:27:18.030 So if we already have a minus 1 and a plus 1, and we know we 00:27:18.030 --> 00:27:20.740 have no charge in the molecule, and we only have one 00:27:20.740 --> 00:27:23.640 type of atom left to talk about, that formal charge had 00:27:23.640 --> 00:27:25.240 better be 0. 00:27:25.240 --> 00:27:25.690 OK. 00:27:25.690 --> 00:27:27.940 So this looks pretty good in terms of a Lewis structure, we 00:27:27.940 --> 00:27:29.760 figured out our formal charges. 00:27:29.760 --> 00:27:32.080 These also look pretty good, too, we don't have too much 00:27:32.080 --> 00:27:33.800 charge separation. 00:27:33.800 --> 00:27:37.640 But what actually it turns out is that if you experimentally 00:27:37.640 --> 00:27:41.060 look at what type of bonds you have, it turns out that all 00:27:41.060 --> 00:27:44.840 three of the b f bonds are equal in length, and they all 00:27:44.840 --> 00:27:48.160 have a length that would correspond to a single bond. 00:27:48.160 --> 00:27:51.790 So, experimentally, we know we have to throw out this Lewis 00:27:51.790 --> 00:27:54.430 structure here, we have some more information, let's think 00:27:54.430 --> 00:27:57.430 about how this could happen. 00:27:57.430 --> 00:28:00.330 So this could happen, for example, is if we take this 00:28:00.330 --> 00:28:03.490 two of the electrons that are in the b f double bond and we 00:28:03.490 --> 00:28:06.810 put it right on to the fluorine here, so now we have 00:28:06.810 --> 00:28:08.280 all single bonds. 00:28:08.280 --> 00:28:11.630 And let's think about what the formal charge situation would 00:28:11.630 --> 00:28:13.970 be in this case here. 00:28:13.970 --> 00:28:16.990 What happens here is now we would have a formal charge of 00:28:16.990 --> 00:28:20.770 0 on the boron, we'd have a formal charge of 0 on all of 00:28:20.770 --> 00:28:22.760 the fluorine molecules as well. 00:28:22.760 --> 00:28:25.480 So, it turns out that actually looking at formal charge, even 00:28:25.480 --> 00:28:28.220 though the first case didn't look too bad, this case 00:28:28.220 --> 00:28:28.970 actually looks a lot better. 00:28:28.970 --> 00:28:32.320 We have absolutely no formal charge separation whatsoever. 00:28:32.320 --> 00:28:35.390 It turns out again, boron and aluminum, those are the two 00:28:35.390 --> 00:28:36.890 that you want to look out for. 00:28:36.890 --> 00:28:39.970 They can be perfectly happy without a full octet, they're 00:28:39.970 --> 00:28:44.510 perfectly happy with 6 instead of 8 in terms of electrons in 00:28:44.510 --> 00:28:45.410 their valence shell. 00:28:45.410 --> 00:28:48.650 So that is our exception the number two. 00:28:48.650 --> 00:28:51.230 We have one more exception and this is a valence shell 00:28:51.230 --> 00:28:53.950 expansion, and this can be the hardest to look out for, 00:28:53.950 --> 00:28:57.770 students tend to forget to look for this one, but it's 00:28:57.770 --> 00:28:59.860 very important as well, because there are a lot of 00:28:59.860 --> 00:29:01.610 structures that are affected for this . 00:29:01.610 --> 00:29:04.660 And this is only applicable if we're talking about a central 00:29:04.660 --> 00:29:08.520 atom that has an n value or a principle quantum number 00:29:08.520 --> 00:29:11.420 that's equal to or greater than three. 00:29:11.420 --> 00:29:16.110 What happens when we have n that's equal to or greater to 00:29:16.110 --> 00:29:19.790 three, is that now, in addition to s orbitals and p 00:29:19.790 --> 00:29:23.090 orbitals, what else do we have available to us? 00:29:23.090 --> 00:29:24.350 D orbitals, great. 00:29:24.350 --> 00:29:28.470 So what we see is we have some empty d orbitals, which means 00:29:28.470 --> 00:29:31.310 that we can have more than eight electrons that fit 00:29:31.310 --> 00:29:33.400 around that central atom. 00:29:33.400 --> 00:29:35.380 If you're looking to see if this is going to happen, do 00:29:35.380 --> 00:29:37.020 you think this would happen with a large or 00:29:37.020 --> 00:29:41.350 small central atom? 00:29:41.350 --> 00:29:43.430 So think of it in terms of just fitting. 00:29:43.430 --> 00:29:47.320 We've got to fit more than 8 electrons around here. 00:29:47.320 --> 00:29:50.860 Yeah, so it's going to be, we need to have a large central 00:29:50.860 --> 00:29:53.050 atom in order for this to take place. 00:29:53.050 --> 00:29:55.470 Literally, we just need to fit everything around is probably 00:29:55.470 --> 00:29:58.050 the easiest way to think about it. 00:29:58.050 --> 00:30:02.160 And what happens is it also tends to have small atoms that 00:30:02.160 --> 00:30:02.860 it's bonded to. 00:30:02.860 --> 00:30:06.900 Again, just think of it in terms of all fitting in there. 00:30:06.900 --> 00:30:10.160 So, let's take an example p c l 5. 00:30:10.160 --> 00:30:12.660 The first example is the more straightforward example, 00:30:12.660 --> 00:30:15.240 because let's start to draw the Lewis structure, and what 00:30:15.240 --> 00:30:19.250 we see is that phosphorous has five chlorines around it. 00:30:19.250 --> 00:30:21.980 So we already know if we want to form five bonds we've 00:30:21.980 --> 00:30:23.120 broken our octet rule. 00:30:23.120 --> 00:30:25.570 But let's go through and figure this out and see how 00:30:25.570 --> 00:30:26.660 that happens. 00:30:26.660 --> 00:30:29.900 What we know is we need 40 valence electrons, 00:30:29.900 --> 00:30:31.090 we have those -- 00:30:31.090 --> 00:30:35.410 5 from the phosphorous, and we have 7 from each of the 00:30:35.410 --> 00:30:39.190 chlorine atoms. If we were to fill out all of those octets, 00:30:39.190 --> 00:30:42.850 that would be 48 electrons. 00:30:42.850 --> 00:30:45.710 So what we end up with when we do our Lewis structure 00:30:45.710 --> 00:30:48.270 calculation is that we only have 8 bonding electrons 00:30:48.270 --> 00:30:49.560 available to us. 00:30:49.560 --> 00:30:52.270 So we can fill those in between the phosphorous and 00:30:52.270 --> 00:30:55.240 the chlorine, those 8 bonding electrons. 00:30:55.240 --> 00:30:59.000 So, this is obviously a problem. 00:30:59.000 --> 00:31:04.080 To make 5 p c l bonds we need 10 shared electrons, and we 00:31:04.080 --> 00:31:06.990 know that that's the situation because it's called p c l 5 00:31:06.990 --> 00:31:10.530 and not p c l 4, so we can go right ahead and add in that 00:31:10.530 --> 00:31:13.220 extra electron pair. 00:31:13.220 --> 00:31:16.350 So we've used up 10 for bonding, so that means what we 00:31:16.350 --> 00:31:19.740 have left is 30 lone pair electrons, and I would not 00:31:19.740 --> 00:31:22.120 recommend filling all of these in your notes right now, you 00:31:22.120 --> 00:31:24.500 can go back and do that, but just know the rest end up 00:31:24.500 --> 00:31:27.300 filling up the octets for all of the chlorines. 00:31:27.300 --> 00:31:30.450 So, in this first case where you actually need to make more 00:31:30.450 --> 00:31:33.170 than for bonds, you will immediately know you need to 00:31:33.170 --> 00:31:37.240 use this exception to the Lewis structure octet rule, 00:31:37.240 --> 00:31:39.130 but sometimes it won't be as obvious. 00:31:39.130 --> 00:31:43.090 So, let's look at c r o 4, the 2 minus version here, so a 00:31:43.090 --> 00:31:47.530 chromate ion, and if we draw the skeletal structure, we 00:31:47.530 --> 00:31:51.940 have four things that the chromate needs to bond to. 00:31:51.940 --> 00:31:54.200 So, let's do the Lewis structure again. 00:31:54.200 --> 00:31:57.320 When we figure out the valence electrons, we have total, we 00:31:57.320 --> 00:32:01.760 have 6 from the chromium, we have 6 from each of the 00:32:01.760 --> 00:32:06.450 different oxygens, and where did this 2 come from? 00:32:06.450 --> 00:32:07.780 Yup, the negative charge. 00:32:07.780 --> 00:32:10.600 So, remember, we have 2 extra electrons hanging out in our 00:32:10.600 --> 00:32:12.520 molecule, so we need to include those. 00:32:12.520 --> 00:32:13.670 We have a total of 32. 00:32:13.670 --> 00:32:16.870 40 are needed to fill up octets. 00:32:16.870 --> 00:32:20.820 So again, we have 8 bonding electrons available, so we can 00:32:20.820 --> 00:32:24.560 go ahead and fill these in between each of the bonds. 00:32:24.560 --> 00:32:27.850 What happens is that we then have 24 lone pair electrons 00:32:27.850 --> 00:32:31.810 left, and we can fill those in like this. 00:32:31.810 --> 00:32:34.350 And the problem comes now when we figure 00:32:34.350 --> 00:32:35.810 out the formal charge. 00:32:35.810 --> 00:32:38.310 So, when we do that what we find is that the chromium has 00:32:38.310 --> 00:32:42.510 a formal charge of plus 1, and that each of the oxygens has a 00:32:42.510 --> 00:32:44.040 total charge of minus 1. 00:32:44.040 --> 00:32:47.240 So we actually have a bit of charge separation here. 00:32:47.240 --> 00:32:49.560 Without even doing a calculation, what is the total 00:32:49.560 --> 00:32:53.420 charge of these that are added up? 00:32:53.420 --> 00:32:55.140 OK, it's minus 2, that's right. 00:32:55.140 --> 00:32:57.850 We know that the total charge of each of the formal charges 00:32:57.850 --> 00:33:00.290 has to add up to minus 2, because that's the charge in 00:33:00.290 --> 00:33:01.210 our molecule. 00:33:01.210 --> 00:33:04.130 We can also just calculate it -- the chromate gives us a 00:33:04.130 --> 00:33:07.030 plus 2, then we have 4 times minus 1 for each of the 00:33:07.030 --> 00:33:09.600 oxygens, so we have a minus 2. 00:33:09.600 --> 00:33:11.950 So, we have some charge separation here, and in some 00:33:11.950 --> 00:33:15.380 cases, if we're not at n equals 3 or higher, there's 00:33:15.380 --> 00:33:17.470 really nothing we can do about it, this would be the best 00:33:17.470 --> 00:33:18.580 structure we can do. 00:33:18.580 --> 00:33:22.530 But since we have these d orbitals available, we can use 00:33:22.530 --> 00:33:25.100 them, and it turns out that experimentally this is what's 00:33:25.100 --> 00:33:29.180 found, that the length and the strength are not single bonds, 00:33:29.180 --> 00:33:32.855 but they're actually something between a single bond and a 00:33:32.855 --> 00:33:33.970 double bond. 00:33:33.970 --> 00:33:37.220 So how do we get a 1 and 1/2 bond, for example, what's the 00:33:37.220 --> 00:33:39.710 term that let's us do that? 00:33:39.710 --> 00:33:40.360 Resonance. 00:33:40.360 --> 00:33:40.920 That's right. 00:33:40.920 --> 00:33:43.130 So that's exactly what's happening here. 00:33:43.130 --> 00:33:46.160 So, if we went ahead and drew this structure here where we 00:33:46.160 --> 00:33:51.230 have now two double bonds and two single bonds, that would 00:33:51.230 --> 00:33:54.920 be in resonance with another structure where we have two 00:33:54.920 --> 00:33:58.220 double bonds instead to these two oxygens, and now, single 00:33:58.220 --> 00:34:00.020 bonds to these two oxygens. 00:34:00.020 --> 00:34:02.810 We can actually also have several other resonance 00:34:02.810 --> 00:34:04.140 structures as well. 00:34:04.140 --> 00:34:06.500 Remember, the definition of a resonance structure is where 00:34:06.500 --> 00:34:09.470 all the atoms stay the same, but what we can do is move 00:34:09.470 --> 00:34:11.830 around the electrons -- we're moving around those extra two 00:34:11.830 --> 00:34:14.280 electrons that can be in double bonds. 00:34:14.280 --> 00:34:17.140 So, why don't you tell me how many other resonance 00:34:17.140 --> 00:34:27.170 structures you would expect to see for this chromate ion? 00:34:27.170 --> 00:34:50.960 All right, let's take 10 more seconds on this. 00:34:50.960 --> 00:34:51.640 All right. 00:34:51.640 --> 00:34:52.260 This is good. 00:34:52.260 --> 00:34:55.980 I know this is a real split response, but the right answer 00:34:55.980 --> 00:34:58.360 is the one that is indicated in the graph 00:34:58.360 --> 00:34:59.930 here that it's four. 00:34:59.930 --> 00:35:02.380 This takes a little bit of time to get used to thinking 00:35:02.380 --> 00:35:04.890 about all the different Lewis structures you can have. So, 00:35:04.890 --> 00:35:06.990 you guys should all go back home if you can't see it 00:35:06.990 --> 00:35:10.330 immediately right now and try drawing out those four other 00:35:10.330 --> 00:35:12.360 Lewis structures, for chromate, 00:35:12.360 --> 00:35:13.700 there are four others. 00:35:13.700 --> 00:35:16.240 You'll probably get a chance to literally do this example 00:35:16.240 --> 00:35:19.420 in recitation where you draw out all four, but it's even 00:35:19.420 --> 00:35:21.500 better to make sure you understand it before you get 00:35:21.500 --> 00:35:22.450 to that point. 00:35:22.450 --> 00:35:25.800 So, we can go back to the class notes. 00:35:25.800 --> 00:35:28.280 So it turns out there's four other Lewis structures, so 00:35:28.280 --> 00:35:30.410 basically just think about all the other different 00:35:30.410 --> 00:35:32.910 combinations where you can have single and double bonds, 00:35:32.910 --> 00:35:35.350 and when you draw those out, you end up with four. 00:35:35.350 --> 00:35:37.730 So, for every single one of these Lewis structures, we 00:35:37.730 --> 00:35:40.680 could figure out what the formal charges are, and what 00:35:40.680 --> 00:35:43.600 we would find is that it's 0 on the chromium, it's 0 for 00:35:43.600 --> 00:35:46.630 the double bonded oxygens, and it's going to be negative 1 00:35:46.630 --> 00:35:48.320 for the single bonded oxygens. 00:35:48.320 --> 00:35:52.050 So, what you can see is that in this situation, we end up 00:35:52.050 --> 00:35:55.310 having less formal charge separation, and that's what 00:35:55.310 --> 00:35:58.180 we're looking for, that's the more stable structure. 00:35:58.180 --> 00:36:02.570 So any time you can have an expanded octet -- an expanded 00:36:02.570 --> 00:36:05.940 valence shell, where you have n is equal to or greater than 00:36:05.940 --> 00:36:09.510 3, and by expanding and adding more electrons into that 00:36:09.510 --> 00:36:12.380 valence shell, you lower the charge separation, 00:36:12.380 --> 00:36:13.560 you want to do that. 00:36:13.560 --> 00:36:17.000 I also want to point out, I basically said there's 6 00:36:17.000 --> 00:36:19.600 different ways we can draw this in terms of drawing all 00:36:19.600 --> 00:36:20.880 the resonance structures. 00:36:20.880 --> 00:36:23.340 You might be wondering if you have to figure out the formal 00:36:23.340 --> 00:36:25.810 charge for each structure individually, and the answer 00:36:25.810 --> 00:36:28.560 is no, you can pick any single structure and the formal 00:36:28.560 --> 00:36:30.360 charges will work out the same. 00:36:30.360 --> 00:36:32.900 So, for example, if you pick this structure and your friend 00:36:32.900 --> 00:36:35.590 picks this structure, you'll both get the right answer that 00:36:35.590 --> 00:36:38.930 there's just the negative 1 on the oxygens and no other 00:36:38.930 --> 00:36:42.130 formal charges in the molecule. 00:36:42.130 --> 00:36:42.480 All right. 00:36:42.480 --> 00:36:45.880 So those are the end of our exceptions to the octet rule 00:36:45.880 --> 00:36:48.080 for Lewis structures, that's everything we're going to say 00:36:48.080 --> 00:36:49.670 about Lewis structures. 00:36:49.670 --> 00:36:52.040 And remember, that when we talk about Lewis structures, 00:36:52.040 --> 00:36:55.140 what they tell us is the electron configuration in 00:36:55.140 --> 00:36:57.520 covalent bonds, so that valence shell electron 00:36:57.520 --> 00:36:58.740 configuration. 00:36:58.740 --> 00:37:02.100 So we talked a lot about covalent bonds before we got 00:37:02.100 --> 00:37:05.290 into Lewis structures, and then how to represent covalent 00:37:05.290 --> 00:37:07.490 bonds by Lewis structures. 00:37:07.490 --> 00:37:10.730 So now I'll say a little bit about ionic bonds, which are 00:37:10.730 --> 00:37:14.080 the other extreme, and when you have an ionic bond, what 00:37:14.080 --> 00:37:18.420 you have now is a complete transfer of either one or many 00:37:18.420 --> 00:37:22.370 electrons between two atoms. So the key word for covalent 00:37:22.370 --> 00:37:25.440 bond was electron sharing, the key word for ionic bonds is 00:37:25.440 --> 00:37:27.660 electron transfer. 00:37:27.660 --> 00:37:30.700 And the bonding between the two atoms ends up resulting 00:37:30.700 --> 00:37:33.700 from an attraction that we're very familiar with, which is 00:37:33.700 --> 00:37:36.130 the Coulomb or the electrostatic attraction 00:37:36.130 --> 00:37:37.930 between the negatively charged and the 00:37:37.930 --> 00:37:41.610 positively charged ions. 00:37:41.610 --> 00:37:42.690 So let's take an example. 00:37:42.690 --> 00:37:45.940 The easiest one to think about is where we have a negative 1 00:37:45.940 --> 00:37:47.360 and a positive 1 ion. 00:37:47.360 --> 00:37:51.460 So this is salt, n a c l -- actually lots of things are 00:37:51.460 --> 00:37:54.610 call salt, but this is what we think of a table salt. 00:37:54.610 --> 00:37:57.970 So, let's think about what we have to do if we want the form 00:37:57.970 --> 00:38:01.690 sodium chloride from the neutral sodium and chlorine 00:38:01.690 --> 00:38:04.570 atoms. So, the first thing that we're going to need to do 00:38:04.570 --> 00:38:08.540 is we need to convert sodium into sodium plus. 00:38:08.540 --> 00:38:10.610 What does this process look like to you? 00:38:10.610 --> 00:38:13.470 Is this one of those periodic trends, perhaps? 00:38:13.470 --> 00:38:17.150 Can anyone name what we're looking at here? 00:38:17.150 --> 00:38:18.930 Exactly, ionization energy. 00:38:18.930 --> 00:38:21.050 So, if we're going to talk about the energy difference 00:38:21.050 --> 00:38:23.420 here, what we're going to be talking about is the 00:38:23.420 --> 00:38:27.150 ionization energy, or the energy it takes to rip off an 00:38:27.150 --> 00:38:32.370 electron from sodium in order to form the sodium plus ion. 00:38:32.370 --> 00:38:34.080 So, we can just put right here, that's 494 00:38:34.080 --> 00:38:37.200 kilojoules per mole. 00:38:37.200 --> 00:38:39.880 The next thing that we want to look at is chlorine, so in 00:38:39.880 --> 00:38:42.680 terms of chlorine we need to go to chlorine minus, so we 00:38:42.680 --> 00:38:44.860 actually need to add an electron. 00:38:44.860 --> 00:38:48.370 This is actually the reverse of one of the periodic trends 00:38:48.370 --> 00:38:49.120 we talked about. 00:38:49.120 --> 00:38:53.160 Which trend is that this is the reverse of? 00:38:53.160 --> 00:38:54.500 Electron affinity, right. 00:38:54.500 --> 00:38:57.330 Because if we go backwards we're saying how badly does 00:38:57.330 --> 00:38:59.310 chlorine want to grab an electron? 00:38:59.310 --> 00:39:02.360 Chlorine wants to do this very badly, and it turns out the 00:39:02.360 --> 00:39:05.060 electron affinity for chlorine is huge, it's 349 kilojoules 00:39:05.060 --> 00:39:08.790 per mole, but remember, we're going in reverse, so we need 00:39:08.790 --> 00:39:13.820 to talk about it as negative 349 kilojoules per mole. 00:39:13.820 --> 00:39:17.060 So if we talk about the sum of what's happening here, what we 00:39:17.060 --> 00:39:20.480 need to do is think about going from the neutrals to the 00:39:20.480 --> 00:39:24.420 ions, so we can just add those two energies together, and 00:39:24.420 --> 00:39:28.690 what we end up with is plus 145 kilojoules per mole, in 00:39:28.690 --> 00:39:30.590 order to go from neutral sodium in 00:39:30.590 --> 00:39:33.350 chlorine to the ions. 00:39:33.350 --> 00:39:36.440 So, the problem here is that we have to actually put energy 00:39:36.440 --> 00:39:39.730 into our system, so this doesn't seem favorable, right. 00:39:39.730 --> 00:39:42.200 What's favorable is when we actually get energy out and 00:39:42.200 --> 00:39:45.420 our energy gets lower, but what we're saying here is that 00:39:45.420 --> 00:39:47.850 we actually need to put in energy. 00:39:47.850 --> 00:39:49.720 So another way to say this is this process 00:39:49.720 --> 00:39:51.820 actually requires energy. 00:39:51.820 --> 00:39:55.290 It does not emit energy, it does not give off excess 00:39:55.290 --> 00:39:57.340 energy, it requires energy. 00:39:57.340 --> 00:40:00.230 So, we need to think about how can we solve this problem in 00:40:00.230 --> 00:40:03.240 terms of thinking about ionic bonds, and the answer is 00:40:03.240 --> 00:40:04.590 Coulomb attraction. 00:40:04.590 --> 00:40:07.460 So there's one more force that we need to talk about, and 00:40:07.460 --> 00:40:10.160 that is when we talk about the attraction between the 00:40:10.160 --> 00:40:13.520 negatively and the positively charged ions, such that we 00:40:13.520 --> 00:40:15.150 form sodium chloride. 00:40:15.150 --> 00:40:18.900 So this process here has a delta energy, a change in 00:40:18.900 --> 00:40:22.570 energy of negative 589 kilojoules per mole. 00:40:22.570 --> 00:40:25.230 So that's huge, we're giving off a lot of energy by this 00:40:25.230 --> 00:40:26.140 attraction. 00:40:26.140 --> 00:40:30.820 So if we add up the net energy for all of this process, all 00:40:30.820 --> 00:40:35.220 we need to do is add negative 589 to plus 145. 00:40:35.220 --> 00:40:38.480 So what we end up getting is the net energy change is going 00:40:38.480 --> 00:40:42.700 to be negative 444 kilojoules per mole, so you can see that, 00:40:42.700 --> 00:40:46.490 in fact, it is very favorable for neutral sodium and neutral 00:40:46.490 --> 00:40:53.090 chloride to form sodium chloride in an ionic bond. 00:40:53.090 --> 00:40:56.680 And the net increase then, is a decrease in energy. 00:40:56.680 --> 00:40:59.880 So, I just gave you the number in terms of what that Coulomb 00:40:59.880 --> 00:41:02.710 potential would be in attraction, but we can I 00:41:02.710 --> 00:41:06.480 easily calculate it as well using this equation here where 00:41:06.480 --> 00:41:10.440 the energy is equal to the charge on each of the ions, 00:41:10.440 --> 00:41:13.070 and this is just multiplied by the value of charge for an 00:41:13.070 --> 00:41:17.880 electron divided by 4 pi epsilon nought times r, are r 00:41:17.880 --> 00:41:21.420 is just the distance in terms of the bond length we could 00:41:21.420 --> 00:41:22.330 talk about. 00:41:22.330 --> 00:41:24.790 So, let's calculate and make sure that I didn't tell you a 00:41:24.790 --> 00:41:25.690 false number here. 00:41:25.690 --> 00:41:28.440 Let's say we do the calculation with the bond 00:41:28.440 --> 00:41:30.460 length that we've looked up, which is 2 . 00:41:30.460 --> 00:41:33.040 3 6 angstroms for the bond length 00:41:33.040 --> 00:41:34.520 between sodium and chloride. 00:41:34.520 --> 00:41:37.230 So we should be able to figure out the Coulombic 00:41:37.230 --> 00:41:37.710 attraction for this. 00:41:37.710 --> 00:41:47.540 So, if we talk about the energy of attraction, we need 00:41:47.540 --> 00:41:52.140 to multiply plus 1, that's the charge on the sodium, times 00:41:52.140 --> 00:41:56.370 minus 1, the charge on the chlorine, times the charge in 00:41:56.370 --> 00:41:58.030 an electron, 1 . 00:41:58.030 --> 00:42:04.640 6 0 2 times 10 the negative 19 Coulombs, and that's all 00:42:04.640 --> 00:42:12.470 divided by 4 pi, and then I've written out epsilon nought in 00:42:12.470 --> 00:42:14.610 your notes, so I won't write it on the board. 00:42:14.610 --> 00:42:17.910 And then r, so r is going to be 2 . 00:42:17.910 --> 00:42:23.170 3 6 and times -- what is angstrom, everyone? 00:42:23.170 --> 00:42:26.080 Yup, 10 to the negative 10. 00:42:26.080 --> 00:42:30.190 So 10 to the negative 10 meters. 00:42:30.190 --> 00:42:34.780 So, if we do this calculation here, what we end up with is 00:42:34.780 --> 00:42:42.770 negative 9.774 times 10 to the negative 19 joules. 00:42:42.770 --> 00:42:46.310 So that's what we have in terms of our energy. 00:42:46.310 --> 00:42:49.330 That does not look the same as what we saw -- yup, do you 00:42:49.330 --> 00:42:49.910 have a question? 00:42:49.910 --> 00:42:54.720 STUDENT: [INAUDIBLE] 00:42:54.720 --> 00:42:56.690 PROFESSOR: OK. 00:42:56.690 --> 00:42:58.770 Luckily, although, I did not write it in my own notes, I 00:42:58.770 --> 00:43:01.090 did it when I put in my calculator, thank you. 00:43:01.090 --> 00:43:04.040 So you need to square this value here and then you should 00:43:04.040 --> 00:43:07.600 get this value right here, negative 9.77. 00:43:07.600 --> 00:43:11.320 All right, so what we need to do though is convert from 00:43:11.320 --> 00:43:13.560 joules into kilojoules per mole, because that's what we 00:43:13.560 --> 00:43:14.650 were using. 00:43:14.650 --> 00:43:20.580 So if we multiply that number there by kilojoules per mole 00:43:20.580 --> 00:43:25.540 -- or excuse me, first kilojoules per joule, so we 00:43:25.540 --> 00:43:30.430 have 1,000 joules in every kilojoule. 00:43:30.430 --> 00:43:35.830 And then we multiply that by Avagadro's number, 6.022 times 00:43:35.830 --> 00:43:40.490 10 to the 23 per mole. 00:43:40.490 --> 00:43:45.770 What we end up with is negative 589 00:43:45.770 --> 00:43:48.090 kilojoules per mole. 00:43:48.090 --> 00:43:51.570 So this is that same Coulombic attraction that we saw in the 00:43:51.570 --> 00:43:53.290 first place. 00:43:53.290 --> 00:43:57.100 So, notice that you will naturally get out a negative 00:43:57.100 --> 00:43:59.760 charge here, remember negative means an attractive force in 00:43:59.760 --> 00:44:02.450 this case, because you have the plus and 00:44:02.450 --> 00:44:05.390 the minus 1 in here. 00:44:05.390 --> 00:44:09.630 So we should be able to easily do that calculation, and what 00:44:09.630 --> 00:44:12.060 we end up getting matches up with what I just told you, 00:44:12.060 --> 00:44:14.470 luckily, and thank you for catching the square, that's an 00:44:14.470 --> 00:44:17.070 important part in getting the right answer. 00:44:17.070 --> 00:44:21.120 So, experimentally then, what we find is that the change in 00:44:21.120 --> 00:44:23.070 energy for this reaction is negative 444 00:44:23.070 --> 00:44:25.850 kilojoules per mole. 00:44:25.850 --> 00:44:28.980 If we look experimentally what we see, it's actually a little 00:44:28.980 --> 00:44:32.800 bit different, it's negative 411 kilojoules per mole. 00:44:32.800 --> 00:44:36.530 So, in terms of this class, this is the method that we're 00:44:36.530 --> 00:44:38.470 going to use, and we're going to say this gets us close 00:44:38.470 --> 00:44:41.070 enough such that we can make comparisons and have a 00:44:41.070 --> 00:44:43.870 meaningful conversations about different types of ionic bonds 00:44:43.870 --> 00:44:46.070 and the attraction between them. 00:44:46.070 --> 00:44:49.500 But let's think about where this discrepancy comes from, 00:44:49.500 --> 00:44:53.710 and before I do that I want to point out, one term we use a 00:44:53.710 --> 00:44:57.190 lot is change in energy for a reaction where, for example, 00:44:57.190 --> 00:44:58.640 you break a bond. 00:44:58.640 --> 00:45:02.520 Remember that the negative of the change in energy is what's 00:45:02.520 --> 00:45:04.570 called delta e sub d. 00:45:04.570 --> 00:45:07.630 We first saw this when we first introduced the idea of 00:45:07.630 --> 00:45:08.330 covalent bonds. 00:45:08.330 --> 00:45:14.600 Do you remember what this term here means, delta e sub d? 00:45:14.600 --> 00:45:17.750 A little bit and some no's, which this was pre-exam, I 00:45:17.750 --> 00:45:19.550 understand, you still need to review those notes, it's 00:45:19.550 --> 00:45:21.320 dissociation energy. 00:45:21.320 --> 00:45:24.950 So you get a negative energy out by breaking the bond. 00:45:24.950 --> 00:45:29.360 The dissociation energy means how much energy that bond is 00:45:29.360 --> 00:45:31.870 worth in terms of strength, so it's the opposite of the 00:45:31.870 --> 00:45:35.350 energy you get out of breaking the bond -- or excuse me, the 00:45:35.350 --> 00:45:37.680 energy that you get out of forming the bond. 00:45:37.680 --> 00:45:40.090 It's the amount of energy you need to put in to break the 00:45:40.090 --> 00:45:42.620 bond is dissociation energy. 00:45:42.620 --> 00:45:44.470 It takes this much energy to dissociate your 00:45:44.470 --> 00:45:44.860 bond, excuse me. 00:45:44.860 --> 00:45:45.230 All right. 00:45:45.230 --> 00:45:48.670 So, let's take a look here at our predictions, so I just put 00:45:48.670 --> 00:45:51.070 them both ways so we don't get confused. 00:45:51.070 --> 00:45:53.730 The dissociation energy is 444. 00:45:53.730 --> 00:45:55.570 The change in energy for forming the 00:45:55.570 --> 00:45:57.630 bond is negative 444. 00:45:57.630 --> 00:46:00.220 We made the following approximations, which explain 00:46:00.220 --> 00:46:03.350 why, in fact, we got a different experimental energy, 00:46:03.350 --> 00:46:04.540 if we look at that. 00:46:04.540 --> 00:46:07.060 The first thing is that we ignored any repulsive 00:46:07.060 --> 00:46:07.730 interactions. 00:46:07.730 --> 00:46:12.220 If you think about salt, it's not just two single atoms that 00:46:12.220 --> 00:46:13.390 you are talking about. 00:46:13.390 --> 00:46:16.090 It's actually in a whole network or whole lattice of 00:46:16.090 --> 00:46:18.920 other molecules, so you actually have some other 00:46:18.920 --> 00:46:21.730 chlorines around that are going to be having repulsive 00:46:21.730 --> 00:46:25.310 interactions with our chlorine that we're talking about. 00:46:25.310 --> 00:46:27.630 We're going to ignore those, make the approximation that 00:46:27.630 --> 00:46:30.840 those don't matter, at this point, in these calculations. 00:46:30.840 --> 00:46:33.370 And the result for that is that we end up with a larger 00:46:33.370 --> 00:46:36.670 dissociation energy than the experimental value. 00:46:36.670 --> 00:46:38.750 That's because the bond is going to be a little bit more 00:46:38.750 --> 00:46:41.780 broken than it was in our calculation, because we do 00:46:41.780 --> 00:46:44.870 have these repulsive interactions. 00:46:44.870 --> 00:46:48.040 The other thing that we did is that we treated both sodium 00:46:48.040 --> 00:46:51.250 and the chlorine as point charges. 00:46:51.250 --> 00:46:53.380 And this is what actually allowed us to make this 00:46:53.380 --> 00:46:56.290 calculation and calculate the Coulomb potential so easily, 00:46:56.290 --> 00:46:58.320 we just treated them as if they're point charges. 00:46:58.320 --> 00:47:01.270 We're ignoring quantum mechanics in this -- this is 00:47:01.270 --> 00:47:04.020 sort of the class where we ignore quantum mechanics, we 00:47:04.020 --> 00:47:05.550 ignored it for Lewis structures, 00:47:05.550 --> 00:47:06.730 we're ignoring it here. 00:47:06.730 --> 00:47:10.170 We will be back to paying a lot of attention to quantum 00:47:10.170 --> 00:47:13.630 mechanics in lecture 14 when we talk about MO theory, but 00:47:13.630 --> 00:47:15.560 for now, these are approximations, these are 00:47:15.560 --> 00:47:18.220 models where we don't take it into consideration. 00:47:18.220 --> 00:47:21.010 And I think you'll agree that we come reasonably close such 00:47:21.010 --> 00:47:23.360 that we'll be able to make comparisons between different 00:47:23.360 --> 00:47:24.580 kinds of ionic bonds. 00:47:24.580 --> 00:47:27.050 All right. 00:47:27.050 --> 00:47:29.690 So, the last thing I want to introduce today is talking 00:47:29.690 --> 00:47:31.840 about polar covalent bonds. 00:47:31.840 --> 00:47:34.150 We've now covered the two extremes. 00:47:34.150 --> 00:47:38.070 One extreme is complete total electron sharing -- if we have 00:47:38.070 --> 00:47:41.560 a perfectly covalent bond, we have perfect sharing. 00:47:41.560 --> 00:47:45.870 The other is electron transfer in terms of ionic bonds. 00:47:45.870 --> 00:47:49.340 So when we talk about a polar covalent bond, what we're now 00:47:49.340 --> 00:47:53.010 talking about is an unequal sharing of electrons between 00:47:53.010 --> 00:47:54.100 two atoms. 00:47:54.100 --> 00:47:56.450 So, this is essentially something we've seen before, 00:47:56.450 --> 00:47:59.350 we just never formally talked about what we would call it. 00:47:59.350 --> 00:48:02.830 This is any time you have a bond forming between two 00:48:02.830 --> 00:48:06.340 non-metals that have different electronegativities, so, for 00:48:06.340 --> 00:48:10.200 example, hydrogen choride, h c l. 00:48:10.200 --> 00:48:13.880 The electronegativity for hydrogen is 2.2, for 00:48:13.880 --> 00:48:15.940 chlorine it's 3.2. 00:48:15.940 --> 00:48:19.120 And in general, what we say is we consider a difference in 00:48:19.120 --> 00:48:22.330 terms of a first approximation if the difference in 00:48:22.330 --> 00:48:24.280 electronegativity is more than 0. 00:48:24.280 --> 00:48:27.560 5, so this is on the Pauling electronegativity scale. 00:48:27.560 --> 00:48:31.850 So what we end up having is we sort of have a kind of, and 00:48:31.850 --> 00:48:34.840 what we call it is a partial negative charge on the 00:48:34.840 --> 00:48:37.220 chlorine, and a partial positive 00:48:37.220 --> 00:48:38.720 charge in the hydrogen. 00:48:38.720 --> 00:48:41.150 The reason we have that is because the chlorine's more 00:48:41.150 --> 00:48:43.970 electronegative, it wants to pull more of that shared 00:48:43.970 --> 00:48:45.770 electron density to itself. 00:48:45.770 --> 00:48:47.900 If it has more electron density, it's going to have a 00:48:47.900 --> 00:48:50.350 little bit of a negative charge and the hydrogen's 00:48:50.350 --> 00:48:53.150 going to be left with a little bit of a positive charge. 00:48:53.150 --> 00:48:56.420 So, we can compare this, for example to, molecular hydrogen 00:48:56.420 --> 00:48:58.940 where they're going to have that complete sharing, so 00:48:58.940 --> 00:49:02.380 there's not going to be a delta plus or a delta minus, 00:49:02.380 --> 00:49:05.300 delta is going to be equal to zero on each of the atoms. 00:49:05.300 --> 00:49:09.670 They are completely sharing their electrons. 00:49:09.670 --> 00:49:13.000 And we can also explain this in another way by talking 00:49:13.000 --> 00:49:16.670 about a dipole moment where we have a charged distribution 00:49:16.670 --> 00:49:19.340 that results in this dipole, this electric dipole. 00:49:19.340 --> 00:49:22.540 And we talk about this using the term mu, which is a 00:49:22.540 --> 00:49:24.290 measurement of what the dipole is. 00:49:24.290 --> 00:49:28.610 A dipole is always written in terms of writing an arrow from 00:49:28.610 --> 00:49:31.040 the positive charge to the negative charge. 00:49:31.040 --> 00:49:33.780 In chemistry, we are always incredibly interested in what 00:49:33.780 --> 00:49:36.900 the electrons are doing, so we tend to pay attention to them 00:49:36.900 --> 00:49:37.900 in terms of arrows. 00:49:37.900 --> 00:49:40.500 Oh, the electrons are going over to the chlorine, so we're 00:49:40.500 --> 00:49:43.290 going to draw our arrow toward the chlorine atom. 00:49:43.290 --> 00:49:47.530 So, we measure this here, so mu is equal to q times r, the 00:49:47.530 --> 00:49:49.040 distance between the two. 00:49:49.040 --> 00:49:52.960 And q, that charge is just equal to the partial negative 00:49:52.960 --> 00:49:57.460 or the partial positive times the charge on the electron. 00:49:57.460 --> 00:50:01.540 So this is measured in Coulomb meters, you won't ever see a 00:50:01.540 --> 00:50:03.990 measurement of electronegativity in Coulomb 00:50:03.990 --> 00:50:06.700 meters -- we tend to talk about it in terms of debye or 00:50:06.700 --> 00:50:12.310 1 d, or sometimes there's no units at all, so the d is just 00:50:12.310 --> 00:50:15.970 assumed, and it's because 1 debye is just equal to this 00:50:15.970 --> 00:50:18.770 very tiny number of Coulomb meters and it's a lot easier 00:50:18.770 --> 00:50:21.850 to work with debye's here. 00:50:21.850 --> 00:50:25.470 So, when we talk about polar molecules, we can actually 00:50:25.470 --> 00:50:29.040 extend our idea of talking about polar bonds to talking 00:50:29.040 --> 00:50:30.170 about polar molecules. 00:50:30.170 --> 00:50:32.860 So, actually let's start with that on Monday. 00:50:32.860 --> 00:50:35.380 So everyone have a great weekend.