WEBVTT 00:00:00.000 --> 00:00:00.272 The following content is provided under a Creative 00:00:00.272 --> 00:00:00.374 Commons license. 00:00:00.374 --> 00:00:00.646 Your support will help MIT OpenCourseWare continue to 00:00:00.646 --> 00:00:00.918 offer high quality educational resources for free. 00:00:00.918 --> 00:00:01.224 To make a donation or view additional materials from 00:00:01.224 --> 00:00:01.496 hundreds of MIT courses, visit MIT OpenCourseWare at 00:00:01.496 --> 00:00:01.550 ocw.mit.edu. 00:00:01.550 --> 00:00:22.550 PROFESSOR: All right. 00:00:22.550 --> 00:00:25.240 It's 12:05, so why don't you go ahead and take 10 more 00:00:25.240 --> 00:00:27.540 seconds on the clicker question today. 00:00:30.170 --> 00:00:32.070 This is about the periodic trends that we 00:00:32.070 --> 00:00:36.720 discussed on Wednesday. 00:00:36.720 --> 00:00:40.430 So specifically, what we're asking here is as we go across 00:00:40.430 --> 00:00:44.170 the periodic table, we want to consider which has the smaller 00:00:44.170 --> 00:00:45.440 ionization energy. 00:00:45.440 --> 00:00:46.540 All right. 00:00:46.540 --> 00:00:49.555 So, let's focus our attention up here now, whether it's 00:00:49.555 --> 00:00:52.890 between aluminum or whether it's between phosphorous. 00:00:52.890 --> 00:00:55.130 And I also wanted you to identify why it's also 00:00:55.130 --> 00:00:58.350 important to understand why we have these trends, not just to 00:00:58.350 --> 00:01:00.010 memorize the trend itself. 00:01:00.010 --> 00:01:02.610 So, it turns out that the majority of you got the 00:01:02.610 --> 00:01:05.150 correct answer, which is that it's aluminum. 00:01:05.150 --> 00:01:08.850 The reason it's aluminum is because aluminum has a lower z 00:01:08.850 --> 00:01:13.920 effective, so it's not being pulled in as tightly by the 00:01:13.920 --> 00:01:16.610 nucleus, and if it's not being pulled in as tightly, you're 00:01:16.610 --> 00:01:19.330 going to have to put in less energy in order to ionize it, 00:01:19.330 --> 00:01:21.860 so that's why it's actually going to have the smaller 00:01:21.860 --> 00:01:23.260 ionization energy. 00:01:23.260 --> 00:01:26.300 So it looks like not too many more than half of you got this 00:01:26.300 --> 00:01:29.270 correct, so make sure you can look at your periodic table 00:01:29.270 --> 00:01:32.120 and figure out how to think about ionization energy in 00:01:32.120 --> 00:01:35.760 terms of z effective, not just in terms of memorizing what 00:01:35.760 --> 00:01:38.240 that trend is. 00:01:38.240 --> 00:01:44.460 All right, so we can go to today's notes, and in terms of 00:01:44.460 --> 00:01:46.550 the notes, what we're going to start with is finishing 00:01:46.550 --> 00:01:49.700 material that's going to be relevant for exam 1, and I 00:01:49.700 --> 00:01:51.810 told you on Wednesday that actually I'd give you some 00:01:51.810 --> 00:01:55.030 information today in terms of what you need to do to prepare 00:01:55.030 --> 00:01:56.270 for exam 1. 00:01:56.270 --> 00:01:59.240 So you should have gotten two handouts as you came in, and 00:01:59.240 --> 00:02:01.830 if you didn't, please raise your hand and a TA will come 00:02:01.830 --> 00:02:04.380 to you and get you that second handout. 00:02:04.380 --> 00:02:08.340 But the second one says "exam instructions and logistics." 00:02:08.340 --> 00:02:10.510 So, if everyone can pull that out. 00:02:10.510 --> 00:02:12.540 So this is going to tell you pretty much everything you 00:02:12.540 --> 00:02:15.270 need to know in terms of getting ready for the exam, 00:02:15.270 --> 00:02:17.210 which is next Wednesday. 00:02:17.210 --> 00:02:20.310 So when you go home today or some time this weekend, make 00:02:20.310 --> 00:02:22.130 sure you read this page in detail. 00:02:22.130 --> 00:02:25.100 I'm just going to go over a few of the main points here. 00:02:25.100 --> 00:02:29.180 So, the first is that what we're going over notes today 00:02:29.180 --> 00:02:32.900 and also on Monday, the exam material ends at the end 00:02:32.900 --> 00:02:34.860 lecture notes from lecture 9. 00:02:34.860 --> 00:02:38.080 So that was Wednesday's class -- not at the end of 00:02:38.080 --> 00:02:40.350 Wednesday's class, but at the end of the lecture notes. 00:02:40.350 --> 00:02:42.130 So we're going to finish up with those today. 00:02:42.130 --> 00:02:44.850 I'll be really, really clear when we get through them, and 00:02:44.850 --> 00:02:47.640 that's where you can stop in terms of studying for this. 00:02:47.640 --> 00:02:50.680 Also, on everything that was on problem-sets 1 through 3. 00:02:50.680 --> 00:02:54.220 So, you turned in p-set 3 today, but we'll have the 00:02:54.220 --> 00:02:56.860 answers posted for you this afternoon, so you can start 00:02:56.860 --> 00:03:01.280 studying from p-set 3, even as early as tonight, if you want 00:03:01.280 --> 00:03:04.700 to, because those answers will be there for you. 00:03:04.700 --> 00:03:07.920 So, in terms of what it is that you need to prepare and 00:03:07.920 --> 00:03:11.670 bring with you for the exam, you need to bring your MIT ID, 00:03:11.670 --> 00:03:14.380 especially if you haven't been showing up regularly to your 00:03:14.380 --> 00:03:18.070 recitations and you aren't 100% sure if your TA knows 00:03:18.070 --> 00:03:20.270 exactly who you are, you need to make sure you have your MIT 00:03:20.270 --> 00:03:21.500 ID with you. 00:03:21.500 --> 00:03:23.680 You can't take the exam unless you're registered for the 00:03:23.680 --> 00:03:26.600 class, and we need to make sure we can verify that. 00:03:26.600 --> 00:03:29.190 You also need to bring a calculator, of course, because 00:03:29.190 --> 00:03:32.020 we'll be solving problems that involve calculations. 00:03:32.020 --> 00:03:34.440 You can bring any calculator you want, we don't actually 00:03:34.440 --> 00:03:37.440 have restrictions for calculator types here, but 00:03:37.440 --> 00:03:40.230 what you can't do, is you can't program any relevant 00:03:40.230 --> 00:03:44.110 chemical or information about constants in there. 00:03:44.110 --> 00:03:46.970 It's OK to use certain fundamental constants that 00:03:46.970 --> 00:03:49.860 come in a lot of calculators, so there's nothing we can do 00:03:49.860 --> 00:03:50.740 about that. 00:03:50.740 --> 00:03:51.760 That's OK. 00:03:51.760 --> 00:03:54.690 If you're wondering what's OK or what's not OK, it's very 00:03:54.690 --> 00:03:57.180 clearly written out in this handout, so make sure you read 00:03:57.180 --> 00:03:59.850 through it, because it is your responsibility to make sure 00:03:59.850 --> 00:04:02.420 that your calculator does not have anything extra 00:04:02.420 --> 00:04:03.480 programmed in it. 00:04:03.480 --> 00:04:06.115 And if you have your calculator all set up as you 00:04:06.115 --> 00:04:08.120 love and you don't want to change it, then maybe you 00:04:08.120 --> 00:04:11.010 should just go and get an $8.00 scientific calculator 00:04:11.010 --> 00:04:13.200 that doesn't have any of the graphing functions, because 00:04:13.200 --> 00:04:15.580 you don't actually need them, so that's a better option for 00:04:15.580 --> 00:04:18.330 you, you can do that as well. 00:04:18.330 --> 00:04:21.010 And I had mentioned several times that you do not need to 00:04:21.010 --> 00:04:23.800 memorize the majority of the equations and you don't need 00:04:23.800 --> 00:04:26.080 to memorize any physical constants. 00:04:26.080 --> 00:04:29.950 So if you flip the info page over on the back here, what 00:04:29.950 --> 00:04:33.340 you'll see is the periodic table, this is the same one 00:04:33.340 --> 00:04:35.925 that I've handed out in the last two lectures -- the 00:04:35.925 --> 00:04:38.490 periodic table without any electron configurations. 00:04:38.490 --> 00:04:40.740 This is exactly the sheet here, it's exactly what you'll 00:04:40.740 --> 00:04:42.000 get on exam day. 00:04:42.000 --> 00:04:44.870 You'll also see that they have all the physical constants 00:04:44.870 --> 00:04:48.660 that you're going to need, and also a bunch of the actual 00:04:48.660 --> 00:04:51.090 equations that we've been using in the first couple 00:04:51.090 --> 00:04:52.040 weeks here. 00:04:52.040 --> 00:04:54.300 So you don't need to memorize any of this, you're actually 00:04:54.300 --> 00:04:55.430 going to be handed this. 00:04:55.430 --> 00:04:59.060 There are a few equations that you need to memorize -- those 00:04:59.060 --> 00:05:01.960 are the very simple -- very, very simple equations, such as 00:05:01.960 --> 00:05:04.820 e equals h times nu -- hopefully you don't have to 00:05:04.820 --> 00:05:06.880 sit down and try to memorize that, hopefully we all know 00:05:06.880 --> 00:05:07.730 that already. 00:05:07.730 --> 00:05:10.280 But just to be really clear, I've written out exactly which 00:05:10.280 --> 00:05:13.190 equations you do have to memorize on the front here, so 00:05:13.190 --> 00:05:17.730 long as you know those, the rest you can just look up. 00:05:17.730 --> 00:05:21.870 And in terms of using these equations in solving problems 00:05:21.870 --> 00:05:25.460 on the exam, and also using these constants, make sure if 00:05:25.460 --> 00:05:27.870 you think there might be any chance you're going to get any 00:05:27.870 --> 00:05:30.210 little part of a problem wrong or do a calculation 00:05:30.210 --> 00:05:33.280 inaccurately, you need to write out every single step of 00:05:33.280 --> 00:05:36.340 your thinking as you write out these problems. We can't give 00:05:36.340 --> 00:05:38.700 you any partial credit whatsoever if we can't see 00:05:38.700 --> 00:05:39.610 your thought process. 00:05:39.610 --> 00:05:42.460 So it's important to write out the equation you use, you need 00:05:42.460 --> 00:05:44.720 to write out the constants that you use to 00:05:44.720 --> 00:05:45.800 fill in that equation. 00:05:45.800 --> 00:05:48.710 And that we need to see your work to get full credit, and 00:05:48.710 --> 00:05:51.400 then especially if you get things wrong, we need to know 00:05:51.400 --> 00:05:53.640 where it went wrong, because we do try to give as much 00:05:53.640 --> 00:05:56.910 partial credit as possible in these exams, since there are a 00:05:56.910 --> 00:05:59.250 lot of places where small mistakes can result in the 00:05:59.250 --> 00:06:01.220 wrong answer. 00:06:01.220 --> 00:06:04.620 So also, along those lines in terms of test taking, make 00:06:04.620 --> 00:06:07.780 sure you also box your answers and that you keep track of 00:06:07.780 --> 00:06:10.370 significant figures and that you also remember to include 00:06:10.370 --> 00:06:10.700 your units. 00:06:10.700 --> 00:06:13.380 These are just little things that can add up, so you just 00:06:13.380 --> 00:06:15.100 want to make sure you're on top of those. 00:06:15.100 --> 00:06:17.580 And your TAs on Tuesday are going to share a lot of other 00:06:17.580 --> 00:06:21.140 types of sort of exam strategies in thinking about 00:06:21.140 --> 00:06:23.900 how you can approach an exam when we're in a time situation 00:06:23.900 --> 00:06:27.160 like we are, so they'll share some of their experience with 00:06:27.160 --> 00:06:31.290 you in terms of taking these timed exams. 00:06:31.290 --> 00:06:34.980 So, in terms of practicing this weekend, I mentioned that 00:06:34.980 --> 00:06:37.180 instead of getting a problem-set today, what I am 00:06:37.180 --> 00:06:40.410 going to be posting is optional extra problems. So 00:06:40.410 --> 00:06:42.700 they're optional, but they're very, very, very highly 00:06:42.700 --> 00:06:45.150 encouraged that you do these, because this is going to give 00:06:45.150 --> 00:06:47.390 you practice for the types of problems that are going to be 00:06:47.390 --> 00:06:48.410 on the exam. 00:06:48.410 --> 00:06:52.170 We're also posting a practice exam for you to take, so after 00:06:52.170 --> 00:06:54.960 you're completely done your studying, it's good to have 00:06:54.960 --> 00:06:57.510 everything done before you take the practice exam, and 00:06:57.510 --> 00:06:59.410 then sit down just with this sheet here and your 00:06:59.410 --> 00:07:02.330 calculator, and ideally a timer, and make sure you can 00:07:02.330 --> 00:07:05.260 do the practice exam in the allotted amount of time. 00:07:05.260 --> 00:07:07.960 So that way you can have an idea if, oh, I do really 00:07:07.960 --> 00:07:10.220 understand this but I'm a little bit slow, maybe I need 00:07:10.220 --> 00:07:13.930 to practice this one type of problem a little bit longer so 00:07:13.930 --> 00:07:15.860 I can get up to speed so I'm going to be able to get 00:07:15.860 --> 00:07:19.550 through all this in terms of the exam time. 00:07:19.550 --> 00:07:19.830 All right. 00:07:19.830 --> 00:07:22.700 So let's move on to today's topics. 00:07:22.700 --> 00:07:25.420 So, as I said, we're finishing up with what we left off with 00:07:25.420 --> 00:07:28.180 yesterday -- or excuse me, on Wednesday. 00:07:28.180 --> 00:07:31.340 This includes atomic radius and the idea of isoelectronic 00:07:31.340 --> 00:07:34.450 atoms. So that's going to be the end of the exam 1 00:07:34.450 --> 00:07:38.710 material, and then we'll move on to exam 2 material, which 00:07:38.710 --> 00:07:41.600 is kind of exciting, because we've been talking about just 00:07:41.600 --> 00:07:45.080 individual atoms and ions up to this point, and now we can 00:07:45.080 --> 00:07:46.700 talk about molecules, so we're going to start 00:07:46.700 --> 00:07:48.100 talking about bonding. 00:07:48.100 --> 00:07:51.220 So for some you that are less interested in maybe the 00:07:51.220 --> 00:07:54.010 physical structure of an individual atom, now some more 00:07:54.010 --> 00:07:56.350 exciting material for you might be coming up if you like 00:07:56.350 --> 00:07:59.780 to think about how, instead, molecules behave, either 00:07:59.780 --> 00:08:01.660 within bonding, within themselves, or with other 00:08:01.660 --> 00:08:03.650 molecules, that's what we're going to be heading to 00:08:03.650 --> 00:08:06.780 in this next unit. 00:08:06.780 --> 00:08:09.920 So, we need to finish up with periodic trends. 00:08:09.920 --> 00:08:12.050 And first, on your lecture notes, I 00:08:12.050 --> 00:08:13.920 start with atomic radius. 00:08:13.920 --> 00:08:16.430 I was so proud of myself getting the lecture notes 00:08:16.430 --> 00:08:18.750 finished early and handing them in to CopyTech, and then 00:08:18.750 --> 00:08:20.440 I realized we didn't do 00:08:20.440 --> 00:08:22.370 electronegativity on Wednesday. 00:08:22.370 --> 00:08:25.880 So, if you can flip your lecture notes over and just 00:08:25.880 --> 00:08:27.690 write on the blank space, we're going to cover 00:08:27.690 --> 00:08:32.870 electronegativity first here, and specifically, you can go 00:08:32.870 --> 00:08:35.870 back and fill this in to your lecture 9 notes, if you want 00:08:35.870 --> 00:08:38.300 to stay organized, but I just suggest just writing it on 00:08:38.300 --> 00:08:41.880 lecture 10 notes now and going back. 00:08:41.880 --> 00:08:45.160 You can still keep organized, which hopefully most of you 00:08:45.160 --> 00:08:48.770 like to do, and get it in the right place in the notes. 00:08:48.770 --> 00:08:57.510 So, when we're talking about the idea of electronegativity, 00:08:57.510 --> 00:09:00.540 essentially what we're talking about is the ability for an 00:09:00.540 --> 00:09:05.300 atom to attract electron density from another atom. 00:09:05.300 --> 00:09:08.330 So it's just a measure of how much does one given atom want 00:09:08.330 --> 00:09:10.715 to pull away electron density from, let's 00:09:10.715 --> 00:09:12.470 say, an adjacent atom. 00:09:12.470 --> 00:09:14.960 So it's actually very related to what we're talking about 00:09:14.960 --> 00:09:16.890 when we said electron affinity, and it's also 00:09:16.890 --> 00:09:21.020 related to ionization energy, and we can call 00:09:21.020 --> 00:09:23.930 electronegativity by symbol here, and it turns out that 00:09:23.930 --> 00:09:28.780 it's going to be proportional to 1/2 of the electron 00:09:28.780 --> 00:09:34.740 affinity of a given atom, plus the ionization energy. 00:09:34.740 --> 00:09:36.340 So, in other words, we can just think of 00:09:36.340 --> 00:09:39.290 electronegativity as being the average of that ionization 00:09:39.290 --> 00:09:41.290 energy and the electron affinity. 00:09:41.290 --> 00:09:43.550 This should make sense, because if an atom has a very 00:09:43.550 --> 00:09:47.360 high electron affinity, that means it's really happy taking 00:09:47.360 --> 00:09:50.440 an electron from another atom, or taking a free electron -- 00:09:50.440 --> 00:09:51.970 that that's very favorable. 00:09:51.970 --> 00:09:54.720 If something has a high ionization energy, it means 00:09:54.720 --> 00:09:56.730 that it really, really, really does not want 00:09:56.730 --> 00:09:58.280 to give up an electron. 00:09:58.280 --> 00:10:00.830 So you can think about how these 2 things combined are 00:10:00.830 --> 00:10:03.890 going to be electronegativity, which is a measure of how much 00:10:03.890 --> 00:10:06.430 an atom wants to pull electron density away 00:10:06.430 --> 00:10:08.300 from another atom. 00:10:08.300 --> 00:10:12.100 So, if we think about electronegativity as a 00:10:12.100 --> 00:10:16.450 periodic trend, we can just draw our nice periodic table 00:10:16.450 --> 00:10:19.650 here, and let's separate it into quadrants. 00:10:19.650 --> 00:10:22.320 So if we think about the upper right hand part of the 00:10:22.320 --> 00:10:25.280 quadrant, well, this is where we're going to have high 00:10:25.280 --> 00:10:28.960 electron affinity and high ionization energy, so we're 00:10:28.960 --> 00:10:34.340 also going to see high electronegativity here. 00:10:34.340 --> 00:10:37.980 And in contrast, in the lower left hand part of the periodic 00:10:37.980 --> 00:10:41.200 table, these 2 quantities are low, so also what we're going 00:10:41.200 --> 00:10:46.740 to see is low electronegativity. 00:10:46.740 --> 00:10:49.680 And if we talk about what's going on in areas, or with 00:10:49.680 --> 00:10:52.210 atoms that have high electronegativity, and we 00:10:52.210 --> 00:10:56.130 think about whether they're electron donors or electron 00:10:56.130 --> 00:10:59.500 acceptors, what would you expect for an atom that has 00:10:59.500 --> 00:11:00.690 high electronegativity? 00:11:00.690 --> 00:11:04.280 Is it going to be an electron donor or acceptor? 00:11:04.280 --> 00:11:04.650 Great. 00:11:04.650 --> 00:11:07.030 Yup, it's going to be an electron acceptor, it wants to 00:11:07.030 --> 00:11:09.730 accept electrons, it wants to accept electron density. 00:11:09.730 --> 00:11:14.310 So, in contrast, if it has a low electronegativity, this 00:11:14.310 --> 00:11:21.540 then is going to be an electron donor. 00:11:21.540 --> 00:11:24.020 All right, so it's very common to talk about 00:11:24.020 --> 00:11:26.470 electronegativity of different atoms, and you can look up 00:11:26.470 --> 00:11:27.580 tables of these. 00:11:27.580 --> 00:11:29.760 Often what you'll see is not a table based on this 00:11:29.760 --> 00:11:32.330 definition, but something that's called the Pauling 00:11:32.330 --> 00:11:34.000 definition of electronegativity, but it's 00:11:34.000 --> 00:11:37.350 exactly the same idea and the same trend as this more 00:11:37.350 --> 00:11:39.320 numerical way to think about what the meaning of 00:11:39.320 --> 00:11:40.730 electronegativity is. 00:11:40.730 --> 00:11:44.620 All right, so now we can move on to the start of today's 00:11:44.620 --> 00:11:46.740 notes, which is atomic radius. 00:11:46.740 --> 00:11:50.870 So this, in fact, is going to be our last principle that 00:11:50.870 --> 00:11:54.690 we're going to talk about in terms of periodic trends. 00:11:54.690 --> 00:11:57.500 So this is actually the most straightforward, so sometimes 00:11:57.500 --> 00:11:59.880 it's nice to end with the easiest concept, and that's 00:11:59.880 --> 00:12:01.110 what we're doing here. 00:12:01.110 --> 00:12:03.520 And if we're talking about atomic radius, essentially 00:12:03.520 --> 00:12:06.000 we're talking about atomic size. 00:12:06.000 --> 00:12:08.930 And immediately it should probably come into your head 00:12:08.930 --> 00:12:12.070 that we don't actually have an atomic radius that we can talk 00:12:12.070 --> 00:12:12.530 about, right? 00:12:12.530 --> 00:12:15.130 What I just spent many lectures discussing is the 00:12:15.130 --> 00:12:19.500 fact that we can not know how far away an electron is from 00:12:19.500 --> 00:12:22.350 the nucleus, so we can't actually know the radius of a 00:12:22.350 --> 00:12:23.520 certain atom. 00:12:23.520 --> 00:12:25.010 And that's true. 00:12:25.010 --> 00:12:28.030 An atom's not a defined sphere, for example. 00:12:28.030 --> 00:12:31.320 We can't define it as an exact radius in terms of the 00:12:31.320 --> 00:12:33.840 definition we might think of classically. 00:12:33.840 --> 00:12:36.470 So, keep that in mind when we're talking about atomic 00:12:36.470 --> 00:12:39.400 radius, I'm not suddenly changing my story and saying, 00:12:39.400 --> 00:12:41.270 yes, we do have a distinct radius. 00:12:41.270 --> 00:12:43.890 Instead, what people have done is come up with different ways 00:12:43.890 --> 00:12:46.160 to think about how they can define a radius. 00:12:46.160 --> 00:12:48.680 And one common way to think about it, is to think about 00:12:48.680 --> 00:12:52.780 the value of r, or the radius, below which 90% of that 00:12:52.780 --> 00:12:55.060 electron density is going to be contained. 00:12:55.060 --> 00:12:57.060 So we're not saying it's all the electron 00:12:57.060 --> 00:12:58.830 density, it's just 90%. 00:12:58.830 --> 00:13:01.700 Because we know as we go to infinity, even though the 00:13:01.700 --> 00:13:04.890 density gets smaller and smaller and smaller, we still 00:13:04.890 --> 00:13:07.980 have electron density very far away from the nucleus. 00:13:07.980 --> 00:13:11.130 So, what we're going to define is just let's just capture 90% 00:13:11.130 --> 00:13:12.950 of that electron density. 00:13:12.950 --> 00:13:15.280 So, that's one way to think about it, and there's also 00:13:15.280 --> 00:13:16.620 another way, and this is the way that your 00:13:16.620 --> 00:13:17.790 book presents it. 00:13:17.790 --> 00:13:21.510 If you, in fact, have two of the same atom right next to 00:13:21.510 --> 00:13:23.650 each other, let's say you have a crystal, or let's say you're 00:13:23.650 --> 00:13:27.350 talking about a metal, what you can do is just look at the 00:13:27.350 --> 00:13:31.010 distance between the two nuclei, and split that in 1/2, 00:13:31.010 --> 00:13:33.410 and take the atomic radius that way. 00:13:33.410 --> 00:13:35.760 So, these are two different definitions of how to think 00:13:35.760 --> 00:13:38.500 about atomic radius, but really what you find when 00:13:38.500 --> 00:13:40.500 these are measured is they come up with almost the 00:13:40.500 --> 00:13:43.570 identical values, so there are tables you can look up of 00:13:43.570 --> 00:13:47.430 atomic radii and see these values, and you can trust them 00:13:47.430 --> 00:13:50.670 that, yes, they work for both this definition and for this 00:13:50.670 --> 00:13:54.770 definition here, in most cases. 00:13:54.770 --> 00:13:56.710 And what we've been talking about with all of these 00:13:56.710 --> 00:14:00.390 properties are, of course, how can we figure out what that is 00:14:00.390 --> 00:14:02.850 for a certain atom by looking at the periodic table, so we 00:14:02.850 --> 00:14:04.130 want to think about the periodic 00:14:04.130 --> 00:14:06.190 trend for atomic radius. 00:14:06.190 --> 00:14:09.460 And we know as we go across a row in the periodic table, 00:14:09.460 --> 00:14:12.340 what's happening is that z effective or the effective 00:14:12.340 --> 00:14:14.920 pull on the nucleus is increasing. 00:14:14.920 --> 00:14:18.100 So would you expect, therefore, as we go across a 00:14:18.100 --> 00:14:23.040 row for the atomic radius, to increase or to decrease? 00:14:23.040 --> 00:14:23.350 Good. 00:14:23.350 --> 00:14:24.020 OK, yes. 00:14:24.020 --> 00:14:26.880 We are expecting to see that it decreases because it's 00:14:26.880 --> 00:14:30.430 feeling a stronger pull, all the electrons are being pulled 00:14:30.430 --> 00:14:33.860 in closer to the nucleus, so that atomic size is going to 00:14:33.860 --> 00:14:35.530 get smaller. 00:14:35.530 --> 00:14:38.530 This is in contrast to what's happening as we go down a 00:14:38.530 --> 00:14:39.800 periodic table. 00:14:39.800 --> 00:14:42.660 So as we go down we're now adding electrons to further 00:14:42.660 --> 00:14:45.240 and further away shells, so what we're going to see is 00:14:45.240 --> 00:14:47.580 that the atomic radius is going to increase as we're 00:14:47.580 --> 00:14:52.290 going down the periodic table. 00:14:52.290 --> 00:14:54.610 And we can look at an example here. 00:14:54.610 --> 00:14:57.410 If we start in the upper left hand corner of the periodic 00:14:57.410 --> 00:15:00.470 table with lithium, you can see that as we go down the 00:15:00.470 --> 00:15:03.270 table, what you're seeing is that that atomic radius is 00:15:03.270 --> 00:15:05.860 actually increasing, as we would expect. 00:15:05.860 --> 00:15:09.060 Whereas, if we go across a row, what we see is that the 00:15:09.060 --> 00:15:11.330 atomic radius is decreasing. 00:15:11.330 --> 00:15:14.210 So, again, this is one of the more straightforward trends. 00:15:14.210 --> 00:15:15.960 You just need to remember what's happening to z 00:15:15.960 --> 00:15:18.450 effective, which really tells us what's happening with all 00:15:18.450 --> 00:15:21.170 the trends, and once you know z effective, you can figure 00:15:21.170 --> 00:15:24.140 out, for example, what direction the atomic radius 00:15:24.140 --> 00:15:26.130 should be going into. 00:15:26.130 --> 00:15:28.190 So, that's it for periodic trends. 00:15:28.190 --> 00:15:29.950 We have talked about four different ones. 00:15:29.950 --> 00:15:33.780 We talked about ionization energy, electron affinity, we 00:15:33.780 --> 00:15:35.530 talked about electronegativity, which is 00:15:35.530 --> 00:15:38.610 just kind of a combination of the first two, and then ended 00:15:38.610 --> 00:15:41.190 with atomic radius here. 00:15:41.190 --> 00:15:44.190 And what you might have noted is although we described how 00:15:44.190 --> 00:15:46.950 to make predictions about these properties, I didn't 00:15:46.950 --> 00:15:49.090 talk too much about what it actually means, what the 00:15:49.090 --> 00:15:51.680 ramifications of these different properties are. 00:15:51.680 --> 00:15:53.830 And the reason we didn't do that is because we're actually 00:15:53.830 --> 00:15:56.940 going to spend much of the rest of the course relating 00:15:56.940 --> 00:16:00.360 these different properties to the properties of molecules in 00:16:00.360 --> 00:16:03.950 terms of bonding, and also in terms of chemical reactions. 00:16:03.950 --> 00:16:07.890 So, for example, if we have a very electronegative atom 00:16:07.890 --> 00:16:10.820 within a certain molecule, what you'll actually find is 00:16:10.820 --> 00:16:15.740 that it does affect how the molecule is going to take part 00:16:15.740 --> 00:16:17.820 in different chemical or biological reactions. 00:16:17.820 --> 00:16:20.420 And this will become more and more clear as we actually talk 00:16:20.420 --> 00:16:22.740 about these reactions and talk about bonding. 00:16:22.740 --> 00:16:24.930 But you need to be able to predict what kind of 00:16:24.930 --> 00:16:27.690 properties a certain atom's going to have within a 00:16:27.690 --> 00:16:29.500 molecule, whether you're talking about something, for 00:16:29.500 --> 00:16:31.730 example, that's very electronegative, or something 00:16:31.730 --> 00:16:33.960 that is not electronegative at all, it is going to make a 00:16:33.960 --> 00:16:36.560 difference in terms of thinking about how molecules 00:16:36.560 --> 00:16:37.790 are structured and also how they 00:16:37.790 --> 00:16:41.310 interact with other molecules. 00:16:41.310 --> 00:16:43.780 However, I can give you at least one example while we're 00:16:43.780 --> 00:16:47.100 still on just talking about atoms. So we haven't gotten to 00:16:47.100 --> 00:16:50.150 molecules yet, we're just talking about single atoms or 00:16:50.150 --> 00:16:53.350 single ions, but what's nice is just talking about this 00:16:53.350 --> 00:16:56.130 very straightforward principle of atomic radius. 00:16:56.130 --> 00:17:00.110 We can already use that in terms of single ions to think 00:17:00.110 --> 00:17:03.240 about a really complex biological issue, which is to 00:17:03.240 --> 00:17:05.030 talk about ion channels. 00:17:05.030 --> 00:17:07.310 So, that is just a quick example for some of you, you 00:17:07.310 --> 00:17:10.070 might be very familiar with ion channels, others might not 00:17:10.070 --> 00:17:12.870 know what these are, so I'll just tell you quite briefly 00:17:12.870 --> 00:17:15.570 that ion channels are these very massive 00:17:15.570 --> 00:17:17.540 transmembrane proteins. 00:17:17.540 --> 00:17:21.600 Essentially, what they are is it's a protein that spans the 00:17:21.600 --> 00:17:23.760 membrane of a cell. 00:17:23.760 --> 00:17:26.560 And what they do is they regulate the influx of ions 00:17:26.560 --> 00:17:27.920 across that cell. 00:17:27.920 --> 00:17:30.420 So the influx of ions from the outside of the cell to the 00:17:30.420 --> 00:17:32.440 inside of the cell, for example. 00:17:32.440 --> 00:17:35.980 And you can think of ion channels as being gated, by 00:17:35.980 --> 00:17:39.340 gated it means the gate can be closed and no ions are going 00:17:39.340 --> 00:17:41.500 through, as in this case here. 00:17:41.500 --> 00:17:45.660 Or you can talk about the gate being open, and in this case, 00:17:45.660 --> 00:17:48.670 you can see that you will have an influx of ions. 00:17:48.670 --> 00:17:52.060 So, ion channels are important for maintaining a voltage 00:17:52.060 --> 00:17:54.820 difference between the inside of the cell and outside of the 00:17:54.820 --> 00:17:57.560 cell, and they're found in all sorts of cell types in your 00:17:57.560 --> 00:18:00.520 body, but if you think about where they're most prevalent, 00:18:00.520 --> 00:18:04.380 it turns out that they're most prevalent in muscle cells and 00:18:04.380 --> 00:18:07.360 also in nerve cells, so in your neurons. 00:18:07.360 --> 00:18:10.810 And essentially, what they do in neurons is they underlie 00:18:10.810 --> 00:18:14.250 those nerve impulses, or those, essentially what we 00:18:14.250 --> 00:18:17.450 call electrical signaling between neurons -- you might 00:18:17.450 --> 00:18:21.020 also call that the action potential of the neurons. 00:18:21.020 --> 00:18:24.510 So essentially, they regulate this action potential, and 00:18:24.510 --> 00:18:28.160 they do so by helping to establish and then control the 00:18:28.160 --> 00:18:30.810 voltage gradient within the cells. so, essentially, 00:18:30.810 --> 00:18:34.070 they're establishing or controlling or changing the 00:18:34.070 --> 00:18:37.640 difference between the charge inside the cell and the charge 00:18:37.640 --> 00:18:39.480 outside cell. 00:18:39.480 --> 00:18:42.320 And when we talk about any type of ion channel, there are 00:18:42.320 --> 00:18:44.590 just tons of different kinds of ion channels, and you can 00:18:44.590 --> 00:18:46.620 characterize them in a few different ways. 00:18:46.620 --> 00:18:48.700 So, for example, you could characterize them in terms of 00:18:48.700 --> 00:18:51.370 how they're gated, and basically how they open or 00:18:51.370 --> 00:18:54.030 close -- that's one way to talk about different types. 00:18:54.030 --> 00:18:56.420 Another way to talk about different types is to think 00:18:56.420 --> 00:18:58.920 about which ion they're selected for. 00:18:58.920 --> 00:19:02.090 And all ion channels are selective for a single type of 00:19:02.090 --> 00:19:06.030 ion, and we can think about how that selectivity takes 00:19:06.030 --> 00:19:08.640 place, and that's where this idea of atomic radius is going 00:19:08.640 --> 00:19:10.280 to become very important. 00:19:10.280 --> 00:19:13.780 So, for example, if we look at sodium channels, and sodium 00:19:13.780 --> 00:19:16.340 channels are some of the particularly prevalent ones 00:19:16.340 --> 00:19:19.010 when we're talking about neurons, if you think about 00:19:19.010 --> 00:19:22.880 the cell membrane, and this little green cartoon is me 00:19:22.880 --> 00:19:26.090 trying to show a sodium channel here, and in this 00:19:26.090 --> 00:19:28.910 case, you can see that it's closed, such that no ions are 00:19:28.910 --> 00:19:30.080 getting through. 00:19:30.080 --> 00:19:33.240 However, when that gate is opened, the sodium channel is 00:19:33.240 --> 00:19:36.570 now going to be incredibly selective and only let through 00:19:36.570 --> 00:19:39.320 sodium ions and no other type of ion. 00:19:39.320 --> 00:19:41.480 And this is really interesting to think about because you can 00:19:41.480 --> 00:19:44.300 imagine in our body we have concentrations of all types of 00:19:44.300 --> 00:19:47.780 ions, and specifically, some seem very, very similar to 00:19:47.780 --> 00:19:48.460 each other. 00:19:48.460 --> 00:19:51.520 So we could think about comparing the potassium ion to 00:19:51.520 --> 00:19:52.850 a sodium ion. 00:19:52.850 --> 00:19:54.740 They have the same charge of plus one. 00:19:54.740 --> 00:19:57.130 The only thing that's different is that they're one 00:19:57.130 --> 00:20:02.000 down on the periodic table, potassium is down one row, so 00:20:02.000 --> 00:20:03.890 it's going to be a little bigger, but when we're 00:20:03.890 --> 00:20:06.900 thinking about size, it maybe does not seem that significant 00:20:06.900 --> 00:20:08.370 to talk about the size. 00:20:08.370 --> 00:20:10.500 But what we find out is that it is. 00:20:10.500 --> 00:20:13.690 So, what happens, this is another view of a sodium 00:20:13.690 --> 00:20:15.930 channel, so this is actually looking a little bit more at 00:20:15.930 --> 00:20:17.330 the protein structure. 00:20:17.330 --> 00:20:19.980 What all of these channels have is what's called a 00:20:19.980 --> 00:20:23.100 selectivity filter, so this filter filters out the type of 00:20:23.100 --> 00:20:25.990 ion that's going to be allowed through. 00:20:25.990 --> 00:20:27.560 And there's two parts of the filter. 00:20:27.560 --> 00:20:30.310 First we need to select for actual charge. 00:20:30.310 --> 00:20:32.780 So the way that it does this is the filter is actually 00:20:32.780 --> 00:20:35.490 lined with all of this negative charge, and for those 00:20:35.490 --> 00:20:38.330 of you that are more into biology or biochemistry, 00:20:38.330 --> 00:20:41.020 that's because of negatively charged amino acid residues, 00:20:41.020 --> 00:20:44.680 but all you need to think about is that it has negative 00:20:44.680 --> 00:20:48.360 charge in the inside of this pore, and what happens then is 00:20:48.360 --> 00:20:50.650 that if something has a positive charge, it's going to 00:20:50.650 --> 00:20:54.000 be stabilized to enter this pore, whereas any negative 00:20:54.000 --> 00:20:55.470 ions are going to be repelled. 00:20:55.470 --> 00:20:58.510 So that's the first step in being selective, but now how 00:20:58.510 --> 00:20:59.920 do we differentiate between these sodium 00:20:59.920 --> 00:21:00.920 and potassium ions. 00:21:00.920 --> 00:21:04.640 And the answer is just really beautifully simple, and it's 00:21:04.640 --> 00:21:07.340 just that the pore gets really, really tiny, to the 00:21:07.340 --> 00:21:10.000 point that it gets so small that all that can fit through 00:21:10.000 --> 00:21:14.880 this pore is one a single ion, one single sodium ion, 00:21:14.880 --> 00:21:18.760 solvated by one single water molecule. 00:21:18.760 --> 00:21:21.360 And that's all that's big enough to pass through or 00:21:21.360 --> 00:21:23.120 small enough to pass through. 00:21:23.120 --> 00:21:26.820 And if we go up even just one row on the periodic table to 00:21:26.820 --> 00:21:30.200 potassium, what we actually see is now that it's going to 00:21:30.200 --> 00:21:33.070 be too large, and, in fact, a potassium solvated with one 00:21:33.070 --> 00:21:36.620 water molecule won't go through our channel. 00:21:36.620 --> 00:21:39.480 So, this is just one example of how these properties can 00:21:39.480 --> 00:21:41.760 already, even our understanding just talking 00:21:41.760 --> 00:21:44.760 about single atoms, can already make an impact in 00:21:44.760 --> 00:21:47.600 these biological systems. And actually, a question that 00:21:47.600 --> 00:21:50.400 might come up, I just explained, the sodium channel, 00:21:50.400 --> 00:21:53.260 you might say, well, how do potassium channels work then, 00:21:53.260 --> 00:21:55.830 because I can understand how you can filter something big 00:21:55.830 --> 00:21:57.820 out, but how do you filter out something small. 00:21:57.820 --> 00:22:00.590 And it turns out that size exclusion is also the 00:22:00.590 --> 00:22:04.420 principle that's in play with potassium channels as well, 00:22:04.420 --> 00:22:06.800 but in this case it's a little more complex, because what 00:22:06.800 --> 00:22:10.780 happens is these negative residues the are in the pore 00:22:10.780 --> 00:22:13.120 need to stabilize the potassium as it goes through, 00:22:13.120 --> 00:22:16.920 and the potassium is large enough to make all the 00:22:16.920 --> 00:22:19.930 contacts it needs, but the sodium, which you can picture 00:22:19.930 --> 00:22:23.410 being smaller, actually can't reach all of the stabilizing 00:22:23.410 --> 00:22:25.770 charges that it needs to to get through the pore. 00:22:25.770 --> 00:22:28.380 So, again, it is based on size, it's a little bit less 00:22:28.380 --> 00:22:30.790 intuitive than the idea of just straining out all of the 00:22:30.790 --> 00:22:31.340 potassium ions. 00:22:31.340 --> 00:22:34.950 But again, many of these ion channels have this size 00:22:34.950 --> 00:22:37.700 exclusion pore, it's a very important part of them. 00:22:37.700 --> 00:22:41.890 All right, so that was a quick aside on thinking about how 00:22:41.890 --> 00:22:43.960 these properties can, in fact, relate to 00:22:43.960 --> 00:22:45.800 something in our body. 00:22:45.800 --> 00:22:49.340 Let's move on to the last topic in terms of this first 00:22:49.340 --> 00:22:51.650 exam, which is thinking about the idea of isoelectronic 00:22:51.650 --> 00:22:56.000 atoms, or isoelectronic ions. 00:22:56.000 --> 00:22:58.950 And isoelectronic is very straightforward, it just means 00:22:58.950 --> 00:23:01.520 having the same electron configuration. 00:23:01.520 --> 00:23:04.890 The easiest way to look at this is just to do an example. 00:23:04.890 --> 00:23:07.220 So let's take the example of neon. 00:23:07.220 --> 00:23:10.680 This has the electron configuration of 1 s 2, 2 00:23:10.680 --> 00:23:12.530 s 2, and 2 p 6. 00:23:12.530 --> 00:23:16.140 It looks like we're cut off the screen a little bit here, 00:23:16.140 --> 00:23:18.600 but you can see I've just circled it there. 00:23:18.600 --> 00:23:21.150 So we can go ahead and think about, well, are there any 00:23:21.150 --> 00:23:23.130 other atoms that are going to have the same electron 00:23:23.130 --> 00:23:23.790 configuration? 00:23:23.790 --> 00:23:26.110 The answer to that is definitely no -- if they had 00:23:26.110 --> 00:23:27.732 the same electron configuration, they would, in 00:23:27.732 --> 00:23:29.200 fact, be neon. 00:23:29.200 --> 00:23:31.590 But we can think about different ions that have this 00:23:31.590 --> 00:23:33.070 electron configuration. 00:23:33.070 --> 00:23:36.510 So for example, if we think about fluorine, that has an 00:23:36.510 --> 00:23:41.570 electron configuration of 1 s 2, 2 s 2, 2 p 5, so all we 00:23:41.570 --> 00:23:44.210 would need to do is add one more electron to get the same 00:23:44.210 --> 00:23:46.270 configuration as for neon. 00:23:46.270 --> 00:23:49.300 So if we want to write out what that would be, it would 00:23:49.300 --> 00:23:55.980 just be to say that f minus is isoelectronic with neon. 00:23:55.980 --> 00:24:00.770 So, we can say that -- if we have neon here and we want to 00:24:00.770 --> 00:24:03.860 think about what's isoelectronic, f minus would 00:24:03.860 --> 00:24:04.170 be isoelectronic. 00:24:04.170 --> 00:24:10.230 We also have oxygen -- what would the charge on oxygen be? 00:24:10.230 --> 00:24:10.690 Um-hmm, right. 00:24:10.690 --> 00:24:12.240 2 minus. 00:24:12.240 --> 00:24:16.110 Then also, nitrogen, 3 minus -- these are all going to be 00:24:16.110 --> 00:24:18.080 isoelectronic with neon. 00:24:18.080 --> 00:24:20.540 We can go in the other direction, so let's go to 00:24:20.540 --> 00:24:24.020 sodium, but we would need to take away an electron to make 00:24:24.020 --> 00:24:25.670 it isoelectronic. 00:24:25.670 --> 00:24:30.710 So we would say sodium plus, or magnesium 2 plus, we can 00:24:30.710 --> 00:24:36.960 just keep going -- aluminum 3 plus, silicone 4 plus, and we 00:24:36.960 --> 00:24:40.080 can go on and on in either direction all the way across 00:24:40.080 --> 00:24:42.670 and down the periodic table. 00:24:42.670 --> 00:24:45.380 So, that's the idea of isoelectronic ions. 00:24:45.380 --> 00:24:48.260 These are all isoelectronic, they all have the same 00:24:48.260 --> 00:24:50.980 electron configuration. 00:24:50.980 --> 00:24:53.110 And we can also think about going back to 00:24:53.110 --> 00:24:54.830 atomic size for a second. 00:24:54.830 --> 00:24:58.130 What the relationship is between these ions and their 00:24:58.130 --> 00:25:03.470 parent atoms. So, for example, if we think of the fluorine 00:25:03.470 --> 00:25:08.190 minus case, would you expect fluorine minus to be larger or 00:25:08.190 --> 00:25:11.940 smaller than neutral fluorine? 00:25:11.940 --> 00:25:12.350 Okay. 00:25:12.350 --> 00:25:14.840 I heard mostly larger, but a little bit of a mix in there, 00:25:14.840 --> 00:25:17.780 and it turns out that larger is correct. 00:25:17.780 --> 00:25:20.750 And we can think about why -- essentially we have fluorine 00:25:20.750 --> 00:25:22.980 and now we're adding another electron. 00:25:22.980 --> 00:25:25.260 So you can picture that fluorine is going to get 00:25:25.260 --> 00:25:27.060 larger in this case. 00:25:27.060 --> 00:25:28.330 And that would be true for all of the 00:25:28.330 --> 00:25:30.320 negatively charged ions. 00:25:30.320 --> 00:25:32.970 So, by the same logic, that means that all of our 00:25:32.970 --> 00:25:36.330 positively charged ions are, in fact, going to be smaller 00:25:36.330 --> 00:25:39.370 in terms of radius, compared to their neutral parents. 00:25:39.370 --> 00:25:42.640 Not only are we taking away an electron here, but we're also 00:25:42.640 --> 00:25:45.810 going to decrease shielding, so the electrons that are 00:25:45.810 --> 00:25:48.220 already in there are going to feel a higher z effective and 00:25:48.220 --> 00:25:53.640 will be pulling and the atom will be getting smaller. 00:25:53.640 --> 00:25:56.670 And this is just a picture showing some of these sizes 00:25:56.670 --> 00:25:57.500 with their parent. 00:25:57.500 --> 00:26:01.620 So, for example, a lithium here, you can see how lithium 00:26:01.620 --> 00:26:05.200 plus is smaller than the actual lithium atom in its 00:26:05.200 --> 00:26:06.190 neutral state. 00:26:06.190 --> 00:26:10.610 Whereas for fluorine, fluorine is smaller than f minus is the 00:26:10.610 --> 00:26:13.570 one that's the outer shell shown here. 00:26:13.570 --> 00:26:15.960 So, let's do a clicker question on isoelectronic 00:26:15.960 --> 00:26:22.450 atoms. And now we're asking you to look at krypton, so the 00:26:22.450 --> 00:26:24.240 atomic mass is 36. 00:26:24.240 --> 00:26:26.760 You can actually just grab that handout, the second 00:26:26.760 --> 00:26:29.830 handout on the exam and look at the periodic table there. 00:26:29.830 --> 00:26:32.200 So, which of the following ions listed is isoelectronic 00:26:32.200 --> 00:26:43.950 with krypton? 00:26:43.950 --> 00:26:58.050 OK, let's take 10 seconds on that. 00:26:58.050 --> 00:26:58.520 OK, good. 00:26:58.520 --> 00:27:03.170 This might be our all-time high, 89% got this right. 00:27:03.170 --> 00:27:04.170 This is great. 00:27:04.170 --> 00:27:07.980 So, selenium 2 minus is what's going to be isoelectronic, 00:27:07.980 --> 00:27:10.610 because if you add two electrons to selenium, you'll 00:27:10.610 --> 00:27:13.490 get the same electron configuration that you have 00:27:13.490 --> 00:27:14.470 for krypton here. 00:27:14.470 --> 00:27:18.070 OK, I think we can safely go back to notes. 00:27:18.070 --> 00:27:20.345 So, I said I would announce it, that's the 00:27:20.345 --> 00:27:22.000 end of exam 1 material. 00:27:22.000 --> 00:27:24.350 So, if you compartmentalize things in your brain in 00:27:24.350 --> 00:27:28.250 certain ways, put that off into the end of the exam 1 00:27:28.250 --> 00:27:29.470 part of your brain, and now we're going to 00:27:29.470 --> 00:27:31.010 move on to exam 2. 00:27:31.010 --> 00:27:33.220 Remember, for exam 2, you still need to know and 00:27:33.220 --> 00:27:36.290 understand everything you learned in exam 1, but you can 00:27:36.290 --> 00:27:41.400 put off learning it completely until we get through, at 00:27:41.400 --> 00:27:44.660 least, next Wednesday before we start maybe spending time 00:27:44.660 --> 00:27:47.580 on these concepts outside of class. 00:27:47.580 --> 00:27:51.035 So, we're going to start with talking about bonding, and any 00:27:51.035 --> 00:27:54.090 time we have a chemical bond, basically what we're talking 00:27:54.090 --> 00:27:56.860 about is having two atoms where the arrangement of their 00:27:56.860 --> 00:28:00.640 nuclei and they're electrons are such that the bonded atoms 00:28:00.640 --> 00:28:04.720 results in a lower energy than for the separate atoms. So we 00:28:04.720 --> 00:28:07.480 know we always want to have our systems in as low an 00:28:07.480 --> 00:28:10.450 energy as possible, so it makes sense that a bond would 00:28:10.450 --> 00:28:13.740 happen any time we got a lower energy when we combine two 00:28:13.740 --> 00:28:18.110 atoms, versus when we keep them separate. 00:28:18.110 --> 00:28:20.290 So specifically, today we're going to talk 00:28:20.290 --> 00:28:22.020 about covalent bonds. 00:28:22.020 --> 00:28:25.560 A covalent bond is any time we have a pair of electrons that 00:28:25.560 --> 00:28:28.470 is shared between two different atoms. And the key 00:28:28.470 --> 00:28:31.900 word for covalent bonds is the idea of being shared. 00:28:31.900 --> 00:28:37.130 The two electrons, for example, we see in the h 2 00:28:37.130 --> 00:28:39.830 molecule, they don't belong to one or the other atom, they're 00:28:39.830 --> 00:28:41.050 actually shared. 00:28:41.050 --> 00:28:44.040 And what we'll see later, is that the sharing is not always 00:28:44.040 --> 00:28:46.180 equal -- in the case of h 2, it is 00:28:46.180 --> 00:28:47.420 completely equal sharing. 00:28:47.420 --> 00:28:50.950 In some cases, because of things like electronegativity, 00:28:50.950 --> 00:28:53.970 one atom will take away more of the electron density than 00:28:53.970 --> 00:28:56.490 the other atom, but they're still shared, even if they're 00:28:56.490 --> 00:28:58.710 not always evenly shared. 00:28:58.710 --> 00:29:01.350 So, in talking about covalent bonds, we should be able to 00:29:01.350 --> 00:29:04.920 still apply a more general definition of a chemical bond, 00:29:04.920 --> 00:29:08.270 which should tell us that the h 2 molecule is going to be 00:29:08.270 --> 00:29:11.990 lower in energy than if we looked at 2 separate hydrogen 00:29:11.990 --> 00:29:12.870 atom molecules. 00:29:12.870 --> 00:29:15.480 So, let's see if that's actually the case. 00:29:15.480 --> 00:29:18.150 So if I tell you that the energy for single hydrogen 00:29:18.150 --> 00:29:22.540 atom is negative 13 12 kilojoules per mole. 00:29:22.540 --> 00:29:25.240 If we want to talk about two hydrogen atoms, then we just 00:29:25.240 --> 00:29:29.130 need to double that, so that's going to be negative 2 6 2 4 00:29:29.130 --> 00:29:32.230 kilojoules per mole that we're talking about in terms of a 00:29:32.230 --> 00:29:33.940 single hydrogen atom. 00:29:33.940 --> 00:29:37.310 So, let's compare this to the energy of the h 2 molecule, 00:29:37.310 --> 00:29:39.630 and we find that that's negative 3,048 00:29:39.630 --> 00:29:40.380 kilojoules per mole. 00:29:40.380 --> 00:29:45.130 So, in fact, yes, we did confirm that these covalent 00:29:45.130 --> 00:29:48.610 bond, at least in the case of hydrogen, we have confirmed by 00:29:48.610 --> 00:29:51.510 the numbers that we are at a lower energy state when we 00:29:51.510 --> 00:29:55.660 talk about the bonded atom versus the individual atom. 00:29:55.660 --> 00:29:58.460 And when we talk about covalent bonds, there's 2 00:29:58.460 --> 00:30:00.990 properties that we'll mostly focus on, and that's going to 00:30:00.990 --> 00:30:04.860 be thinking about the bond strength or the energy by 00:30:04.860 --> 00:30:07.450 which it stabilized when it bonds. 00:30:07.450 --> 00:30:10.140 And we can also talk about the bond length, so we might be 00:30:10.140 --> 00:30:13.270 interested in what the bond length is, what the distance 00:30:13.270 --> 00:30:15.450 between these two nuclei are. 00:30:15.450 --> 00:30:18.910 And we can actually better visualize this if we plot how 00:30:18.910 --> 00:30:21.540 that energy changes as a function of 00:30:21.540 --> 00:30:24.180 internuclear distance. 00:30:24.180 --> 00:30:27.000 And when I say internuclear distance, we actually call 00:30:27.000 --> 00:30:28.670 this r here. 00:30:28.670 --> 00:30:31.020 It's kind of ironic that we put this in the same lecture 00:30:31.020 --> 00:30:34.320 as we talk about atomic radii, which we also call r, but 00:30:34.320 --> 00:30:36.260 they're two different r's, so you need to keep them 00:30:36.260 --> 00:30:38.760 separated in terms of what you're talking about. 00:30:38.760 --> 00:30:42.400 When we're talking about r for internuclear distance, we're 00:30:42.400 --> 00:30:45.180 talking about the distance between two different nuclei 00:30:45.180 --> 00:30:48.900 in a bond, in a covalent bond. 00:30:48.900 --> 00:30:52.000 So, if we look at this graph where what we're charting is 00:30:52.000 --> 00:30:54.820 the internuclear distance, so the distance between these two 00:30:54.820 --> 00:30:58.700 hydrogen atoms, as a function of energy, what we are going 00:30:58.700 --> 00:31:00.810 to see is a curve that looks like this -- this is the 00:31:00.810 --> 00:31:04.430 general curve that you'll see for any covalent bond, and 00:31:04.430 --> 00:31:07.020 we'll explain where that comes from in a minute. 00:31:07.020 --> 00:31:10.430 I want to point out that the zero energy is defined as when 00:31:10.430 --> 00:31:15.040 you have a naked proton where the electron has popped out -- 00:31:15.040 --> 00:31:17.970 that's what we've defined as zero energy up to this point 00:31:17.970 --> 00:31:20.740 when we're talking about single atoms. So, for starters 00:31:20.740 --> 00:31:22.880 we'll keep that as our zero energy, we're going to change 00:31:22.880 --> 00:31:26.150 it soon to make something that makes more sense in terms of 00:31:26.150 --> 00:31:28.610 bonding, but we'll keep that as zero for now. 00:31:28.610 --> 00:31:32.370 So, we see that the two h atoms separate have a certain 00:31:32.370 --> 00:31:34.650 energy that's lower than when the electron's 00:31:34.650 --> 00:31:36.040 not with the atom. 00:31:36.040 --> 00:31:38.650 And then even lower down, we have our 00:31:38.650 --> 00:31:42.120 bonded hydrogen molecule. 00:31:42.120 --> 00:31:43.940 So, we can think about the different kinds of 00:31:43.940 --> 00:31:45.620 interactions that are taking place. 00:31:45.620 --> 00:31:48.820 I said what hold the bonds together, what holds two atoms 00:31:48.820 --> 00:31:52.510 together is the attractive force we have between each 00:31:52.510 --> 00:31:55.200 electron and the other nucleus. 00:31:55.200 --> 00:31:57.840 That's the huge force that we're talking about in terms 00:31:57.840 --> 00:32:01.120 of making a bond stable, but there are also repulsive 00:32:01.120 --> 00:32:03.360 forces, so you can imagine we're going to have 00:32:03.360 --> 00:32:06.800 electron-electron repulsion between the two electrons if 00:32:06.800 --> 00:32:08.460 we're bringing them closer together. 00:32:08.460 --> 00:32:11.370 And the real killer is if we get too close we're even going 00:32:11.370 --> 00:32:14.750 to have nuclear-nuclear repulsion between the nuclei 00:32:14.750 --> 00:32:16.320 of the two atoms. 00:32:16.320 --> 00:32:19.090 So, this makes this chart shown in pink make a lot more 00:32:19.090 --> 00:32:22.720 sense, because if we're way out at very far distances, 00:32:22.720 --> 00:32:25.310 essentially what we have here is we're talking about two 00:32:25.310 --> 00:32:28.270 separate atoms. They're not interacting at all so that's 00:32:28.270 --> 00:32:31.870 why the energy is the same as that for two individual atoms, 00:32:31.870 --> 00:32:33.430 that's what we're dealing with. 00:32:33.430 --> 00:32:36.350 As we get closer together, we start get lower 00:32:36.350 --> 00:32:38.040 and lower in energy. 00:32:38.040 --> 00:32:40.550 The reason is because the predominant force at this 00:32:40.550 --> 00:32:43.400 point is going to be the attraction that's being felt 00:32:43.400 --> 00:32:46.930 between the nuclei and the electrons in each of the 00:32:46.930 --> 00:32:51.120 atoms. At some point you're going to hit a well here, 00:32:51.120 --> 00:32:56.140 which is the point where it's most stabilized or at it's 00:32:56.140 --> 00:32:56.520 lowest energy. 00:32:56.520 --> 00:32:58.790 So, when we think about a bond length, this is going to be 00:32:58.790 --> 00:33:01.470 the length of our bond here, that makes sense because it's 00:33:01.470 --> 00:33:03.320 going to want to be at that distance that 00:33:03.320 --> 00:33:05.140 minimizes the energy. 00:33:05.140 --> 00:33:08.010 But as we keep getting closer, even though as we get closer, 00:33:08.010 --> 00:33:10.390 the attraction is going to get stronger between the two 00:33:10.390 --> 00:33:11.990 nuclei and the electrons. 00:33:11.990 --> 00:33:14.610 We're also going to start to have the repulsive forces 00:33:14.610 --> 00:33:17.640 become more prominent here, and, in fact, they take over 00:33:17.640 --> 00:33:21.050 at some point, becoming the more prevalent of the forces, 00:33:21.050 --> 00:33:22.910 so as you get closer, the electron-electron repulsions, 00:33:22.910 --> 00:33:26.950 and eventually the nucleus-nucleus repulsion is 00:33:26.950 --> 00:33:29.070 going to mean that your energy is just absolutely 00:33:29.070 --> 00:33:32.550 skyrocketing, so it just keeps going up and up as you get 00:33:32.550 --> 00:33:36.300 closer to zero here. 00:33:36.300 --> 00:33:39.030 So, when we want to talk about the information that we can 00:33:39.030 --> 00:33:41.480 get out of looking at a chart like this, well, the first 00:33:41.480 --> 00:33:43.720 thing I did tell you was that this is going to be the bond 00:33:43.720 --> 00:33:46.890 length, so the distance r where the energy is lowest, 00:33:46.890 --> 00:33:49.720 but we can also talk about something called dissociation 00:33:49.720 --> 00:33:54.170 energy, that's going to be this distance right here or 00:33:54.170 --> 00:33:56.520 the energy that is this value. 00:33:56.520 --> 00:33:59.790 And the dissociation energy is very intuitive in terms of 00:33:59.790 --> 00:34:02.570 what it means, it means how much energy you need to put 00:34:02.570 --> 00:34:05.320 into the molecule in order to disassociate it into its 00:34:05.320 --> 00:34:09.650 individual atoms. And so we can actually think about how 00:34:09.650 --> 00:34:12.460 do we calculate what the dissociation energy should be 00:34:12.460 --> 00:34:16.270 for h 2, so let's go ahead and do this. 00:34:16.270 --> 00:34:20.760 So, if we talk about dissociating h 2, we're going 00:34:20.760 --> 00:34:25.260 from the h 2 molecule, and breaking this bond right in 00:34:25.260 --> 00:34:28.540 half, so we now have two individual 00:34:28.540 --> 00:34:31.090 hydrogen atoms here. 00:34:31.090 --> 00:34:35.020 So we need to take the energy for the two atoms, which we 00:34:35.020 --> 00:34:39.070 know is -- so let's take our dissociation energy is going 00:34:39.070 --> 00:34:48.360 to be equal to negative 2 6 2 4 kilojoules per mole, and we 00:34:48.360 --> 00:34:52.420 want to subtract the energy of the hydrogen molecule itself, 00:34:52.420 --> 00:35:00.560 so that's going to be negative 3 0 4 8 kilojoules per mole. 00:35:00.560 --> 00:35:03.770 So, what we get for the disassociation energy for a 00:35:03.770 --> 00:35:11.880 hydrogen atom is 424 kilojoules per mole. 00:35:11.880 --> 00:35:14.680 So what that means is that's how much energy we would have 00:35:14.680 --> 00:35:17.890 to put in to a hydrogen molecule in order to get it to 00:35:17.890 --> 00:35:25.110 split apart into its two atoms. 00:35:25.110 --> 00:35:28.280 So, another way to talk about dissociation energy is simply 00:35:28.280 --> 00:35:32.150 to call it bond strength, it's the same thing, they're equal 00:35:32.150 --> 00:35:32.690 to each other. 00:35:32.690 --> 00:35:36.150 If we know that this is it the dissociation energy for a 00:35:36.150 --> 00:35:39.250 hydrogen atom, we can also say the bond strength for hydrogen 00:35:39.250 --> 00:35:43.090 molecule is 424. 00:35:43.090 --> 00:35:45.520 So, there's actually another way to graph it where we can 00:35:45.520 --> 00:35:48.550 directly graph the dissociation energy or the 00:35:48.550 --> 00:35:49.620 bond strengths. 00:35:49.620 --> 00:35:53.000 So I said before when we were talking about single atoms, we 00:35:53.000 --> 00:35:55.770 always define the zero energy as when an electron was 00:35:55.770 --> 00:36:00.120 actually ejected, but now, when we talk about chemical 00:36:00.120 --> 00:36:03.010 reactions taking place, it's very, very rare that we're 00:36:03.010 --> 00:36:05.940 actually going to be talking about anything that gets to 00:36:05.940 --> 00:36:06.660 this point here. 00:36:06.660 --> 00:36:10.990 It's much more relevant to set our zero point energy as the 00:36:10.990 --> 00:36:14.790 separation of a bond in terms of talking about the reactions 00:36:14.790 --> 00:36:16.850 that we'll usually be dealing with here. 00:36:16.850 --> 00:36:20.300 So, let's change our graph where we now have this zero 00:36:20.300 --> 00:36:23.700 point set as the two individuals hydrogen atoms, 00:36:23.700 --> 00:36:27.780 and then we see that our h 2 molecule is at the negative of 00:36:27.780 --> 00:36:30.640 the dissociation energy, or the negative what that bond 00:36:30.640 --> 00:36:31.600 strength is. 00:36:31.600 --> 00:36:33.630 So we know what that number would be, it would be negative 00:36:33.630 --> 00:36:37.710 424 kilojoules per mole that we see here. 00:36:37.710 --> 00:36:41.380 So, what this let's us do now is directly compare, for 00:36:41.380 --> 00:36:45.420 example, the strength of a bond in terms of a hydrogen 00:36:45.420 --> 00:36:48.760 atom and hydrogen molecule, compared to any kind of 00:36:48.760 --> 00:36:51.350 molecule that we want to graph on top of it. 00:36:51.350 --> 00:36:54.040 So, let's, for example, look at nitrogen. 00:36:54.040 --> 00:36:58.020 So n 2, we can do the chart here in green, so it's the 00:36:58.020 --> 00:37:02.120 green dotted line, and what we see is that we have now 00:37:02.120 --> 00:37:04.000 defined this energy as where the 2 00:37:04.000 --> 00:37:05.870 nitrogen atoms are separated. 00:37:05.870 --> 00:37:08.710 So what we can actually directly compare is the 00:37:08.710 --> 00:37:12.230 dissociation energy or the bond strength of nitrogen 00:37:12.230 --> 00:37:14.350 versus hydrogen. 00:37:14.350 --> 00:37:16.960 So, if we think about this, which would you say has a 00:37:16.960 --> 00:37:17.930 stronger bond? 00:37:17.930 --> 00:37:21.810 Is it going to be hydrogen or nitrogen? 00:37:21.810 --> 00:37:23.530 Yup, it's going to be nitrogen. 00:37:23.530 --> 00:37:26.140 And the reason we can see that by looking at this graph is 00:37:26.140 --> 00:37:28.940 that we see that nitrogen when it's bonded is in an even 00:37:28.940 --> 00:37:31.200 lower well than we saw for hydrogen. 00:37:31.200 --> 00:37:33.420 It's going to be a stronger bond because it's more 00:37:33.420 --> 00:37:38.130 stabilized when it when it comes together as a molecule. 00:37:38.130 --> 00:37:42.020 We can also think about the distance, the bond distance. 00:37:42.020 --> 00:37:43.350 So, which would you say is going to be 00:37:43.350 --> 00:37:44.350 shorter in this case? 00:37:44.350 --> 00:37:47.980 Is a hydrogen bond shorter, or is a nitrogen-nitrogen triple 00:37:47.980 --> 00:37:50.310 bond going to be shorter? 00:37:50.310 --> 00:37:52.790 Um-hmm, again, we can get this information 00:37:52.790 --> 00:37:54.370 directly from our graph. 00:37:54.370 --> 00:37:57.450 We see that the radius is shorter, so that means that 00:37:57.450 --> 00:38:00.460 the nitrogen-nitrogen bond is going to be shorter. 00:38:00.460 --> 00:38:03.340 We can know this information even if we just knew that the 00:38:03.340 --> 00:38:05.880 bond was stronger, we wouldn't need to look at a graph here, 00:38:05.880 --> 00:38:08.890 because it turns out that if you have a stronger bond, that 00:38:08.890 --> 00:38:11.650 also means that you have a shorter bond -- those ywo are 00:38:11.650 --> 00:38:12.260 correlated. 00:38:12.260 --> 00:38:15.810 And something that we'll see later on is that triple bonds, 00:38:15.810 --> 00:38:18.880 for example, are going to be stronger than a corresponding 00:38:18.880 --> 00:38:21.130 double bond or a corresponding single bond. 00:38:21.130 --> 00:38:23.720 So, if we talked about a nitrogen-nitrogen single 00:38:23.720 --> 00:38:26.550 versus double versus triple bond, the triple bond will be 00:38:26.550 --> 00:38:29.600 the shortest and it will be the strongest. 00:38:29.600 --> 00:38:32.300 So, that's basically the idea of how we are going to be 00:38:32.300 --> 00:38:33.870 thinking about covalent bonds. 00:38:33.870 --> 00:38:37.420 It's also important, once we start talking about molecules, 00:38:37.420 --> 00:38:39.840 to have a way to represent them, and also to be able to 00:38:39.840 --> 00:38:43.410 look at a shorthand notation for a certain molecule and 00:38:43.410 --> 00:38:45.310 understand what the bond is. 00:38:45.310 --> 00:38:48.370 So, for example, down here I wrote that it was n 2 and that 00:38:48.370 --> 00:38:51.300 it was h 2, but when I re-wrote the molecules up 00:38:51.300 --> 00:38:54.490 here, you saw that it's an h h single bond where it's a 00:38:54.490 --> 00:38:56.630 nitrogen-nitrogen triple bond. 00:38:56.630 --> 00:39:00.140 So any chemist should be able to just look at n 2 and know 00:39:00.140 --> 00:39:02.500 that it's a triple bond, but that's not something that 00:39:02.500 --> 00:39:04.950 we've learned how did to do yet, so let's go ahead and 00:39:04.950 --> 00:39:07.950 start a new topic that's going to allow us to have some sort 00:39:07.950 --> 00:39:11.150 of sense of what the valence electron configuration, which 00:39:11.150 --> 00:39:13.770 includes whether something's a single or double or a triple 00:39:13.770 --> 00:39:17.850 bond can be figured out for any given molecule. 00:39:17.850 --> 00:39:20.320 So, to do this, what I'm going to do is introduce the topic 00:39:20.320 --> 00:39:21.510 of Lewis structures. 00:39:21.510 --> 00:39:24.290 We're going to really get into this next class, but I just 00:39:24.290 --> 00:39:27.250 want to introduce it to you to give us a start, and many of 00:39:27.250 --> 00:39:30.310 you have used Lewis structures in high school, but we'll be 00:39:30.310 --> 00:39:33.340 doing some much more challenging Lewis structures, 00:39:33.340 --> 00:39:37.140 I can assure you, in this class here. 00:39:37.140 --> 00:39:41.160 So, a Lewis structure is basically an organizing 00:39:41.160 --> 00:39:46.740 property of bonding, of molecules, which is the idea 00:39:46.740 --> 00:39:49.890 that when we're thinking about bonding, the key is to achieve 00:39:49.890 --> 00:39:53.420 a full valence shell in each of the individual atoms. So we 00:39:53.420 --> 00:39:58.090 want to have in an h h bond, for example, a full shell for 00:39:58.090 --> 00:40:01.550 each of the hydrogen atoms. And G.N. Lewis is the 00:40:01.550 --> 00:40:05.050 scientist that is credited, and who did, in fact, come up 00:40:05.050 --> 00:40:08.690 with this idea for the way to represent this, so the other 00:40:08.690 --> 00:40:11.630 parts of this idea, another way to phrase it is that the 00:40:11.630 --> 00:40:14.300 electrons are going to be distributed in such a way that 00:40:14.300 --> 00:40:18.650 we have what are called full octets for each of the atoms, 00:40:18.650 --> 00:40:21.790 and basically that's the same thing as saying we have a full 00:40:21.790 --> 00:40:24.130 valence shell, and this is something that Lewis was able 00:40:24.130 --> 00:40:28.320 to recognize very, very early, way before we had quantum 00:40:28.320 --> 00:40:31.390 mechanics to describe what these orbitals were, but it 00:40:31.390 --> 00:40:34.630 makes sense a full valence shell means for most atoms 00:40:34.630 --> 00:40:38.820 that we have a full s orbital plus a full p orbital, so 00:40:38.820 --> 00:40:42.200 we're going to have a total of four orbitals that are each 00:40:42.200 --> 00:40:45.300 filled with eight electrons, so that's why we see that we 00:40:45.300 --> 00:40:48.570 need an octet here. 00:40:48.570 --> 00:40:52.320 And the idea is that when you do these Lewis dot structures, 00:40:52.320 --> 00:40:55.180 we're representing electrons with dots, which we'll see in 00:40:55.180 --> 00:40:57.260 a minute, and each dot is going to 00:40:57.260 --> 00:40:59.600 represent a valence electron. 00:40:59.600 --> 00:41:02.080 So, hopefully, you remember what we mean by valence 00:41:02.080 --> 00:41:04.570 electrons versus core electrons. 00:41:04.570 --> 00:41:07.340 Core electrons are all those electrons held in really tight 00:41:07.340 --> 00:41:10.640 with the nucleus in the inner shells, whereas the valence 00:41:10.640 --> 00:41:12.830 electrons are only those electrons that are in the 00:41:12.830 --> 00:41:16.720 outer-most shell, or at your highest value of n of the 00:41:16.720 --> 00:41:19.110 principal quantum number. 00:41:19.110 --> 00:41:22.740 So, Lewis structures are really a model for a way to 00:41:22.740 --> 00:41:26.220 think about what the valence electron configuration is, and 00:41:26.220 --> 00:41:29.130 as I said, it's not based on quantum mechanics, it's 00:41:29.130 --> 00:41:33.140 something that Lewis observed far, far before quantum 00:41:33.140 --> 00:41:34.860 mechanics were discovered. 00:41:34.860 --> 00:41:38.230 So he came up with the ideas that led to the idea of Lewis 00:41:38.230 --> 00:41:41.230 structures in the very early 1900's. 00:41:41.230 --> 00:41:44.440 So you might ask well, why are we using this model if it 00:41:44.440 --> 00:41:47.000 clearly doesn't take into account quantum mechanics? 00:41:47.000 --> 00:41:49.420 And the reason that we use it is that it is incredibly 00:41:49.420 --> 00:41:52.580 accurate, and allows us to very, very quickly predict and 00:41:52.580 --> 00:41:55.810 to predict accurately, in most cases, what the electron 00:41:55.810 --> 00:41:58.760 configuration of molecules are going to be. 00:41:58.760 --> 00:42:00.530 So this is really useful. 00:42:00.530 --> 00:42:02.730 We don't always want to go and solve the Schrodinger 00:42:02.730 --> 00:42:05.790 equation, and in fact, once we start talking about molecules, 00:42:05.790 --> 00:42:08.270 I can imagine none of you, as much as you love math or 00:42:08.270 --> 00:42:10.110 physics, want to be trying to solve this Schrodinger 00:42:10.110 --> 00:42:12.310 equation in that case either. 00:42:12.310 --> 00:42:15.260 So, what Lewis structures allow us to do is over 90% of 00:42:15.260 --> 00:42:18.180 the time be correct in terms of figuring out what the 00:42:18.180 --> 00:42:20.130 electron configuration is. 00:42:20.130 --> 00:42:22.150 And we won't just use them in this class. 00:42:22.150 --> 00:42:25.760 If you actually go to any of the chemistry labs at MIT, if 00:42:25.760 --> 00:42:28.670 you go over to building 18 and look in the organic labs where 00:42:28.670 --> 00:42:31.670 they're synthesizing new molecules or making up new 00:42:31.670 --> 00:42:35.680 reactions, what you'll see if you open anyone's notebook, 00:42:35.680 --> 00:42:39.210 their lab notebook, assuming they keep a nice lab notebook, 00:42:39.210 --> 00:42:42.150 is that they will have Lewis structures drawn in there that 00:42:42.150 --> 00:42:44.280 explain the reactions that they're going to be 00:42:44.280 --> 00:42:45.680 doing for that day. 00:42:45.680 --> 00:42:48.900 And I mean this means way past all the chemistry they've 00:42:48.900 --> 00:42:51.160 taken, they're now graduate students or they're now 00:42:51.160 --> 00:42:53.900 professors, and they're still writing out Lewis structures. 00:42:53.900 --> 00:42:56.420 Now they're writing a more abbreviated form, which you'll 00:42:56.420 --> 00:42:59.950 probably get to if you take organic chemistry, but really 00:42:59.950 --> 00:43:02.130 it's the exact same idea. 00:43:02.130 --> 00:43:05.220 And this goes all the way back to 1902. 00:43:05.220 --> 00:43:09.240 In fact, Lewis was an American scientist, so he was trained 00:43:09.240 --> 00:43:13.340 in America, and he actually was a professor here at MIT 00:43:13.340 --> 00:43:18.660 from 1905 all the way to about 1911 or 1912, and these are 00:43:18.660 --> 00:43:23.590 some notes from 1902, and you can't see them very well, but 00:43:23.590 --> 00:43:27.520 this was essentially an early form of Lewis structures, and 00:43:27.520 --> 00:43:29.640 this was called the cubicle atom. 00:43:29.640 --> 00:43:32.030 So, basically what he's showing in these cubes is that 00:43:32.030 --> 00:43:34.510 there are eight spaces that need to be filled up 00:43:34.510 --> 00:43:35.960 to have a full cube. 00:43:35.960 --> 00:43:38.700 So in order to fill them, he would have to have eight 00:43:38.700 --> 00:43:41.350 electrons or an octet around the cubes. 00:43:41.350 --> 00:43:45.460 So what we're seeing is this is notes from 1902 -- he 00:43:45.460 --> 00:43:48.750 actually didn't publish any of this work or these ideas that 00:43:48.750 --> 00:43:52.120 led to Lewis structures until 1916, but his early class 00:43:52.120 --> 00:43:55.590 notes were used as evidence about how long ago he actually 00:43:55.590 --> 00:43:57.080 came up with the idea of it. 00:43:57.080 --> 00:43:59.690 So it's really neat to think that your counterparts 100 00:43:59.690 --> 00:44:02.230 years ago right here at MIT could have been sitting in a 00:44:02.230 --> 00:44:05.570 class where they had Lewis as their lecturer, and he's 00:44:05.570 --> 00:44:07.980 putting forth these ideas -- these are actually his lecture 00:44:07.980 --> 00:44:11.160 notes, even though it wasn't even published yet, and giving 00:44:11.160 --> 00:44:13.680 this idea of Lewis structure, which is exactly what we keep 00:44:13.680 --> 00:44:16.720 using today in order to make a lot of these predictions. 00:44:16.720 --> 00:44:19.910 So, let's see how some of this works, and hopefully your 00:44:19.910 --> 00:44:22.540 counterparts from 100 years ago would also be able to 00:44:22.540 --> 00:44:25.700 think about how this works, even if they don't have the 00:44:25.700 --> 00:44:29.410 quantum mechanics behind the individual electron 00:44:29.410 --> 00:44:32.500 configurations for atoms. So I said that we want to be 00:44:32.500 --> 00:44:35.280 talking about valence electrons here, so that means 00:44:35.280 --> 00:44:38.700 if we're talking about, for example, the octet rule for an 00:44:38.700 --> 00:44:42.350 f f molecule where we have two fluorine atoms, we need to 00:44:42.350 --> 00:44:45.610 write the valence electrons as dots around them. 00:44:45.610 --> 00:44:48.220 So let's do a quick clicker question, and you tell me how 00:44:48.220 --> 00:44:52.100 many valence electrons does fluorine have? 00:44:52.100 --> 00:44:54.850 Remember, valence electrons are different from core, 00:44:54.850 --> 00:44:57.030 they're only the outer-most electrons in 00:44:57.030 --> 00:45:03.410 the outer-most shell. 00:45:03.410 --> 00:45:04.960 So, 10 seconds on this, this should 00:45:04.960 --> 00:45:17.510 be fast. OK, great. 00:45:17.510 --> 00:45:19.340 Good job on the clicker questions today. 00:45:19.340 --> 00:45:21.840 So we have seven valence electrons. 00:45:21.840 --> 00:45:24.570 So, let's go back to the notes, and let's fill these 00:45:24.570 --> 00:45:26.570 in, seven electrons. 00:45:26.570 --> 00:45:28.780 Another way you could have known them was to look at 00:45:28.780 --> 00:45:32.040 Lewis' notes here, where if look at this box carefully you 00:45:32.040 --> 00:45:35.510 see there are seven dots around the cube, so there are 00:45:35.510 --> 00:45:38.080 his seven valence electrons. 00:45:38.080 --> 00:45:41.660 So, we see is when we use the octet rule to look at fluorine 00:45:41.660 --> 00:45:45.000 molecule, we're combining two fluorine atoms, and what we 00:45:45.000 --> 00:45:48.380 end up with is an f f molecule where they're sharing two 00:45:48.380 --> 00:45:51.360 electrons, so making that covalent bond. 00:45:51.360 --> 00:45:54.890 But that each individual fluorine atom has eight 00:45:54.890 --> 00:45:56.860 electrons, or full octet around it. 00:45:56.860 --> 00:45:59.710 We can think about where those electrons came from, so we got 00:45:59.710 --> 00:46:03.610 seven from the blue electrons here, seven as shown in green 00:46:03.610 --> 00:46:07.470 here, but each individual fluorine atom has eight, even 00:46:07.470 --> 00:46:10.640 though two of those are being shared between both of them. 00:46:10.640 --> 00:46:13.250 So, the octet rule is a general rule that you'll for 00:46:13.250 --> 00:46:16.700 all of the atoms. There are some exceptions, which we'll 00:46:16.700 --> 00:46:20.160 get to later, but the only a big exception here is with 00:46:20.160 --> 00:46:23.090 hydrogen, which has a special stability that's associated 00:46:23.090 --> 00:46:24.470 with two electrons. 00:46:24.470 --> 00:46:26.420 This should make a lot of sense, because we know that a 00:46:26.420 --> 00:46:32.940 hydrogen has 1 s as it's outer-most or valence orbital, 00:46:32.940 --> 00:46:37.690 so it can be filled up just with two 1 s electrons. 00:46:37.690 --> 00:46:40.320 And we give different names, depending on what kind of 00:46:40.320 --> 00:46:43.280 electrons we're dealing with, so, for example, with h c l 00:46:43.280 --> 00:46:45.760 here, we can talk about having bonded 00:46:45.760 --> 00:46:48.060 versus lone pair electrons. 00:46:48.060 --> 00:46:50.990 So, in terms of the c l atom, we need to talk about each 00:46:50.990 --> 00:46:52.470 atom individually. 00:46:52.470 --> 00:47:01.030 How many bonding electrons does c l have? 00:47:01.030 --> 00:47:01.540 All right. 00:47:01.540 --> 00:47:05.000 Let's see, we've got a mixed response here, it turns out it 00:47:05.000 --> 00:47:06.540 has two bonding electrons. 00:47:06.540 --> 00:47:09.290 I heard some people say one, and that's a good guess, 00:47:09.290 --> 00:47:11.300 remember they're actually sharing. 00:47:11.300 --> 00:47:14.630 So these two electrons, they belong to chlorine, they also 00:47:14.630 --> 00:47:17.250 belong to hydrogen, but they do, in fact, belong to 00:47:17.250 --> 00:47:17.920 chlorine as well. 00:47:17.920 --> 00:47:20.750 There's no one person owning them, so they both have two 00:47:20.750 --> 00:47:22.480 electrons here that are bonding. 00:47:22.480 --> 00:47:26.510 So how many lone pair electrons do we have? 00:47:26.510 --> 00:47:26.850 OK. 00:47:26.850 --> 00:47:29.810 I hear six and three, so both are sort of right, we have 6 00:47:29.810 --> 00:47:31.400 lone pair electrons, which means that we 00:47:31.400 --> 00:47:35.180 have three lone pairs. 00:47:35.180 --> 00:47:39.560 So, in terms of thinking about how to draw a Lewis structure, 00:47:39.560 --> 00:47:42.740 I won't go through this today or any day in terms of just 00:47:42.740 --> 00:47:45.230 reading through the rules, you can read that yourself. 00:47:45.230 --> 00:47:47.760 But what we'll do is go through each of these rules in 00:47:47.760 --> 00:47:48.900 terms of an example. 00:47:48.900 --> 00:47:52.060 So, what will start with on Monday is doing the most 00:47:52.060 --> 00:47:54.670 simple example of methane using these 00:47:54.670 --> 00:47:56.130 Lewis structure rules. 00:47:56.130 --> 00:47:59.400 So, don't forget to study this weekend and get those extra 00:47:59.400 --> 00:48:02.840 practice problems from the course website.