WEBVTT 00:00:00.000 --> 00:00:00.104 The following content is provided under a Creative 00:00:00.104 --> 00:00:00.143 Commons license. 00:00:00.143 --> 00:00:00.247 Your support will help MIT OpenCourseWare continue to 00:00:00.247 --> 00:00:00.351 offer high quality educational resources for free. 00:00:00.351 --> 00:00:00.468 To make a donation or view additional materials from 00:00:00.468 --> 00:00:00.572 hundreds of MIT courses, visit MIT OpenCourseWare at 00:00:00.572 --> 00:00:00.600 ocw.mit.edu. 00:00:00.600 --> 00:00:22.340 PROFESSOR: All right, let's get started. 00:00:22.340 --> 00:00:25.880 Could everyone take 10 more seconds 00:00:25.880 --> 00:00:27.360 on the clicker question. 00:00:27.360 --> 00:00:35.250 And as a reminder, hopefully I don't need to remind any of 00:00:35.250 --> 00:00:38.810 you, but exam 1 is on Wednesday, so rather than our 00:00:38.810 --> 00:00:41.210 clicker question being on something from last class, 00:00:41.210 --> 00:00:44.120 which is exam 2 material, let's just make sure everyone 00:00:44.120 --> 00:00:48.250 remembers some small topic from exam 1 material, which is 00:00:48.250 --> 00:00:50.170 the idea of angular nodes. 00:00:50.170 --> 00:00:54.160 So I was hoping to see more like 99% on this, but you do 00:00:54.160 --> 00:00:56.590 still have two more days before the exam. 00:00:56.590 --> 00:00:59.410 Remember, we're talking about angular nodes here, so you 00:00:59.410 --> 00:01:01.800 need to read the question carefully. 00:01:01.800 --> 00:01:05.380 For an angular node, we're just talking about what the l 00:01:05.380 --> 00:01:08.290 value is, so whatever l is equal to is equal to the 00:01:08.290 --> 00:01:10.410 number of angular nodes you have. 00:01:10.410 --> 00:01:14.800 For an f orbital, what is the quantum number l equal to? 00:01:14.800 --> 00:01:15.340 Three. 00:01:15.340 --> 00:01:18.370 Good, so everyone that recognized that probably got 00:01:18.370 --> 00:01:20.880 the right answer of three angular nodes here. 00:01:20.880 --> 00:01:23.370 So let's switch to today's notes. 00:01:23.370 --> 00:01:26.580 Two more quick things about the exam, the first is that 00:01:26.580 --> 00:01:29.020 just remember on Wednesday, don't come here, the exam is 00:01:29.020 --> 00:01:30.980 not here, don't come here. 00:01:30.980 --> 00:01:33.580 It's in Walker, so make sure you go to Walker. 00:01:33.580 --> 00:01:36.480 And also, keep in mind that I have office hours 00:01:36.480 --> 00:01:38.220 today from 3 to 5. 00:01:38.220 --> 00:01:40.460 All your TA's have either already have their extra 00:01:40.460 --> 00:01:43.090 office hours, or there are some that will be going on 00:01:43.090 --> 00:01:45.650 tonight or tomorrow, so keep those in mind as your 00:01:45.650 --> 00:01:48.250 finishing up your studying for the exam. 00:01:48.250 --> 00:01:51.340 So, today we're moving on, we're talking about Lewis 00:01:51.340 --> 00:01:52.340 structures. 00:01:52.340 --> 00:01:55.550 This is a really good topic to do the class before an exam 00:01:55.550 --> 00:01:57.700 because it's a little bit of a lighter topic. 00:01:57.700 --> 00:02:00.250 We remember that Lewis structures are an idea that 00:02:00.250 --> 00:02:02.190 are pre-quantum mechanics. 00:02:02.190 --> 00:02:05.130 So that means that we don't have to worry about things 00:02:05.130 --> 00:02:07.170 like wave functions when we're talking about Lewis 00:02:07.170 --> 00:02:09.980 structures, but because they're so simple to use and 00:02:09.980 --> 00:02:13.930 because they so often predict the electron configuration of 00:02:13.930 --> 00:02:17.100 molecules accurately, we end up using them all the time in 00:02:17.100 --> 00:02:19.610 chemistry, so it's very valuable to know how to draw 00:02:19.610 --> 00:02:22.830 them correctly and to know how to work with them. 00:02:22.830 --> 00:02:25.760 So we'll talk specifically about drawing Lewis structures 00:02:25.760 --> 00:02:28.520 and then about formal charge and resonance, which are 00:02:28.520 --> 00:02:31.060 within Lewis structures. 00:02:31.060 --> 00:02:33.460 So remember, that when we talked about Lewis structure, 00:02:33.460 --> 00:02:36.640 the organizing principle behind Lewis structures is the 00:02:36.640 --> 00:02:40.420 idea that within the molecule the atoms are going to arrange 00:02:40.420 --> 00:02:43.930 their valence electrons, such that each atom within the 00:02:43.930 --> 00:02:48.320 molecule has a complete octet or full outer shell. 00:02:48.320 --> 00:02:51.780 So this is the idea of the octet rule that Lewis came up 00:02:51.780 --> 00:02:54.360 with way back in 1902. 00:02:54.360 --> 00:02:58.480 So, at the end of the class on Friday, we saw that list of 00:02:58.480 --> 00:03:00.650 eight steps that you always need to go through when you 00:03:00.650 --> 00:03:02.300 draw a Lewis structure. 00:03:02.300 --> 00:03:06.140 Once you're doing this on your own, especially, for example, 00:03:06.140 --> 00:03:09.010 on exam 2, which is a ways down the road, you won't be 00:03:09.010 --> 00:03:11.510 able to look at those steps, so you need to make sure that 00:03:11.510 --> 00:03:13.800 you can go through them without looking at them, but 00:03:13.800 --> 00:03:16.820 for now we can look at them as we are actually learning how 00:03:16.820 --> 00:03:19.050 to draw the Lewis structures, and rather just go through 00:03:19.050 --> 00:03:21.250 them step-by-step, it's more interesting to 00:03:21.250 --> 00:03:22.700 do it with an example. 00:03:22.700 --> 00:03:25.770 So let's hydrogen cyanide as our first example. 00:03:25.770 --> 00:03:27.780 So we have h c n. 00:03:27.780 --> 00:03:31.360 So, in this example here, we start with the first step -- 00:03:31.360 --> 00:03:34.310 the first step in any Lewis structure is drawing the 00:03:34.310 --> 00:03:35.940 skeletal structure. 00:03:35.940 --> 00:03:39.520 So, essentially drawing how the atoms are arranged within 00:03:39.520 --> 00:03:40.830 our molecule. 00:03:40.830 --> 00:03:43.860 And in this case we have three choices here in terms of 00:03:43.860 --> 00:03:46.730 what's going to be in the middle, so we need to decide 00:03:46.730 --> 00:03:48.040 that first. 00:03:48.040 --> 00:03:51.640 In terms of where different atoms are in a molecule, if 00:03:51.640 --> 00:03:55.270 you have a hydrogen atom or a fluorine atom, you can pretty 00:03:55.270 --> 00:03:58.120 much guarantee they're always going to be terminal atoms. 00:03:58.120 --> 00:04:00.620 By terminal I mean they're only bonded to one thing. 00:04:00.620 --> 00:04:03.080 So, for example, hydrogen or fluorine they'll never be in 00:04:03.080 --> 00:04:05.500 the middle, they'll always be on the end of a molecule. 00:04:05.500 --> 00:04:07.750 So that takes care of the hydrogen, what about between 00:04:07.750 --> 00:04:09.250 the carbon and the nitrogen? 00:04:09.250 --> 00:04:12.100 In terms of picking a Lewis structure that's going to be 00:04:12.100 --> 00:04:15.340 the lowest energy, what you want to do is put the atom 00:04:15.340 --> 00:04:17.730 with the lowest ionization energy in the 00:04:17.730 --> 00:04:19.300 center of your atom. 00:04:19.300 --> 00:04:21.840 This should make sense because if something has a low 00:04:21.840 --> 00:04:25.160 ionization energy, that means it's not very electronegative, 00:04:25.160 --> 00:04:27.500 which means it's going to be a lot happier giving up electron 00:04:27.500 --> 00:04:30.020 density, which is essentially what you're doing when you're 00:04:30.020 --> 00:04:32.450 forming covalent bonds is you're sharing some of your 00:04:32.450 --> 00:04:33.740 electron density. 00:04:33.740 --> 00:04:37.000 So, we keep the atoms with the lowest ionization energy in 00:04:37.000 --> 00:04:37.890 the center. 00:04:37.890 --> 00:04:41.300 So, why don't you go ahead and tell me, keeping that in mind, 00:04:41.300 --> 00:04:45.910 which atom in terms of h c or n would you expect to be in 00:04:45.910 --> 00:04:49.550 the center of hydrogen cyanide? 00:04:49.550 --> 00:04:52.670 And I put a periodic table up there, just the part you might 00:04:52.670 --> 00:04:53.450 need to look at. 00:04:53.450 --> 00:05:07.950 So this should be fast, so let's take 10 seconds. 00:05:07.950 --> 00:05:09.280 All right, good job everyone. 00:05:09.280 --> 00:05:13.190 So most of you saw that carbon should be in the center. 00:05:13.190 --> 00:05:15.680 Carbon should be in the center because it has the lowest 00:05:15.680 --> 00:05:17.130 ionization energy. 00:05:17.130 --> 00:05:21.000 We know that ionization energy is going to increase as we go 00:05:21.000 --> 00:05:24.770 across the periodic table, so that means carbon has a lower 00:05:24.770 --> 00:05:26.810 ionization energy than nitrogen, which is 00:05:26.810 --> 00:05:28.170 right next to us. 00:05:28.170 --> 00:05:30.920 So as I just said, we want to put that one in the middle. 00:05:30.920 --> 00:05:34.230 You got an extra hint here in terms of the order, so even if 00:05:34.230 --> 00:05:36.720 you had just forgotten what I said, sometimes it's not a 00:05:36.720 --> 00:05:38.970 terrible idea just to put it in the order it's written, 00:05:38.970 --> 00:05:41.240 that can give you a lot of clues as well. 00:05:41.240 --> 00:05:43.820 So, either of those ways of figuring this out is the first 00:05:43.820 --> 00:05:46.490 guess of what goes in the middle will work pretty well. 00:05:46.490 --> 00:05:49.570 So, let's go ahead and draw our Lewis structure based on 00:05:49.570 --> 00:05:53.800 the rest of the rules now that we have a skeleton. 00:05:53.800 --> 00:05:57.210 So our skeleton tells us that carbon is in the middle, so 00:05:57.210 --> 00:06:00.310 we'll put the h on one side, and the n on 00:06:00.310 --> 00:06:02.650 the other side there. 00:06:02.650 --> 00:06:06.070 So, our second step, as we go through our Lewis structure 00:06:06.070 --> 00:06:09.420 rules, is to figure out how many valence electrons we have 00:06:09.420 --> 00:06:11.510 in our entire molecule. 00:06:11.510 --> 00:06:14.340 So if we talk about hydrogen, how many valence electrons are 00:06:14.340 --> 00:06:16.390 we talking about? 00:06:16.390 --> 00:06:17.380 1. 00:06:17.380 --> 00:06:20.550 What about carbon? 00:06:20.550 --> 00:06:21.760 I heard 4 and 6. 00:06:21.760 --> 00:06:23.770 And remember, we're only talking about valence 00:06:23.770 --> 00:06:26.360 electrons, so the outer-most shells. 00:06:26.360 --> 00:06:30.360 So we're talking about four valence electrons for carbon. 00:06:30.360 --> 00:06:34.010 And then for nitrogen? 00:06:34.010 --> 00:06:37.260 Lots of options I have to choose from from these 00:06:37.260 --> 00:06:38.870 answers, but it's 5. 00:06:38.870 --> 00:06:42.000 So, if you can't immediately know, and you don't all have 00:06:42.000 --> 00:06:44.210 periodic tables in front of you, so that's fine, but if 00:06:44.210 --> 00:06:46.940 you have a periodic table in front of you, you need to be 00:06:46.940 --> 00:06:49.210 able to count valence electrons, so work on that if 00:06:49.210 --> 00:06:51.450 it doesn't come naturally to you in terms of 00:06:51.450 --> 00:06:52.300 figuring that out. 00:06:52.300 --> 00:06:56.250 So then in order to figure out the complete number of valence 00:06:56.250 --> 00:07:00.300 electrons in our molecule, we just add 5 plus 4 plus 1. 00:07:00.300 --> 00:07:02.610 So we end up having 10 valence electrons. 00:07:02.610 --> 00:07:09.100 Step three in our Lewis structure rules is to figure 00:07:09.100 --> 00:07:12.560 out how many electronis we would need in order for every 00:07:12.560 --> 00:07:16.260 single atom in our molecule to have a full valence shell. 00:07:16.260 --> 00:07:18.980 So, if we're talking about hydrogen, that's our one 00:07:18.980 --> 00:07:21.520 exception so far to the octet rule. 00:07:21.520 --> 00:07:24.670 So we actually only need two electrons to fill up the 00:07:24.670 --> 00:07:27.110 valence shell of hydrogen, remember that's because all we 00:07:27.110 --> 00:07:29.640 need to fill up is the 1 s. 00:07:29.640 --> 00:07:34.090 However, for carbon and nitrogen we need 8 each. 00:07:34.090 --> 00:07:37.050 So in terms of total numbers that we would need to complete 00:07:37.050 --> 00:07:40.510 our octets and fill our valence shells, we would need 00:07:40.510 --> 00:07:48.650 18 electrons. 00:07:48.650 --> 00:07:48.980 All right. 00:07:48.980 --> 00:07:51.970 So let's bring down our middle slide here. 00:07:51.970 --> 00:07:54.890 So we have 18 electrons, and the next thing that we need to 00:07:54.890 --> 00:07:57.780 figure out is how many bonding electrons we have. 00:07:57.780 --> 00:08:00.500 So to figure out bonding electrons, what we take is 00:08:00.500 --> 00:08:04.220 that number 18, which is our total number of electrons we 00:08:04.220 --> 00:08:07.450 need to fill valence shells, and we subtract it from our 00:08:07.450 --> 00:08:11.110 number of valence electrons, which is 10. 00:08:11.110 --> 00:08:18.570 And what we find that we have is 8 bonding electrons. 00:08:18.570 --> 00:08:23.810 And hopefully on your paper, you can actually reach your h 00:08:23.810 --> 00:08:24.750 c n skeleton -- 00:08:24.750 --> 00:08:27.940 I think I should probably re-draw mine here. 00:08:27.940 --> 00:08:31.710 Because step five is that we need to fill in our bonding 00:08:31.710 --> 00:08:33.920 electrons, and we start it with filling in two 00:08:33.920 --> 00:08:35.850 electrons per bond. 00:08:35.850 --> 00:08:39.800 So I'm just going to re-draw my skeleton. 00:08:39.800 --> 00:08:44.420 So, the first thing we do is put two electrons between h 00:08:44.420 --> 00:08:47.970 and c, and then two electrons between c and n. 00:08:47.970 --> 00:08:50.210 Remember, every time we have two electrons that are being 00:08:50.210 --> 00:08:54.400 shared, that's a single bond. 00:08:54.400 --> 00:08:56.930 The next thing that we want to do is figure out do we have 00:08:56.930 --> 00:08:59.400 any bonding electrons left. 00:08:59.400 --> 00:09:03.090 So let's see, we started with 8 bonding electrons, and we 00:09:03.090 --> 00:09:08.990 used up only 4, so the answer is yes, we have 4 bonding 00:09:08.990 --> 00:09:10.790 electrons left. 00:09:10.790 --> 00:09:13.350 So what we need to do is fill in those extra bonding 00:09:13.350 --> 00:09:15.810 electrons into our bonds. 00:09:15.810 --> 00:09:17.850 Should I put an extra pair of electrons 00:09:17.850 --> 00:09:19.890 here, does anyone think? 00:09:19.890 --> 00:09:20.590 No. 00:09:20.590 --> 00:09:22.850 The reason is because we already have a full valence 00:09:22.850 --> 00:09:24.220 shell for our hydrogen, it doesn't 00:09:24.220 --> 00:09:25.590 want anymore electrons. 00:09:25.590 --> 00:09:28.600 What about between the carbon and nitrogen? 00:09:28.600 --> 00:09:29.080 Yes. 00:09:29.080 --> 00:09:31.840 Definitely, because both of these are not anywhere near 00:09:31.840 --> 00:09:33.670 filling up their octets yet. 00:09:33.670 --> 00:09:37.820 So we can put actually all 4 of our extra electrons in 00:09:37.820 --> 00:09:39.780 between the carbon and the nitrogen. 00:09:39.780 --> 00:09:44.960 Now we have 6 things around the nitrogen, and we have 8 00:09:44.960 --> 00:09:47.700 around the carbon. 00:09:47.700 --> 00:09:51.970 So, what we do as our seventh step is then figure out if we 00:09:51.970 --> 00:09:56.840 have any extra valence electrons left at all. 00:09:56.840 --> 00:10:01.280 So we started with 10 valence electrons, we used up 8 of 00:10:01.280 --> 00:10:04.600 those electrons in terms of making bonds. 00:10:04.600 --> 00:10:08.380 So it turns out that we have 2 valence electrons left. 00:10:08.380 --> 00:10:11.920 So we need to add those 2 valence electrons left as lone 00:10:11.920 --> 00:10:14.010 pair electrons in our structure. 00:10:14.010 --> 00:10:17.070 So, which atom is in need of those lone pair electrons? 00:10:17.070 --> 00:10:19.290 The nitrogen. 00:10:19.290 --> 00:10:21.500 The reason being that's the only one that didn't have a 00:10:21.500 --> 00:10:22.930 full octet yet. 00:10:22.930 --> 00:10:26.940 So now we're done, actually there is one more step, which 00:10:26.940 --> 00:10:29.650 is to determine the formal charge. 00:10:29.650 --> 00:10:32.760 This is a good way to actually check if your Lewis structure 00:10:32.760 --> 00:10:34.170 is correct or not. 00:10:34.170 --> 00:10:37.490 We haven't actually learned how to calculate the formal 00:10:37.490 --> 00:10:39.830 charge yet, we'll learn it soon. 00:10:39.830 --> 00:10:42.650 So we won't do it for this molecule, but we'll go back 00:10:42.650 --> 00:10:44.730 and do it for some of our other examples, and you can go 00:10:44.730 --> 00:10:46.380 back and do it for this one. 00:10:46.380 --> 00:10:48.560 The other thing is that we can re-write our h c 00:10:48.560 --> 00:10:50.860 n in terms of bonds. 00:10:50.860 --> 00:10:53.970 So we know every time we have two electrons, that's a bond. 00:10:53.970 --> 00:10:57.130 So we have h, then we can draw our bond as a line. 00:10:57.130 --> 00:10:59.870 And then we have a triple bond there because we have 3 pairs 00:10:59.870 --> 00:11:01.750 of electrons. 00:11:01.750 --> 00:11:07.010 So it looks a lot less messy if we just draw our Lewis 00:11:07.010 --> 00:11:11.540 structure like this for h c n, where we have h bonded to c 00:11:11.540 --> 00:11:14.650 triple, bonded to n, and then a lone pair on 00:11:14.650 --> 00:11:17.020 the nitrogen there. 00:11:17.020 --> 00:11:19.220 All right, so this is the same procedure that we're going to 00:11:19.220 --> 00:11:21.940 go through, regardless of what kind of Lewis structure we're 00:11:21.940 --> 00:11:22.850 going to draw. 00:11:22.850 --> 00:11:25.810 What you'll actually find in terms of asking your TAs about 00:11:25.810 --> 00:11:28.590 the Lewis structure rules is that sometimes they won't be 00:11:28.590 --> 00:11:31.350 as good at them as you are, and the reason is once you've 00:11:31.350 --> 00:11:33.480 drawn enough of these structures, you start to get a 00:11:33.480 --> 00:11:36.280 lot of chemical intuition about what's right or what's 00:11:36.280 --> 00:11:39.090 not right -- it just looks wrong to you if it's wrong. 00:11:39.090 --> 00:11:41.600 So your TA might take a minute, so be patient with 00:11:41.600 --> 00:11:43.510 them if they see your structure and they say oh, no, 00:11:43.510 --> 00:11:45.580 no, no, no that's wrong, that's terrible, and they 00:11:45.580 --> 00:11:46.860 don't immediately know why. 00:11:46.860 --> 00:11:48.840 They might need to go through the rules with you, you might 00:11:48.840 --> 00:11:49.540 need to remind them. 00:11:49.540 --> 00:11:51.130 Hopefully, they'll all study them again, so 00:11:51.130 --> 00:11:52.470 this will be an issue. 00:11:52.470 --> 00:11:55.920 But what really happens is as you go on in chemistry, you 00:11:55.920 --> 00:11:58.180 draw so many of these you can just draw them without 00:11:58.180 --> 00:11:59.730 following the rules. 00:11:59.730 --> 00:12:01.920 Some of you might get almost to that point, or you might be 00:12:01.920 --> 00:12:05.010 at that point now, but I recommend this for you and for 00:12:05.010 --> 00:12:08.640 me and for the TAs, go through the rules because there'll be 00:12:08.640 --> 00:12:11.790 cases where it's a little bit tricky and it's always much 00:12:11.790 --> 00:12:15.310 faster to have gone through step-by-step, than to try to 00:12:15.310 --> 00:12:18.170 just kind of hit or miss figure out what's going to be 00:12:18.170 --> 00:12:19.680 right or wrong. 00:12:19.680 --> 00:12:24.480 So, let's try another example here, and let's try a case now 00:12:24.480 --> 00:12:27.260 where instead of dealing with a neutral molecule we have an 00:12:27.260 --> 00:12:30.700 ion, so we have c n minus. 00:12:30.700 --> 00:12:33.360 And what I'll mention to you just in terms of the fact that 00:12:33.360 --> 00:12:37.390 we're finally dealing with real molecules, which is -- or 00:12:37.390 --> 00:12:39.770 molecules that are made up of more than one atom, which is 00:12:39.770 --> 00:12:42.680 kind of exciting for me and maybe for some other of you 00:12:42.680 --> 00:12:46.390 that like to move into thinking about what some of 00:12:46.390 --> 00:12:49.760 the consequences of these molecules reacting might be. 00:12:49.760 --> 00:12:51.750 A lot of the examples that we're going to give you in 00:12:51.750 --> 00:12:55.450 terms of trying out your Lewis structures will be molecule 00:12:55.450 --> 00:12:58.010 that are used in organic synthesis, or maybe they're 00:12:58.010 --> 00:13:00.360 molecules that react in interesting ways with 00:13:00.360 --> 00:13:03.630 biomolecules in your body or proteins in your body. 00:13:03.630 --> 00:13:06.660 So, you already will have a head start when you get on to 00:13:06.660 --> 00:13:09.120 later classes, like organic chemistry or if you're 00:13:09.120 --> 00:13:12.480 thinking about biochemistry where being able to draw the 00:13:12.480 --> 00:13:15.490 Lewis structure allows you to think about, eventually, the 00:13:15.490 --> 00:13:18.290 reactivity of the molecule, which becomes very interesting 00:13:18.290 --> 00:13:20.760 in thinking about how you're going to synthesize a more 00:13:20.760 --> 00:13:23.790 complex molecule, or how that molecule is going to interact 00:13:23.790 --> 00:13:26.580 with an active site in a protein in the body. 00:13:26.580 --> 00:13:30.300 So, for example, just talking about hydrogen cyanide or the 00:13:30.300 --> 00:13:34.750 cyanide anion, these are both molecules which are used in 00:13:34.750 --> 00:13:37.780 organic synthesis, so particularly the cyanide, 00:13:37.780 --> 00:13:41.320 anion and salts of the cyanide anion. 00:13:41.320 --> 00:13:45.920 So either a potassium cyanide or sodium cyanide, these are 00:13:45.920 --> 00:13:49.980 used in synthesis in terms of making carbon-carbon bonds. 00:13:49.980 --> 00:13:52.120 So if you're trying to make a more complicated organic 00:13:52.120 --> 00:13:55.160 molecule, carbon-carbon bonds are one of the most difficult 00:13:55.160 --> 00:13:57.880 things to make an organic chemistry, and it turns out 00:13:57.880 --> 00:14:00.900 that c n minus is a very reactive molecule, so it's a 00:14:00.900 --> 00:14:04.460 good way, even though we'll go over some drawbacks in a 00:14:04.460 --> 00:14:07.450 second, it is a good way to make carbon-carbon bonds. 00:14:07.450 --> 00:14:08.550 It's very reactive. 00:14:08.550 --> 00:14:12.100 And because, of course, we have this carbon here what you 00:14:12.100 --> 00:14:15.580 end up doing is adding a carbon to your molecule. 00:14:15.580 --> 00:14:19.460 So, when you think about cyanide, you might not think 00:14:19.460 --> 00:14:21.560 about organic reagents. 00:14:21.560 --> 00:14:23.370 Does anyone have something else they think about when 00:14:23.370 --> 00:14:24.230 they think about cyanide? 00:14:24.230 --> 00:14:25.600 STUDENT: Death. 00:14:25.600 --> 00:14:28.370 PROFESSOR: Death -- that's a good thought. 00:14:28.370 --> 00:14:31.980 Yes, cyanide and death, very closely related as well. 00:14:31.980 --> 00:14:34.490 Cyanide is incredibly toxic, it's a poison. 00:14:34.490 --> 00:14:37.280 That might be how you're more familiar with cyanide. 00:14:37.280 --> 00:14:40.110 So if you're working with cyanide in the lab as 00:14:40.110 --> 00:14:43.320 potassium cyanide or sodium cyanide, those are what are 00:14:43.320 --> 00:14:46.660 called p h s's, or particularly hazardous 00:14:46.660 --> 00:14:49.100 substances -- it's a rating for 00:14:49.100 --> 00:14:50.780 different kinds of chemicals. 00:14:50.780 --> 00:14:52.950 And what that means it's there's all sorts of 00:14:52.950 --> 00:14:55.930 precautions and procedures you take that are special when you 00:14:55.930 --> 00:14:56.980 deal with these. 00:14:56.980 --> 00:14:59.120 They're kept away from other chemicals. 00:14:59.120 --> 00:15:02.880 You handle them very special in terms of being extra 00:15:02.880 --> 00:15:07.330 careful in a very high ventilation area, in hoods is 00:15:07.330 --> 00:15:08.490 how you handle them. 00:15:08.490 --> 00:15:13.050 So, yes, they're very poisonous, and in fact, there 00:15:13.050 --> 00:15:16.880 are areas where you find this toxic compound, cyanide. 00:15:16.880 --> 00:15:22.330 Other than just in poisons and in organic synthesis shells, 00:15:22.330 --> 00:15:24.870 you might also find them in some things we're more 00:15:24.870 --> 00:15:26.360 familiar with, such as almonds. 00:15:26.360 --> 00:15:29.260 I don't know how many know that there are trace , trace 00:15:29.260 --> 00:15:31.900 amounts, of cyanide in almonds. 00:15:31.900 --> 00:15:34.530 I don't know if there any big almond eaters out there. 00:15:34.530 --> 00:15:37.070 You don't have to worry, we're definitely talking about 00:15:37.070 --> 00:15:38.160 trace, trace amounts. 00:15:38.160 --> 00:15:39.790 It's not going to hurt you. 00:15:39.790 --> 00:15:42.720 And actually, what we usually eat are what are called sweet 00:15:42.720 --> 00:15:47.090 almonds, and there aren't actual cyanide in the sweet 00:15:47.090 --> 00:15:49.350 almonds we eat, there's precursors to cyanide, which 00:15:49.350 --> 00:15:50.640 might not make you more comfortable. 00:15:50.640 --> 00:15:52.840 But the fact is there are trace, trace amounts. 00:15:52.840 --> 00:15:56.150 This is nothing that we need to worry in our food supply. 00:15:56.150 --> 00:15:59.130 However, some people aren't so lucky. 00:15:59.130 --> 00:16:01.100 I don't know how many of you are familiar with the Cassava 00:16:01.100 --> 00:16:05.070 plant, which is a kind of woody shrub that's first been 00:16:05.070 --> 00:16:08.050 cultivated in South America, but it's grown throughout 00:16:08.050 --> 00:16:12.020 Africa, the Caribbean, South America still, many places 00:16:12.020 --> 00:16:15.450 around the world, and this is a major source of 00:16:15.450 --> 00:16:18.590 carbohydrates for much of the world, because Cassava root is 00:16:18.590 --> 00:16:21.140 very, very rich in carbohydrates. 00:16:21.140 --> 00:16:24.940 It's not the best form of food in that they're actually very 00:16:24.940 --> 00:16:28.820 poor in protein, and unfortunately very, very rich 00:16:28.820 --> 00:16:30.180 in cyanide. 00:16:30.180 --> 00:16:32.080 So, these roots can be very dangerous. 00:16:32.080 --> 00:16:35.010 There's different types of the root that you can get called 00:16:35.010 --> 00:16:37.120 the bitter and the sweet. 00:16:37.120 --> 00:16:39.710 Hopefully you would all choose the sweet if two are put in 00:16:39.710 --> 00:16:40.580 front of you. 00:16:40.580 --> 00:16:42.200 The bitter, of course, are the ones that are 00:16:42.200 --> 00:16:43.800 very high in cyanide. 00:16:43.800 --> 00:16:46.450 If you eat these raw, which they do in many places around 00:16:46.450 --> 00:16:49.740 the world, if you eat a bitter one, you could, in fact, get 00:16:49.740 --> 00:16:51.670 enough cyanide to kill you. 00:16:51.670 --> 00:16:54.390 And there are ways to prepare these, so it's important -- 00:16:54.390 --> 00:16:57.560 this kind of thinking more along the food science idea. 00:16:57.560 --> 00:16:59.930 There's a way to actually grind down and prepare the 00:16:59.930 --> 00:17:03.180 flower, so that you promote the enzymes within the plant 00:17:03.180 --> 00:17:05.950 to breakdown the cyanide precursors. 00:17:05.950 --> 00:17:08.080 And if you put this in the well-ventilated area, if you 00:17:08.080 --> 00:17:11.980 prepare this outside, the h c n gas will actually be 00:17:11.980 --> 00:17:14.270 released into the air, so you're safe, 00:17:14.270 --> 00:17:15.730 you can eat it later. 00:17:15.730 --> 00:17:18.970 About 80% of the cyanide at that point is gone, so it does 00:17:18.970 --> 00:17:20.720 render the root much more safe. 00:17:20.720 --> 00:17:23.540 But you do, in fact, have to worry about long-term 00:17:23.540 --> 00:17:26.950 exposure, cyanide poisoning in terms of long-term effects in 00:17:26.950 --> 00:17:29.070 certain populations that do get the bulk of their 00:17:29.070 --> 00:17:31.460 carbohydrates from this root, from the root 00:17:31.460 --> 00:17:33.080 of the Cassava plant. 00:17:33.080 --> 00:17:35.680 But in terms of us going to the grocery store and thinking 00:17:35.680 --> 00:17:39.010 about things, probably we're all breathing sighs of relief. 00:17:39.010 --> 00:17:40.470 I just told you almonds are not a 00:17:40.470 --> 00:17:42.280 problem, no worries there. 00:17:42.280 --> 00:17:45.830 We probably don't find any forms of the Cassava plant 00:17:45.830 --> 00:17:48.540 ever in the U.S. that are going to have that high 00:17:48.540 --> 00:17:52.630 cyanide content, so we should all be relieved. 00:17:52.630 --> 00:17:55.910 Unless, of course, you're a smoker, or you're thinking of 00:17:55.910 --> 00:17:58.670 becoming a smoker, and then maybe you should worry, 00:17:58.670 --> 00:18:01.850 because this is one of the advertisements that was airing 00:18:01.850 --> 00:18:04.260 in terms of anti-smoking campaign. 00:18:04.260 --> 00:18:08.730 Hydrogen cyanide is found in cigarettes. 00:18:08.730 --> 00:18:11.740 So, if you're looking for another reason to quit, if 00:18:11.740 --> 00:18:14.490 you're looking for a reason not to start smoking, here's 00:18:14.490 --> 00:18:15.330 another good one. 00:18:15.330 --> 00:18:18.970 As I said, it's a particularly hazardous substance, this is 00:18:18.970 --> 00:18:20.040 worked with in fume hoods. 00:18:20.040 --> 00:18:22.230 You don't want to inhale it, it's definitely not 00:18:22.230 --> 00:18:23.750 recommended. 00:18:23.750 --> 00:18:27.260 The way, in the simplest terms that cyanide can kill you, is 00:18:27.260 --> 00:18:30.110 it basically out-competes your oxygen for the 00:18:30.110 --> 00:18:31.500 heme in your blood. 00:18:31.500 --> 00:18:34.580 So instead of carrying oxygen to your cells, you're carrying 00:18:34.580 --> 00:18:36.700 cyanide to your cells. 00:18:36.700 --> 00:18:39.170 Obviously, the amounts that are in cigarettes are not 00:18:39.170 --> 00:18:41.210 enough that people are dropping dead of cyanide 00:18:41.210 --> 00:18:44.800 poisoning, but still it's not a good idea if you can avoid 00:18:44.800 --> 00:18:47.730 eating or inhaling cyanide -- you definitely want to 00:18:47.730 --> 00:18:49.610 minimize your exposure. 00:18:49.610 --> 00:18:53.920 And in terms of thinking about it for organic chemistry or if 00:18:53.920 --> 00:18:56.880 you're interested in thinking about the mechanism maybe by 00:18:56.880 --> 00:19:00.230 which it is toxic, a first step would be to draw its 00:19:00.230 --> 00:19:01.770 Lewis structure. 00:19:01.770 --> 00:19:05.470 So, let's go ahead and make sure we can draw that, if we 00:19:05.470 --> 00:19:08.200 have interest either in the area of organic chemistry or 00:19:08.200 --> 00:19:10.670 biochemistry or biology here. 00:19:10.670 --> 00:19:14.300 So in terms of the first step of skeletal structure, this is 00:19:14.300 --> 00:19:17.370 actually going to be easier because we don't have a 00:19:17.370 --> 00:19:23.440 central atom, we just have carbon and nitrogen here. 00:19:23.440 --> 00:19:27.160 Our next step is thinking about valence electrons. 00:19:27.160 --> 00:19:32.440 So we have 4 plus 5, but we're actually not done yet, because 00:19:32.440 --> 00:19:36.190 it's c n minus, so if we have minus, we actually have an 00:19:36.190 --> 00:19:38.780 extra electron in our molecule. 00:19:38.780 --> 00:19:40.650 So we need to add 1 more. 00:19:40.650 --> 00:19:45.660 If instead we had a positive ion, a cation, what we would 00:19:45.660 --> 00:19:47.890 have to do is subtract 1. 00:19:47.890 --> 00:19:50.340 But here we're going to add 1, so again, we 00:19:50.340 --> 00:19:54.100 have 10 valence electrons. 00:19:54.100 --> 00:19:57.150 And if we go on to step three where we figure out how many 00:19:57.150 --> 00:20:00.810 we would need for full octets, it's just going to be 2 times 00:20:00.810 --> 00:20:04.780 8, so we have 16. 00:20:04.780 --> 00:20:09.710 And step four is going to have us figure out how many bonding 00:20:09.710 --> 00:20:16.230 electrons we have, so we have 16 minus 10, is going to be 6 00:20:16.230 --> 00:20:21.150 bonding electrons. 00:20:21.150 --> 00:20:29.210 So, step five tells us to add 2 electrons between each atom, 00:20:29.210 --> 00:20:32.260 so we add two there. 00:20:32.260 --> 00:20:35.290 And step six asks us, well, do we have any 00:20:35.290 --> 00:20:36.630 bonding electrons left? 00:20:36.630 --> 00:20:40.120 So how many bonding electrons do we have left? 00:20:40.120 --> 00:20:42.770 Yup, so we do, we have 4 left. 00:20:42.770 --> 00:20:45.600 We started with 6, we only used 2. 00:20:45.600 --> 00:20:48.330 This is very easy molecule because we know exactly where 00:20:48.330 --> 00:20:50.600 to put them without even having to think, we only have 00:20:50.600 --> 00:20:54.790 one option, and we'll make a triple bond between the carbon 00:20:54.790 --> 00:20:56.850 and the nitrogen. 00:20:56.850 --> 00:21:00.710 So, seven asks us if we have any valence electrons left, 00:21:00.710 --> 00:21:03.970 and how many valence electrons do we have left? 00:21:03.970 --> 00:21:06.070 Yeah, so also 4. 00:21:06.070 --> 00:21:09.540 We started with 10 valence electrons, we used up 6 of 00:21:09.540 --> 00:21:13.090 those as bonding electrons, so we have 4 left, which will be 00:21:13.090 --> 00:21:14.980 lone pair electrons. 00:21:14.980 --> 00:21:19.150 So, in order to fill our octet, what we do is put two 00:21:19.150 --> 00:21:23.900 on the nitrogen and two on the carbon. 00:21:23.900 --> 00:21:27.030 So, in terms of finishing our Lewis structure, we're 00:21:27.030 --> 00:21:29.720 actually not done yet here, even though we have full 00:21:29.720 --> 00:21:33.610 octets, and we've used up all of our valence electrons, and 00:21:33.610 --> 00:21:36.400 the reason is because it's c n minus, so we need to make sure 00:21:36.400 --> 00:21:39.290 that that's reflected in our Lewis structure, so let's put 00:21:39.290 --> 00:21:43.600 it in brackets here, and put a minus 1. 00:21:43.600 --> 00:21:45.920 And also I wanted to mention in terms of checking your 00:21:45.920 --> 00:21:49.230 Lewis structures, regardless of what they are, you should 00:21:49.230 --> 00:21:51.080 always go back and say how many valence 00:21:51.080 --> 00:21:52.370 electrons did I have -- 00:21:52.370 --> 00:21:57.290 I had 10, and then count 2, 4, 6, 8, 10, because you always 00:21:57.290 --> 00:21:59.770 need to make sure you have the same number of valence 00:21:59.770 --> 00:22:02.590 electrons that you calculated in your actual structure. 00:22:02.590 --> 00:22:05.270 That'll catch a lot of just silly mistakes for you if you 00:22:05.270 --> 00:22:08.190 go back and see it and you don't have all of that. 00:22:08.190 --> 00:22:11.260 Let's re-draw this, so it looks a little bit neater, 00:22:11.260 --> 00:22:16.350 where we have a triple bond in the middle instead, and again, 00:22:16.350 --> 00:22:19.240 we need our negative 1 charge there. 00:22:19.240 --> 00:22:23.010 And our eigth step in the process, again, is formal 00:22:23.010 --> 00:22:32.880 charge, which we will talk about very soon. 00:22:32.880 --> 00:22:33.200 All right. 00:22:33.200 --> 00:22:35.850 So let's try one more example of drawing Lewis structures 00:22:35.850 --> 00:22:38.100 before we talk about formal charge. 00:22:38.100 --> 00:22:40.800 And the last example that we're going to talk about is 00:22:40.800 --> 00:22:45.250 thionyl chloride, so it's s o c l 2. 00:22:45.250 --> 00:22:47.830 This is another good step forward, because now we 00:22:47.830 --> 00:22:51.890 actually have four different atoms in our molecule. 00:22:51.890 --> 00:22:55.220 I'll tell you a little about thionyl chloride as well. 00:22:55.220 --> 00:22:58.100 This is another organic chemistry reagent, it's also 00:22:58.100 --> 00:23:01.460 used extensively in the pharmaceutical industry. 00:23:01.460 --> 00:23:03.790 And what it's used is to convert one type of group, 00:23:03.790 --> 00:23:07.170 what's called a carboxylic acid into another type of very 00:23:07.170 --> 00:23:10.400 reactive intermediate, which is called an acid chloride. 00:23:10.400 --> 00:23:13.590 So I show that here, so in green, you have what's called 00:23:13.590 --> 00:23:18.170 a carboxcylic acid group, a c o o h, which gets converted by 00:23:18.170 --> 00:23:23.280 s o c l 2 to a c double bond o c l or an acid chloride. 00:23:23.280 --> 00:23:25.020 This is the very reactive intermediate. 00:23:25.020 --> 00:23:27.250 You'll learn a lot more about this if you take organic 00:23:27.250 --> 00:23:30.470 chemistry, but, In fact, you can then go on and make a 00:23:30.470 --> 00:23:33.730 bunch of other different kinds of very interesting molecules. 00:23:33.730 --> 00:23:36.920 So, for example, this is the synthesis of novacaine. 00:23:36.920 --> 00:23:38.020 This is what's used in industry to 00:23:38.020 --> 00:23:39.650 actually make novacaine. 00:23:39.650 --> 00:23:41.430 Has anyone had a novacaine procedure? 00:23:41.430 --> 00:23:42.060 Yes. 00:23:42.060 --> 00:23:47.910 I've had it also many times, you usually get 00:23:47.910 --> 00:23:49.530 novacaine for cavities. 00:23:49.530 --> 00:23:51.480 There's some alternatives that are used now as well. 00:23:51.480 --> 00:23:54.260 It's also used as a local anesthetic for other types of 00:23:54.260 --> 00:23:55.580 small procedures. 00:23:55.580 --> 00:23:58.250 So, this is, in fact, what's used to make 00:23:58.250 --> 00:23:59.780 novacaine in industry. 00:23:59.780 --> 00:24:02.540 You'll notice that a lot of different kinds medications do 00:24:02.540 --> 00:24:05.810 you have chlorine in them, you'll see that c l group. 00:24:05.810 --> 00:24:09.160 So, for example, you might be familiar with Wellbutrin here, 00:24:09.160 --> 00:24:12.170 this is a type of anti-depressant that a lot of 00:24:12.170 --> 00:24:14.070 people use right now that are taking anti-depressants. 00:24:14.070 --> 00:24:19.200 It's on the market, very popular in terms of your 00:24:19.200 --> 00:24:22.230 choices right now as an option as an anti-depressant. 00:24:22.230 --> 00:24:24.070 Also, Lunesta. 00:24:24.070 --> 00:24:27.390 This was very big in an ad campaign at least last year, 00:24:27.390 --> 00:24:30.010 I'm not sure if it still is, with the little butterfly. 00:24:30.010 --> 00:24:32.430 This is the structure of Lunesta, and you see the c l 00:24:32.430 --> 00:24:33.620 in it as well. 00:24:33.620 --> 00:24:36.700 I just wanted to point out that although you see these 00:24:36.700 --> 00:24:41.100 chlorine atoms in these drugs, what you almost never see is 00:24:41.100 --> 00:24:44.310 an acid chloride -- in fact, I don't think I've ever seen an 00:24:44.310 --> 00:24:47.360 acid chloride in a final pharmaceutical product or drug 00:24:47.360 --> 00:24:50.650 that we take, and the reason is because they're so reactive 00:24:50.650 --> 00:24:53.340 that you wouldn't want to have that in something you digest. 00:24:53.340 --> 00:24:56.420 So just keep in mind when you do see the chlorine in these 00:24:56.420 --> 00:24:58.550 drugs, it's very different from the acid chloride. 00:24:58.550 --> 00:25:02.290 So, for example, Wellbutrin, it is very unlikely that it 00:25:02.290 --> 00:25:06.140 would have thionyl chloride in order to make it, and if 00:25:06.140 --> 00:25:08.990 thionyl chloride was used at some point in the synthesis, 00:25:08.990 --> 00:25:11.150 it was not to put that chlorine atom on, it was to 00:25:11.150 --> 00:25:12.680 put something else on. 00:25:12.680 --> 00:25:16.190 But in terms of drugs that don't look like maybe this 00:25:16.190 --> 00:25:19.230 compound was used in the synthesis, many of them might 00:25:19.230 --> 00:25:23.170 have used thionyl chloride, because it generates such a 00:25:23.170 --> 00:25:25.560 nice reactive intermediate that you can go on and make a 00:25:25.560 --> 00:25:28.200 bunch of different compounds from that intermediate. 00:25:28.200 --> 00:25:28.490 All right. 00:25:28.490 --> 00:25:30.760 So let's think about how to draw the Lewis structure for 00:25:30.760 --> 00:25:34.620 thionyl chloride -- oh, actually, let me let you tell 00:25:34.620 --> 00:25:37.050 me how we should start this Lewis structure. 00:25:37.050 --> 00:25:39.890 So, which atom would you expect to be in the center of 00:25:39.890 --> 00:25:50.170 a Lewis structure for thionyl chloride? 00:25:50.170 --> 00:25:50.410 All right. 00:25:50.410 --> 00:25:59.080 Let's take 10 seconds on that. 00:25:59.080 --> 00:26:01.780 Looks like we have some fast thinking here, a lot of last 00:26:01.780 --> 00:26:03.650 minute answers coming in. 00:26:03.650 --> 00:26:04.880 OK. 00:26:04.880 --> 00:26:08.850 We have a split decision, so -- you know what, actually, 00:26:08.850 --> 00:26:10.510 let's think about this for a second. 00:26:10.510 --> 00:26:14.940 So hopefully, it was a time issue in terms of looking at 00:26:14.940 --> 00:26:18.670 the periodic table, because let's have you tell me what 00:26:18.670 --> 00:26:22.480 are we looking for here? 00:26:22.480 --> 00:26:22.770 Yeah. 00:26:22.770 --> 00:26:22.980 OK. 00:26:22.980 --> 00:26:25.440 We're looking for the lowest ionization energy. 00:26:25.440 --> 00:26:28.230 So, this one can be tricky because oxygen looks like it's 00:26:28.230 --> 00:26:30.540 in the middle because of the way it's written, but we need 00:26:30.540 --> 00:26:33.250 to start by looking at the lowest ionization energy. 00:26:33.250 --> 00:26:37.170 So, if we look on the periodic table, comparing, for example, 00:26:37.170 --> 00:26:41.720 s to o, if we have s it's below o, what happens to 00:26:41.720 --> 00:26:44.760 ionization energy as we go down a table? 00:26:44.760 --> 00:26:46.070 It decreases. 00:26:46.070 --> 00:26:48.550 If you're still not completely up on the periodic trends, 00:26:48.550 --> 00:26:51.130 that is stuff that's going to be on the first exam, so make 00:26:51.130 --> 00:26:54.280 sure that you're able to do this without taking too much 00:26:54.280 --> 00:26:55.340 time to think about it. 00:26:55.340 --> 00:26:58.420 We would expect the ionization energy to decrease, that means 00:26:58.420 --> 00:27:00.880 that sulfur has our lowest ionization energy. 00:27:00.880 --> 00:27:01.480 All right. 00:27:01.480 --> 00:27:05.950 So, let's go ahead and draw our Lewis structure here with 00:27:05.950 --> 00:27:08.390 sulfur in the middle. 00:27:08.390 --> 00:27:13.030 So, we can put our sulfur in the middle, and then it 00:27:13.030 --> 00:27:16.120 doesn't really matter how we draw the rest of it, where we 00:27:16.120 --> 00:27:18.930 put our c l's versus where we put our oxygens. 00:27:18.930 --> 00:27:23.680 We'll just put them in some way around our sulfur atom. 00:27:23.680 --> 00:27:26.050 So that's our step one. 00:27:26.050 --> 00:27:28.600 For our step two, what we need is 00:27:28.600 --> 00:27:31.130 number of valence electrons. 00:27:31.130 --> 00:27:33.600 So we have 2 for each of the chlorine. 00:27:33.600 --> 00:27:35.570 How many valence electrons are in chlorine? 00:27:35.570 --> 00:27:39.230 All right. 00:27:39.230 --> 00:27:41.860 So it's 7 that are in chlorine, it's the same as 00:27:41.860 --> 00:27:45.500 fluorine or any of the others in that row or 00:27:45.500 --> 00:27:46.440 in that group rather. 00:27:46.440 --> 00:27:51.990 2 times 7, plus we have 6 in the sulfur, and oxygen is 00:27:51.990 --> 00:27:55.000 right above sulfur, so that also has 6. 00:27:55.000 --> 00:27:59.240 So we end up having 26 valence electrons that we're 00:27:59.240 --> 00:28:01.220 dealing with here. 00:28:01.220 --> 00:28:04.870 Our step three is to figure out how many bonding electrons 00:28:04.870 --> 00:28:08.070 that we need, or excuse me, how many total electrons that 00:28:08.070 --> 00:28:11.630 we need to fill up our octets, so that's just going to be 4 00:28:11.630 --> 00:28:15.730 times 8, which is 32. 00:28:15.730 --> 00:28:20.810 And then we take 32 minus 26. 00:28:20.810 --> 00:28:25.360 So what we end up with in terms of our bonding electrons 00:28:25.360 --> 00:28:27.540 is going to be 6 bonding electrons. 00:28:27.540 --> 00:28:30.550 So we can go right ahead and fill these in. 00:28:30.550 --> 00:28:36.060 1 2, 3 4, 5 and 6. 00:28:36.060 --> 00:28:37.910 And that was step five. 00:28:37.910 --> 00:28:40.080 Step six is thinking about do we have any 00:28:40.080 --> 00:28:42.970 bonding electrons left? 00:28:42.970 --> 00:28:44.240 Nope, we used them all up. 00:28:44.240 --> 00:28:46.920 So we don't need to put any more bonds in there. 00:28:46.920 --> 00:28:52.280 And step seven is how many electrons do we have left over 00:28:52.280 --> 00:28:54.130 that are going to go into lone pairs? 00:28:54.130 --> 00:28:55.800 How many? 00:28:55.800 --> 00:28:56.290 20. 00:28:56.290 --> 00:29:01.580 26 minus 6, so that tells us 20, and these are all going to 00:29:01.580 --> 00:29:02.770 be lone pairs. 00:29:02.770 --> 00:29:04.780 Well, we're talking about a pretty high number here, so to 00:29:04.780 --> 00:29:08.330 make counting easier, we'll just say 10 lone pairs, 00:29:08.330 --> 00:29:10.480 because 20 lone pair electrons is the same 00:29:10.480 --> 00:29:12.130 thing as 10 lone pairs. 00:29:12.130 --> 00:29:14.310 And all we need to do is go over here now 00:29:14.310 --> 00:29:16.270 and fill up our octets. 00:29:16.270 --> 00:29:23.670 So oxygen gets 3 pairs, and each chlorine gets 3 pairs, so 00:29:23.670 --> 00:29:26.900 now we're up to 9 pairs. 00:29:26.900 --> 00:29:31.840 And what we have left here is the sulfur, which will also 00:29:31.840 --> 00:29:33.800 get a pair. 00:29:33.800 --> 00:29:36.320 So, if you look at all of these, we have full octets for 00:29:36.320 --> 00:29:39.100 all of them, and if we count up all of the valence 00:29:39.100 --> 00:29:44.520 electrons, it's going to be equal to our number 26 here. 00:29:44.520 --> 00:29:47.400 And the last thing we do for any of our structures to check 00:29:47.400 --> 00:29:50.080 them and figure out are these valid or not valid, are these 00:29:50.080 --> 00:29:54.380 good Lewis structures is to check the formal charge. 00:29:54.380 --> 00:29:56.520 So now that we have enough practice drawing Lewis 00:29:56.520 --> 00:29:59.450 structures, let's talk about actually figuring out this 00:29:59.450 --> 00:30:02.670 formal charge. 00:30:02.670 --> 00:30:05.500 So when we talk about formal charge, basically formal 00:30:05.500 --> 00:30:09.990 charge is the measure of the extent to which an individual 00:30:09.990 --> 00:30:12.060 atom within your molecule has either 00:30:12.060 --> 00:30:14.400 gained or lost an electron. 00:30:14.400 --> 00:30:17.880 So as we said when we first introduced covalent bonds, 00:30:17.880 --> 00:30:20.510 it's a sharing of electrons, but it's not 00:30:20.510 --> 00:30:22.120 always an equal sharing. 00:30:22.120 --> 00:30:24.700 Sometimes we have a very electronegative atom that's 00:30:24.700 --> 00:30:27.330 going to take more of its equal 00:30:27.330 --> 00:30:29.270 share of electron density. 00:30:29.270 --> 00:30:31.500 So for example, that might have a formal charge of 00:30:31.500 --> 00:30:35.980 negative 1, because to some extent it has gained that much 00:30:35.980 --> 00:30:38.210 electron density that it now has a formal 00:30:38.210 --> 00:30:40.000 charge that's negative. 00:30:40.000 --> 00:30:44.450 So, when we think about any type of formal charges, we 00:30:44.450 --> 00:30:48.520 have to assign these based on a formula here, which is very 00:30:48.520 --> 00:30:50.260 easy to follow. 00:30:50.260 --> 00:30:54.310 Formal charge equals v minus l minus 1/2 s. 00:30:54.310 --> 00:30:57.200 It's even easier to follow if we know what all 00:30:57.200 --> 00:30:59.820 of those stand for. 00:30:59.820 --> 00:31:04.960 So, f c, I think you all know is formal charge. 00:31:04.960 --> 00:31:10.940 Does anyone have a guess for v? 00:31:10.940 --> 00:31:12.530 Everyone has a guess, great. 00:31:12.530 --> 00:31:14.930 Valence electrons. 00:31:14.930 --> 00:31:17.610 What about l? 00:31:17.610 --> 00:31:20.120 Lone pairs. 00:31:20.120 --> 00:31:23.530 So, lone pair electrons, actually, not lone pairs 00:31:23.530 --> 00:31:25.780 themselves. 00:31:25.780 --> 00:31:28.440 And then s? 00:31:28.440 --> 00:31:28.710 Good. 00:31:28.710 --> 00:31:30.760 That's a tricky one, shared electrons. 00:31:30.760 --> 00:31:35.930 All right. 00:31:35.930 --> 00:31:38.900 So this means we can actually calculate this for any 00:31:38.900 --> 00:31:41.680 molecule that we've drawn the Lewis structure for, because 00:31:41.680 --> 00:31:44.560 we actually do need to draw the Lewis structure before we 00:31:44.560 --> 00:31:47.290 know, for example, how many of each of these we have, or at 00:31:47.290 --> 00:31:50.080 least go through the rules. 00:31:50.080 --> 00:31:53.090 And what's important to keep in mind about formal charge is 00:31:53.090 --> 00:31:56.570 if we have a neutral atom, such as we did in thionyl 00:31:56.570 --> 00:32:00.650 chloride here, the sum of the individual formal charges on 00:32:00.650 --> 00:32:04.840 individual atoms within the molecule have to equal 0. 00:32:04.840 --> 00:32:07.620 So if we add them all up, there should be no net charge 00:32:07.620 --> 00:32:11.240 on the molecule, if the molecule is neutral. 00:32:11.240 --> 00:32:15.460 So, if we think about the second case here where we have 00:32:15.460 --> 00:32:18.700 c n minus, now we're talking about a molecule with a net 00:32:18.700 --> 00:32:20.370 charge of negative 1. 00:32:20.370 --> 00:32:23.350 So that means if we add up all of the formal charges within 00:32:23.350 --> 00:32:26.500 the molecule, what we would expect to see is that they sum 00:32:26.500 --> 00:32:29.940 up to give a net charge of negative 1. 00:32:29.940 --> 00:32:33.840 So we can do this for any final charge we have, if we a 00:32:33.840 --> 00:32:37.110 molecule that has a charge of plus 2, then all of the formal 00:32:37.110 --> 00:32:41.470 charges should add up to plus 2 and so on. 00:32:41.470 --> 00:32:44.470 So, let's just figure this out for some of the examples we 00:32:44.470 --> 00:32:45.820 did, so for the cyanide anion. 00:32:45.820 --> 00:32:49.250 So, if we want to figure out the formal charge on the 00:32:49.250 --> 00:32:52.810 carbon, we need to take the number of valence electrons, 00:32:52.810 --> 00:32:54.110 so that's 4. 00:32:54.110 --> 00:32:58.530 We need to subtract the lone pair, what number is that? 00:32:58.530 --> 00:32:59.600 It's 2. 00:32:59.600 --> 00:33:03.140 And then 1/2 of the number of shared electrons. 00:33:03.140 --> 00:33:06.050 So, shared electrons are the ones that are shared between 00:33:06.050 --> 00:33:10.770 the carbon and the nitrogen, so we have 6 shared electrons, 00:33:10.770 --> 00:33:12.120 and we want to take 1/2 of that. 00:33:12.120 --> 00:33:13.770 So we end up with a formal charge on 00:33:13.770 --> 00:33:16.370 carbon of negative 1. 00:33:16.370 --> 00:33:18.330 We can do the same thing for nitrogen. 00:33:18.330 --> 00:33:21.410 So in terms of nitrogen that starts off with a valence 00:33:21.410 --> 00:33:25.630 number of 5, again we have 2 lone pair electrons in the 00:33:25.630 --> 00:33:29.900 nitrogen, and again, we have 6 electrons that are shared. 00:33:29.900 --> 00:33:31.750 So what we see is that the formal charge on 00:33:31.750 --> 00:33:34.000 the nitrogen is 0. 00:33:34.000 --> 00:33:37.270 Also, formal charges can be checked, as I just said. 00:33:37.270 --> 00:33:41.110 Negative 1 plus 0 should add up to negative 1, if in fact, 00:33:41.110 --> 00:33:43.270 we're correct for the c n anion. 00:33:43.270 --> 00:33:48.040 And it does, so we know that we're probably on target in 00:33:48.040 --> 00:33:50.990 terms of calculating our formal charge. 00:33:50.990 --> 00:33:53.440 So, let's think about our second example -- actually our 00:33:53.440 --> 00:33:55.290 third, but the second one we're going to talk about in 00:33:55.290 --> 00:33:58.490 terms of formal charge, which is thionyl chloride. 00:33:58.490 --> 00:34:01.220 So why don't you tell me what the formal charge should be on 00:34:01.220 --> 00:34:03.910 the sulfur atom of thionyl chloride? 00:34:03.910 --> 00:34:38.050 All right. 00:34:38.050 --> 00:34:53.610 Let's take 10 more seconds. 00:34:53.610 --> 00:34:56.970 OK, so the majority got it. 00:34:56.970 --> 00:35:00.460 So hopefully next time we do a formal charge question, we'll 00:35:00.460 --> 00:35:01.740 get everyone back up to speed. 00:35:01.740 --> 00:35:04.030 But we've just introduced it, so let's go back to the class 00:35:04.030 --> 00:35:06.890 notes and explain why this is the correct answer. 00:35:06.890 --> 00:35:10.400 So if we look at sulfur, what we need to do is take the 00:35:10.400 --> 00:35:13.850 valence electrons in sulfur, and there are 6. 00:35:13.850 --> 00:35:16.190 By looking at the periodic table it's right underneath 00:35:16.190 --> 00:35:19.700 oxygen, so those both have 6 valence electrons. 00:35:19.700 --> 00:35:23.890 There are 2 lone pair electrons on sulfur -- we only 00:35:23.890 --> 00:35:29.140 have 2 lone pairs -- or 1 lone pair, 2 lone pair electrons. 00:35:29.140 --> 00:35:32.870 And then we end up having 6 shared electrons, 2 from each 00:35:32.870 --> 00:35:35.710 of the bonds, so we end up with a formal charge on 00:35:35.710 --> 00:35:37.980 sulfur of plus 1. 00:35:37.980 --> 00:35:42.040 If we go to the oxygen atom, now we're talking about 00:35:42.040 --> 00:35:45.810 starting with 6 in terms of valence electrons again, but 00:35:45.810 --> 00:35:50.040 instead of 2, you can see we have 6 lone pair electrons 00:35:50.040 --> 00:35:54.180 around the oxygen minus 1/2 of 2, so we have minus 1 is our 00:35:54.180 --> 00:35:55.360 formal charge. 00:35:55.360 --> 00:35:58.380 And if we talk about chlorine, and both of the chlorines are 00:35:58.380 --> 00:36:01.870 the same in this case, we start with a valence number of 00:36:01.870 --> 00:36:06.140 7 for chlorine, and then we subtract 6, because it had 6 00:36:06.140 --> 00:36:09.340 lone pair electrons around each of the chlorine atoms. 00:36:09.340 --> 00:36:14.700 Then minus 1/2 of 2, because we only have one bond or 2 00:36:14.700 --> 00:36:16.460 electrons in a bond. 00:36:16.460 --> 00:36:19.140 So again, we should be able to check all of our formal 00:36:19.140 --> 00:36:23.010 charges and make sure they add up to 0, which they do, and 00:36:23.010 --> 00:36:25.390 that makes sense, because we have a neutral atom in terms 00:36:25.390 --> 00:36:28.140 of thionyl chloride. 00:36:28.140 --> 00:36:30.700 Another thing to mention in terms of thinking about if you 00:36:30.700 --> 00:36:34.090 have a good or a bad Lewis structure, is that when you 00:36:34.090 --> 00:36:37.550 figure out the formal charge on each of the atoms, it's the 00:36:37.550 --> 00:36:40.290 more electronegative atoms that you would expect to have 00:36:40.290 --> 00:36:42.850 that negative charge, and that should make sense to you 00:36:42.850 --> 00:36:46.310 because electronegative atoms want to have electron density, 00:36:46.310 --> 00:36:49.350 they want to pull in electron density to them, so it would 00:36:49.350 --> 00:36:51.730 make sense that they have more of it, which would give them a 00:36:51.730 --> 00:36:53.630 negative charge. 00:36:53.630 --> 00:36:58.030 So, if we compare the sulfur to the oxygen, the oxygen it 00:36:58.030 --> 00:37:00.120 turns out is more electronegative and that is 00:37:00.120 --> 00:37:04.690 what holds the negative charge in this molecule. 00:37:04.690 --> 00:37:07.570 Another thing I want to point out, and for some of you just 00:37:07.570 --> 00:37:09.930 ignore what I'm saying, if you haven't thought about 00:37:09.930 --> 00:37:12.450 oxidation number, if you haven't heard of that before, 00:37:12.450 --> 00:37:14.230 don't worry we'll get to it in the second half 00:37:14.230 --> 00:37:15.440 with Professor Drennan. 00:37:15.440 --> 00:37:17.520 But for those of you from high school that have learned about 00:37:17.520 --> 00:37:20.570 oxidation number and maybe are starting to think about it 00:37:20.570 --> 00:37:23.230 when you look at these molecules, formal charge is -- 00:37:23.230 --> 00:37:25.890 it's not the same thing as oxidation number, so separate 00:37:25.890 --> 00:37:27.300 those two things in your head. 00:37:27.300 --> 00:37:29.760 We'll get to oxidation number in the second half of this 00:37:29.760 --> 00:37:31.690 course, but it's not in any way the same 00:37:31.690 --> 00:37:35.190 idea as formal charge. 00:37:35.190 --> 00:37:35.450 All right. 00:37:35.450 --> 00:37:38.000 So formal charge can actually help us out when we're trying 00:37:38.000 --> 00:37:41.200 to decide between several Lewis structures that look 00:37:41.200 --> 00:37:43.870 like they might be comparable in terms of which might be the 00:37:43.870 --> 00:37:46.170 lower energy or the more stable structure. 00:37:46.170 --> 00:37:48.540 The examples we've done so far have been pretty 00:37:48.540 --> 00:37:51.480 straightforward, so we haven't needed to use formal charge to 00:37:51.480 --> 00:37:53.120 make this kind of decision. 00:37:53.120 --> 00:37:56.620 But we could, for example, look at a case where we have 00:37:56.620 --> 00:37:59.620 several different structures that look pretty good, and the 00:37:59.620 --> 00:38:02.290 one we want to determine as being the lowest energy 00:38:02.290 --> 00:38:05.740 structure is the one in which the absolute values of the 00:38:05.740 --> 00:38:08.940 formal charges are going to be lower, so essentially that 00:38:08.940 --> 00:38:11.120 they have less charge separation. 00:38:11.120 --> 00:38:14.140 Those are going to be the more stable or the lower energy 00:38:14.140 --> 00:38:16.530 structures. 00:38:16.530 --> 00:38:19.780 So, for example, let's look at thiocyanate ion, we 00:38:19.780 --> 00:38:22.380 have c s and n. 00:38:22.380 --> 00:38:25.220 What we've learned so far is as a first approximation, what 00:38:25.220 --> 00:38:28.540 we want to do is put the atom with the lowest ionization 00:38:28.540 --> 00:38:30.270 energy in the middle here. 00:38:30.270 --> 00:38:33.670 So let's compare those ionization energies. 00:38:33.670 --> 00:38:38.800 We have 10 90 for carbon, 1,000 for sulfur, and 1,400 00:38:38.800 --> 00:38:39.950 for nitrogen. 00:38:39.950 --> 00:38:42.650 So, thinking about ionization energy, which atom would you 00:38:42.650 --> 00:38:46.590 put in the middle here? 00:38:46.590 --> 00:38:48.530 So, a lot of people I hear are saying 00:38:48.530 --> 00:38:50.070 sulfur, and that's right. 00:38:50.070 --> 00:38:52.530 So, in terms of ionization energy, we would expect to see 00:38:52.530 --> 00:38:53.450 sulfur in the middle. 00:38:53.450 --> 00:38:56.780 So if we went through and drew out our Lewis structure 00:38:56.780 --> 00:38:59.690 following each of our steps, what we would get is this as 00:38:59.690 --> 00:39:02.230 our Lewis structure here, and we could figure out all of the 00:39:02.230 --> 00:39:03.380 formal charges. 00:39:03.380 --> 00:39:05.830 But what I'm going to tell you already is this is a case 00:39:05.830 --> 00:39:08.710 where, in fact, it's an exception to the idea that the 00:39:08.710 --> 00:39:11.600 lowest energy structure has the lowest ionization energy 00:39:11.600 --> 00:39:14.180 in the middle, and we can figure this out when we look 00:39:14.180 --> 00:39:15.380 at formal charge. 00:39:15.380 --> 00:39:17.880 It's always a good first approximation, because you 00:39:17.880 --> 00:39:19.940 need to start somewhere in terms of drawing Lewis 00:39:19.940 --> 00:39:22.500 structures, but then if you go and figure out the formal 00:39:22.500 --> 00:39:25.750 charge and you just have lots of charge separation or very 00:39:25.750 --> 00:39:29.360 high charges, like a plus 2 and a minus 2 and a minus 1 00:39:29.360 --> 00:39:31.850 all different places in the atom, what it should tell you 00:39:31.850 --> 00:39:33.690 is maybe there's a better structure. 00:39:33.690 --> 00:39:36.070 So, let's think of all of the combinations that we could 00:39:36.070 --> 00:39:38.120 have in terms of this molecule. 00:39:38.120 --> 00:39:40.620 So in one case, we could actually put carbon in the 00:39:40.620 --> 00:39:43.270 middle, in one place, we could put sulfur in the middle, and 00:39:43.270 --> 00:39:44.960 in one case we could put nitrogen. 00:39:44.960 --> 00:39:47.280 And then we can go ahead and let's quickly write out what 00:39:47.280 --> 00:39:49.720 the formal charges for all of these will be. 00:39:49.720 --> 00:39:53.410 So in our first structure, we would find for the nitrogen we 00:39:53.410 --> 00:39:56.980 have a formal charge 5 minus 4 minus 2, because we're 00:39:56.980 --> 00:40:00.840 starting with 5 valence electrons, so that is a formal 00:40:00.840 --> 00:40:03.940 charge of minus 1. 00:40:03.940 --> 00:40:08.070 For the carbon, we start with 4 valence electrons, we have 0 00:40:08.070 --> 00:40:12.370 lone pair electrons minus 4, and we end up with a formal 00:40:12.370 --> 00:40:14.790 charge of 0. 00:40:14.790 --> 00:40:18.220 For the sulfur, we start off with 6 valence electrons, 00:40:18.220 --> 00:40:22.730 minus 4 lone pair electrons, minus 2, taking in account our 00:40:22.730 --> 00:40:26.820 bonding electrons, so we end up with a formal charge of 0. 00:40:26.820 --> 00:40:28.080 All right, so this looks pretty good. 00:40:28.080 --> 00:40:30.180 We don't actually have much charge separation 00:40:30.180 --> 00:40:31.530 in this case here. 00:40:31.530 --> 00:40:34.650 Let's take a look at the lowest ionization energy in 00:40:34.650 --> 00:40:35.950 the center case. 00:40:35.950 --> 00:40:39.030 And what you find out if you do these calculations, is that 00:40:39.030 --> 00:40:42.980 you have a negative 1 for your formal charge on nitrogen, you 00:40:42.980 --> 00:40:47.210 have a negative 2 for your formal charge on carbon, and 00:40:47.210 --> 00:40:51.330 you have a positive 2 for your formal charge on sulfur. 00:40:51.330 --> 00:40:53.940 If we look at our last structure here where we have 00:40:53.940 --> 00:40:56.230 nitrogen the middle, we can also figure out all those 00:40:56.230 --> 00:40:59.640 formal charges, and in this case we have plus 1 on the 00:40:59.640 --> 00:41:04.560 nitrogen, we have minus 2 on the carbon, and then we end up 00:41:04.560 --> 00:41:08.450 with a 0 on the sulfur there. 00:41:08.450 --> 00:41:10.460 So let's go to a clicker question. 00:41:10.460 --> 00:41:13.950 If we call this structure a, b, and c -- you can look on 00:41:13.950 --> 00:41:16.260 your notes, it also says structure a, b, and c on your 00:41:16.260 --> 00:41:17.480 notes, so you didn't need to have just 00:41:17.480 --> 00:41:19.250 memorized that slide. 00:41:19.250 --> 00:41:23.080 But I want you to tell me in terms of thinking about formal 00:41:23.080 --> 00:41:25.420 charge, which Lewis structure would you predict to be the 00:41:25.420 --> 00:41:29.670 most stable? 00:41:29.670 --> 00:41:43.780 And this should be fast, so let's take 10 seconds on this. 00:41:43.780 --> 00:41:44.600 Okay, great. 00:41:44.600 --> 00:41:47.050 So let's go back to our notes here. 00:41:47.050 --> 00:41:48.920 And the reason that this should be so fast is we 00:41:48.920 --> 00:41:50.210 already did all the calculations 00:41:50.210 --> 00:41:51.650 for the formal charges. 00:41:51.650 --> 00:41:54.740 So what we see is that structure a is the most stable 00:41:54.740 --> 00:41:57.510 because we have the least separation of charge in the 00:41:57.510 --> 00:42:00.130 case of structure a. 00:42:00.130 --> 00:42:03.360 And you can do this any time if you have Lewis structures 00:42:03.360 --> 00:42:04.980 that you're choosing between. 00:42:04.980 --> 00:42:08.750 You won't always draw out every single possibility that 00:42:08.750 --> 00:42:09.890 you have to start with. 00:42:09.890 --> 00:42:12.070 Often a good thing to start with is to put the lowest 00:42:12.070 --> 00:42:14.960 ionization energy atom in the middle, and if you don't have 00:42:14.960 --> 00:42:18.290 charge separation then go with that structure, but if you do 00:42:18.290 --> 00:42:20.450 find you have a lot of separation, such as the case 00:42:20.450 --> 00:42:23.570 in negative 2, positive 2, and minus 1, then you want to say 00:42:23.570 --> 00:42:26.320 wait a second, this is really bad in terms of formal charge, 00:42:26.320 --> 00:42:30.880 let me go ahead and see what other options I have here. 00:42:30.880 --> 00:42:36.260 So, we can also get into a case where we have similar 00:42:36.260 --> 00:42:39.270 values in terms of absolute values of formal charge 00:42:39.270 --> 00:42:41.280 between two different molecules we're deciding 00:42:41.280 --> 00:42:43.320 between in their Lewis structures. 00:42:43.320 --> 00:42:45.580 And in this case, the tie-breaker goes to the 00:42:45.580 --> 00:42:48.660 molecule in which the negative charge is on the most 00:42:48.660 --> 00:42:51.030 electronegative atom. 00:42:51.030 --> 00:42:53.440 So, let's look at an example of this here. 00:42:53.440 --> 00:42:56.740 So we could say, for example, this molecule here, this -- 00:42:56.740 --> 00:42:59.130 now we're dealing with a lot of different atoms in the 00:42:59.130 --> 00:43:01.340 molecule, much more complicated than the initial 00:43:01.340 --> 00:43:04.080 case of the cyanide ion where we only had two. 00:43:04.080 --> 00:43:06.660 So, how do we figure out first how to draw the skeletal 00:43:06.660 --> 00:43:09.270 structure of this molecule here? 00:43:09.270 --> 00:43:13.360 And one thing I want to tell you to start out with is 00:43:13.360 --> 00:43:17.380 something about this c h 3 group here. 00:43:17.380 --> 00:43:23.210 Any time you see a c h 3, this means a methyl group. 00:43:23.210 --> 00:43:27.510 And if you draw out what a methyl group is -- hopefully 00:43:27.510 --> 00:43:35.880 you won't have to spell it -- then what you have is a carbon 00:43:35.880 --> 00:43:40.910 in the middle with three hydrogens around it, and then 00:43:40.910 --> 00:43:43.360 it can only be bonded to one other thing. 00:43:43.360 --> 00:43:46.990 So any time you see c h 3 here, remember that that's 00:43:46.990 --> 00:43:49.410 methyl and that's going to be a terminal group. 00:43:49.410 --> 00:43:51.740 So, you already have a hint that methyl groups are never 00:43:51.740 --> 00:43:54.150 in the middle, they always have to be on the outside. 00:43:54.150 --> 00:43:57.190 So that's a good start for us putting together a skeletal 00:43:57.190 --> 00:43:59.320 structure for this compound here. 00:43:59.320 --> 00:44:01.460 The other tip I'm going to give you is any time you see a 00:44:01.460 --> 00:44:05.080 chain molecule, by chain I just mean many different atoms 00:44:05.080 --> 00:44:06.480 written out in a row. 00:44:06.480 --> 00:44:10.440 A convention is that typically you will put a terminal atom. 00:44:10.440 --> 00:44:13.600 We know that h is always terminal, it's always on the 00:44:13.600 --> 00:44:15.720 end, never in the center. 00:44:15.720 --> 00:44:19.190 Right after the molecule that it's attached to. 00:44:19.190 --> 00:44:22.370 So, for instance, this would suggest to us by the way it's 00:44:22.370 --> 00:44:25.810 written, that the hydrogen is attached to the nitrogen and 00:44:25.810 --> 00:44:27.440 not the oxygen. 00:44:27.440 --> 00:44:30.390 So, if we use those two tips to try to figure out a 00:44:30.390 --> 00:44:33.300 structure, a skeletal structure, we would get this 00:44:33.300 --> 00:44:36.960 structure here if we write out the full Lewis structure. 00:44:36.960 --> 00:44:41.640 We could I think well, maybe this isn't written out in 00:44:41.640 --> 00:44:44.090 terms of that convention, which sometimes it's not, so 00:44:44.090 --> 00:44:46.790 let's also try writing it, such that we have the hydrogen 00:44:46.790 --> 00:44:48.600 and the oxygen atom there. 00:44:48.600 --> 00:44:50.370 So that would give us this structure here. 00:44:50.370 --> 00:44:53.532 So notice a difference in these structures, is this has 00:44:53.532 --> 00:44:57.550 an n h bond whereas this has an o h bond. 00:44:57.550 --> 00:45:01.120 And if we were to think about which one of these is better, 00:45:01.120 --> 00:45:03.750 it turns out that it's the same in terms of formal 00:45:03.750 --> 00:45:05.790 charges, so that doesn't help us out. 00:45:05.790 --> 00:45:08.490 So we need to go to this second case where we're 00:45:08.490 --> 00:45:11.650 instead going to think about electronegativity, and we want 00:45:11.650 --> 00:45:13.660 to think about which atom is the most electronegative. 00:45:13.660 --> 00:45:18.550 So, in this case, we see that our formal charge is negative 00:45:18.550 --> 00:45:22.200 on the nitrogen, in this case it's negative on oxygen. 00:45:22.200 --> 00:45:25.660 Which of those two is more electronegative? 00:45:25.660 --> 00:45:26.390 The oxygen. 00:45:26.390 --> 00:45:28.700 So you should be able to look at your periodic table and see 00:45:28.700 --> 00:45:31.250 this, or also I've written the trend here. 00:45:31.250 --> 00:45:35.630 So that means that the more stable molecule is going to be 00:45:35.630 --> 00:45:38.210 this molecule here, which actually puts the negative 00:45:38.210 --> 00:45:41.520 charge on be more electronegative atom. 00:45:41.520 --> 00:45:46.290 So this is our lower energy structure. 00:45:46.290 --> 00:45:49.190 So, these are the different ways that we can actually go 00:45:49.190 --> 00:45:51.790 ahead and use formal charge when we're choosing between 00:45:51.790 --> 00:45:53.650 different types of Lewis structures. 00:45:53.650 --> 00:45:56.520 So one last concept that I want introduces is this idea 00:45:56.520 --> 00:45:58.080 of resonance. 00:45:58.080 --> 00:46:01.670 And resonance is the idea that sometimes one single Lewis 00:46:01.670 --> 00:46:04.340 structure does not adequately describe the electron 00:46:04.340 --> 00:46:09.320 configuration around a given molecule, so instead you need 00:46:09.320 --> 00:46:12.320 to draw two different Lewis structures to describe that 00:46:12.320 --> 00:46:13.740 more appropriately. 00:46:13.740 --> 00:46:17.810 So, let's quickly go through the Lewis structure for ozone. 00:46:17.810 --> 00:46:20.280 I have the skeletal structure written up there, I've written 00:46:20.280 --> 00:46:22.140 it twice and you'll see why in a minute. 00:46:22.140 --> 00:46:23.910 It's easy to write the skeletal structure, because 00:46:23.910 --> 00:46:26.030 it's all oxygen, we don't have to worry about what's going to 00:46:26.030 --> 00:46:27.540 go in the middle. 00:46:27.540 --> 00:46:30.790 In this case, we're going to have 3 times 6 for valence 00:46:30.790 --> 00:46:31.090 electrons -- 00:46:31.090 --> 00:46:35.550 6 valence electrons for each oxygen, so we have 18 total 00:46:35.550 --> 00:46:37.860 valence electrons. 00:46:37.860 --> 00:46:41.740 So, in order to fill up our shell, what we need is 3 times 00:46:41.740 --> 00:46:44.090 8 or 24 electrons. 00:46:44.090 --> 00:46:48.820 This leaves us with 24 minus 18, or 6 00:46:48.820 --> 00:46:51.260 bonding electrons left. 00:46:51.260 --> 00:46:53.960 So what we can do is fill that in here. 00:46:53.960 --> 00:46:59.360 Clearly, we put 2 for each bond, and now we end up having 00:46:59.360 --> 00:47:01.770 2 remaining bonding electrons left. 00:47:01.770 --> 00:47:04.100 So here is where our question comes, because 00:47:04.100 --> 00:47:04.960 where do we put it? 00:47:04.960 --> 00:47:08.640 There's absolutely nothing that tells us which atoms we 00:47:08.640 --> 00:47:11.680 should put it between, because they're both oxygen-oxygen. 00:47:11.680 --> 00:47:15.320 So, let's just arbitrarily put it between these two in this 00:47:15.320 --> 00:47:18.880 case here, but actually there's no reason we couldn't 00:47:18.880 --> 00:47:21.730 also put it between oxygen b and c, so I'm going to draw 00:47:21.730 --> 00:47:25.020 another structure where we have it here. 00:47:25.020 --> 00:47:28.500 So in terms of remaining valence electrons we have 12, 00:47:28.500 --> 00:47:31.330 so we can finish off each of our Lewis structures, so 00:47:31.330 --> 00:47:34.120 that's our first structure there, and our second 00:47:34.120 --> 00:47:35.080 structure there. 00:47:35.080 --> 00:47:37.690 We could also figure out the formal charges, and obviously 00:47:37.690 --> 00:47:40.820 the formal charges between these two atoms, they're going 00:47:40.820 --> 00:47:42.500 to be identical, we're only dealing with 00:47:42.500 --> 00:47:44.810 oxygen atoms here. 00:47:44.810 --> 00:47:47.230 So, we need to think about what this means -- which is 00:47:47.230 --> 00:47:50.060 the more stable structure, because we have two different 00:47:50.060 --> 00:47:50.710 structures here. 00:47:50.710 --> 00:47:54.290 In this case we have a double bond between a and b, and in 00:47:54.290 --> 00:47:56.870 this case we have it between b and c. 00:47:56.870 --> 00:48:00.370 So, presumably, if we follow our rules so far only one of 00:48:00.370 --> 00:48:02.420 these should be correct. 00:48:02.420 --> 00:48:05.620 And again, if we figure out the formal charges, let's go 00:48:05.620 --> 00:48:10.230 through this quickly, we get 0 plus 1 and negative 1 for 00:48:10.230 --> 00:48:11.720 structure 1. 00:48:11.720 --> 00:48:16.270 We get negative 1 plus 1 and 0 for structure 2. 00:48:16.270 --> 00:48:18.780 So as I said, they're going to be identical in terms of 00:48:18.780 --> 00:48:21.120 making the decision that way. 00:48:21.120 --> 00:48:24.040 And what it turns out is experimental evidence tells us 00:48:24.040 --> 00:48:26.410 that these two structures are equivalent. 00:48:26.410 --> 00:48:28.850 And by that what we mean is that they're absolutely 00:48:28.850 --> 00:48:31.980 identical, and it turns out that this here is not a double 00:48:31.980 --> 00:48:34.730 bond, it's not a single bond either, it's actually 00:48:34.730 --> 00:48:35.670 something in between. 00:48:35.670 --> 00:48:38.980 So if we look at its length, it's actually shorter than a 00:48:38.980 --> 00:48:41.190 single bond, but longer than a double bond. 00:48:41.190 --> 00:48:44.170 Or if we look at how strong it is, it's actually stronger 00:48:44.170 --> 00:48:47.010 than a single bond, but weaker than a double bond. 00:48:47.010 --> 00:48:50.420 And we find the same thing for these two atoms here, it's not 00:48:50.420 --> 00:48:54.490 actually a double bond, it's somewhere between a single 00:48:54.490 --> 00:48:56.360 bond and a double bond. 00:48:56.360 --> 00:48:59.620 So this is a case where we have resonance structures, or 00:48:59.620 --> 00:49:01.820 we call this a resonance hybrid. 00:49:01.820 --> 00:49:03.950 So the reality of the situation is that it's a 00:49:03.950 --> 00:49:06.780 combination between these 2 structures. 00:49:06.780 --> 00:49:08.800 And an important thing to remember when we talk about 00:49:08.800 --> 00:49:12.530 resonance hybrids is that the structure it's not 1/2 the 00:49:12.530 --> 00:49:15.910 time this structure, and 1/2 of the time this structure, 00:49:15.910 --> 00:49:19.390 it's actually some combination or some average between the 00:49:19.390 --> 00:49:20.440 two structures. 00:49:20.440 --> 00:49:22.970 But since in drawing Lewis structures we don't have a way 00:49:22.970 --> 00:49:26.120 to represent that -- actually, in some cases you do, you can 00:49:26.120 --> 00:49:30.200 draw a dotted line that means a 1 and 1/2 bond, but most in 00:49:30.200 --> 00:49:33.570 most cases, we just draw out both resonance structures, and 00:49:33.570 --> 00:49:35.730 the way that we say it's a resonance structure is that we 00:49:35.730 --> 00:49:38.750 put it in the brackets and we put an arrow between it. 00:49:38.750 --> 00:49:40.860 So, when you think about resonance structures, some 00:49:40.860 --> 00:49:43.540 students tend to just get confused and be picturing this 00:49:43.540 --> 00:49:45.470 flickering back and forth. 00:49:45.470 --> 00:49:49.270 A good example to keep in mind is the idea of a mule. 00:49:49.270 --> 00:49:52.780 So most of you know, hopefully, that a mule is a 00:49:52.780 --> 00:49:57.670 combination of a donkey and a horse. 00:49:57.670 --> 00:50:00.590 A mule is not spending 1/2 of its time as a donkey, and 1/2 00:50:00.590 --> 00:50:04.620 of its time as a horse, we don't see it flickering back 00:50:04.620 --> 00:50:07.520 and forth between the two, that's not what we see. 00:50:07.520 --> 00:50:10.845 Instead what we see is it's an average, it's part like a 00:50:10.845 --> 00:50:12.680 horse, it's part like a donkey. 00:50:12.680 --> 00:50:16.940 So if we want to put that in chemical terms, we want to 00:50:16.940 --> 00:50:19.580 make sure we put these in brackets here, and remember, 00:50:19.580 --> 00:50:22.980 this is the resonance arrow, it's not a reaction arrow, 00:50:22.980 --> 00:50:25.140 it's a resonance arrow, so make sure you mark it up 00:50:25.140 --> 00:50:27.080 correctly like that. 00:50:27.080 --> 00:50:30.050 When we talk about resonance structures, the key word is 00:50:30.050 --> 00:50:33.120 that the electrons are de-localized. 00:50:33.120 --> 00:50:36.060 So they're not just between two atoms here. 00:50:36.060 --> 00:50:38.380 Now, for example, in our structure with ozone it's 00:50:38.380 --> 00:50:41.340 between all three atoms. 00:50:41.340 --> 00:50:44.240 And the final point I want to make today, and this is very, 00:50:44.240 --> 00:50:47.720 very important, so make sure that you do understand this. 00:50:47.720 --> 00:50:50.590 When we talk about resonance structures, we're talking 00:50:50.590 --> 00:50:54.560 about cases that have the same arrangement of atoms, the key 00:50:54.560 --> 00:50:57.365 is the atoms are the same, and the thing that is different is 00:50:57.365 --> 00:51:00.080 the arrangement of electrons here.