1 00:00:00 --> 00:00:01 2 00:00:01 --> 00:00:02 The following content is provided under a Creative 3 00:00:02 --> 00:00:03 Commons license. 4 00:00:03 --> 00:00:06 Your support will help MIT OpenCourseWare continue to 5 00:00:06 --> 00:00:10 offer high-quality educational resources for free. 6 00:00:10 --> 00:00:13 To make a donation or view additional materials from 7 00:00:13 --> 00:00:15 hundreds of MIT courses, visit MIT OpenCourseWare 8 00:00:15 --> 00:00:17 at ocw.mit.edu. 9 00:00:17 --> 00:00:22 PROFESSOR: All right, let's get started. 10 00:00:22 --> 00:00:26 Could everyone take 10 more seconds on the 11 00:00:26 --> 00:00:27 clicker question. 12 00:00:27 --> 00:00:35 And as a reminder, hopefully I don't need to remind any of 13 00:00:35 --> 00:00:38 you, but exam 1 is on Wednesday, so rather than our 14 00:00:38 --> 00:00:41 clicker question being on something from last class, 15 00:00:41 --> 00:00:44 which is exam 2 material, let's just make sure everyone 16 00:00:44 --> 00:00:48 remembers some small topic from exam 1 material, which is the 17 00:00:48 --> 00:00:50 idea of angular nodes. 18 00:00:50 --> 00:00:53 So I was hoping to see more like 99% on this, but you 19 00:00:53 --> 00:00:56 do still have two more days before the exam. 20 00:00:56 --> 00:00:59 Remember, we're talking about angular nodes here, so you need 21 00:00:59 --> 00:01:01 to read the question carefully. 22 00:01:01 --> 00:01:05 For an angular node, we're just talking about what the l value 23 00:01:05 --> 00:01:08 is, so whatever l is equal to is equal to the number of 24 00:01:08 --> 00:01:10 angular nodes you have. 25 00:01:10 --> 00:01:14 For an f orbital, what is the quantum number l equal to? 26 00:01:14 --> 00:01:15 Three. 27 00:01:15 --> 00:01:18 Good, so everyone that recognized that probably got 28 00:01:18 --> 00:01:20 the right answer of three angular nodes here. 29 00:01:20 --> 00:01:23 So let's switch to today's notes. 30 00:01:23 --> 00:01:26 Two more quick things about the exam, the first is that just 31 00:01:26 --> 00:01:29 remember on Wednesday, don't come here, the exam is not 32 00:01:29 --> 00:01:30 here, don't come here. 33 00:01:30 --> 00:01:33 It's in Walker, so make sure you go to Walker. 34 00:01:33 --> 00:01:36 And also, keep in mind that I have office hours 35 00:01:36 --> 00:01:38 today from 3 to 5. 36 00:01:38 --> 00:01:40 All your TA's have either already have their extra office 37 00:01:40 --> 00:01:43 hours, or there are some that will be going on tonight or 38 00:01:43 --> 00:01:46 tomorrow, so keep those in mind as your finishing up your 39 00:01:46 --> 00:01:48 studying for the exam. 40 00:01:48 --> 00:01:51 So, today we're moving on, we're talking about 41 00:01:51 --> 00:01:52 Lewis structures. 42 00:01:52 --> 00:01:55 This is a really good topic to do the class before an 43 00:01:55 --> 00:01:57 exam because it's a little bit of a lighter topic. 44 00:01:57 --> 00:02:00 We remember that Lewis structures are an idea that 45 00:02:00 --> 00:02:02 are pre-quantum mechanics. 46 00:02:02 --> 00:02:05 So that means that we don't have to worry about things like 47 00:02:05 --> 00:02:08 wave functions when we're talking about Lewis structures, 48 00:02:08 --> 00:02:10 but because they're so simple to use and because they so 49 00:02:10 --> 00:02:14 often predict the electron configuration of molecules 50 00:02:14 --> 00:02:17 accurately, we end up using them all the time in chemistry, 51 00:02:17 --> 00:02:20 so it's very valuable to know how to draw them correctly and 52 00:02:20 --> 00:02:22 to know how to work with them. 53 00:02:22 --> 00:02:25 So we'll talk specifically about drawing Lewis structures 54 00:02:25 --> 00:02:28 and then about formal charge and resonance, which are 55 00:02:28 --> 00:02:31 within Lewis structures. 56 00:02:31 --> 00:02:33 So remember, that when we talked about Lewis structure, 57 00:02:33 --> 00:02:37 the organizing principle behind Lewis structures is the idea 58 00:02:37 --> 00:02:40 that within the molecule the atoms are going to arrange 59 00:02:40 --> 00:02:43 their valence electrons, such that each atom within the 60 00:02:43 --> 00:02:48 molecule has a complete octet or full outer shell. 61 00:02:48 --> 00:02:51 So this is the idea of the octet rule that Lewis came 62 00:02:51 --> 00:02:54 up with way back in 1902. 63 00:02:54 --> 00:02:58 So, at the end of the class on Friday, we saw that list of 64 00:02:58 --> 00:03:00 eight steps that you always need to go through when you 65 00:03:00 --> 00:03:02 draw a Lewis structure. 66 00:03:02 --> 00:03:06 Once you're doing this on your own, especially, for example, 67 00:03:06 --> 00:03:09 on exam 2, which is a ways down the road, you won't be able to 68 00:03:09 --> 00:03:11 look at those steps, so you need to make sure that you can 69 00:03:11 --> 00:03:14 go through them without looking at them, but for now we can 70 00:03:14 --> 00:03:17 look at them as we are actually learning how to draw the Lewis 71 00:03:17 --> 00:03:20 structures, and rather just go through them step-by-step, it's 72 00:03:20 --> 00:03:22 more interesting to do it with an example. 73 00:03:22 --> 00:03:25 So let's hydrogen cyanide as our first example. 74 00:03:25 --> 00:03:27 So we have h c n. 75 00:03:27 --> 00:03:31 So, in this example here, we start with the first step -- 76 00:03:31 --> 00:03:34 the first step in any Lewis structure is drawing the 77 00:03:34 --> 00:03:35 skeletal structure. 78 00:03:35 --> 00:03:39 So, essentially drawing how the atoms are arranged 79 00:03:39 --> 00:03:40 within our molecule. 80 00:03:40 --> 00:03:44 And in this case we have three choices here in terms of what's 81 00:03:44 --> 00:03:48 going to be in the middle, so we need to decide that first. 82 00:03:48 --> 00:03:51 In terms of where different atoms are in a molecule, if you 83 00:03:51 --> 00:03:55 have a hydrogen atom or a fluorine atom, you can pretty 84 00:03:55 --> 00:03:58 much guarantee they're always going to be terminal atoms. 85 00:03:58 --> 00:04:00 By terminal I mean they're only bonded to one thing. 86 00:04:00 --> 00:04:03 So, for example, hydrogen or fluorine they'll never be in 87 00:04:03 --> 00:04:05 the middle, they'll always be on the end of a molecule. 88 00:04:05 --> 00:04:07 So that takes care of the hydrogen, what about between 89 00:04:07 --> 00:04:09 the carbon and the nitrogen? 90 00:04:09 --> 00:04:12 In terms of picking a Lewis structure that's going to be 91 00:04:12 --> 00:04:15 the lowest energy, what you want to do is put the atom with 92 00:04:15 --> 00:04:19 the lowest ionization energy in the center of your atom. 93 00:04:19 --> 00:04:21 This should make sense because if something has a low 94 00:04:21 --> 00:04:25 ionization energy, that means it's not very electronegative, 95 00:04:25 --> 00:04:27 which means it's going to be a lot happier giving up electron 96 00:04:27 --> 00:04:30 density, which is essentially what you're doing when you're 97 00:04:30 --> 00:04:32 forming covalent bonds is you're sharing some of 98 00:04:32 --> 00:04:33 your electron density. 99 00:04:33 --> 00:04:36 So, we keep the atoms with the lowest ionization 100 00:04:36 --> 00:04:37 energy in the center. 101 00:04:37 --> 00:04:41 So, why don't you go ahead and tell me, keeping that in mind, 102 00:04:41 --> 00:04:46 which atom in terms of h c or n would you expect to be in the 103 00:04:46 --> 00:04:49 center of hydrogen cyanide? 104 00:04:49 --> 00:04:52 And I put a periodic table up there, just the part 105 00:04:52 --> 00:04:53 you might need to look at. 106 00:04:53 --> 00:05:07 So this should be fast, so let's take 10 seconds. 107 00:05:07 --> 00:05:09 All right, good job everyone. 108 00:05:09 --> 00:05:13 So most of you saw that carbon should be in the center. 109 00:05:13 --> 00:05:15 Carbon should be in the center because it has the lowest 110 00:05:15 --> 00:05:17 ionization energy. 111 00:05:17 --> 00:05:21 We know that ionization energy is going to increase as we go 112 00:05:21 --> 00:05:24 across the periodic table, so that means carbon has a lower 113 00:05:24 --> 00:05:26 ionization energy than nitrogen, which is 114 00:05:26 --> 00:05:28 right next to us. 115 00:05:28 --> 00:05:30 So as I just said, we want to put that one in the middle. 116 00:05:30 --> 00:05:34 You got an extra hint here in terms of the order, so even if 117 00:05:34 --> 00:05:36 you had just forgotten what I said, sometimes it's not a 118 00:05:36 --> 00:05:39 terrible idea just to put it in the order it's written, that 119 00:05:39 --> 00:05:41 can give you a lot of clues as well. 120 00:05:41 --> 00:05:43 So, either of those ways of figuring this out is the first 121 00:05:43 --> 00:05:46 guess of what goes in the middle will work pretty well. 122 00:05:46 --> 00:05:49 So, let's go ahead and draw our Lewis structure based on 123 00:05:49 --> 00:05:53 the rest of the rules now that we have a skeleton. 124 00:05:53 --> 00:05:57 So our skeleton tells us that carbon is in the middle, so 125 00:05:57 --> 00:06:00 we'll put the h on one side, and the n on 126 00:06:00 --> 00:06:02 the other side there. 127 00:06:02 --> 00:06:06 So, our second step, as we go through our Lewis structure 128 00:06:06 --> 00:06:09 rules, is to figure out how many valence electrons we 129 00:06:09 --> 00:06:11 have in our entire molecule. 130 00:06:11 --> 00:06:14 So if we talk about hydrogen, how many valence electrons 131 00:06:14 --> 00:06:16 are we talking about? 132 00:06:16 --> 00:06:17 1. 133 00:06:17 --> 00:06:20 What about carbon? 134 00:06:20 --> 00:06:21 I heard 4 and 6. 135 00:06:21 --> 00:06:23 And remember, we're only talking about valence 136 00:06:23 --> 00:06:26 electrons, so the outer-most shells. 137 00:06:26 --> 00:06:30 So we're talking about four valence electrons for carbon. 138 00:06:30 --> 00:06:34 And then for nitrogen? 139 00:06:34 --> 00:06:37 Lots of options I have to choose from from these 140 00:06:37 --> 00:06:38 answers, but it's 5. 141 00:06:38 --> 00:06:42 So, if you can't immediately know, and you don't all have 142 00:06:42 --> 00:06:44 periodic tables in front of you, so that's fine, but if you 143 00:06:44 --> 00:06:47 have a periodic table in front of you, you need to be able to 144 00:06:47 --> 00:06:49 count valence electrons, so work on that if it doesn't come 145 00:06:49 --> 00:06:52 naturally to you in terms of figuring that out. 146 00:06:52 --> 00:06:56 So then in order to figure out the complete number of valence 147 00:06:56 --> 00:07:00 electrons in our molecule, we just add 5 plus 4 plus 1. 148 00:07:00 --> 00:07:02 So we end up having 10 valence electrons. 149 00:07:02 --> 00:07:09 Step three in our Lewis structure rules is to figure 150 00:07:09 --> 00:07:12 out how many electronis we would need in order for every 151 00:07:12 --> 00:07:16 single atom in our molecule to have a full valence shell. 152 00:07:16 --> 00:07:18 So, if we're talking about hydrogen, that's our one 153 00:07:18 --> 00:07:21 exception so far to the octet rule. 154 00:07:21 --> 00:07:24 So we actually only need two electrons to fill up the 155 00:07:24 --> 00:07:27 valence shell of hydrogen, remember that's because all we 156 00:07:27 --> 00:07:29 need to fill up is the 1 s. 157 00:07:29 --> 00:07:34 However, for carbon and nitrogen we need 8 each. 158 00:07:34 --> 00:07:37 So in terms of total numbers that we would need to complete 159 00:07:37 --> 00:07:40 our octets and fill our valence shells, we would 160 00:07:40 --> 00:07:48 need 18 electrons. 161 00:07:48 --> 00:07:48 All right. 162 00:07:48 --> 00:07:51 So let's bring down our middle slide here. 163 00:07:51 --> 00:07:54 So we have 18 electrons, and the next thing that we need 164 00:07:54 --> 00:07:57 to figure out is how many bonding electrons we have. 165 00:07:57 --> 00:08:00 So to figure out bonding electrons, what we take is that 166 00:08:00 --> 00:08:04 number 18, which is our total number of electrons we need to 167 00:08:04 --> 00:08:07 fill valence shells, and we subtract it from our number of 168 00:08:07 --> 00:08:11 valence electrons, which is 10. 169 00:08:11 --> 00:08:18 And what we find that we have is 8 bonding electrons. 170 00:08:18 --> 00:08:23 And hopefully on your paper, you can actually reach your h c 171 00:08:23 --> 00:08:27 n skeleton -- I think I should probably re-draw mine here. 172 00:08:27 --> 00:08:31 Because step five is that we need to fill in our bonding 173 00:08:31 --> 00:08:33 electrons, and we start it with filling in two 174 00:08:33 --> 00:08:35 electrons per bond. 175 00:08:35 --> 00:08:39 So I'm just going to re-draw my skeleton. 176 00:08:39 --> 00:08:43 So, the first thing we do is put two electrons between 177 00:08:43 --> 00:08:47 h and c, and then two electrons between c and n. 178 00:08:47 --> 00:08:50 Remember, every time we have two electrons that are being 179 00:08:50 --> 00:08:54 shared, that's a single bond. 180 00:08:54 --> 00:08:56 The next thing that we want to do is figure out do we have 181 00:08:56 --> 00:08:59 any bonding electrons left. 182 00:08:59 --> 00:09:03 So let's see, we started with 8 bonding electrons, and we used 183 00:09:03 --> 00:09:08 up only 4, so the answer is yes, we have 4 bonding 184 00:09:08 --> 00:09:10 electrons left. 185 00:09:10 --> 00:09:13 So what we need to do is fill in those extra bonding 186 00:09:13 --> 00:09:15 electrons into our bonds. 187 00:09:15 --> 00:09:18 Should I put an extra pair of electrons here, 188 00:09:18 --> 00:09:19 does anyone think? 189 00:09:19 --> 00:09:20 No. 190 00:09:20 --> 00:09:22 The reason is because we already have a full valence 191 00:09:22 --> 00:09:25 shell for our hydrogen, it doesn't want anymore electrons. 192 00:09:25 --> 00:09:28 What about between the carbon and nitrogen? 193 00:09:28 --> 00:09:29 Yes. 194 00:09:29 --> 00:09:31 Definitely, because both of these are not anywhere near 195 00:09:31 --> 00:09:33 filling up their octets yet. 196 00:09:33 --> 00:09:38 So we can put actually all 4 of our extra electrons in between 197 00:09:38 --> 00:09:39 the carbon and the nitrogen. 198 00:09:39 --> 00:09:44 Now we have 6 things around the nitrogen, and we have 199 00:09:44 --> 00:09:47 8 around the carbon. 200 00:09:47 --> 00:09:51 So, what we do as our seventh step is then figure out if we 201 00:09:51 --> 00:09:56 have any extra valence electrons left at all. 202 00:09:56 --> 00:10:01 So we started with 10 valence electrons, we used up 8 203 00:10:01 --> 00:10:04 of those electrons in terms of making bonds. 204 00:10:04 --> 00:10:08 So it turns out that we have 2 valence electrons left. 205 00:10:08 --> 00:10:11 So we need to add those 2 valence electrons left as 206 00:10:11 --> 00:10:14 lone pair electrons in our structure. 207 00:10:14 --> 00:10:17 So, which atom is in need of those lone pair electrons? 208 00:10:17 --> 00:10:19 The nitrogen. 209 00:10:19 --> 00:10:21 The reason being that's the only one that didn't 210 00:10:21 --> 00:10:22 have a full octet yet. 211 00:10:22 --> 00:10:26 So now we're done, actually there is one more step, 212 00:10:26 --> 00:10:29 which is to determine the formal charge. 213 00:10:29 --> 00:10:32 This is a good way to actually check if your Lewis 214 00:10:32 --> 00:10:34 structure is correct or not. 215 00:10:34 --> 00:10:37 We haven't actually learned how to calculate the formal charge 216 00:10:37 --> 00:10:39 yet, we'll learn it soon. 217 00:10:39 --> 00:10:42 So we won't do it for this molecule, but we'll go back and 218 00:10:42 --> 00:10:45 do it for some of our other examples, and you can go back 219 00:10:45 --> 00:10:46 and do it for this one. 220 00:10:46 --> 00:10:48 The other thing is that we can re-write our h 221 00:10:48 --> 00:10:50 c n in terms of bonds. 222 00:10:50 --> 00:10:53 So we know every time we have two electrons, that's a bond. 223 00:10:53 --> 00:10:57 So we have h, then we can draw our bond as a line. 224 00:10:57 --> 00:10:59 And then we have a triple bond there because we have 225 00:10:59 --> 00:11:01 3 pairs of electrons. 226 00:11:01 --> 00:11:07 So it looks a lot less messy if we just draw our Lewis 227 00:11:07 --> 00:11:11 structure like this for h c n, where we have h bonded to c 228 00:11:11 --> 00:11:14 triple, bonded to n, and then a lone pair on the 229 00:11:14 --> 00:11:17 nitrogen there. 230 00:11:17 --> 00:11:19 All right, so this is the same procedure that we're going to 231 00:11:19 --> 00:11:21 go through, regardless of what kind of Lewis structure 232 00:11:21 --> 00:11:22 we're going to draw. 233 00:11:22 --> 00:11:25 What you'll actually find in terms of asking your TAs about 234 00:11:25 --> 00:11:28 the Lewis structure rules is that sometimes they won't be as 235 00:11:28 --> 00:11:31 good at them as you are, and the reason is once you've drawn 236 00:11:31 --> 00:11:34 enough of these structures, you start to get a lot of chemical 237 00:11:34 --> 00:11:36 intuition about what's right or what's not right -- it just 238 00:11:36 --> 00:11:39 looks wrong to you if it's wrong. 239 00:11:39 --> 00:11:42 So your TA might take a minute, so be patient with them if they 240 00:11:42 --> 00:11:44 see your structure and they say oh, no, no, no, no that's 241 00:11:44 --> 00:11:46 wrong, that's terrible, and they don't immediately 242 00:11:46 --> 00:11:46 know why. 243 00:11:46 --> 00:11:48 They might need to go through the rules with you, you 244 00:11:48 --> 00:11:49 might need to remind them. 245 00:11:49 --> 00:11:51 Hopefully, they'll all study them again, so 246 00:11:51 --> 00:11:52 this will be an issue. 247 00:11:52 --> 00:11:55 But what really happens is as you go on in chemistry, you 248 00:11:55 --> 00:11:58 draw so many of these you can just draw them without 249 00:11:58 --> 00:11:59 following the rules. 250 00:11:59 --> 00:12:02 Some of you might get almost to that point, or you might be at 251 00:12:02 --> 00:12:06 that point now, but I recommend this for you and for me and for 252 00:12:06 --> 00:12:09 the TAs, go through the rules because there'll be cases where 253 00:12:09 --> 00:12:12 it's a little bit tricky and it's always much faster to have 254 00:12:12 --> 00:12:16 gone through step-by-step, than to try to just kind of hit or 255 00:12:16 --> 00:12:19 miss figure out what's going to be right or wrong. 256 00:12:19 --> 00:12:24 So, let's try another example here, and let's try a case now 257 00:12:24 --> 00:12:27 where instead of dealing with a neutral molecule we have an 258 00:12:27 --> 00:12:30 ion, so we have c n minus. 259 00:12:30 --> 00:12:33 And what I'll mention to you just in terms of the fact that 260 00:12:33 --> 00:12:37 we're finally dealing with real molecules, which is -- or 261 00:12:37 --> 00:12:39 molecules that are made up of more than one atom, which is 262 00:12:39 --> 00:12:42 kind of exciting for me and maybe for some other of you 263 00:12:42 --> 00:12:47 that like to move into thinking about what some of the 264 00:12:47 --> 00:12:49 consequences of these molecules reacting might be. 265 00:12:49 --> 00:12:51 A lot of the examples that we're going to give you in 266 00:12:51 --> 00:12:55 terms of trying out your Lewis structures will be molecule 267 00:12:55 --> 00:12:58 that are used in organic synthesis, or maybe they're 268 00:12:58 --> 00:13:00 molecules that react in interesting ways with 269 00:13:00 --> 00:13:03 biomolecules in your body or proteins in your body. 270 00:13:03 --> 00:13:06 So, you already will have a head start when you get on to 271 00:13:06 --> 00:13:09 later classes, like organic chemistry or if you're thinking 272 00:13:09 --> 00:13:12 about biochemistry where being able to draw the Lewis 273 00:13:12 --> 00:13:15 structure allows you to think about, eventually, the 274 00:13:15 --> 00:13:18 reactivity of the molecule, which becomes very interesting 275 00:13:18 --> 00:13:20 in thinking about how you're going to synthesize a more 276 00:13:20 --> 00:13:23 complex molecule, or how that molecule is going to interact 277 00:13:23 --> 00:13:26 with an active site in a protein in the body. 278 00:13:26 --> 00:13:30 So, for example, just talking about hydrogen cyanide or the 279 00:13:30 --> 00:13:34 cyanide anion, these are both molecules which are used 280 00:13:34 --> 00:13:37 in organic synthesis, so particularly the cyanide, anion 281 00:13:37 --> 00:13:41 and salts of the cyanide anion. 282 00:13:41 --> 00:13:45 So either a potassium cyanide or sodium cyanide, these are 283 00:13:45 --> 00:13:49 used in synthesis in terms of making carbon-carbon bonds. 284 00:13:49 --> 00:13:52 So if you're trying to make a more complicated organic 285 00:13:52 --> 00:13:55 molecule, carbon-carbon bonds are one of the most difficult 286 00:13:55 --> 00:13:57 things to make an organic chemistry, and it turns out 287 00:13:57 --> 00:14:00 that c n minus is a very reactive molecule, so it's a 288 00:14:00 --> 00:14:04 good way, even though we'll go over some drawbacks in a 289 00:14:04 --> 00:14:07 second, it is a good way to make carbon-carbon bonds. 290 00:14:07 --> 00:14:08 It's very reactive. 291 00:14:08 --> 00:14:12 And because, of course, we have this carbon here what you 292 00:14:12 --> 00:14:15 end up doing is adding a carbon to your molecule. 293 00:14:15 --> 00:14:19 So, when you think about cyanide, you might not think 294 00:14:19 --> 00:14:21 about organic reagents. 295 00:14:21 --> 00:14:23 Does anyone have something else they think about when 296 00:14:23 --> 00:14:24 they think about cyanide? 297 00:14:24 --> 00:14:25 STUDENT: Death. 298 00:14:25 --> 00:14:28 PROFESSOR: Death -- that's a good thought. 299 00:14:28 --> 00:14:31 Yes, cyanide and death, very closely related as well. 300 00:14:31 --> 00:14:34 Cyanide is incredibly toxic, it's a poison. 301 00:14:34 --> 00:14:37 That might be how you're more familiar with cyanide. 302 00:14:37 --> 00:14:40 So if you're working with cyanide in the lab as potassium 303 00:14:40 --> 00:14:44 cyanide or sodium cyanide, those are what are called p h 304 00:14:44 --> 00:14:49 s's, or particularly hazardous substances -- it's a rating for 305 00:14:49 --> 00:14:50 different kinds of chemicals. 306 00:14:50 --> 00:14:52 And what that means it's there's all sorts of 307 00:14:52 --> 00:14:55 precautions and procedures you take that are special when 308 00:14:55 --> 00:14:56 you deal with these. 309 00:14:56 --> 00:14:59 They're kept away from other chemicals. 310 00:14:59 --> 00:15:03 You handle them very special in terms of being extra careful in 311 00:15:03 --> 00:15:07 a very high ventilation area, in hoods is 312 00:15:07 --> 00:15:08 how you handle them. 313 00:15:08 --> 00:15:13 So, yes, they're very poisonous, and in fact, there 314 00:15:13 --> 00:15:16 are areas where you find this toxic compound, cyanide. 315 00:15:16 --> 00:15:22 Other than just in poisons and in organic synthesis shells, 316 00:15:22 --> 00:15:25 you might also find them in some things we're more familiar 317 00:15:25 --> 00:15:26 with, such as almonds. 318 00:15:26 --> 00:15:29 I don't know how many know that there are trace , trace 319 00:15:29 --> 00:15:31 amounts, of cyanide in almonds. 320 00:15:31 --> 00:15:34 I don't know if there any big almond eaters out there. 321 00:15:34 --> 00:15:37 You don't have to worry, we're definitely talking about 322 00:15:37 --> 00:15:38 trace, trace amounts. 323 00:15:38 --> 00:15:39 It's not going to hurt you. 324 00:15:39 --> 00:15:42 And actually, what we usually eat are what are called sweet 325 00:15:42 --> 00:15:47 almonds, and there aren't actual cyanide in the sweet 326 00:15:47 --> 00:15:49 almonds we eat, there's precursors to cyanide, which 327 00:15:49 --> 00:15:50 might not make you more comfortable. 328 00:15:50 --> 00:15:52 But the fact is there are trace, trace amounts. 329 00:15:52 --> 00:15:56 This is nothing that we need to worry in our food supply. 330 00:15:56 --> 00:15:59 However, some people aren't so lucky. 331 00:15:59 --> 00:16:01 I don't know how many of you are familiar with the Cassava 332 00:16:01 --> 00:16:05 plant, which is a kind of woody shrub that's first been 333 00:16:05 --> 00:16:08 cultivated in South America, but it's grown throughout 334 00:16:08 --> 00:16:12 Africa, the Caribbean, South America still, many places 335 00:16:12 --> 00:16:16 around the world, and this is a major source of carbohydrates 336 00:16:16 --> 00:16:18 for much of the world, because Cassava root is very, very 337 00:16:18 --> 00:16:21 rich in carbohydrates. 338 00:16:21 --> 00:16:24 It's not the best form of food in that they're actually very 339 00:16:24 --> 00:16:28 poor in protein, and unfortunately very, 340 00:16:28 --> 00:16:30 very rich in cyanide. 341 00:16:30 --> 00:16:32 So, these roots can be very dangerous. 342 00:16:32 --> 00:16:35 There's different types of the root that you can get called 343 00:16:35 --> 00:16:37 the bitter and the sweet. 344 00:16:37 --> 00:16:39 Hopefully you would all choose the sweet if two 345 00:16:39 --> 00:16:40 are put in front of you. 346 00:16:40 --> 00:16:42 The bitter, of course, are the ones that are 347 00:16:42 --> 00:16:43 very high in cyanide. 348 00:16:43 --> 00:16:46 If you eat these raw, which they do in many places around 349 00:16:46 --> 00:16:49 the world, if you eat a bitter one, you could, in fact, get 350 00:16:49 --> 00:16:51 enough cyanide to kill you. 351 00:16:51 --> 00:16:54 And there are ways to prepare these, so it's important -- 352 00:16:54 --> 00:16:57 this kind of thinking more along the food science idea. 353 00:16:57 --> 00:17:00 There's a way to actually grind down and prepare the flower, so 354 00:17:00 --> 00:17:04 that you promote the enzymes within the plant to breakdown 355 00:17:04 --> 00:17:05 the cyanide precursors. 356 00:17:05 --> 00:17:08 And if you put this in the well-ventilated area, if you 357 00:17:08 --> 00:17:12 prepare this outside, the h c n gas will actually be released 358 00:17:12 --> 00:17:15 into the air, so you're safe, you can eat it later. 359 00:17:15 --> 00:17:18 About 80% of the cyanide at that point is gone, so it does 360 00:17:18 --> 00:17:20 render the root much more safe. 361 00:17:20 --> 00:17:24 But you do, in fact, have to worry about long-term exposure, 362 00:17:24 --> 00:17:27 cyanide poisoning in terms of long-term effects in certain 363 00:17:27 --> 00:17:30 populations that do get the bulk of their carbohydrates 364 00:17:30 --> 00:17:33 from this root, from the root of the Cassava plant. 365 00:17:33 --> 00:17:35 But in terms of us going to the grocery store and thinking 366 00:17:35 --> 00:17:39 about things, probably we're all breathing sighs of relief. 367 00:17:39 --> 00:17:42 I just told you almonds are not a problem, no worries there. 368 00:17:42 --> 00:17:46 We probably don't find any forms of the Cassava plant ever 369 00:17:46 --> 00:17:49 in the U.S. that are going to have that high cyanide content, 370 00:17:49 --> 00:17:52 so we should all be relieved. 371 00:17:52 --> 00:17:55 Unless, of course, you're a smoker, or you're thinking of 372 00:17:55 --> 00:17:58 becoming a smoker, and then maybe you should worry, 373 00:17:58 --> 00:18:01 because this is one of the advertisements that was 374 00:18:01 --> 00:18:04 airing in terms of anti-smoking campaign. 375 00:18:04 --> 00:18:08 Hydrogen cyanide is found in cigarettes. 376 00:18:08 --> 00:18:11 So, if you're looking for another reason to quit, if 377 00:18:11 --> 00:18:14 you're looking for a reason not to start smoking, 378 00:18:14 --> 00:18:15 here's another good one. 379 00:18:15 --> 00:18:18 As I said, it's a particularly hazardous substance, this is 380 00:18:18 --> 00:18:20 worked with in fume hoods. 381 00:18:20 --> 00:18:22 You don't want to inhale it, it's definitely 382 00:18:22 --> 00:18:23 not recommended. 383 00:18:23 --> 00:18:27 The way, in the simplest terms that cyanide can kill you, is 384 00:18:27 --> 00:18:30 it basically out-competes your oxygen for the heme 385 00:18:30 --> 00:18:31 in your blood. 386 00:18:31 --> 00:18:34 So instead of carrying oxygen to your cells, you're carrying 387 00:18:34 --> 00:18:36 cyanide to your cells. 388 00:18:36 --> 00:18:39 Obviously, the amounts that are in cigarettes are not enough 389 00:18:39 --> 00:18:42 that people are dropping dead of cyanide poisoning, but still 390 00:18:42 --> 00:18:46 it's not a good idea if you can avoid eating or inhaling 391 00:18:46 --> 00:18:49 cyanide -- you definitely want to minimize your exposure. 392 00:18:49 --> 00:18:53 And in terms of thinking about it for organic chemistry or if 393 00:18:53 --> 00:18:56 you're interested in thinking about the mechanism maybe by 394 00:18:56 --> 00:19:00 which it is toxic, a first step would be to draw its 395 00:19:00 --> 00:19:01 Lewis structure. 396 00:19:01 --> 00:19:05 So, let's go ahead and make sure we can draw that, if we 397 00:19:05 --> 00:19:08 have interest either in the area of organic chemistry or 398 00:19:08 --> 00:19:10 biochemistry or biology here. 399 00:19:10 --> 00:19:14 So in terms of the first step of skeletal structure, this is 400 00:19:14 --> 00:19:17 actually going to be easier because we don't have a central 401 00:19:17 --> 00:19:23 atom, we just have carbon and nitrogen here. 402 00:19:23 --> 00:19:27 Our next step is thinking about valence electrons. 403 00:19:27 --> 00:19:32 So we have 4 plus 5, but we're actually not done yet, because 404 00:19:32 --> 00:19:36 it's c n minus, so if we have minus, we actually have an 405 00:19:36 --> 00:19:38 extra electron in our molecule. 406 00:19:38 --> 00:19:40 So we need to add 1 more. 407 00:19:40 --> 00:19:45 If instead we had a positive ion, a cation, what we would 408 00:19:45 --> 00:19:47 have to do is subtract 1. 409 00:19:47 --> 00:19:50 But here we're going to add 1, so again, we have 410 00:19:50 --> 00:19:54 10 valence electrons. 411 00:19:54 --> 00:19:57 And if we go on to step three where we figure out how many we 412 00:19:57 --> 00:20:00 would need for full octets, it's just going to be 2 413 00:20:00 --> 00:20:04 times 8, so we have 16. 414 00:20:04 --> 00:20:09 And step four is going to have us figure out how many bonding 415 00:20:09 --> 00:20:15 electrons we have, so we have 16 minus 10, is going to 416 00:20:15 --> 00:20:21 be 6 bonding electrons. 417 00:20:21 --> 00:20:26 So, step five tells us to add 2 electrons between each 418 00:20:26 --> 00:20:32 atom, so we add two there. 419 00:20:32 --> 00:20:35 And step six asks us, well, do we have any bonding 420 00:20:35 --> 00:20:36 electrons left? 421 00:20:36 --> 00:20:40 So how many bonding electrons do we have left? 422 00:20:40 --> 00:20:42 Yup, so we do, we have 4 left. 423 00:20:42 --> 00:20:45 We started with 6, we only used 2. 424 00:20:45 --> 00:20:48 This is very easy molecule because we know exactly where 425 00:20:48 --> 00:20:50 to put them without even having to think, we only have one 426 00:20:50 --> 00:20:54 option, and we'll make a triple bond between the carbon 427 00:20:54 --> 00:20:56 and the nitrogen. 428 00:20:56 --> 00:21:00 So, seven asks us if we have any valence electrons left, and 429 00:21:00 --> 00:21:03 how many valence electrons do we have left? 430 00:21:03 --> 00:21:06 Yeah, so also 4. 431 00:21:06 --> 00:21:09 We started with 10 valence electrons, we used up 6 of 432 00:21:09 --> 00:21:12 those as bonding electrons, so we have 4 left, which will 433 00:21:12 --> 00:21:14 be lone pair electrons. 434 00:21:14 --> 00:21:19 So, in order to fill our octet, what we do is put two on the 435 00:21:19 --> 00:21:23 nitrogen and two on the carbon. 436 00:21:23 --> 00:21:27 So, in terms of finishing our Lewis structure, we're actually 437 00:21:27 --> 00:21:30 not done yet here, even though we have full octets, and we've 438 00:21:30 --> 00:21:34 used up all of our valence electrons, and the reason is 439 00:21:34 --> 00:21:36 because it's c n minus, so we need to make sure that that's 440 00:21:36 --> 00:21:39 reflected in our Lewis structure, so let's put it in 441 00:21:39 --> 00:21:43 brackets here, and put a minus 1. 442 00:21:43 --> 00:21:46 And also I wanted to mention in terms of checking your Lewis 443 00:21:46 --> 00:21:49 structures, regardless of what they are, you should always go 444 00:21:49 --> 00:21:52 back and say how many valence electrons did I have -- I had 445 00:21:52 --> 00:21:57 10, and then count 2, 4, 6, 8, 10, because you always need to 446 00:21:57 --> 00:21:59 make sure you have the same number of valence electrons 447 00:21:59 --> 00:22:02 that you calculated in your actual structure. 448 00:22:02 --> 00:22:05 That'll catch a lot of just silly mistakes for you if you 449 00:22:05 --> 00:22:08 go back and see it and you don't have all of that. 450 00:22:08 --> 00:22:11 Let's re-draw this, so it looks a little bit neater, where we 451 00:22:11 --> 00:22:16 have a triple bond in the middle instead, and again, we 452 00:22:16 --> 00:22:19 need our negative 1 charge there. 453 00:22:19 --> 00:22:23 And our eigth step in the process, again, is formal 454 00:22:23 --> 00:22:32 charge, which we will talk about very soon. 455 00:22:32 --> 00:22:33 All right. 456 00:22:33 --> 00:22:35 So let's try one more example of drawing Lewis structures 457 00:22:35 --> 00:22:38 before we talk about formal charge. 458 00:22:38 --> 00:22:41 And the last example that we're going to talk about is thionyl 459 00:22:41 --> 00:22:45 chloride, so it's s o c l 2. 460 00:22:45 --> 00:22:47 This is another good step forward, because now we 461 00:22:47 --> 00:22:51 actually have four different atoms in our molecule. 462 00:22:51 --> 00:22:55 I'll tell you a little about thionyl chloride as well. 463 00:22:55 --> 00:22:58 This is another organic chemistry reagent, it's also 464 00:22:58 --> 00:23:01 used extensively in the pharmaceutical industry. 465 00:23:01 --> 00:23:03 And what it's used is to convert one type of group, 466 00:23:03 --> 00:23:07 what's called a carboxylic acid into another type of very 467 00:23:07 --> 00:23:10 reactive intermediate, which is called an acid chloride. 468 00:23:10 --> 00:23:13 So I show that here, so in green, you have what's called a 469 00:23:13 --> 00:23:18 carboxcylic acid group, a c o o h, which gets converted by s o 470 00:23:18 --> 00:23:23 c l 2 to a c double bond o c l or an acid chloride. 471 00:23:23 --> 00:23:25 This is the very reactive intermediate. 472 00:23:25 --> 00:23:27 You'll learn a lot more about this if you take organic 473 00:23:27 --> 00:23:30 chemistry, but, In fact, you can then go on and make a bunch 474 00:23:30 --> 00:23:33 of other different kinds of very interesting molecules. 475 00:23:33 --> 00:23:36 So, for example, this is the synthesis of novacaine. 476 00:23:36 --> 00:23:39 This is what's used in industry to actually make novacaine. 477 00:23:39 --> 00:23:41 Has anyone had a novacaine procedure? 478 00:23:41 --> 00:23:42 Yes. 479 00:23:42 --> 00:23:47 I've had it also many times, you usually get 480 00:23:47 --> 00:23:49 novacaine for cavities. 481 00:23:49 --> 00:23:51 There's some alternatives that are used now as well. 482 00:23:51 --> 00:23:54 It's also used as a local anesthetic for other types 483 00:23:54 --> 00:23:55 of small procedures. 484 00:23:55 --> 00:23:58 So, this is, in fact, what's used to make novacaine 485 00:23:58 --> 00:23:59 in industry. 486 00:23:59 --> 00:24:02 You'll notice that a lot of different kinds medications do 487 00:24:02 --> 00:24:05 you have chlorine in them, you'll see that c l group. 488 00:24:05 --> 00:24:08 So, for example, you might be familiar with Wellbutrin 489 00:24:08 --> 00:24:12 here, this is a type of anti-depressant that a lot of 490 00:24:12 --> 00:24:14 people use right now that are taking anti-depressants. 491 00:24:14 --> 00:24:19 It's on the market, very popular in terms of your 492 00:24:19 --> 00:24:22 choices right now as an option as an anti-depressant. 493 00:24:22 --> 00:24:24 Also, Lunesta. 494 00:24:24 --> 00:24:27 This was very big in an ad campaign at least last year, 495 00:24:27 --> 00:24:30 I'm not sure if it still is, with the little butterfly. 496 00:24:30 --> 00:24:31 This is the structure of Lunesta, and you see 497 00:24:31 --> 00:24:33 the c l in it as well. 498 00:24:33 --> 00:24:37 I just wanted to point out that although you see these chlorine 499 00:24:37 --> 00:24:41 atoms in these drugs, what you almost never see is an acid 500 00:24:41 --> 00:24:44 chloride -- in fact, I don't think I've ever seen an 501 00:24:44 --> 00:24:47 acid chloride in a final pharmaceutical product or drug 502 00:24:47 --> 00:24:50 that we take, and the reason is because they're so reactive 503 00:24:50 --> 00:24:53 that you wouldn't want to have that in something you digest. 504 00:24:53 --> 00:24:56 So just keep in mind when you do see the chlorine in these 505 00:24:56 --> 00:24:58 drugs, it's very different from the acid chloride. 506 00:24:58 --> 00:25:02 So, for example, Wellbutrin, it is very unlikely that it would 507 00:25:02 --> 00:25:06 have thionyl chloride in order to make it, and if thionyl 508 00:25:06 --> 00:25:09 chloride was used at some point in the synthesis, it was not to 509 00:25:09 --> 00:25:12 put that chlorine atom on, it was to put something else on. 510 00:25:12 --> 00:25:16 But in terms of drugs that don't look like maybe this 511 00:25:16 --> 00:25:19 compound was used in the synthesis, many of them might 512 00:25:19 --> 00:25:23 have used thionyl chloride, because it generates such a 513 00:25:23 --> 00:25:25 nice reactive intermediate that you can go on and make a 514 00:25:25 --> 00:25:28 bunch of different compounds from that intermediate. 515 00:25:28 --> 00:25:28 All right. 516 00:25:28 --> 00:25:30 So let's think about how to draw the Lewis structure for 517 00:25:30 --> 00:25:34 thionyl chloride -- oh, actually, let me let you tell 518 00:25:34 --> 00:25:37 me how we should start this Lewis structure. 519 00:25:37 --> 00:25:40 So, which atom would you expect to be in the center of a Lewis 520 00:25:40 --> 00:25:50 structure for thionyl chloride? 521 00:25:50 --> 00:25:50 All right. 522 00:25:50 --> 00:25:59 Let's take 10 seconds on that. 523 00:25:59 --> 00:26:01 Looks like we have some fast thinking here, a lot of last 524 00:26:01 --> 00:26:03 minute answers coming in. 525 00:26:03 --> 00:26:04 OK. 526 00:26:04 --> 00:26:09 We have a split decision, so -- you know what, actually, let's 527 00:26:09 --> 00:26:10 think about this for a second. 528 00:26:10 --> 00:26:14 So hopefully, it was a time issue in terms of looking at 529 00:26:14 --> 00:26:18 the periodic table, because let's have you tell me what 530 00:26:18 --> 00:26:22 are we looking for here? 531 00:26:22 --> 00:26:22 Yeah. 532 00:26:22 --> 00:26:22 OK. 533 00:26:22 --> 00:26:25 We're looking for the lowest ionization energy. 534 00:26:25 --> 00:26:28 So, this one can be tricky because oxygen looks like it's 535 00:26:28 --> 00:26:30 in the middle because of the way it's written, but we need 536 00:26:30 --> 00:26:33 to start by looking at the lowest ionization energy. 537 00:26:33 --> 00:26:37 So, if we look on the periodic table, comparing, for example, 538 00:26:37 --> 00:26:41 s to o, if we have s it's below o, what happens to ionization 539 00:26:41 --> 00:26:44 energy as we go down a table? 540 00:26:44 --> 00:26:46 It decreases. 541 00:26:46 --> 00:26:48 If you're still not completely up on the periodic trends, that 542 00:26:48 --> 00:26:51 is stuff that's going to be on the first exam, so make sure 543 00:26:51 --> 00:26:54 that you're able to do this without taking too much 544 00:26:54 --> 00:26:55 time to think about it. 545 00:26:55 --> 00:26:58 We would expect the ionization energy to decrease, that means 546 00:26:58 --> 00:27:00 that sulfur has our lowest ionization energy. 547 00:27:00 --> 00:27:01 All right. 548 00:27:01 --> 00:27:05 So, let's go ahead and draw our Lewis structure here 549 00:27:05 --> 00:27:08 with sulfur in the middle. 550 00:27:08 --> 00:27:13 So, we can put our sulfur in the middle, and then it doesn't 551 00:27:13 --> 00:27:16 really matter how we draw the rest of it, where we put our c 552 00:27:16 --> 00:27:18 l's versus where we put our oxygens. 553 00:27:18 --> 00:27:23 We'll just put them in some way around our sulfur atom. 554 00:27:23 --> 00:27:26 So that's our step one. 555 00:27:26 --> 00:27:31 For our step two, what we need is number of valence electrons. 556 00:27:31 --> 00:27:33 So we have 2 for each of the chlorine. 557 00:27:33 --> 00:27:35 How many valence electrons are in chlorine? 558 00:27:35 --> 00:27:39 All right. 559 00:27:39 --> 00:27:42 So it's 7 that are in chlorine, it's the same as fluorine or 560 00:27:42 --> 00:27:46 any of the others in that row or in that group rather. 561 00:27:46 --> 00:27:51 2 times 7, plus we have 6 in the sulfur, and oxygen is 562 00:27:51 --> 00:27:55 right above sulfur, so that also has 6. 563 00:27:55 --> 00:27:59 So we end up having 26 valence electrons that 564 00:27:59 --> 00:28:01 we're dealing with here. 565 00:28:01 --> 00:28:05 Our step three is to figure out how many bonding electrons that 566 00:28:05 --> 00:28:08 we need, or excuse me, how many total electrons that we need to 567 00:28:08 --> 00:28:11 fill up our octets, so that's just going to be 4 568 00:28:11 --> 00:28:15 times 8, which is 32. 569 00:28:15 --> 00:28:20 And then we take 32 minus 26. 570 00:28:20 --> 00:28:25 So what we end up with in terms of our bonding electrons 571 00:28:25 --> 00:28:27 is going to be 6 bonding electrons. 572 00:28:27 --> 00:28:30 So we can go right ahead and fill these in. 573 00:28:30 --> 00:28:36 1 2, 3 4, 5 and 6. 574 00:28:36 --> 00:28:37 And that was step five. 575 00:28:37 --> 00:28:40 Step six is thinking about do we have any bonding 576 00:28:40 --> 00:28:42 electrons left? 577 00:28:42 --> 00:28:44 Nope, we used them all up. 578 00:28:44 --> 00:28:46 So we don't need to put any more bonds in there. 579 00:28:46 --> 00:28:51 And step seven is how many electrons do we have left 580 00:28:51 --> 00:28:54 over that are going to go into lone pairs? 581 00:28:54 --> 00:28:55 How many? 582 00:28:55 --> 00:28:56 20. 583 00:28:56 --> 00:29:01 26 minus 6, so that tells us 20, and these are all 584 00:29:01 --> 00:29:02 going to be lone pairs. 585 00:29:02 --> 00:29:04 Well, we're talking about a pretty high number here, so to 586 00:29:04 --> 00:29:08 make counting easier, we'll just say 10 lone pairs, because 587 00:29:08 --> 00:29:12 20 lone pair electrons is the same thing as 10 lone pairs. 588 00:29:12 --> 00:29:14 And all we need to do is go over here now and 589 00:29:14 --> 00:29:16 fill up our octets. 590 00:29:16 --> 00:29:23 So oxygen gets 3 pairs, and each chlorine gets 3 pairs, 591 00:29:23 --> 00:29:26 so now we're up to 9 pairs. 592 00:29:26 --> 00:29:31 And what we have left here is the sulfur, which 593 00:29:31 --> 00:29:33 will also get a pair. 594 00:29:33 --> 00:29:36 So, if you look at all of these, we have full octets for 595 00:29:36 --> 00:29:39 all of them, and if we count up all of the valence electrons, 596 00:29:39 --> 00:29:44 it's going to be equal to our number 26 here. 597 00:29:44 --> 00:29:47 And the last thing we do for any of our structures to check 598 00:29:47 --> 00:29:50 them and figure out are these valid or not valid, are these 599 00:29:50 --> 00:29:54 good Lewis structures is to check the formal charge. 600 00:29:54 --> 00:29:56 So now that we have enough practice drawing Lewis 601 00:29:56 --> 00:29:59 structures, let's talk about actually figuring out 602 00:29:59 --> 00:30:02 this formal charge. 603 00:30:02 --> 00:30:05 So when we talk about formal charge, basically formal charge 604 00:30:05 --> 00:30:10 is the measure of the extent to which an individual atom within 605 00:30:10 --> 00:30:14 your molecule has either gained or lost an electron. 606 00:30:14 --> 00:30:18 So as we said when we first introduced covalent bonds, it's 607 00:30:18 --> 00:30:21 a sharing of electrons, but it's not always an 608 00:30:21 --> 00:30:22 equal sharing. 609 00:30:22 --> 00:30:24 Sometimes we have a very electronegative atom that's 610 00:30:24 --> 00:30:29 going to take more of its equal share of electron density. 611 00:30:29 --> 00:30:32 So for example, that might have a formal charge of negative 1, 612 00:30:32 --> 00:30:36 because to some extent it has gained that much electron 613 00:30:36 --> 00:30:40 density that it now has a formal charge that's negative. 614 00:30:40 --> 00:30:44 So, when we think about any type of formal charges, we have 615 00:30:44 --> 00:30:48 to assign these based on a formula here, which is 616 00:30:48 --> 00:30:50 very easy to follow. 617 00:30:50 --> 00:30:54 Formal charge equals v minus l minus 1/2 s. 618 00:30:54 --> 00:30:57 It's even easier to follow if we know what all 619 00:30:57 --> 00:30:59 of those stand for. 620 00:30:59 --> 00:31:04 So, f c, I think you all know is formal charge. 621 00:31:04 --> 00:31:10 Does anyone have a guess for v? 622 00:31:10 --> 00:31:12 Everyone has a guess, great. 623 00:31:12 --> 00:31:14 Valence electrons. 624 00:31:14 --> 00:31:17 What about l? 625 00:31:17 --> 00:31:20 Lone pairs. 626 00:31:20 --> 00:31:23 So, lone pair electrons, actually, not lone 627 00:31:23 --> 00:31:25 pairs themselves. 628 00:31:25 --> 00:31:28 And then s? 629 00:31:28 --> 00:31:28 Good. 630 00:31:28 --> 00:31:30 That's a tricky one, shared electrons. 631 00:31:30 --> 00:31:35 All right. 632 00:31:35 --> 00:31:39 So this means we can actually calculate this for any molecule 633 00:31:39 --> 00:31:41 that we've drawn the Lewis structure for, because we 634 00:31:41 --> 00:31:44 actually do need to draw the Lewis structure before we know, 635 00:31:44 --> 00:31:47 for example, how many of each of these we have, or at 636 00:31:47 --> 00:31:50 least go through the rules. 637 00:31:50 --> 00:31:53 And what's important to keep in mind about formal charge is if 638 00:31:53 --> 00:31:57 we have a neutral atom, such as we did in thionyl chloride 639 00:31:57 --> 00:32:01 here, the sum of the individual formal charges on individual 640 00:32:01 --> 00:32:04 atoms within the molecule have to equal 0. 641 00:32:04 --> 00:32:07 So if we add them all up, there should be no net charge on 642 00:32:07 --> 00:32:11 the molecule, if the molecule is neutral. 643 00:32:11 --> 00:32:15 So, if we think about the second case here where we have 644 00:32:15 --> 00:32:18 c n minus, now we're talking about a molecule with a 645 00:32:18 --> 00:32:20 net charge of negative 1. 646 00:32:20 --> 00:32:23 So that means if we add up all of the formal charges within 647 00:32:23 --> 00:32:26 the molecule, what we would expect to see is that they 648 00:32:26 --> 00:32:29 sum up to give a net charge of negative 1. 649 00:32:29 --> 00:32:33 So we can do this for any final charge we have, if we a 650 00:32:33 --> 00:32:37 molecule that has a charge of plus 2, then all of the formal 651 00:32:37 --> 00:32:41 charges should add up to plus 2 and so on. 652 00:32:41 --> 00:32:44 So, let's just figure this out for some of the examples we 653 00:32:44 --> 00:32:45 did, so for the cyanide anion. 654 00:32:45 --> 00:32:49 So, if we want to figure out the formal charge on the 655 00:32:49 --> 00:32:51 carbon, we need to take the number of valence 656 00:32:51 --> 00:32:54 electrons, so that's 4. 657 00:32:54 --> 00:32:58 We need to subtract the lone pair, what number is that? 658 00:32:58 --> 00:32:59 It's 2. 659 00:32:59 --> 00:33:03 And then 1/2 of the number of shared electrons. 660 00:33:03 --> 00:33:06 So, shared electrons are the ones that are shared between 661 00:33:06 --> 00:33:10 the carbon and the nitrogen, so we have 6 shared electrons, and 662 00:33:10 --> 00:33:12 we want to take 1/2 of that. 663 00:33:12 --> 00:33:16 So we end up with a formal charge on carbon of negative 1. 664 00:33:16 --> 00:33:18 We can do the same thing for nitrogen. 665 00:33:18 --> 00:33:21 So in terms of nitrogen that starts off with a valence 666 00:33:21 --> 00:33:25 number of 5, again we have 2 lone pair electrons in the 667 00:33:25 --> 00:33:29 nitrogen, and again, we have 6 electrons that are shared. 668 00:33:29 --> 00:33:31 So what we see is that the formal charge on 669 00:33:31 --> 00:33:34 the nitrogen is 0. 670 00:33:34 --> 00:33:37 Also, formal charges can be checked, as I just said. 671 00:33:37 --> 00:33:41 Negative 1 plus 0 should add up to negative 1, if in fact, 672 00:33:41 --> 00:33:43 we're correct for the c n anion. 673 00:33:43 --> 00:33:48 And it does, so we know that we're probably on target in 674 00:33:48 --> 00:33:50 terms of calculating our formal charge. 675 00:33:50 --> 00:33:53 So, let's think about our second example -- actually our 676 00:33:53 --> 00:33:55 third, but the second one we're going to talk about in terms of 677 00:33:55 --> 00:33:58 formal charge, which is thionyl chloride. 678 00:33:58 --> 00:34:01 So why don't you tell me what the formal charge should be on 679 00:34:01 --> 00:34:03 the sulfur atom of thionyl chloride? 680 00:34:03 --> 00:34:38 All right. 681 00:34:38 --> 00:34:53 Let's take 10 more seconds. 682 00:34:53 --> 00:34:56 OK, so the majority got it. 683 00:34:56 --> 00:35:00 So hopefully next time we do a formal charge question, we'll 684 00:35:00 --> 00:35:01 get everyone back up to speed. 685 00:35:01 --> 00:35:04 But we've just introduced it, so let's go back to the class 686 00:35:04 --> 00:35:06 notes and explain why this is the correct answer. 687 00:35:06 --> 00:35:09 So if we look at sulfur, what we need to do is take 688 00:35:09 --> 00:35:13 the valence electrons in sulfur, and there are 6. 689 00:35:13 --> 00:35:16 By looking at the periodic table it's right underneath 690 00:35:16 --> 00:35:19 oxygen, so those both have 6 valence electrons. 691 00:35:19 --> 00:35:24 There are 2 lone pair electrons on sulfur -- we only have 2 692 00:35:24 --> 00:35:29 lone pairs -- or 1 lone pair, 2 lone pair electrons. 693 00:35:29 --> 00:35:32 And then we end up having 6 shared electrons, 2 from each 694 00:35:32 --> 00:35:35 of the bonds, so we end up with a formal charge 695 00:35:35 --> 00:35:37 on sulfur of plus 1. 696 00:35:37 --> 00:35:42 If we go to the oxygen atom, now we're talking about 697 00:35:42 --> 00:35:45 starting with 6 in terms of valence electrons again, but 698 00:35:45 --> 00:35:50 instead of 2, you can see we have 6 lone pair electrons 699 00:35:50 --> 00:35:53 around the oxygen minus 1/2 of 2, so we have minus 700 00:35:53 --> 00:35:55 1 is our formal charge. 701 00:35:55 --> 00:35:58 And if we talk about chlorine, and both of the chlorines are 702 00:35:58 --> 00:36:02 the same in this case, we start with a valence number of 7 for 703 00:36:02 --> 00:36:06 chlorine, and then we subtract 6, because it had 6 lone 704 00:36:06 --> 00:36:09 pair electrons around each of the chlorine atoms. 705 00:36:09 --> 00:36:14 Then minus 1/2 of 2, because we only have one bond or 706 00:36:14 --> 00:36:16 2 electrons in a bond. 707 00:36:16 --> 00:36:19 So again, we should be able to check all of our formal charges 708 00:36:19 --> 00:36:23 and make sure they add up to 0, which they do, and that makes 709 00:36:23 --> 00:36:25 sense, because we have a neutral atom in terms 710 00:36:25 --> 00:36:28 of thionyl chloride. 711 00:36:28 --> 00:36:30 Another thing to mention in terms of thinking about if you 712 00:36:30 --> 00:36:34 have a good or a bad Lewis structure, is that when you 713 00:36:34 --> 00:36:37 figure out the formal charge on each of the atoms, it's the 714 00:36:37 --> 00:36:40 more electronegative atoms that you would expect to have that 715 00:36:40 --> 00:36:42 negative charge, and that should make sense to you 716 00:36:42 --> 00:36:46 because electronegative atoms want to have electron density, 717 00:36:46 --> 00:36:49 they want to pull in electron density to them, so it would 718 00:36:49 --> 00:36:51 make sense that they have more of it, which would give 719 00:36:51 --> 00:36:53 them a negative charge. 720 00:36:53 --> 00:36:58 So, if we compare the sulfur to the oxygen, the oxygen it turns 721 00:36:58 --> 00:37:01 out is more electronegative and that is what holds the negative 722 00:37:01 --> 00:37:04 charge in this molecule. 723 00:37:04 --> 00:37:07 Another thing I want to point out, and for some of you just 724 00:37:07 --> 00:37:10 ignore what I'm saying, if you haven't thought about oxidation 725 00:37:10 --> 00:37:13 number, if you haven't heard of that before, don't worry we'll 726 00:37:13 --> 00:37:15 get to it in the second half with Professor Drennan. 727 00:37:15 --> 00:37:17 But for those of you from high school that have learned about 728 00:37:17 --> 00:37:20 oxidation number and maybe are starting to think about it when 729 00:37:20 --> 00:37:23 you look at these molecules, formal charge is -- it's not 730 00:37:23 --> 00:37:26 the same thing as oxidation number, so separate those 731 00:37:26 --> 00:37:27 two things in your head. 732 00:37:27 --> 00:37:29 We'll get to oxidation number in the second half of this 733 00:37:29 --> 00:37:35 course, but it's not in any way the same idea as formal charge. 734 00:37:35 --> 00:37:35 All right. 735 00:37:35 --> 00:37:38 So formal charge can actually help us out when we're trying 736 00:37:38 --> 00:37:41 to decide between several Lewis structures that look like they 737 00:37:41 --> 00:37:44 might be comparable in terms of which might be the lower energy 738 00:37:44 --> 00:37:46 or the more stable structure. 739 00:37:46 --> 00:37:48 The examples we've done so far have been pretty 740 00:37:48 --> 00:37:51 straightforward, so we haven't needed to use formal charge to 741 00:37:51 --> 00:37:53 make this kind of decision. 742 00:37:53 --> 00:37:57 But we could, for example, look at a case where we have several 743 00:37:57 --> 00:37:59 different structures that look pretty good, and the one we 744 00:37:59 --> 00:38:03 want to determine as being the lowest energy structure is the 745 00:38:03 --> 00:38:06 one in which the absolute values of the formal charges 746 00:38:06 --> 00:38:09 are going to be lower, so essentially that they have 747 00:38:09 --> 00:38:11 less charge separation. 748 00:38:11 --> 00:38:13 Those are going to be the more stable or the lower 749 00:38:13 --> 00:38:16 energy structures. 750 00:38:16 --> 00:38:18 So, for example, let's look at thiocyanate 751 00:38:18 --> 00:38:22 ion, we have c s and n. 752 00:38:22 --> 00:38:25 What we've learned so far is as a first approximation, what we 753 00:38:25 --> 00:38:28 want to do is put the atom with the lowest ionization 754 00:38:28 --> 00:38:30 energy in the middle here. 755 00:38:30 --> 00:38:33 So let's compare those ionization energies. 756 00:38:33 --> 00:38:37 We have 10 90 for carbon, 1,000 for sulfur, and 757 00:38:37 --> 00:38:39 1,400 for nitrogen. 758 00:38:39 --> 00:38:42 So, thinking about ionization energy, which atom would 759 00:38:42 --> 00:38:46 you put in the middle here? 760 00:38:46 --> 00:38:49 So, a lot of people I hear are saying sulfur, 761 00:38:49 --> 00:38:50 and that's right. 762 00:38:50 --> 00:38:52 So, in terms of ionization energy, we would expect to 763 00:38:52 --> 00:38:53 see sulfur in the middle. 764 00:38:53 --> 00:38:56 So if we went through and drew out our Lewis structure 765 00:38:56 --> 00:38:59 following each of our steps, what we would get is this as 766 00:38:59 --> 00:39:02 our Lewis structure here, and we could figure out all 767 00:39:02 --> 00:39:03 of the formal charges. 768 00:39:03 --> 00:39:05 But what I'm going to tell you already is this is a case 769 00:39:05 --> 00:39:08 where, in fact, it's an exception to the idea that the 770 00:39:08 --> 00:39:11 lowest energy structure has the lowest ionization energy in the 771 00:39:11 --> 00:39:14 middle, and we can figure this out when we look 772 00:39:14 --> 00:39:15 at formal charge. 773 00:39:15 --> 00:39:18 It's always a good first approximation, because you need 774 00:39:18 --> 00:39:20 to start somewhere in terms of drawing Lewis structures, but 775 00:39:20 --> 00:39:23 then if you go and figure out the formal charge and you just 776 00:39:23 --> 00:39:26 have lots of charge separation or very high charges, like a 777 00:39:26 --> 00:39:30 plus 2 and a minus 2 and a minus 1 all different places in 778 00:39:30 --> 00:39:32 the atom, what it should tell you is maybe there's 779 00:39:32 --> 00:39:33 a better structure. 780 00:39:33 --> 00:39:36 So, let's think of all of the combinations that we could have 781 00:39:36 --> 00:39:38 in terms of this molecule. 782 00:39:38 --> 00:39:40 So in one case, we could actually put carbon in the 783 00:39:40 --> 00:39:43 middle, in one place, we could put sulfur in the middle, and 784 00:39:43 --> 00:39:44 in one case we could put nitrogen. 785 00:39:44 --> 00:39:47 And then we can go ahead and let's quickly write out what 786 00:39:47 --> 00:39:49 the formal charges for all of these will be. 787 00:39:49 --> 00:39:53 So in our first structure, we would find for the nitrogen we 788 00:39:53 --> 00:39:57 have a formal charge 5 minus 4 minus 2, because we're starting 789 00:39:57 --> 00:40:00 with 5 valence electrons, so that is a formal 790 00:40:00 --> 00:40:03 charge of minus 1. 791 00:40:03 --> 00:40:08 For the carbon, we start with 4 valence electrons, we have 0 792 00:40:08 --> 00:40:11 lone pair electrons minus 4, and we end up with a 793 00:40:11 --> 00:40:14 formal charge of 0. 794 00:40:14 --> 00:40:18 For the sulfur, we start off with 6 valence electrons, minus 795 00:40:18 --> 00:40:23 4 lone pair electrons, minus 2, taking in account our bonding 796 00:40:23 --> 00:40:26 electrons, so we end up with a formal charge of 0. 797 00:40:26 --> 00:40:28 All right, so this looks pretty good. 798 00:40:28 --> 00:40:30 We don't actually have much charge separation 799 00:40:30 --> 00:40:31 in this case here. 800 00:40:31 --> 00:40:34 Let's take a look at the lowest ionization energy 801 00:40:34 --> 00:40:35 in the center case. 802 00:40:35 --> 00:40:39 And what you find out if you do these calculations, is that you 803 00:40:39 --> 00:40:42 have a negative 1 for your formal charge on nitrogen, you 804 00:40:42 --> 00:40:47 have a negative 2 for your formal charge on carbon, and 805 00:40:47 --> 00:40:51 you have a positive 2 for your formal charge on sulfur. 806 00:40:51 --> 00:40:53 If we look at our last structure here where we have 807 00:40:53 --> 00:40:56 nitrogen the middle, we can also figure out all those 808 00:40:56 --> 00:40:59 formal charges, and in this case we have plus 1 on the 809 00:40:59 --> 00:41:04 nitrogen, we have minus 2 on the carbon, and then we end up 810 00:41:04 --> 00:41:08 with a 0 on the sulfur there. 811 00:41:08 --> 00:41:10 So let's go to a clicker question. 812 00:41:10 --> 00:41:14 If we call this structure a, b, and c -- you can look on your 813 00:41:14 --> 00:41:16 notes, it also says structure a, b, and c on your notes, 814 00:41:16 --> 00:41:19 so you didn't need to have just memorized that slide. 815 00:41:19 --> 00:41:23 But I want you to tell me in terms of thinking about formal 816 00:41:23 --> 00:41:25 charge, which Lewis structure would you predict to 817 00:41:25 --> 00:41:29 be the most stable? 818 00:41:29 --> 00:41:43 And this should be fast, so let's take 10 seconds on this. 819 00:41:43 --> 00:41:44 Okay, great. 820 00:41:44 --> 00:41:47 So let's go back to our notes here. 821 00:41:47 --> 00:41:49 And the reason that this should be so fast is we already did 822 00:41:49 --> 00:41:51 all the calculations for the formal charges. 823 00:41:51 --> 00:41:54 So what we see is that structure a is the most stable 824 00:41:54 --> 00:41:57 because we have the least separation of charge in 825 00:41:57 --> 00:42:00 the case of structure a. 826 00:42:00 --> 00:42:03 And you can do this any time if you have Lewis structures 827 00:42:03 --> 00:42:04 that you're choosing between. 828 00:42:04 --> 00:42:08 You won't always draw out every single possibility 829 00:42:08 --> 00:42:09 that you have to start with. 830 00:42:09 --> 00:42:12 Often a good thing to start with is to put the lowest 831 00:42:12 --> 00:42:14 ionization energy atom in the middle, and if you don't have 832 00:42:14 --> 00:42:18 charge separation then go with that structure, but if you 833 00:42:18 --> 00:42:20 do find you have a lot of separation, such as the case in 834 00:42:20 --> 00:42:23 negative 2, positive 2, and minus 1, then you want to say 835 00:42:23 --> 00:42:26 wait a second, this is really bad in terms of formal charge, 836 00:42:26 --> 00:42:30 let me go ahead and see what other options I have here. 837 00:42:30 --> 00:42:36 So, we can also get into a case where we have similar values in 838 00:42:36 --> 00:42:39 terms of absolute values of formal charge between two 839 00:42:39 --> 00:42:42 different molecules we're deciding between in 840 00:42:42 --> 00:42:43 their Lewis structures. 841 00:42:43 --> 00:42:45 And in this case, the tie-breaker goes to the 842 00:42:45 --> 00:42:48 molecule in which the negative charge is on the most 843 00:42:48 --> 00:42:51 electronegative atom. 844 00:42:51 --> 00:42:53 So, let's look at an example of this here. 845 00:42:53 --> 00:42:57 So we could say, for example, this molecule here, this -- now 846 00:42:57 --> 00:42:59 we're dealing with a lot of different atoms in the 847 00:42:59 --> 00:43:01 molecule, much more complicated than the initial case of the 848 00:43:01 --> 00:43:04 cyanide ion where we only had two. 849 00:43:04 --> 00:43:06 So, how do we figure out first how to draw the skeletal 850 00:43:06 --> 00:43:09 structure of this molecule here? 851 00:43:09 --> 00:43:13 And one thing I want to tell you to start out with is 852 00:43:13 --> 00:43:17 something about this c h 3 group here. 853 00:43:17 --> 00:43:23 Any time you see a c h 3, this means a methyl group. 854 00:43:23 --> 00:43:27 And if you draw out what a methyl group is -- hopefully 855 00:43:27 --> 00:43:35 you won't have to spell it -- then what you have is a carbon 856 00:43:35 --> 00:43:40 in the middle with three hydrogens around it, and then 857 00:43:40 --> 00:43:43 it can only be bonded to one other thing. 858 00:43:43 --> 00:43:47 So any time you see c h 3 here, remember that that's methyl and 859 00:43:47 --> 00:43:49 that's going to be a terminal group. 860 00:43:49 --> 00:43:51 So, you already have a hint that methyl groups are never 861 00:43:51 --> 00:43:54 in the middle, they always have to be on the outside. 862 00:43:54 --> 00:43:57 So that's a good start for us putting together a skeletal 863 00:43:57 --> 00:43:59 structure for this compound here. 864 00:43:59 --> 00:44:01 The other tip I'm going to give you is any time you see a chain 865 00:44:01 --> 00:44:05 molecule, by chain I just mean many different atoms 866 00:44:05 --> 00:44:06 written out in a row. 867 00:44:06 --> 00:44:10 A convention is that typically you will put a terminal atom. 868 00:44:10 --> 00:44:13 We know that h is always terminal, it's always on the 869 00:44:13 --> 00:44:15 end, never in the center. 870 00:44:15 --> 00:44:19 Right after the molecule that it's attached to. 871 00:44:19 --> 00:44:22 So, for instance, this would suggest to us by the way it's 872 00:44:22 --> 00:44:25 written, that the hydrogen is attached to the nitrogen 873 00:44:25 --> 00:44:27 and not the oxygen. 874 00:44:27 --> 00:44:31 So, if we use those two tips to try to figure out a structure, 875 00:44:31 --> 00:44:34 a skeletal structure, we would get this structure here if we 876 00:44:34 --> 00:44:36 write out the full Lewis structure. 877 00:44:36 --> 00:44:41 We could I think well, maybe this isn't written out in terms 878 00:44:41 --> 00:44:44 of that convention, which sometimes it's not, so let's 879 00:44:44 --> 00:44:46 also try writing it, such that we have the hydrogen and 880 00:44:46 --> 00:44:48 the oxygen atom there. 881 00:44:48 --> 00:44:50 So that would give us this structure here. 882 00:44:50 --> 00:44:53 So notice a difference in these structures, is this has an n h 883 00:44:53 --> 00:44:57 bond whereas this has an o h bond. 884 00:44:57 --> 00:45:01 And if we were to think about which one of these is better, 885 00:45:01 --> 00:45:04 it turns out that it's the same in terms of formal charges, 886 00:45:04 --> 00:45:05 so that doesn't help us out. 887 00:45:05 --> 00:45:08 So we need to go to this second case where we're instead 888 00:45:08 --> 00:45:11 going to think about electronegativity, and we want 889 00:45:11 --> 00:45:13 to think about which atom is the most electronegative. 890 00:45:13 --> 00:45:18 So, in this case, we see that our formal charge is negative 891 00:45:18 --> 00:45:22 on the nitrogen, in this case it's negative on oxygen. 892 00:45:22 --> 00:45:25 Which of those two is more electronegative? 893 00:45:25 --> 00:45:26 The oxygen. 894 00:45:26 --> 00:45:28 So you should be able to look at your periodic table and 895 00:45:28 --> 00:45:31 see this, or also I've written the trend here. 896 00:45:31 --> 00:45:35 So that means that the more stable molecule is going to be 897 00:45:35 --> 00:45:38 this molecule here, which actually puts the negative 898 00:45:38 --> 00:45:41 charge on be more electronegative atom. 899 00:45:41 --> 00:45:46 So this is our lower energy structure. 900 00:45:46 --> 00:45:49 So, these are the different ways that we can actually go 901 00:45:49 --> 00:45:51 ahead and use formal charge when we're choosing between 902 00:45:51 --> 00:45:53 different types of Lewis structures. 903 00:45:53 --> 00:45:56 So one last concept that I want introduces is this 904 00:45:56 --> 00:45:58 idea of resonance. 905 00:45:58 --> 00:46:01 And resonance is the idea that sometimes one single Lewis 906 00:46:01 --> 00:46:04 structure does not adequately describe the electron 907 00:46:04 --> 00:46:09 configuration around a given molecule, so instead you need 908 00:46:09 --> 00:46:12 to draw two different Lewis structures to describe 909 00:46:12 --> 00:46:13 that more appropriately. 910 00:46:13 --> 00:46:17 So, let's quickly go through the Lewis structure for ozone. 911 00:46:17 --> 00:46:19 I have the skeletal structure written up there, I've 912 00:46:19 --> 00:46:22 written it twice and you'll see why in a minute. 913 00:46:22 --> 00:46:24 It's easy to write the skeletal structure, because it's all 914 00:46:24 --> 00:46:26 oxygen, we don't have to worry about what's going to 915 00:46:26 --> 00:46:27 go in the middle. 916 00:46:27 --> 00:46:30 In this case, we're going to have 3 times 6 for valence 917 00:46:30 --> 00:46:33 electrons -- 6 valence electrons for each oxygen, 918 00:46:33 --> 00:46:37 so we have 18 total valence electrons. 919 00:46:37 --> 00:46:41 So, in order to fill up our shell, what we need is 3 920 00:46:41 --> 00:46:44 times 8 or 24 electrons. 921 00:46:44 --> 00:46:49 This leaves us with 24 minus 18, or 6 bonding 922 00:46:49 --> 00:46:51 electrons left. 923 00:46:51 --> 00:46:53 So what we can do is fill that in here. 924 00:46:53 --> 00:46:58 Clearly, we put 2 for each bond, and now we end 925 00:46:58 --> 00:47:01 up having 2 remaining bonding electrons left. 926 00:47:01 --> 00:47:04 So here is where our question comes, because 927 00:47:04 --> 00:47:04 where do we put it? 928 00:47:04 --> 00:47:08 There's absolutely nothing that tells us which atoms we should 929 00:47:08 --> 00:47:11 put it between, because they're both oxygen-oxygen. 930 00:47:11 --> 00:47:15 So, let's just arbitrarily put it between these two in this 931 00:47:15 --> 00:47:19 case here, but actually there's no reason we couldn't also put 932 00:47:19 --> 00:47:21 it between oxygen b and c, so I'm going to draw 933 00:47:21 --> 00:47:25 another structure where we have it here. 934 00:47:25 --> 00:47:28 So in terms of remaining valence electrons we have 12, 935 00:47:28 --> 00:47:31 so we can finish off each of our Lewis structures, so that's 936 00:47:31 --> 00:47:35 our first structure there, and our second structure there. 937 00:47:35 --> 00:47:37 We could also figure out the formal charges, and obviously 938 00:47:37 --> 00:47:40 the formal charges between these two atoms, they're going 939 00:47:40 --> 00:47:44 to be identical, we're only dealing with oxygen atoms here. 940 00:47:44 --> 00:47:47 So, we need to think about what this means -- which is the more 941 00:47:47 --> 00:47:50 stable structure, because we have two different 942 00:47:50 --> 00:47:50 structures here. 943 00:47:50 --> 00:47:54 In this case we have a double bond between a and b, and in 944 00:47:54 --> 00:47:56 this case we have it between b and c. 945 00:47:56 --> 00:48:00 So, presumably, if we follow our rules so far only one of 946 00:48:00 --> 00:48:02 these should be correct. 947 00:48:02 --> 00:48:05 And again, if we figure out the formal charges, let's go 948 00:48:05 --> 00:48:09 through this quickly, we get 0 plus 1 and negative 949 00:48:09 --> 00:48:11 1 for structure 1. 950 00:48:11 --> 00:48:16 We get negative 1 plus 1 and 0 for structure 2. 951 00:48:16 --> 00:48:18 So as I said, they're going to be identical in terms of 952 00:48:18 --> 00:48:21 making the decision that way. 953 00:48:21 --> 00:48:23 And what it turns out is experimental evidence 954 00:48:23 --> 00:48:26 tells us that these two structures are equivalent. 955 00:48:26 --> 00:48:28 And by that what we mean is that they're absolutely 956 00:48:28 --> 00:48:31 identical, and it turns out that this here is not a double 957 00:48:31 --> 00:48:34 bond, it's not a single bond either, it's actually 958 00:48:34 --> 00:48:35 something in between. 959 00:48:35 --> 00:48:38 So if we look at its length, it's actually shorter than a 960 00:48:38 --> 00:48:41 single bond, but longer than a double bond. 961 00:48:41 --> 00:48:44 Or if we look at how strong it is, it's actually stronger than 962 00:48:44 --> 00:48:47 a single bond, but weaker than a double bond. 963 00:48:47 --> 00:48:50 And we find the same thing for these two atoms here, it's not 964 00:48:50 --> 00:48:54 actually a double bond, it's somewhere between a single 965 00:48:54 --> 00:48:56 bond and a double bond. 966 00:48:56 --> 00:48:59 So this is a case where we have resonance structures, or we 967 00:48:59 --> 00:49:01 call this a resonance hybrid. 968 00:49:01 --> 00:49:04 So the reality of the situation is that it's a combination 969 00:49:04 --> 00:49:06 between these 2 structures. 970 00:49:06 --> 00:49:08 And an important thing to remember when we talk about 971 00:49:08 --> 00:49:12 resonance hybrids is that the structure it's not 1/2 the time 972 00:49:12 --> 00:49:16 this structure, and 1/2 of the time this structure, it's 973 00:49:16 --> 00:49:19 actually some combination or some average between 974 00:49:19 --> 00:49:20 the two structures. 975 00:49:20 --> 00:49:22 But since in drawing Lewis structures we don't have a way 976 00:49:22 --> 00:49:26 to represent that -- actually, in some cases you do, you can 977 00:49:26 --> 00:49:30 draw a dotted line that means a 1 and 1/2 bond, but most in 978 00:49:30 --> 00:49:33 most cases, we just draw out both resonance structures, and 979 00:49:33 --> 00:49:35 the way that we say it's a resonance structure is that we 980 00:49:35 --> 00:49:38 put it in the brackets and we put an arrow between it. 981 00:49:38 --> 00:49:40 So, when you think about resonance structures, some 982 00:49:40 --> 00:49:43 students tend to just get confused and be picturing this 983 00:49:43 --> 00:49:45 flickering back and forth. 984 00:49:45 --> 00:49:49 A good example to keep in mind is the idea of a mule. 985 00:49:49 --> 00:49:54 So most of you know, hopefully, that a mule is a combination 986 00:49:54 --> 00:49:57 of a donkey and a horse. 987 00:49:57 --> 00:50:00 A mule is not spending 1/2 of its time as a donkey, and 1/2 988 00:50:00 --> 00:50:04 of its time as a horse, we don't see it flickering back 989 00:50:04 --> 00:50:07 and forth between the two, that's not what we see. 990 00:50:07 --> 00:50:10 Instead what we see is it's an average, it's part like a 991 00:50:10 --> 00:50:12 horse, it's part like a donkey. 992 00:50:12 --> 00:50:17 So if we want to put that in chemical terms, we want to make 993 00:50:17 --> 00:50:20 sure we put these in brackets here, and remember, this is the 994 00:50:20 --> 00:50:23 resonance arrow, it's not a reaction arrow, it's a 995 00:50:23 --> 00:50:25 resonance arrow, so make sure you mark it up 996 00:50:25 --> 00:50:27 correctly like that. 997 00:50:27 --> 00:50:30 When we talk about resonance structures, the key word is 998 00:50:30 --> 00:50:33 that the electrons are de-localized. 999 00:50:33 --> 00:50:36 So they're not just between two atoms here. 1000 00:50:36 --> 00:50:38 Now, for example, in our structure with ozone it's 1001 00:50:38 --> 00:50:41 between all three atoms. 1002 00:50:41 --> 00:50:44 And the final point I want to make today, and this is very, 1003 00:50:44 --> 00:50:47 very important, so make sure that you do understand this. 1004 00:50:47 --> 00:50:50 When we talk about resonance structures, we're talking about 1005 00:50:50 --> 00:50:54 cases that have the same arrangement of atoms, the key 1006 00:50:54 --> 00:50:57 is the atoms are the same, and the thing that is different is 1007 00:50:57 --> 00:51:00 the arrangement of electrons here.