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 --> 00:00:23 PROFESSOR: OK, let's get started here. 10 00:00:23 --> 00:00:26 Go ahead and take 10 more seconds on the clicker 11 00:00:26 --> 00:00:29 question, which probably looks all too familiar at this 12 00:00:29 --> 00:00:35 point, if you went to recitation yesterday. 13 00:00:35 --> 00:00:38 All right, and let's see how we do here. 14 00:00:38 --> 00:00:38 OK. 15 00:00:38 --> 00:00:41 So, let's talk about this for one second. 16 00:00:41 --> 00:00:44 So what we're asking here, if we can settle down and listen 17 00:00:44 --> 00:00:47 up, is which equations can be used if we're talking about 18 00:00:47 --> 00:00:50 converting wavelength to energy for an electron. 19 00:00:50 --> 00:00:53 Remember, the key word here is electron. 20 00:00:53 --> 00:00:56 This might look familiar to the first part of problem one on 21 00:00:56 --> 00:00:59 the exam, and problem one on the exam is what tended to be 22 00:00:59 --> 00:01:02 the huge problem on the exam. 23 00:01:02 --> 00:01:06 I think over 2/3 of you decided on the exam to use this first 24 00:01:06 --> 00:01:09 equation, e equals h c over wavelength. 25 00:01:09 --> 00:01:13 So I just want to reiterate one more time, why can we not 26 00:01:13 --> 00:01:17 use this equation if we're talking about an electron? 27 00:01:17 --> 00:01:17 C. 28 00:01:17 --> 00:01:19 OK, good, good, I'm hearing it. 29 00:01:19 --> 00:01:20 So the answer is c. 30 00:01:20 --> 00:01:23 What you need to do is you need to ask yourself if you're 31 00:01:23 --> 00:01:25 trying to convert from wavelength to energy for an 32 00:01:25 --> 00:01:29 electron, and you are tempted, because we are all tempted to 33 00:01:29 --> 00:01:33 use this equation, and if you were tempted, say, does 34 00:01:33 --> 00:01:35 an electron travel at the speed of light? 35 00:01:35 --> 00:01:38 And if the answer is no, an electron does not travel at the 36 00:01:38 --> 00:01:40 speed of light, light travels at the speed of light, then you 37 00:01:40 --> 00:01:43 want to stay away from using this equation. 38 00:01:43 --> 00:01:46 And I know how tempting it is to do that, but we have other 39 00:01:46 --> 00:01:49 equations we can use -- the DeBroglie wavelength, and this 40 00:01:49 --> 00:01:52 is just a combination of energy equals 1/2 m v squared, and the 41 00:01:52 --> 00:01:55 definition of momentum, so we can combine those 42 00:01:55 --> 00:01:56 things to get it. 43 00:01:56 --> 00:01:59 You might be wondering why I'm telling you this now, you've 44 00:01:59 --> 00:02:01 already -- if you've lost points on that, lost the points 45 00:02:01 --> 00:02:04 on it, and what I'm saying to you is if there are parts of 46 00:02:04 --> 00:02:08 exam 1 that you did not do well on, you will have a chance to 47 00:02:08 --> 00:02:10 show us again that you now understand that 48 00:02:10 --> 00:02:12 material on the final. 49 00:02:12 --> 00:02:15 One quarter of the final is going to be exam 1 material, 50 00:02:15 --> 00:02:18 and what that means is when we look at your grade at the end 51 00:02:18 --> 00:02:21 of the semester, and we take a look at what you got on exam 1, 52 00:02:21 --> 00:02:24 and you're right at that borderline, and we say well, 53 00:02:24 --> 00:02:27 what happened, did they understand more at the end of 54 00:02:27 --> 00:02:30 the semester, did the concepts kind of solidify 55 00:02:30 --> 00:02:31 over the semester? 56 00:02:31 --> 00:02:34 And if they did and if you showed us that they did, then 57 00:02:34 --> 00:02:36 you're going to get bumped up into that next grade category. 58 00:02:36 --> 00:02:39 So keep that in mind as you're reviewing the exam, sometimes 59 00:02:39 --> 00:02:42 if things don't go as well as you want them to, the 60 00:02:42 --> 00:02:45 temptation is just to put that exam away forever and ever. 61 00:02:45 --> 00:02:47 But the reality is that new material builds on that 62 00:02:47 --> 00:02:55 material, and specifically exam 1 a, question 1 a that deals 63 00:02:55 --> 00:02:58 with converting wavelength to energy for an electron. 64 00:02:58 --> 00:03:01 I really want you guys know this and to understand it, so 65 00:03:01 --> 00:03:04 I can guarantee you that you will see this on the final. 66 00:03:04 --> 00:03:08 Specifically, question 1, part a. 67 00:03:08 --> 00:03:10 You will see something very, very similar to 68 00:03:10 --> 00:03:11 this on the final. 69 00:03:11 --> 00:03:14 If you are thinking about 1 thing to go back and study 70 00:03:14 --> 00:03:20 on exam 1, 1 a is a really good choice for that. 71 00:03:20 --> 00:03:22 This is important to me, so you're going to 72 00:03:22 --> 00:03:24 see it on the final. 73 00:03:24 --> 00:03:27 So if you have friends that aren't here, you might want to 74 00:03:27 --> 00:03:30 mention it to them, or maybe not, maybe this is your reward 75 00:03:30 --> 00:03:33 for coming to class, which is fine with me as well. 76 00:03:33 --> 00:03:33 All right. 77 00:03:33 --> 00:03:36 So I want to talk a little bit about exam 1. 78 00:03:36 --> 00:03:39 I know most you picked up your examine in recitation. 79 00:03:39 --> 00:03:43 If you didn't, any extra exams can be picked up in the 80 00:03:43 --> 00:03:47 Chemistry Education office, that's room 2204. 81 00:03:47 --> 00:03:51 So, the class average for the exam was a 68%, which is 82 00:03:51 --> 00:03:54 actually a strong, solid average for an exam 1 grade in 83 00:03:54 --> 00:03:57 the fall semester of 511-1. 84 00:03:57 --> 00:04:01 What we typically see is something right in this range, 85 00:04:01 --> 00:04:05 either ranging from the 50's for an exam 1 average to 86 00:04:05 --> 00:04:08 occasionally getting into the 70's, but most commonly what 87 00:04:08 --> 00:04:13 we've seen for exam 1 averages is 60, 61 -- those low 60's. 88 00:04:13 --> 00:04:18 So in many ways, seeing this 68 here, this is a great sign 89 00:04:18 --> 00:04:21 that we are off to a good start for this semester. 90 00:04:21 --> 00:04:24 And I do want to address, because I know many of you, 91 00:04:24 --> 00:04:27 this is only your second exam at MIT, and perhaps you've 92 00:04:27 --> 00:04:31 never gotten an exam back that didn't start with a 90 or start 93 00:04:31 --> 00:04:33 with an 80 in terms of the grades. 94 00:04:33 --> 00:04:36 So one thing you need to keep in mind is don't just 95 00:04:36 --> 00:04:37 look at the number grade. 96 00:04:37 --> 00:04:40 The reason that we give you these letters grade categories 97 00:04:40 --> 00:04:44 is that you can understand what it actually means, what your 98 00:04:44 --> 00:04:47 exam score actually says in terms of how we perceive you as 99 00:04:47 --> 00:04:49 understanding the material. 100 00:04:49 --> 00:04:53 So, for example, and this is the same categories that were 101 00:04:53 --> 00:04:56 shared in recitation, so I apologize for repeating, but I 102 00:04:56 --> 00:04:59 know sometimes when you get an exam back, no more information 103 00:04:59 --> 00:05:02 comes into your head except obsessing over the exam, so I'm 104 00:05:02 --> 00:05:05 just going to say it one more time, and that is between 105 00:05:05 --> 00:05:07 88 and 100, so that's 20% of you got an A. 106 00:05:07 --> 00:05:10 This is just absolutely fantastic, you really nailed 107 00:05:10 --> 00:05:13 this very hard material and these hard questions on the 108 00:05:13 --> 00:05:17 exam where you had to both use equations and solve problems, 109 00:05:17 --> 00:05:19 but also understand the concept in order to get yourself 110 00:05:19 --> 00:05:21 started on solving the problem. 111 00:05:21 --> 00:05:26 The same with the B, the B range was between 69 and 87 -- 112 00:05:26 --> 00:05:30 anywhere in between those ranges, you've got a B, some 113 00:05:30 --> 00:05:32 sort of B on the exam. 114 00:05:32 --> 00:05:35 So again, if you're in the A or the B category here, this is 115 00:05:35 --> 00:05:38 really something to be proud of, you really earned 116 00:05:38 --> 00:05:39 these grades. 117 00:05:39 --> 00:05:43 You know these exams, our 511-1 exams, we're not giving you 118 00:05:43 --> 00:05:46 points here, there are no give me, easy points, you earned 119 00:05:46 --> 00:05:47 every single one of these points. 120 00:05:47 --> 00:05:51 So, A and B here, these are refrigerator-worthy grades, 121 00:05:51 --> 00:05:52 hang those up in your dorm. 122 00:05:52 --> 00:05:54 This is something to feel good about. 123 00:05:54 --> 00:05:55 All right. 124 00:05:55 --> 00:05:59 So, for those of you that got between a 51 and a 68, this 125 00:05:59 --> 00:06:01 is somewhere in the C range. 126 00:06:01 --> 00:06:04 For some people, they feel comfortable being in the C 127 00:06:04 --> 00:06:08 range, other people really do not like being in this range. 128 00:06:08 --> 00:06:10 We understand that, there is plenty of room up there 129 00:06:10 --> 00:06:11 with the A's and the B's. 130 00:06:11 --> 00:06:15 You are welcome to come up to these higher ranges starting 131 00:06:15 --> 00:06:17 with the next exam. 132 00:06:17 --> 00:06:20 And what I want to tell you if you are in the C range, and 133 00:06:20 --> 00:06:22 this is not a place that you want to be in, anyone that's 134 00:06:22 --> 00:06:27 got below the class average, so below a 68 -- or a 68 or below, 135 00:06:27 --> 00:06:30 is eligible for free tutoring, and I put the website on the 136 00:06:30 --> 00:06:32 front page of your notes. 137 00:06:32 --> 00:06:35 This means you get a one-on-one tutor paid for by the Chemistry 138 00:06:35 --> 00:06:37 Department to help you if it's concepts you're not quite up 139 00:06:37 --> 00:06:42 on, if it's exam strategy that you need to work on more. 140 00:06:42 --> 00:06:44 Whatever it is that you need to work on, we want to 141 00:06:44 --> 00:06:45 help you get there. 142 00:06:45 --> 00:06:47 So, if you have a grade that you're not happy with, that 143 00:06:47 --> 00:06:52 you're feeling upset or discouraged about, please, I'm 144 00:06:52 --> 00:06:54 happy to talk to all of you about your grades individually. 145 00:06:54 --> 00:06:58 You can come talk to me, bring your exam, and we'll go over 146 00:06:58 --> 00:07:00 what the strategy should be in terms of you succeeding 147 00:07:00 --> 00:07:01 on the next exam. 148 00:07:01 --> 00:07:03 You can do the same thing with all of your TAs are more than 149 00:07:03 --> 00:07:05 happy to meet with each and every one of you. 150 00:07:05 --> 00:07:07 And then in addition to that, we can set you up with a tutor 151 00:07:07 --> 00:07:11 if you are in the C range or below, in terms of 152 00:07:11 --> 00:07:13 this first exam. 153 00:07:13 --> 00:07:13 All right. 154 00:07:13 --> 00:07:16 So 44 to 50, this is going to be in the D range. 155 00:07:16 --> 00:07:19 And then anything below a 44 is going to be 156 00:07:19 --> 00:07:20 failing on this exam. 157 00:07:20 --> 00:07:22 And also keep in mind, for those of you that are 158 00:07:22 --> 00:07:25 freshman, you need at least a C to pass the class. 159 00:07:25 --> 00:07:29 So, if you did get a D or an F on the first exam, you are 160 00:07:29 --> 00:07:32 going to need to really evaluate why that happened and 161 00:07:32 --> 00:07:35 make some changes, and we're absolutely here to 162 00:07:35 --> 00:07:36 help you do that. 163 00:07:36 --> 00:07:39 So the real key is identifying where the problem is -- is 164 00:07:39 --> 00:07:42 it with understanding the concepts, are you in a study 165 00:07:42 --> 00:07:45 group that's dragging you along but you're not understanding? 166 00:07:45 --> 00:07:47 Do you kind of panic when you get in the exam? 167 00:07:47 --> 00:07:49 There are all sorts of scenarios we can talk 168 00:07:49 --> 00:07:51 about and we want to talk about them with you. 169 00:07:51 --> 00:07:55 Seriously, even if we had a huge range in this exam from 17 170 00:07:55 --> 00:07:58 to 100, if you're sitting there and you're the 17, and actually 171 00:07:58 --> 00:08:02 there's more than 1 so don't feel alone, if you're a 17 or 172 00:08:02 --> 00:08:05 you're a 20, it's not time to give up, it's not time to drop 173 00:08:05 --> 00:08:08 the class and say I'm no good at chemistry, I can't do this. 174 00:08:08 --> 00:08:11 You still can, this is your first couple of exams, 175 00:08:11 --> 00:08:14 certainly your first in this class, potentially one of your 176 00:08:14 --> 00:08:17 first at MIT, so there's tons of room to improve 177 00:08:17 --> 00:08:18 from here on out. 178 00:08:18 --> 00:08:21 This is only 100 points out of 750. 179 00:08:21 --> 00:08:23 So, the same thing goes if you did really well, you still 180 00:08:23 --> 00:08:26 have 650 other points that you need to deal with. 181 00:08:26 --> 00:08:28 So, make sure you don't just rest on your high score 182 00:08:28 --> 00:08:31 from this first exam. 183 00:08:31 --> 00:08:34 So, OK, so that's pretty much what I wanted to say about the 184 00:08:34 --> 00:08:38 exam, and in terms of there's tons of resources if things 185 00:08:38 --> 00:08:39 didn't work out quite as you wanted. 186 00:08:39 --> 00:08:42 If you feel upset in any way, please come and talk to me. 187 00:08:42 --> 00:08:44 We want you to love chemistry and feel good about 188 00:08:44 --> 00:08:45 your ability to do it. 189 00:08:45 --> 00:08:48 Nobody get into MIT by mistake, so you all deserve to be 190 00:08:48 --> 00:08:50 sitting here, and you all can pass this class and do well in 191 00:08:50 --> 00:08:53 it, so we can help you get there no matter what. 192 00:08:53 --> 00:08:56 You all absolutely can do this. 193 00:08:56 --> 00:08:58 And then one more time, to reiterate, in case anyone 194 00:08:58 --> 00:09:01 missed it, 1 a, make sure you understand that, I feel 195 00:09:01 --> 00:09:02 like that's important. 196 00:09:02 --> 00:09:04 And actually all of 1 -- I really feel like the 197 00:09:04 --> 00:09:07 photoelectric effect is important for understanding 198 00:09:07 --> 00:09:08 all of these energy concepts. 199 00:09:08 --> 00:09:12 So, as you go on in this class, make sure you don't go on 200 00:09:12 --> 00:09:14 before you go back and make sure you understand 201 00:09:14 --> 00:09:16 that problem. 202 00:09:16 --> 00:09:22 All right, so let's move on to material for exam 2 now, and 203 00:09:22 --> 00:09:25 we're already three lectures into exam 2 material. 204 00:09:25 --> 00:09:29 And I do want to say that in terms of 511-1, what tends to 205 00:09:29 --> 00:09:33 happen is the exam scores go up and up and up, in terms of as 206 00:09:33 --> 00:09:36 we go from exam 1, to exam 2, to exam 3. 207 00:09:36 --> 00:09:38 One of these reasons is we are building on material, the other 208 00:09:38 --> 00:09:40 reason is you'll be shocked at how much better you are at 209 00:09:40 --> 00:09:43 taking an exam just a few weeks from now. 210 00:09:43 --> 00:09:47 So this will be on, starting with the Lewis structures, so 211 00:09:47 --> 00:09:50 go back in your notes -- if this doesn't sound familiar, if 212 00:09:50 --> 00:09:54 you spent too much time -- or not too much time, spent a 213 00:09:54 --> 00:09:56 lot of time studying exam 1 and didn't move on here. 214 00:09:56 --> 00:09:58 Today we're going to talk about the breakdown 215 00:09:58 --> 00:09:59 of the octet rule. 216 00:09:59 --> 00:10:02 Cases where we don't have eight electrons around our Lewis 217 00:10:02 --> 00:10:06 structures, then we'll move on to talking about ionic bonds. 218 00:10:06 --> 00:10:08 We had already talked about covalent bonds, and then we 219 00:10:08 --> 00:10:11 talked about Lewis structures, which describe the electron 220 00:10:11 --> 00:10:13 configuration in covalent bonds. 221 00:10:13 --> 00:10:16 So now let's think about the other extreme of ionic bonds, 222 00:10:16 --> 00:10:19 and then we'll talk about polar covalent bonds to end, if we 223 00:10:19 --> 00:10:23 get there or will start with that in class on Monday. 224 00:10:23 --> 00:10:26 Also, public service announcement for all of you, 225 00:10:26 --> 00:10:28 voter registration in Massachusetts, which is where 226 00:10:28 --> 00:10:32 we are, is on Monday, the deadline if you want 227 00:10:32 --> 00:10:33 to register to vote. 228 00:10:33 --> 00:10:36 There's some websites up there that can guide you through 229 00:10:36 --> 00:10:38 registering and also can guide you through, if you need an 230 00:10:38 --> 00:10:41 absentee ballot for your home state. 231 00:10:41 --> 00:10:44 And I actually saw, and I saw a 5.111 student manning, there's 232 00:10:44 --> 00:10:47 some booths around MIT that will register you or get 233 00:10:47 --> 00:10:48 you an absentee ballot. 234 00:10:48 --> 00:10:52 So, the deadline's coming soon, so patriotic duty, I need to 235 00:10:52 --> 00:10:55 remind you of that as your chemistry teacher -- chemistry 236 00:10:55 --> 00:10:56 issues are important in politics as well. 237 00:10:56 --> 00:11:00 So make sure you get registered to vote. 238 00:11:00 --> 00:11:02 I just remembered one more announcement, too, that I did 239 00:11:02 --> 00:11:05 want to mention, some of you may have friends in 511-2 and 240 00:11:05 --> 00:11:08 have heard their class average for exam 1. 241 00:11:08 --> 00:11:11 And I want to tell you, this happens every year, their 242 00:11:11 --> 00:11:14 average was 15 points higher than our average. 243 00:11:14 --> 00:11:17 Last year, their average was 15 points higher than our average. 244 00:11:17 --> 00:11:18 This is for exam 1. 245 00:11:18 --> 00:11:23 This is what tends to happen to 511-2 grades 246 00:11:23 --> 00:11:24 as the exam goes on. 247 00:11:24 --> 00:11:25 This is what happens to 511-1. 248 00:11:25 --> 00:11:27 You guys are in a good spot. 249 00:11:27 --> 00:11:30 Also, I want to point out that what's not important is just 250 00:11:30 --> 00:11:32 that number grade, but also the letter that goes with it. 251 00:11:32 --> 00:11:36 So, for example, if you got a 69 in this class on this 252 00:11:36 --> 00:11:38 exam, that's a B minus. 253 00:11:38 --> 00:11:41 If you got a 69 on your exam in 511-2, that's a D, you 254 00:11:41 --> 00:11:42 didn't pass the exam. 255 00:11:42 --> 00:11:45 So keep that in mind when your friend might have gotten a 256 00:11:45 --> 00:11:49 higher number grade than you and you know you understand the 257 00:11:49 --> 00:11:51 similar material just as well. 258 00:11:51 --> 00:11:53 Similarly, an 80 in this class on the exam was a 259 00:11:53 --> 00:11:55 B plus, a very high B. 260 00:11:55 --> 00:11:58 An 80 in that class is going to be a C. 261 00:11:58 --> 00:12:01 So, just don't worry so much about exactly where that 262 00:12:01 --> 00:12:03 average lies, you really want to think about what the 263 00:12:03 --> 00:12:04 letter grade means. 264 00:12:04 --> 00:12:05 OK, I've said enough. 265 00:12:05 --> 00:12:07 I just -- I hate to see people discouraged, and I know that a 266 00:12:07 --> 00:12:10 few people have been feeling discouraged, so that's my 267 00:12:10 --> 00:12:14 long-winded explanation of exam 1 grades. 268 00:12:14 --> 00:12:14 All right. 269 00:12:14 --> 00:12:17 So, let's move on with life though, so talking about the 270 00:12:17 --> 00:12:20 breakdown of the octet rule. 271 00:12:20 --> 00:12:22 The first example where we're going to see a breakdown is any 272 00:12:22 --> 00:12:25 time we have an odd number of valence electrons. 273 00:12:25 --> 00:12:28 This is probably the easiest to explain and to think about, 274 00:12:28 --> 00:12:32 because if we have an odd number that means that we can't 275 00:12:32 --> 00:12:34 have our octet rule, because our octet rule works 276 00:12:34 --> 00:12:36 by pairing electrons. 277 00:12:36 --> 00:12:39 And if we have an odd number, we automatically have 278 00:12:39 --> 00:12:41 an odd electron out. 279 00:12:41 --> 00:12:44 So, if we look at an example, the methyl radical, we can 280 00:12:44 --> 00:12:47 first think about how we draw the Lewis structure -- we draw 281 00:12:47 --> 00:12:49 the skeletal structure here. 282 00:12:49 --> 00:12:50 And then what we're going to do is add up our valence electrons 283 00:12:50 --> 00:12:56 -- we have 3 times 1 for the hydrogen atoms, carbon has 4 284 00:12:56 --> 00:12:58 valence electrons, so we have a total of 7. 285 00:12:58 --> 00:13:02 If we want to fill all of our valence shells in each of these 286 00:13:02 --> 00:13:06 atoms, we're going to need a total of 14 electrons. 287 00:13:06 --> 00:13:08 So, what we see we're left with is that we have 288 00:13:08 --> 00:13:10 7 bonding electrons. 289 00:13:10 --> 00:13:14 So we can fill in 6 of those straightforward here, because 290 00:13:14 --> 00:13:16 we know that we need to make 3 different bonds. 291 00:13:16 --> 00:13:19 And now we're left over with 1 electron, we can't make a bond. 292 00:13:19 --> 00:13:24 So, what we'll do is carbon does not have an octet yet. 293 00:13:24 --> 00:13:27 We can't get it one, but we can do the best we can and help it 294 00:13:27 --> 00:13:31 out with adding that extra electron onto the carbon atom, 295 00:13:31 --> 00:13:33 so that at least we're getting as close as possible to 296 00:13:33 --> 00:13:36 filling our octets. 297 00:13:36 --> 00:13:40 This is what we call a radical species or a free radical. 298 00:13:40 --> 00:13:43 Free radical or radical species is essentially any type of a 299 00:13:43 --> 00:13:48 molecule that has this unpaired electron on one of the atoms. 300 00:13:48 --> 00:13:51 This might look really strange, we're used to seeing octets. 301 00:13:51 --> 00:13:54 But you'll realize, if you calculate the formal charge on 302 00:13:54 --> 00:13:57 this molecule, that it's not the worst situation 303 00:13:57 --> 00:13:58 ever for carbon. 304 00:13:58 --> 00:14:01 At least it's formal charge is zero, even if it doesn't have 305 00:14:01 --> 00:14:04 -- it would rather have an extra bond and have 306 00:14:04 --> 00:14:05 a full octet. 307 00:14:05 --> 00:14:08 But it's not the worst scenario that we can imagine. 308 00:14:08 --> 00:14:10 But still, radicals tend to be incredibly reactive 309 00:14:10 --> 00:14:13 because they do want to fill that octet. 310 00:14:13 --> 00:14:16 So, what happens when you have a radical is it tends to react 311 00:14:16 --> 00:14:18 with the first thing that it runs into, especially highly 312 00:14:18 --> 00:14:21 reactive radicals that are not stabilized in some other way, 313 00:14:21 --> 00:14:24 which you'll tend to talk about it organic chemistry -- how 314 00:14:24 --> 00:14:26 you can stabilize radicals. 315 00:14:26 --> 00:14:30 So the term free radical should sound familiar to you, whether 316 00:14:30 --> 00:14:33 you've heard it in chemistry before, or you haven't heard it 317 00:14:33 --> 00:14:36 in chemistry, but maybe have heard it, I don't know, 318 00:14:36 --> 00:14:40 commercials for facial products or other things. 319 00:14:40 --> 00:14:44 People like to talk about free radicals, and they're sort of 320 00:14:44 --> 00:14:46 the hero that gets rid of free radicals, which 321 00:14:46 --> 00:14:47 are antioxidants. 322 00:14:47 --> 00:14:51 So you hear in a lot of different creams or products 323 00:14:51 --> 00:14:54 or vitamins that they have antioxidants in them, which 324 00:14:54 --> 00:14:55 get rid of free radicals. 325 00:14:55 --> 00:14:58 The reason you would want to get rid of free radicals is 326 00:14:58 --> 00:15:00 that free radicals can damage DNA, so they're 327 00:15:00 --> 00:15:01 incredibly reactive. 328 00:15:01 --> 00:15:04 It makes sense that if they hit a strand of DNA, they're going 329 00:15:04 --> 00:15:06 to react with the DNA, you end up breaking the strands of 330 00:15:06 --> 00:15:09 DNA and causing DNA damage. 331 00:15:09 --> 00:15:12 So, this is actually what happens in aging because 332 00:15:12 --> 00:15:14 we have a lot of free radicals in our body. 333 00:15:14 --> 00:15:17 We can introduce them artificially, for example, 334 00:15:17 --> 00:15:20 cigarette smoke has a lot of really dangerous free radicals 335 00:15:20 --> 00:15:24 that get into the cells in your lungs, which damage your 336 00:15:24 --> 00:15:26 lung DNA, which can cause lung cancer. 337 00:15:26 --> 00:15:29 But also, all of us are living and breathing, which means 338 00:15:29 --> 00:15:33 we're having metabolism go on in our body, which means that 339 00:15:33 --> 00:15:37 as we use oxygen and as we metabolize our food, we are 340 00:15:37 --> 00:15:40 actually producing free radicals as well. 341 00:15:40 --> 00:15:44 So it's kind of a paradox because we need them because 342 00:15:44 --> 00:15:47 they are a natural by-product of these important processes, 343 00:15:47 --> 00:15:50 but then they can go on and damage cells, which is what 344 00:15:50 --> 00:15:54 kind of is causing aging and can lead to cancer. 345 00:15:54 --> 00:15:58 We have enzymes in our body that repair damage that is done 346 00:15:58 --> 00:16:00 by free radicals, that will put the strands of DNA 347 00:16:00 --> 00:16:01 back together. 348 00:16:01 --> 00:16:04 And we also have antioxidants in our body. 349 00:16:04 --> 00:16:08 So, you might know that, for example, very brightly colored 350 00:16:08 --> 00:16:12 fruit is full of antioxidants, they're full of chemicals that 351 00:16:12 --> 00:16:14 will neutralize free radicals. 352 00:16:14 --> 00:16:17 Lots of vitamins are also antioxidants, so we have 353 00:16:17 --> 00:16:20 vitamin A on the top there and vitamin E. 354 00:16:20 --> 00:16:22 So, the most common thing we think of when we think of free 355 00:16:22 --> 00:16:25 radicals is very reactive, bad for your body, 356 00:16:25 --> 00:16:28 causes DNA damage. 357 00:16:28 --> 00:16:30 But the reality is that free radicals are also 358 00:16:30 --> 00:16:31 essential for life. 359 00:16:31 --> 00:16:34 So this is kind of interesting to think about. 360 00:16:34 --> 00:16:38 And, for example, certain enzymes or proteins actually 361 00:16:38 --> 00:16:41 use free radicals in order to carry out the reactions that 362 00:16:41 --> 00:16:43 they carry out in your body. 363 00:16:43 --> 00:16:46 So, for example, this is a picture or a snapshot of a 364 00:16:46 --> 00:16:49 protein, this is a crystal structure of ribonucleotide 365 00:16:49 --> 00:16:51 reductase is what it's called. 366 00:16:51 --> 00:16:56 It's an enzyme that catalyzes the reaction of an essential 367 00:16:56 --> 00:16:59 step in both DNA synthesis and also DNA repair, and it 368 00:16:59 --> 00:17:04 requires having radicals within its active site in order to 369 00:17:04 --> 00:17:05 carry out the chemistry. 370 00:17:05 --> 00:17:08 So, this is kind of a neat paradox, because radicals 371 00:17:08 --> 00:17:13 damage DNA, but in order to repair your DNA, you need 372 00:17:13 --> 00:17:15 certain enzymes, and those enzymes require different 373 00:17:15 --> 00:17:17 types of free radicals. 374 00:17:17 --> 00:17:20 So, free radicals are definitely very interesting, 375 00:17:20 --> 00:17:23 and once we get -- or hopefully you will get into organic 376 00:17:23 --> 00:17:24 chemistry at some point and get to really think about 377 00:17:24 --> 00:17:28 what they do in terms of a radical mechanism. 378 00:17:28 --> 00:17:31 We can think about radicals that are also more stable, so 379 00:17:31 --> 00:17:35 let's do another example with the molecule nitric acid. 380 00:17:35 --> 00:17:40 So we can again, draw the skeleton here, and just by 381 00:17:40 --> 00:17:42 looking at it we might not know it's a radical, but as we start 382 00:17:42 --> 00:17:45 to count valence electrons, we should be able to figure it out 383 00:17:45 --> 00:17:50 very quickly, because what we have is 11 valence electrons. 384 00:17:50 --> 00:17:53 We need 16 electrons to have full octets. 385 00:17:53 --> 00:17:56 So, we're left with 5 bonding electrons. 386 00:17:56 --> 00:18:00 We put a double bond in between our nitrogen and our oxygen, so 387 00:18:00 --> 00:18:04 what we're left over with is this single bonding electron, 388 00:18:04 --> 00:18:06 and we'll put that on the nitrogen here. 389 00:18:06 --> 00:18:09 And I'll explain why we put it on the nitrogen and not the 390 00:18:09 --> 00:18:11 oxygen in just a minute. 391 00:18:11 --> 00:18:16 But what we find is then once we fill in the rest of the 392 00:18:16 --> 00:18:19 valence electrons in terms of lone pairs, this is the 393 00:18:19 --> 00:18:21 structure that we get. 394 00:18:21 --> 00:18:24 And if you add up all of the formal charges on the nitrogen 395 00:18:24 --> 00:18:27 and on the oxygen, what you'll see is they're both 0. 396 00:18:27 --> 00:18:31 So if you happen to try drawing this structure and you put the 397 00:18:31 --> 00:18:34 lone pair on oxygen and then you figured out the formal 398 00:18:34 --> 00:18:37 charge and saw that you had a split charge, a plus 1 and a 399 00:18:37 --> 00:18:39 minus 1, the first thing you might want to try is putting it 400 00:18:39 --> 00:18:41 on the other atom, and once you did that you'd see that you had 401 00:18:41 --> 00:18:46 a better structure with no formal charge. 402 00:18:46 --> 00:18:49 I have to mention what nitric oxide does, because it's a 403 00:18:49 --> 00:18:50 very interesting molecule. 404 00:18:50 --> 00:18:54 Don't get it confused with nitrous oxide, which is 405 00:18:54 --> 00:18:56 happy gas, that's n o 2. 406 00:18:56 --> 00:19:00 This is nitric oxide, and it's actually much more interesting 407 00:19:00 --> 00:19:02 than nitrous oxide. 408 00:19:02 --> 00:19:05 It's a signaling molecule in your body, it's one of the very 409 00:19:05 --> 00:19:08 few signaling molecules that is a gas, and obviously, 410 00:19:08 --> 00:19:09 it's also a radical. 411 00:19:09 --> 00:19:12 What happens with n o is that it's produced in the 412 00:19:12 --> 00:19:15 endothelium of your blood vessels, so the inner lining of 413 00:19:15 --> 00:19:20 your blood vessels, and it signals for smooth muscle that 414 00:19:20 --> 00:19:23 line your blood vessels to relax, which causes 415 00:19:23 --> 00:19:25 vasodilation , and by vasodilation, I just mean a 416 00:19:25 --> 00:19:27 widening of the blood vessels. 417 00:19:27 --> 00:19:31 So, n o signals for your blood vessels to get wider and allow 418 00:19:31 --> 00:19:33 more blood to flow through. 419 00:19:33 --> 00:19:35 And if you think about what consequences this could have, 420 00:19:35 --> 00:19:39 in terms of places where they have high altitude, so they 421 00:19:39 --> 00:19:42 have lower oxygen levels, do you think that they produce 422 00:19:42 --> 00:19:47 more or less and n o their body? 423 00:19:47 --> 00:19:48 More? 424 00:19:48 --> 00:19:49 Yeah, it turns out they do produce more. 425 00:19:49 --> 00:19:52 The reason they produce more is that they want to have more 426 00:19:52 --> 00:19:55 blood flowing through their veins so that they can get more 427 00:19:55 --> 00:19:59 oxygenated blood into different parts of their body. 428 00:19:59 --> 00:20:02 N o is also a target in the pharmaceutical industry. 429 00:20:02 --> 00:20:06 A very famous one that became famous I guess over 10 years 430 00:20:06 --> 00:20:10 ago now, and this is from a drug that actually targets one 431 00:20:10 --> 00:20:14 of n o's receptors, and this drug has the net effect of 432 00:20:14 --> 00:20:18 vasodilation or widening of blood vessels in a certain 433 00:20:18 --> 00:20:19 area in the body. 434 00:20:19 --> 00:20:22 So this is viagra, some of you may be familiar, I think 435 00:20:22 --> 00:20:24 everyone's heard of viagra. 436 00:20:24 --> 00:20:27 Now you know how viagra works. 437 00:20:27 --> 00:20:32 Viagra breaks down, or it inhibits the breakdown of n o's 438 00:20:32 --> 00:20:35 binding partner in just certain areas, not everywhere 439 00:20:35 --> 00:20:36 in your body. 440 00:20:36 --> 00:20:39 So, in those areas, what happens is you get more n 441 00:20:39 --> 00:20:42 o signaling, you get more vasodilation, you get 442 00:20:42 --> 00:20:44 increased blood flow. 443 00:20:44 --> 00:20:45 So that's a little bit of pharmacology for 444 00:20:45 --> 00:20:47 you here today. 445 00:20:47 --> 00:20:50 All right, so let's talk about one more example in terms of 446 00:20:50 --> 00:20:52 the breakdown of the octet rule with radicals. 447 00:20:52 --> 00:20:57 Let's think about molecular oxygen. 448 00:20:57 --> 00:21:00 So let's go ahead and quickly draw this Lewis structure. 449 00:21:00 --> 00:21:02 We have o 2. 450 00:21:02 --> 00:21:04 The second thing we need to do is figure out 451 00:21:04 --> 00:21:06 valence electrons. 452 00:21:06 --> 00:21:11 6 plus 6, so we would expect to see 12. 453 00:21:11 --> 00:21:17 For a complete octet we would need 8 electrons each, so 16. 454 00:21:17 --> 00:21:24 So in terms of bonding electrons, what we have 455 00:21:24 --> 00:21:26 is 4 bonding electrons. 456 00:21:26 --> 00:21:29 So, we can go ahead and fill those in as a double bond 457 00:21:29 --> 00:21:32 between the two oxygens. 458 00:21:32 --> 00:21:37 So, what we end up having left, and this would be step six then 459 00:21:37 --> 00:21:42 because five was just filling in that, is 12 minus 4, so we 460 00:21:42 --> 00:21:45 have 8 lone pair electrons left. 461 00:21:45 --> 00:21:51 So we can just fill it in to our oxygens like this. 462 00:21:51 --> 00:21:53 All right, so using everything we've learned about Lewis 463 00:21:53 --> 00:21:58 structures, we here have the structure of molecular oxygen. 464 00:21:58 --> 00:22:01 And I just want to point out for anyone that gets confused, 465 00:22:01 --> 00:22:06 when we talk about oxygen as an atom, that's o, but molecular 466 00:22:06 --> 00:22:08 oxygen is actually o 2, the same for molecular 467 00:22:08 --> 00:22:09 hydrogen, for example. 468 00:22:09 --> 00:22:12 All right, so let's look at what the actual Lewis structure 469 00:22:12 --> 00:22:17 is for molecular oxygen, and it turns out that actually we 470 00:22:17 --> 00:22:19 don't have a double bond, we have a single bond, and 471 00:22:19 --> 00:22:21 we have two radicals. 472 00:22:21 --> 00:22:23 And any time we have two radicals, we talk about 473 00:22:23 --> 00:22:26 what's called a biradical. 474 00:22:26 --> 00:22:30 And while using this exception to the Lewis structure rule, to 475 00:22:30 --> 00:22:34 the octet rule for odd numbers of valence electrons can clue 476 00:22:34 --> 00:22:38 us into the fact that we have a radical, there's really no way 477 00:22:38 --> 00:22:40 for us to use Lewis structures to predict when we have a 478 00:22:40 --> 00:22:43 biradical, right, because we would just predict that we 479 00:22:43 --> 00:22:45 would get this Lewis structure here. 480 00:22:45 --> 00:22:48 So, when I first introduced Lewis structures, I said these 481 00:22:48 --> 00:22:50 are great, they're really easy to use and they work 482 00:22:50 --> 00:22:53 about 90% of the time. 483 00:22:53 --> 00:22:55 This falls into that 10% that Lewis structures 484 00:22:55 --> 00:22:56 don't work for us. 485 00:22:56 --> 00:23:00 It turns out, in order to understand that this is the 486 00:23:00 --> 00:23:04 electron configuration for o 2, we need to use something called 487 00:23:04 --> 00:23:08 molecular orbital theory, and just wait till next Wednesday 488 00:23:08 --> 00:23:10 and we will tell you what that is, and we will, in fact, 489 00:23:10 --> 00:23:12 use it for oxygen. 490 00:23:12 --> 00:23:16 But until that point, I'll just tell you that molecular orbital 491 00:23:16 --> 00:23:19 theory takes into account quantum mechanics, which 492 00:23:19 --> 00:23:21 Lewis theory does not. 493 00:23:21 --> 00:23:23 So that's why, in fact, there are those 10% of cases that 494 00:23:23 --> 00:23:26 Lewis structures don't work for. 495 00:23:26 --> 00:23:29 All right, the second case of exceptions to the octet rule 496 00:23:29 --> 00:23:32 are when we have octet deficient molecules. 497 00:23:32 --> 00:23:34 So basically, this means we're going to have a molecule that's 498 00:23:34 --> 00:23:38 stable, even though it doesn't have a complete octet. 499 00:23:38 --> 00:23:41 And these tend to happen in group 13 molecules, and 500 00:23:41 --> 00:23:44 actually happen almost exclusively in group 13 501 00:23:44 --> 00:23:48 molecules, specifically with boron and aluminum. 502 00:23:48 --> 00:23:51 So, any time you see a Lewis structure with boron or 503 00:23:51 --> 00:23:54 aluminum, you want to just remember that I should look out 504 00:23:54 --> 00:23:57 to make sure that these might have an incomplete octet, so 505 00:23:57 --> 00:24:01 look out for that when you see those atoms. 506 00:24:01 --> 00:24:05 So, let's look at b f 3 as our example here. 507 00:24:05 --> 00:24:09 And what we see for b f 3 is the number of valence electrons 508 00:24:09 --> 00:24:13 that we have are 24, because the valence number of electrons 509 00:24:13 --> 00:24:19 for boron is 3, and then 3 times 7 for each fluorine. 510 00:24:19 --> 00:24:24 For total filled octets we need 32, so that means we need 511 00:24:24 --> 00:24:25 8 bonding electrons. 512 00:24:25 --> 00:24:29 So, let's assign two to each bond here, and then we're going 513 00:24:29 --> 00:24:31 to have two extra bonding electrons, so let's just 514 00:24:31 --> 00:24:35 arbitrarily pick a fluorine to give a double bond to. 515 00:24:35 --> 00:24:38 And then we can fill in the lone pair electrons, 516 00:24:38 --> 00:24:40 we have 16 left over. 517 00:24:40 --> 00:24:42 So thinking about what the formal charge is, if we want to 518 00:24:42 --> 00:24:46 figure out the formal charge for the boron here, what we're 519 00:24:46 --> 00:24:49 talking about is the valence number for boron, which is 3, 520 00:24:49 --> 00:24:53 minus 0 because there are no lone pairs, minus 1/2 of 8 521 00:24:53 --> 00:24:55 because there are eight shared electrons. 522 00:24:55 --> 00:24:58 We get a formal charge of minus 1. 523 00:24:58 --> 00:25:03 What is our formal charge since we learned this on Monday for 524 00:25:03 --> 00:25:07 thinking about the double bonded fluorine in boron? 525 00:25:07 --> 00:25:10 So, look at your notes and look at the fluorine that has a 526 00:25:10 --> 00:25:12 double bond with it, and I want you to go ahead and tell 527 00:25:12 --> 00:25:32 me what that formal charge should be. 528 00:25:32 --> 00:25:46 All right, let's take 10 more seconds on that. 529 00:25:46 --> 00:25:50 OK, so 49%. 530 00:25:50 --> 00:25:54 So, let's go look back at the notes, we'll talk about why 531 00:25:54 --> 00:25:58 about 50% of you are right, and 50% need to review, which I 532 00:25:58 --> 00:26:01 totally understand you haven't had time to do yet, your formal 533 00:26:01 --> 00:26:04 charge rules from Monday's class, there were other 534 00:26:04 --> 00:26:05 things going on. 535 00:26:05 --> 00:26:08 But let's talk about how we figure out formal charge. 536 00:26:08 --> 00:26:11 Formal charge is just the number of valence 537 00:26:11 --> 00:26:13 electrons you have. 538 00:26:13 --> 00:26:14 So fluorine has 7. 539 00:26:14 --> 00:26:16 You should be able to look at a periodic table and see 540 00:26:16 --> 00:26:18 that fluorine has seven. 541 00:26:18 --> 00:26:21 What we subtract from that is the number of lone pair 542 00:26:21 --> 00:26:25 electrons, and there are four lone pair electrons on this 543 00:26:25 --> 00:26:28 double bonded fluorine, so it's minus 4. 544 00:26:28 --> 00:26:32 Then we subtract 1/2 of the shared electrons. 545 00:26:32 --> 00:26:34 Well we have a double bond with boron here, so we have a 546 00:26:34 --> 00:26:37 total of 4 shared electrons. 547 00:26:37 --> 00:26:41 And when we do the subtraction here, what we end up with is a 548 00:26:41 --> 00:26:46 formal charge plus 1 on the double bonded fluorine. 549 00:26:46 --> 00:26:48 Without even doing a calculation, what do you 550 00:26:48 --> 00:26:50 think that the formal charge should be on you single 551 00:26:50 --> 00:26:52 bonded fluorines? 552 00:26:52 --> 00:26:53 Good. 553 00:26:53 --> 00:26:56 OK, it should be 0 and it is 0. 554 00:26:56 --> 00:27:01 The reason it's zero in terms of calculating it is 7 minus 6 555 00:27:01 --> 00:27:04 lone pair electrons minus 1/2 half of 2 shared 556 00:27:04 --> 00:27:05 electrons is 0. 557 00:27:05 --> 00:27:09 The reason that you all told me, I think, and I hope, is 558 00:27:09 --> 00:27:12 that you know that the formal charge on individual atoms 559 00:27:12 --> 00:27:15 has to equal the total charge on the molecule. 560 00:27:15 --> 00:27:18 So if we already have a minus 1 and a plus 1, and we know we 561 00:27:18 --> 00:27:21 have no charge in the molecule, and we only have one type of 562 00:27:21 --> 00:27:25 atom left to talk about, that formal charge had better be 0. 563 00:27:25 --> 00:27:25 OK. 564 00:27:25 --> 00:27:27 So this looks pretty good in terms of a Lewis structure, we 565 00:27:27 --> 00:27:29 figured out our formal charges. 566 00:27:29 --> 00:27:31 These also look pretty good, too, we don't have too 567 00:27:31 --> 00:27:33 much charge separation. 568 00:27:33 --> 00:27:37 But what actually it turns out is that if you experimentally 569 00:27:37 --> 00:27:41 look at what type of bonds you have, it turns out that all 570 00:27:41 --> 00:27:44 three of the b f bonds are equal in length, and they all 571 00:27:44 --> 00:27:48 have a length that would correspond to a single bond. 572 00:27:48 --> 00:27:51 So, experimentally, we know we have to throw out this Lewis 573 00:27:51 --> 00:27:54 structure here, we have some more information, let's think 574 00:27:54 --> 00:27:57 about how this could happen. 575 00:27:57 --> 00:28:00 So this could happen, for example, is if we take this two 576 00:28:00 --> 00:28:03 of the electrons that are in the b f double bond and we put 577 00:28:03 --> 00:28:06 it right on to the fluorine here, so now we have 578 00:28:06 --> 00:28:08 all single bonds. 579 00:28:08 --> 00:28:11 And let's think about what the formal charge situation 580 00:28:11 --> 00:28:13 would be in this case here. 581 00:28:13 --> 00:28:16 What happens here is now we would have a formal charge of 582 00:28:16 --> 00:28:20 0 on the boron, we'd have a formal charge of 0 on all of 583 00:28:20 --> 00:28:22 the fluorine molecules as well. 584 00:28:22 --> 00:28:25 So, it turns out that actually looking at formal charge, even 585 00:28:25 --> 00:28:28 though the first case didn't look too bad, this case 586 00:28:28 --> 00:28:28 actually looks a lot better. 587 00:28:28 --> 00:28:32 We have absolutely no formal charge separation whatsoever. 588 00:28:32 --> 00:28:35 It turns out again, boron and aluminum, those are the two 589 00:28:35 --> 00:28:36 that you want to look out for. 590 00:28:36 --> 00:28:39 They can be perfectly happy without a full octet, they're 591 00:28:39 --> 00:28:44 perfectly happy with 6 instead of 8 in terms of electrons 592 00:28:44 --> 00:28:45 in their valence shell. 593 00:28:45 --> 00:28:48 So that is our exception the number two. 594 00:28:48 --> 00:28:51 We have one more exception and this is a valence shell 595 00:28:51 --> 00:28:53 expansion, and this can be the hardest to look out for, 596 00:28:53 --> 00:28:58 students tend to forget to look for this one, but it's very 597 00:28:58 --> 00:29:00 important as well, because there are a lot of structures 598 00:29:00 --> 00:29:01 that are affected for this . 599 00:29:01 --> 00:29:04 And this is only applicable if we're talking about a central 600 00:29:04 --> 00:29:08 atom that has an n value or a principle quantum number that's 601 00:29:08 --> 00:29:11 equal to or greater than three. 602 00:29:11 --> 00:29:16 What happens when we have n that's equal to or greater to 603 00:29:16 --> 00:29:19 three, is that now, in addition to s orbitals and p orbitals, 604 00:29:19 --> 00:29:23 what else do we have available to us? 605 00:29:23 --> 00:29:24 D orbitals, great. 606 00:29:24 --> 00:29:28 So what we see is we have some empty d orbitals, which means 607 00:29:28 --> 00:29:31 that we can have more than eight electrons that fit 608 00:29:31 --> 00:29:33 around that central atom. 609 00:29:33 --> 00:29:35 If you're looking to see if this is going to happen, do you 610 00:29:35 --> 00:29:41 think this would happen with a large or small central atom? 611 00:29:41 --> 00:29:43 So think of it in terms of just fitting. 612 00:29:43 --> 00:29:47 We've got to fit more than 8 electrons around here. 613 00:29:47 --> 00:29:50 Yeah, so it's going to be, we need to have a large 614 00:29:50 --> 00:29:53 central atom in order for this to take place. 615 00:29:53 --> 00:29:55 Literally, we just need to fit everything around is probably 616 00:29:55 --> 00:29:58 the easiest way to think about it. 617 00:29:58 --> 00:30:02 And what happens is it also tends to have small atoms 618 00:30:02 --> 00:30:02 that it's bonded to. 619 00:30:02 --> 00:30:06 Again, just think of it in terms of all fitting in there. 620 00:30:06 --> 00:30:10 So, let's take an example p c l 5. 621 00:30:10 --> 00:30:12 The first example is the more straightforward example, 622 00:30:12 --> 00:30:15 because let's start to draw the Lewis structure, and what we 623 00:30:15 --> 00:30:19 see is that phosphorous has five chlorines around it. 624 00:30:19 --> 00:30:21 So we already know if we want to form five bonds we've 625 00:30:21 --> 00:30:23 broken our octet rule. 626 00:30:23 --> 00:30:25 But let's go through and figure this out and 627 00:30:25 --> 00:30:26 see how that happens. 628 00:30:26 --> 00:30:30 What we know is we need 40 valence electrons, we have 629 00:30:30 --> 00:30:34 those -- 5 from the phosphorous, and we have 7 from 630 00:30:34 --> 00:30:37 each of the chlorine atoms. 631 00:30:37 --> 00:30:39 If we were to fill out all of those octets, that 632 00:30:39 --> 00:30:42 would be 48 electrons. 633 00:30:42 --> 00:30:45 So what we end up with when we do our Lewis structure 634 00:30:45 --> 00:30:48 calculation is that we only have 8 bonding electrons 635 00:30:48 --> 00:30:49 available to us. 636 00:30:49 --> 00:30:51 So we can fill those in between the phosphorous 637 00:30:51 --> 00:30:55 and the chlorine, those 8 bonding electrons. 638 00:30:55 --> 00:30:59 So, this is obviously a problem. 639 00:30:59 --> 00:31:04 To make 5 p c l bonds we need 10 shared electrons, and we 640 00:31:04 --> 00:31:07 know that that's the situation because it's called p c l 5 and 641 00:31:07 --> 00:31:10 not p c l 4, so we can go right ahead and add in that 642 00:31:10 --> 00:31:13 extra electron pair. 643 00:31:13 --> 00:31:16 So we've used up 10 for bonding, so that means what we 644 00:31:16 --> 00:31:19 have left is 30 lone pair electrons, and I would not 645 00:31:19 --> 00:31:22 recommend filling all of these in your notes right now, you 646 00:31:22 --> 00:31:24 can go back and do that, but just know the rest end up 647 00:31:24 --> 00:31:27 filling up the octets for all of the chlorines. 648 00:31:27 --> 00:31:30 So, in this first case where you actually need to make more 649 00:31:30 --> 00:31:33 than for bonds, you will immediately know you need to 650 00:31:33 --> 00:31:37 use this exception to the Lewis structure octet rule, but 651 00:31:37 --> 00:31:39 sometimes it won't be as obvious. 652 00:31:39 --> 00:31:43 So, let's look at c r o 4, the 2 minus version here, so a 653 00:31:43 --> 00:31:47 chromate ion, and if we draw the skeletal structure, we 654 00:31:47 --> 00:31:51 have four things that the chromate needs to bond to. 655 00:31:51 --> 00:31:54 So, let's do the Lewis structure again. 656 00:31:54 --> 00:31:57 When we figure out the valence electrons, we have total, we 657 00:31:57 --> 00:32:01 have 6 from the chromium, we have 6 from each of the 658 00:32:01 --> 00:32:06 different oxygens, and where did this 2 come from? 659 00:32:06 --> 00:32:07 Yup, the negative charge. 660 00:32:07 --> 00:32:10 So, remember, we have 2 extra electrons hanging out in our 661 00:32:10 --> 00:32:12 molecule, so we need to include those. 662 00:32:12 --> 00:32:13 We have a total of 32. 663 00:32:13 --> 00:32:16 40 are needed to fill up octets. 664 00:32:16 --> 00:32:20 So again, we have 8 bonding electrons available, so we can 665 00:32:20 --> 00:32:24 go ahead and fill these in between each of the bonds. 666 00:32:24 --> 00:32:27 What happens is that we then have 24 lone pair electrons 667 00:32:27 --> 00:32:31 left, and we can fill those in like this. 668 00:32:31 --> 00:32:34 And the problem comes now when we figure out 669 00:32:34 --> 00:32:35 the formal charge. 670 00:32:35 --> 00:32:38 So, when we do that what we find is that the chromium has a 671 00:32:38 --> 00:32:42 formal charge of plus 1, and that each of the oxygens has 672 00:32:42 --> 00:32:44 a total charge of minus 1. 673 00:32:44 --> 00:32:47 So we actually have a bit of charge separation here. 674 00:32:47 --> 00:32:49 Without even doing a calculation, what is the 675 00:32:49 --> 00:32:53 total charge of these that are added up? 676 00:32:53 --> 00:32:55 OK, it's minus 2, that's right. 677 00:32:55 --> 00:32:57 We know that the total charge of each of the formal charges 678 00:32:57 --> 00:32:59 has to add up to minus 2, because that's the 679 00:32:59 --> 00:33:01 charge in our molecule. 680 00:33:01 --> 00:33:04 We can also just calculate it -- the chromate gives us a plus 681 00:33:04 --> 00:33:07 2, then we have 4 times minus 1 for each of the oxygens, 682 00:33:07 --> 00:33:09 so we have a minus 2. 683 00:33:09 --> 00:33:11 So, we have some charge separation here, and in some 684 00:33:11 --> 00:33:15 cases, if we're not at n equals 3 or higher, there's really 685 00:33:15 --> 00:33:17 nothing we can do about it, this would be the best 686 00:33:17 --> 00:33:18 structure we can do. 687 00:33:18 --> 00:33:22 But since we have these d orbitals available, we can use 688 00:33:22 --> 00:33:25 them, and it turns out that experimentally this is what's 689 00:33:25 --> 00:33:29 found, that the length and the strength are not single bonds, 690 00:33:29 --> 00:33:32 but they're actually something between a single bond 691 00:33:32 --> 00:33:33 and a double bond. 692 00:33:33 --> 00:33:37 So how do we get a 1 and 1/2 bond, for example, what's the 693 00:33:37 --> 00:33:39 term that let's us do that? 694 00:33:39 --> 00:33:40 Resonance. 695 00:33:40 --> 00:33:40 That's right. 696 00:33:40 --> 00:33:43 So that's exactly what's happening here. 697 00:33:43 --> 00:33:46 So, if we went ahead and drew this structure here where we 698 00:33:46 --> 00:33:51 have now two double bonds and two single bonds, that would 699 00:33:51 --> 00:33:54 be in resonance with another structure where we have two 700 00:33:54 --> 00:33:58 double bonds instead to these two oxygens, and now, single 701 00:33:58 --> 00:34:00 bonds to these two oxygens. 702 00:34:00 --> 00:34:02 We can actually also have several other resonance 703 00:34:02 --> 00:34:04 structures as well. 704 00:34:04 --> 00:34:06 Remember, the definition of a resonance structure is where 705 00:34:06 --> 00:34:09 all the atoms stay the same, but what we can do is move 706 00:34:09 --> 00:34:11 around the electrons -- we're moving around those extra 707 00:34:11 --> 00:34:14 two electrons that can be in double bonds. 708 00:34:14 --> 00:34:17 So, why don't you tell me how many other resonance structures 709 00:34:17 --> 00:34:27 you would expect to see for this chromate ion? 710 00:34:27 --> 00:34:50 All right, let's take 10 more seconds on this. 711 00:34:50 --> 00:34:51 All right. 712 00:34:51 --> 00:34:52 This is good. 713 00:34:52 --> 00:34:55 I know this is a real split response, but the right answer 714 00:34:55 --> 00:34:59 is the one that is indicated in the graph here that it's four. 715 00:34:59 --> 00:35:02 This takes a little bit of time to get used to thinking about 716 00:35:02 --> 00:35:04 all the different Lewis structures you can have. 717 00:35:04 --> 00:35:06 So, you guys should all go back home if you can't see it 718 00:35:06 --> 00:35:10 immediately right now and try drawing out those four other 719 00:35:10 --> 00:35:13 Lewis structures, for chromate, there are four others. 720 00:35:13 --> 00:35:16 You'll probably get a chance to literally do this example in 721 00:35:16 --> 00:35:19 recitation where you draw out all four, but it's even better 722 00:35:19 --> 00:35:22 to make sure you understand it before you get to that point. 723 00:35:22 --> 00:35:25 So, we can go back to the class notes. 724 00:35:25 --> 00:35:28 So it turns out there's four other Lewis structures, so 725 00:35:28 --> 00:35:30 basically just think about all the other different 726 00:35:30 --> 00:35:33 combinations where you can have single and double bonds, and 727 00:35:33 --> 00:35:35 when you draw those out, you end up with four. 728 00:35:35 --> 00:35:37 So, for every single one of these Lewis structures, we 729 00:35:37 --> 00:35:40 could figure out what the formal charges are, and what we 730 00:35:40 --> 00:35:43 would find is that it's 0 on the chromium, it's 0 for the 731 00:35:43 --> 00:35:46 double bonded oxygens, and it's going to be negative 1 for 732 00:35:46 --> 00:35:48 the single bonded oxygens. 733 00:35:48 --> 00:35:52 So, what you can see is that in this situation, we end up 734 00:35:52 --> 00:35:55 having less formal charge separation, and that's what 735 00:35:55 --> 00:35:58 we're looking for, that's the more stable structure. 736 00:35:58 --> 00:36:02 So any time you can have an expanded octet -- an expanded 737 00:36:02 --> 00:36:06 valence shell, where you have n is equal to or greater than 3, 738 00:36:06 --> 00:36:09 and by expanding and adding more electrons into that 739 00:36:09 --> 00:36:12 valence shell, you lower the charge separation, 740 00:36:12 --> 00:36:13 you want to do that. 741 00:36:13 --> 00:36:17 I also want to point out, I basically said there's 6 742 00:36:17 --> 00:36:19 different ways we can draw this in terms of drawing all 743 00:36:19 --> 00:36:20 the resonance structures. 744 00:36:20 --> 00:36:23 You might be wondering if you have to figure out the formal 745 00:36:23 --> 00:36:25 charge for each structure individually, and the answer is 746 00:36:25 --> 00:36:28 no, you can pick any single structure and the formal 747 00:36:28 --> 00:36:30 charges will work out the same. 748 00:36:30 --> 00:36:32 So, for example, if you pick this structure and your friend 749 00:36:32 --> 00:36:35 picks this structure, you'll both get the right answer that 750 00:36:35 --> 00:36:39 there's just the negative 1 on the oxygens and no other formal 751 00:36:39 --> 00:36:42 charges in the molecule. 752 00:36:42 --> 00:36:42 All right. 753 00:36:42 --> 00:36:45 So those are the end of our exceptions to the octet rule 754 00:36:45 --> 00:36:47 for Lewis structures, that's everything we're going to 755 00:36:47 --> 00:36:49 say about Lewis structures. 756 00:36:49 --> 00:36:52 And remember, that when we talk about Lewis structures, what 757 00:36:52 --> 00:36:55 they tell us is the electron configuration in covalent 758 00:36:55 --> 00:36:58 bonds, so that valence shell electron configuration. 759 00:36:58 --> 00:37:02 So we talked a lot about covalent bonds before we got 760 00:37:02 --> 00:37:05 into Lewis structures, and then how to represent covalent 761 00:37:05 --> 00:37:07 bonds by Lewis structures. 762 00:37:07 --> 00:37:10 So now I'll say a little bit about ionic bonds, which are 763 00:37:10 --> 00:37:14 the other extreme, and when you have an ionic bond, what you 764 00:37:14 --> 00:37:18 have now is a complete transfer of either one or many 765 00:37:18 --> 00:37:20 electrons between two atoms. 766 00:37:20 --> 00:37:24 So the key word for covalent bond was electron sharing, 767 00:37:24 --> 00:37:27 the key word for ionic bonds is electron transfer. 768 00:37:27 --> 00:37:31 And the bonding between the two atoms ends up resulting from an 769 00:37:31 --> 00:37:33 attraction that we're very familiar with, which is the 770 00:37:33 --> 00:37:36 Coulomb or the electrostatic attraction between the 771 00:37:36 --> 00:37:41 negatively charged and the positively charged ions. 772 00:37:41 --> 00:37:42 So let's take an example. 773 00:37:42 --> 00:37:45 The easiest one to think about is where we have a negative 774 00:37:45 --> 00:37:47 1 and a positive 1 ion. 775 00:37:47 --> 00:37:51 So this is salt, n a c l -- actually lots of things are 776 00:37:51 --> 00:37:54 call salt, but this is what we think of a table salt. 777 00:37:54 --> 00:37:57 So, let's think about what we have to do if we want the form 778 00:37:57 --> 00:38:01 sodium chloride from the neutral sodium and 779 00:38:01 --> 00:38:02 chlorine atoms. 780 00:38:02 --> 00:38:05 So, the first thing that we're going to need to do is we need 781 00:38:05 --> 00:38:08 to convert sodium into sodium plus. 782 00:38:08 --> 00:38:10 What does this process look like to you? 783 00:38:10 --> 00:38:13 Is this one of those periodic trends, perhaps? 784 00:38:13 --> 00:38:17 Can anyone name what we're looking at here? 785 00:38:17 --> 00:38:18 Exactly, ionization energy. 786 00:38:18 --> 00:38:21 So, if we're going to talk about the energy difference 787 00:38:21 --> 00:38:24 here, what we're going to be talking about is the ionization 788 00:38:24 --> 00:38:28 energy, or the energy it takes to rip off an electron from 789 00:38:28 --> 00:38:32 sodium in order to form the sodium plus ion. 790 00:38:32 --> 00:38:37 So, we can just put right here, that's 494 kilojoules per mole. 791 00:38:37 --> 00:38:39 The next thing that we want to look at is chlorine, so in 792 00:38:39 --> 00:38:42 terms of chlorine we need to go to chlorine minus, so we 793 00:38:42 --> 00:38:44 actually need to add an electron. 794 00:38:44 --> 00:38:47 This is actually the reverse of one of the periodic 795 00:38:47 --> 00:38:49 trends we talked about. 796 00:38:49 --> 00:38:53 Which trend is that this is the reverse of? 797 00:38:53 --> 00:38:54 Electron affinity, right. 798 00:38:54 --> 00:38:57 Because if we go backwards we're saying how badly 799 00:38:57 --> 00:38:59 does chlorine want to grab an electron? 800 00:38:59 --> 00:39:02 Chlorine wants to do this very badly, and it turns out the 801 00:39:02 --> 00:39:05 electron affinity for chlorine is huge, it's 349 kilojoules 802 00:39:05 --> 00:39:08 per mole, but remember, we're going in reverse, so we need to 803 00:39:08 --> 00:39:13 talk about it as negative 349 kilojoules per mole. 804 00:39:13 --> 00:39:17 So if we talk about the sum of what's happening here, what we 805 00:39:17 --> 00:39:21 need to do is think about going from the neutrals to the ions, 806 00:39:21 --> 00:39:24 so we can just add those two energies together, and what we 807 00:39:24 --> 00:39:28 end up with is plus 145 kilojoules per mole, in order 808 00:39:28 --> 00:39:33 to go from neutral sodium in chlorine to the ions. 809 00:39:33 --> 00:39:36 So, the problem here is that we have to actually put energy 810 00:39:36 --> 00:39:39 into our system, so this doesn't seem favorable, right. 811 00:39:39 --> 00:39:42 What's favorable is when we actually get energy out and our 812 00:39:42 --> 00:39:45 energy gets lower, but what we're saying here is that we 813 00:39:45 --> 00:39:47 actually need to put in energy. 814 00:39:47 --> 00:39:50 So another way to say this is this process actually 815 00:39:50 --> 00:39:51 requires energy. 816 00:39:51 --> 00:39:55 It does not emit energy, it does not give off excess 817 00:39:55 --> 00:39:57 energy, it requires energy. 818 00:39:57 --> 00:40:00 So, we need to think about how can we solve this problem in 819 00:40:00 --> 00:40:03 terms of thinking about ionic bonds, and the answer 820 00:40:03 --> 00:40:04 is Coulomb attraction. 821 00:40:04 --> 00:40:07 So there's one more force that we need to talk about, and that 822 00:40:07 --> 00:40:10 is when we talk about the attraction between the 823 00:40:10 --> 00:40:13 negatively and the positively charged ions, such that 824 00:40:13 --> 00:40:15 we form sodium chloride. 825 00:40:15 --> 00:40:18 So this process here has a delta energy, a change in 826 00:40:18 --> 00:40:22 energy of negative 589 kilojoules per mole. 827 00:40:22 --> 00:40:24 So that's huge, we're giving off a lot of energy 828 00:40:24 --> 00:40:26 by this attraction. 829 00:40:26 --> 00:40:30 So if we add up the net energy for all of this process, 830 00:40:30 --> 00:40:35 all we need to do is add negative 589 to plus 145. 831 00:40:35 --> 00:40:38 So what we end up getting is the net energy change is going 832 00:40:38 --> 00:40:42 to be negative 444 kilojoules per mole, so you can see that, 833 00:40:42 --> 00:40:46 in fact, it is very favorable for neutral sodium and neutral 834 00:40:46 --> 00:40:53 chloride to form sodium chloride in an ionic bond. 835 00:40:53 --> 00:40:56 And the net increase then, is a decrease in energy. 836 00:40:56 --> 00:40:59 So, I just gave you the number in terms of what that Coulomb 837 00:40:59 --> 00:41:03 potential would be in attraction, but we can I easily 838 00:41:03 --> 00:41:07 calculate it as well using this equation here where the energy 839 00:41:07 --> 00:41:11 is equal to the charge on each of the ions, and this is just 840 00:41:11 --> 00:41:14 multiplied by the value of charge for an electron divided 841 00:41:14 --> 00:41:19 by 4 pi epsilon nought times r, are r is just the distance in 842 00:41:19 --> 00:41:22 terms of the bond length we could talk about. 843 00:41:22 --> 00:41:24 So, let's calculate and make sure that I didn't tell 844 00:41:24 --> 00:41:25 you a false number here. 845 00:41:25 --> 00:41:28 Let's say we do the calculation with the bond length that 846 00:41:28 --> 00:41:30 we've looked up, which is 2 . 847 00:41:30 --> 00:41:33 3 6 angstroms for the bond length between 848 00:41:33 --> 00:41:34 sodium and chloride. 849 00:41:34 --> 00:41:37 So we should be able to figure out the Coulombic 850 00:41:37 --> 00:41:37 attraction for this. 851 00:41:37 --> 00:41:47 So, if we talk about the energy of attraction, we need to 852 00:41:47 --> 00:41:52 multiply plus 1, that's the charge on the sodium, times 853 00:41:52 --> 00:41:56 minus 1, the charge on the chlorine, times the charge 854 00:41:56 --> 00:41:58 in an electron, 1 . 855 00:41:58 --> 00:42:04 6 0 2 times 10 the negative 19 Coulombs, and that's all 856 00:42:04 --> 00:42:12 divided by 4 pi, and then I've written out epsilon nought in 857 00:42:12 --> 00:42:14 your notes, so I won't write it on the board. 858 00:42:14 --> 00:42:17 And then r, so r is going to be 2 . 859 00:42:17 --> 00:42:23 3 6 and times -- what is angstrom, everyone? 860 00:42:23 --> 00:42:26 Yup, 10 to the negative 10. 861 00:42:26 --> 00:42:30 So 10 to the negative 10 meters. 862 00:42:30 --> 00:42:34 So, if we do this calculation here, what we end up with is 863 00:42:34 --> 00:42:42 negative 9.774 times 10 to the negative 19 joules. 864 00:42:42 --> 00:42:46 So that's what we have in terms of our energy. 865 00:42:46 --> 00:42:49 That does not look the same as what we saw -- yup, 866 00:42:49 --> 00:42:49 do you have a question? 867 00:42:49 --> 00:42:54 STUDENT: [INAUDIBLE] 868 00:42:54 --> 00:42:56 PROFESSOR: OK. 869 00:42:56 --> 00:42:58 Luckily, although, I did not write it in my own notes, I 870 00:42:58 --> 00:43:01 did it when I put in my calculator, thank you. 871 00:43:01 --> 00:43:03 So you need to square this value here and then you 872 00:43:03 --> 00:43:07 should get this value right here, negative 9.77. 873 00:43:07 --> 00:43:11 All right, so what we need to do though is convert from 874 00:43:11 --> 00:43:13 joules into kilojoules per mole, because that's 875 00:43:13 --> 00:43:14 what we were using. 876 00:43:14 --> 00:43:22 So if we multiply that number there by kilojoules per mole -- 877 00:43:22 --> 00:43:27 or excuse me, first kilojoules per joule, so we have 1 878 00:43:27 --> 00:43:30 joules in every kilojoule. 879 00:43:30 --> 00:43:35 And then we multiply that by Avagadro's number, 6.022 880 00:43:35 --> 00:43:40 times 10 to the 23 per mole. 881 00:43:40 --> 00:43:48 What we end up with is negative 589 kilojoules per mole. 882 00:43:48 --> 00:43:51 So this is that same Coulombic attraction that we saw 883 00:43:51 --> 00:43:53 in the first place. 884 00:43:53 --> 00:43:57 So, notice that you will naturally get out a negative 885 00:43:57 --> 00:43:59 charge here, remember negative means an attractive force in 886 00:43:59 --> 00:44:05 this case, because you have the plus and the minus 1 in here. 887 00:44:05 --> 00:44:09 So we should be able to easily do that calculation, and what 888 00:44:09 --> 00:44:12 we end up getting matches up with what I just told you, 889 00:44:12 --> 00:44:14 luckily, and thank you for catching the square, that's an 890 00:44:14 --> 00:44:17 important part in getting the right answer. 891 00:44:17 --> 00:44:21 So, experimentally then, what we find is that the change in 892 00:44:21 --> 00:44:23 energy for this reaction is negative 444 893 00:44:23 --> 00:44:25 kilojoules per mole. 894 00:44:25 --> 00:44:28 If we look experimentally what we see, it's actually a little 895 00:44:28 --> 00:44:32 bit different, it's negative 411 kilojoules per mole. 896 00:44:32 --> 00:44:36 So, in terms of this class, this is the method that we're 897 00:44:36 --> 00:44:38 going to use, and we're going to say this gets us close 898 00:44:38 --> 00:44:41 enough such that we can make comparisons and have a 899 00:44:41 --> 00:44:43 meaningful conversations about different types of ionic bonds 900 00:44:43 --> 00:44:46 and the attraction between them. 901 00:44:46 --> 00:44:49 But let's think about where this discrepancy comes from, 902 00:44:49 --> 00:44:53 and before I do that I want to point out, one term we use 903 00:44:53 --> 00:44:56 a lot is change in energy for a reaction where, for 904 00:44:56 --> 00:44:58 example, you break a bond. 905 00:44:58 --> 00:45:02 Remember that the negative of the change in energy is 906 00:45:02 --> 00:45:04 what's called delta e sub d. 907 00:45:04 --> 00:45:07 We first saw this when we first introduced the 908 00:45:07 --> 00:45:08 idea of covalent bonds. 909 00:45:08 --> 00:45:14 Do you remember what this term here means, delta e sub d? 910 00:45:14 --> 00:45:17 A little bit and some no's, which this was pre-exam, I 911 00:45:17 --> 00:45:19 understand, you still need to review those notes, 912 00:45:19 --> 00:45:21 it's dissociation energy. 913 00:45:21 --> 00:45:24 So you get a negative energy out by breaking the bond. 914 00:45:24 --> 00:45:29 The dissociation energy means how much energy that bond is 915 00:45:29 --> 00:45:32 worth in terms of strength, so it's the opposite of the energy 916 00:45:32 --> 00:45:35 you get out of breaking the bond -- or excuse me, the 917 00:45:35 --> 00:45:37 energy that you get out of forming the bond. 918 00:45:37 --> 00:45:40 It's the amount of energy you need to put in to break the 919 00:45:40 --> 00:45:42 bond is dissociation energy. 920 00:45:42 --> 00:45:44 It takes this much energy to dissociate your 921 00:45:44 --> 00:45:44 bond, excuse me. 922 00:45:44 --> 00:45:45 All right. 923 00:45:45 --> 00:45:48 So, let's take a look here at our predictions, so I just put 924 00:45:48 --> 00:45:51 them both ways so we don't get confused. 925 00:45:51 --> 00:45:53 The dissociation energy is 444. 926 00:45:53 --> 00:45:55 The change in energy for forming the bond 927 00:45:55 --> 00:45:57 is negative 444. 928 00:45:57 --> 00:46:00 We made the following approximations, which explain 929 00:46:00 --> 00:46:02 why, in fact, we got a different experimental 930 00:46:02 --> 00:46:04 energy, if we look at that. 931 00:46:04 --> 00:46:06 The first thing is that we ignored any 932 00:46:06 --> 00:46:07 repulsive interactions. 933 00:46:07 --> 00:46:12 If you think about salt, it's not just two single atoms 934 00:46:12 --> 00:46:13 that you are talking about. 935 00:46:13 --> 00:46:16 It's actually in a whole network or whole lattice of 936 00:46:16 --> 00:46:18 other molecules, so you actually have some other 937 00:46:18 --> 00:46:21 chlorines around that are going to be having repulsive 938 00:46:21 --> 00:46:25 interactions with our chlorine that we're talking about. 939 00:46:25 --> 00:46:27 We're going to ignore those, make the approximation that 940 00:46:27 --> 00:46:30 those don't matter, at this point, in these calculations. 941 00:46:30 --> 00:46:33 And the result for that is that we end up with a larger 942 00:46:33 --> 00:46:36 dissociation energy than the experimental value. 943 00:46:36 --> 00:46:38 That's because the bond is going to be a little bit more 944 00:46:38 --> 00:46:42 broken than it was in our calculation, because we do have 945 00:46:42 --> 00:46:44 these repulsive interactions. 946 00:46:44 --> 00:46:48 The other thing that we did is that we treated both sodium and 947 00:46:48 --> 00:46:51 the chlorine as point charges. 948 00:46:51 --> 00:46:53 And this is what actually allowed us to make this 949 00:46:53 --> 00:46:56 calculation and calculate the Coulomb potential so easily, 950 00:46:56 --> 00:46:58 we just treated them as if they're point charges. 951 00:46:58 --> 00:47:01 We're ignoring quantum mechanics in this -- this is 952 00:47:01 --> 00:47:04 sort of the class where we ignore quantum mechanics, we 953 00:47:04 --> 00:47:06 ignored it for Lewis structures, we're 954 00:47:06 --> 00:47:06 ignoring it here. 955 00:47:06 --> 00:47:10 We will be back to paying a lot of attention to quantum 956 00:47:10 --> 00:47:13 mechanics in lecture 14 when we talk about MO theory, but for 957 00:47:13 --> 00:47:16 now, these are approximations, these are models where we don't 958 00:47:16 --> 00:47:18 take it into consideration. 959 00:47:18 --> 00:47:21 And I think you'll agree that we come reasonably close such 960 00:47:21 --> 00:47:23 that we'll be able to make comparisons between different 961 00:47:23 --> 00:47:24 kinds of ionic bonds. 962 00:47:24 --> 00:47:27 All right. 963 00:47:27 --> 00:47:29 So, the last thing I want to introduce today is talking 964 00:47:29 --> 00:47:31 about polar covalent bonds. 965 00:47:31 --> 00:47:34 We've now covered the two extremes. 966 00:47:34 --> 00:47:38 One extreme is complete total electron sharing -- if we have 967 00:47:38 --> 00:47:41 a perfectly covalent bond, we have perfect sharing. 968 00:47:41 --> 00:47:45 The other is electron transfer in terms of ionic bonds. 969 00:47:45 --> 00:47:49 So when we talk about a polar covalent bond, what we're now 970 00:47:49 --> 00:47:52 talking about is an unequal sharing of electrons 971 00:47:52 --> 00:47:54 between two atoms. 972 00:47:54 --> 00:47:56 So, this is essentially something we've seen before, we 973 00:47:56 --> 00:47:59 just never formally talked about what we would call it. 974 00:47:59 --> 00:48:02 This is any time you have a bond forming between two 975 00:48:02 --> 00:48:06 non-metals that have different electronegativities, so, for 976 00:48:06 --> 00:48:10 example, hydrogen choride, h c l. 977 00:48:10 --> 00:48:13 The electronegativity for hydrogen is 2.2, for 978 00:48:13 --> 00:48:15 chlorine it's 3.2. 979 00:48:15 --> 00:48:19 And in general, what we say is we consider a difference in 980 00:48:19 --> 00:48:22 terms of a first approximation if the difference in 981 00:48:22 --> 00:48:24 electronegativity is more than 0. 982 00:48:24 --> 00:48:27 5, so this is on the Pauling electronegativity scale. 983 00:48:27 --> 00:48:31 So what we end up having is we sort of have a kind of, and 984 00:48:31 --> 00:48:34 what we call it is a partial negative charge on the 985 00:48:34 --> 00:48:37 chlorine, and a partial positive charge 986 00:48:37 --> 00:48:38 in the hydrogen. 987 00:48:38 --> 00:48:41 The reason we have that is because the chlorine's more 988 00:48:41 --> 00:48:43 electronegative, it wants to pull more of that shared 989 00:48:43 --> 00:48:45 electron density to itself. 990 00:48:45 --> 00:48:47 If it has more electron density, it's going to have a 991 00:48:47 --> 00:48:50 little bit of a negative charge and the hydrogen's going to be 992 00:48:50 --> 00:48:53 left with a little bit of a positive charge. 993 00:48:53 --> 00:48:56 So, we can compare this, for example to, molecular hydrogen 994 00:48:56 --> 00:48:58 where they're going to have that complete sharing, so 995 00:48:58 --> 00:49:02 there's not going to be a delta plus or a delta minus, delta is 996 00:49:02 --> 00:49:05 going to be equal to zero on each of the atoms. 997 00:49:05 --> 00:49:09 They are completely sharing their electrons. 998 00:49:09 --> 00:49:13 And we can also explain this in another way by talking about a 999 00:49:13 --> 00:49:16 dipole moment where we have a charged distribution that 1000 00:49:16 --> 00:49:19 results in this dipole, this electric dipole. 1001 00:49:19 --> 00:49:22 And we talk about this using the term mu, which is a 1002 00:49:22 --> 00:49:24 measurement of what the dipole is. 1003 00:49:24 --> 00:49:28 A dipole is always written in terms of writing an arrow from 1004 00:49:28 --> 00:49:31 the positive charge to the negative charge. 1005 00:49:31 --> 00:49:33 In chemistry, we are always incredibly interested in what 1006 00:49:33 --> 00:49:36 the electrons are doing, so we tend to pay attention to 1007 00:49:36 --> 00:49:37 them in terms of arrows. 1008 00:49:37 --> 00:49:40 Oh, the electrons are going over to the chlorine, so we're 1009 00:49:40 --> 00:49:43 going to draw our arrow toward the chlorine atom. 1010 00:49:43 --> 00:49:47 So, we measure this here, so mu is equal to q times r, the 1011 00:49:47 --> 00:49:49 distance between the two. 1012 00:49:49 --> 00:49:52 And q, that charge is just equal to the partial negative 1013 00:49:52 --> 00:49:57 or the partial positive times the charge on the electron. 1014 00:49:57 --> 00:50:00 So this is measured in Coulomb meters, you won't ever 1015 00:50:00 --> 00:50:03 see a measurement of electronegativity in Coulomb 1016 00:50:03 --> 00:50:08 meters -- we tend to talk about it in terms of debye or 1 d, or 1017 00:50:08 --> 00:50:13 sometimes there's no units at all, so the d is just assumed, 1018 00:50:13 --> 00:50:16 and it's because 1 debye is just equal to this very tiny 1019 00:50:16 --> 00:50:18 number of Coulomb meters and it's a lot easier to 1020 00:50:18 --> 00:50:21 work with debye's here. 1021 00:50:21 --> 00:50:25 So, when we talk about polar molecules, we can actually 1022 00:50:25 --> 00:50:29 extend our idea of talking about polar bonds to talking 1023 00:50:29 --> 00:50:30 about polar molecules. 1024 00:50:30 --> 00:50:32 So, actually let's start with that on Monday. 1025 00:50:32 --> 00:50:35 So everyone have a great weekend.