1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,090 continue to offer high-quality educational resources for free. 5 00:00:10,090 --> 00:00:12,660 To make a donation or to view additional materials 6 00:00:12,660 --> 00:00:16,486 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,486 --> 00:00:17,110 at ocw.mit.edu. 8 00:00:21,330 --> 00:00:25,760 CATHERINE DRENNAN: So that is electron affinity. 9 00:00:25,760 --> 00:00:28,750 But honestly, chemists don't really talk so much 10 00:00:28,750 --> 00:00:31,000 about electron affinity. 11 00:00:31,000 --> 00:00:35,010 They prefer to talk about electronegativity. 12 00:00:35,010 --> 00:00:37,450 And these are highly related terms. 13 00:00:37,450 --> 00:00:42,440 So this was also re-copied, although completely identical, 14 00:00:42,440 --> 00:00:43,750 I think, between the handouts. 15 00:00:43,750 --> 00:00:46,300 I just thought it was weird to have re-copied this and not 16 00:00:46,300 --> 00:00:46,800 this. 17 00:00:46,800 --> 00:00:51,430 So your handout for today is perfect on this point. 18 00:00:51,430 --> 00:00:54,730 So electron negativity, the net ability 19 00:00:54,730 --> 00:00:59,610 of an atom to attract an electron from another atom. 20 00:00:59,610 --> 00:01:04,090 So you can see that electron affinity and electronegativity 21 00:01:04,090 --> 00:01:04,970 are very similar. 22 00:01:04,970 --> 00:01:08,230 In fact, all of these terms are highly related to each other. 23 00:01:08,230 --> 00:01:11,310 And this idea of electronegativity, 24 00:01:11,310 --> 00:01:14,400 of this as a term for a way of thinking about atoms 25 00:01:14,400 --> 00:01:16,460 initiated with Linus Pauling. 26 00:01:16,460 --> 00:01:18,860 But here I have up a different picture. 27 00:01:18,860 --> 00:01:20,790 I have Robert Millikan. 28 00:01:20,790 --> 00:01:22,610 And the reason why I picked this picture 29 00:01:22,610 --> 00:01:26,830 is because he helped, a few years after Linus Pauling came 30 00:01:26,830 --> 00:01:29,790 up with this idea, coming up with an equation 31 00:01:29,790 --> 00:01:33,030 that help people think better about what electronegativity 32 00:01:33,030 --> 00:01:33,760 is. 33 00:01:33,760 --> 00:01:36,360 And another reason why I picked his picture 34 00:01:36,360 --> 00:01:38,890 to put up here instead of Linus Pauling 35 00:01:38,890 --> 00:01:41,470 is that he was an MIT undergraduate, 36 00:01:41,470 --> 00:01:43,390 and he was a chemistry major. 37 00:01:43,390 --> 00:01:46,810 I'm not sure in 1917 or whenever-- 38 00:01:46,810 --> 00:01:50,530 that was the yearbook picture-- when he took-- I don't think 39 00:01:50,530 --> 00:01:52,380 it was 5.111 at that point. 40 00:01:52,380 --> 00:01:56,140 But at some point, he was here studying chemistry just 41 00:01:56,140 --> 00:01:57,140 like you. 42 00:01:57,140 --> 00:02:00,150 And then when he got a faculty position later on, 43 00:02:00,150 --> 00:02:04,070 he did some beautiful work that had to do with bonding that he 44 00:02:04,070 --> 00:02:05,360 got the Nobel Prize. 45 00:02:05,360 --> 00:02:09,539 So some of you may have a Nobel Prize in chemistry one day. 46 00:02:09,539 --> 00:02:12,620 And so you want to make sure your yearbook picture 47 00:02:12,620 --> 00:02:16,720 is at least as good as this one for other generations 48 00:02:16,720 --> 00:02:18,850 of professors to show your picture 49 00:02:18,850 --> 00:02:21,640 and describe the work that you did to contribute 50 00:02:21,640 --> 00:02:25,020 to the field of chemistry. 51 00:02:25,020 --> 00:02:27,110 Oh, and he was born in Massachusetts too, 52 00:02:27,110 --> 00:02:31,730 so he is a native to this area in more than one way. 53 00:02:31,730 --> 00:02:32,230 All right. 54 00:02:32,230 --> 00:02:35,410 So the way that he-- and this is a little bit more 55 00:02:35,410 --> 00:02:37,740 of a squishy definition. 56 00:02:37,740 --> 00:02:42,460 So electron negativity is proportional to a 1/2-- 57 00:02:42,460 --> 00:02:44,334 and IE stands for what? 58 00:02:44,334 --> 00:02:45,250 AUDIENCE: [INAUDIBLE]. 59 00:02:45,250 --> 00:02:46,860 CATHERINE DRENNAN: Ionization energy. 60 00:02:46,860 --> 00:02:51,330 And EA-- our electron affinity that we just discussed. 61 00:02:51,330 --> 00:02:51,830 All right. 62 00:02:51,830 --> 00:02:54,390 So it's related to these other terms 63 00:02:54,390 --> 00:02:57,080 that we have already talked about. 64 00:02:57,080 --> 00:03:01,460 So let's think about then what this means. 65 00:03:01,460 --> 00:03:05,950 So we can consider an atom with high electronegativity 66 00:03:05,950 --> 00:03:08,482 and an atom with low electronegativity. 67 00:03:08,482 --> 00:03:10,550 And we want to think about whether an atom 68 00:03:10,550 --> 00:03:13,730 with high electronegativity is going to be an electron 69 00:03:13,730 --> 00:03:16,080 acceptor or an electron donor. 70 00:03:16,080 --> 00:03:19,060 And that you can tell me, and that is 71 00:03:19,060 --> 00:03:21,690 going to be a clicker question. 72 00:03:21,690 --> 00:03:24,090 So you can try to grab your handout while clicking 73 00:03:24,090 --> 00:03:24,835 at the same time. 74 00:04:10,080 --> 00:04:10,580 All right. 75 00:04:10,580 --> 00:04:11,850 So we'll take 10 more seconds. 76 00:04:27,650 --> 00:04:28,900 Yes, 88%. 77 00:04:28,900 --> 00:04:29,970 That's great. 78 00:04:29,970 --> 00:04:32,570 Of course, if you looked and if you didn't believe 79 00:04:32,570 --> 00:04:35,420 it could be a donor, then that ruled out three of the four, 80 00:04:35,420 --> 00:04:36,670 but that's OK. 81 00:04:36,670 --> 00:04:38,710 Those are good things. 82 00:04:38,710 --> 00:04:41,990 So yes, if it has high electronegativity, 83 00:04:41,990 --> 00:04:44,730 it's going to be an electron acceptor. 84 00:04:44,730 --> 00:04:46,600 And part of the reason for that is 85 00:04:46,600 --> 00:04:49,400 that it has a high affinity for electrons. 86 00:04:49,400 --> 00:04:51,420 And another part of the reason for that 87 00:04:51,420 --> 00:04:53,320 is that if you look at the equation, when 88 00:04:53,320 --> 00:04:55,260 you have a high ionization energy, 89 00:04:55,260 --> 00:05:00,320 something that has a high ionization energy 90 00:05:00,320 --> 00:05:01,970 is not going to be a good donor. 91 00:05:01,970 --> 00:05:03,920 So that wouldn't make sense. 92 00:05:03,920 --> 00:05:07,830 So both of those terms having high in both categories 93 00:05:07,830 --> 00:05:11,050 is consistent then with this trend. 94 00:05:11,050 --> 00:05:14,060 So let's take a little bit more of a look at that 95 00:05:14,060 --> 00:05:15,640 and why this is true. 96 00:05:15,640 --> 00:05:18,070 So high electronegativity, an atom 97 00:05:18,070 --> 00:05:21,570 with high electronegativity is an electron acceptor, 98 00:05:21,570 --> 00:05:24,510 and then low would be a donor. 99 00:05:24,510 --> 00:05:28,012 And so if we think about this-- and this is our periodic table. 100 00:05:28,012 --> 00:05:29,720 And again, it's not going to be including 101 00:05:29,720 --> 00:05:32,100 our noble gases, which really don't want to be accepting 102 00:05:32,100 --> 00:05:33,770 or donating anything. 103 00:05:33,770 --> 00:05:38,070 So in this corner then we had our high ionization energy, 104 00:05:38,070 --> 00:05:41,670 and we had a high electron affinity. 105 00:05:41,670 --> 00:05:44,650 And we saw last class we had high ionization energy. 106 00:05:44,650 --> 00:05:48,160 So it doesn't want to give up an electron, 107 00:05:48,160 --> 00:05:49,910 but it does want to accept one. 108 00:05:49,910 --> 00:05:53,070 So we have things that are going to be good acceptors. 109 00:05:53,070 --> 00:05:56,560 And down here, we have low ionization energy, 110 00:05:56,560 --> 00:05:59,855 so it's easy to donate an electron. 111 00:05:59,855 --> 00:06:01,570 Oh, let me just put these up, sorry. 112 00:06:01,570 --> 00:06:04,940 So we have then if you're high and high up here, 113 00:06:04,940 --> 00:06:07,080 you have something that's a good acceptor, 114 00:06:07,080 --> 00:06:09,950 and it's going to have a high electronegativity. 115 00:06:09,950 --> 00:06:12,980 So high high means high over there. 116 00:06:12,980 --> 00:06:15,330 And then down here, we have low low, 117 00:06:15,330 --> 00:06:18,310 which means we have low electronegativity. 118 00:06:18,310 --> 00:06:21,270 Low ionization energy-- it's easy to give something up. 119 00:06:21,270 --> 00:06:24,310 Low electron affinity-- it doesn't want electrons. 120 00:06:24,310 --> 00:06:26,180 It's happy to give up electrons. 121 00:06:26,180 --> 00:06:28,220 And if it gives up electrons, then you 122 00:06:28,220 --> 00:06:30,390 can get a complete octet. 123 00:06:30,390 --> 00:06:32,191 It can have a noble gas configuration. 124 00:06:32,191 --> 00:06:33,690 So on this side, you need electrons. 125 00:06:33,690 --> 00:06:36,920 This side, it's happy to give them up. 126 00:06:36,920 --> 00:06:41,240 So if we look then just at a periodic table again, 127 00:06:41,240 --> 00:06:42,220 this makes sense. 128 00:06:42,220 --> 00:06:44,140 We gain an electron over here. 129 00:06:44,140 --> 00:06:47,300 We get our happy noble gas configuration. 130 00:06:47,300 --> 00:06:48,970 We lose electrons over here. 131 00:06:48,970 --> 00:06:50,480 We do the same thing. 132 00:06:50,480 --> 00:06:53,491 So that's a way to think about electronegativity. 133 00:06:53,491 --> 00:06:53,990 All right. 134 00:06:53,990 --> 00:06:57,350 So why should we care about electronegativity? 135 00:06:57,350 --> 00:07:02,120 And that's because a lot of atoms that are electronegative 136 00:07:02,120 --> 00:07:05,620 are used in pharmaceutical molecules, 137 00:07:05,620 --> 00:07:08,170 and that this gives them special properties. 138 00:07:08,170 --> 00:07:10,860 So to hear in their own words, we're 139 00:07:10,860 --> 00:07:14,910 going to hear from a former uropper, Kateryna, talking 140 00:07:14,910 --> 00:07:19,187 about why you should care about electronegativity. 141 00:07:26,988 --> 00:07:28,420 [VIDEO PLAYBACK] 142 00:07:28,420 --> 00:07:30,550 - My name is Kateryna Kozyrytska. 143 00:07:30,550 --> 00:07:32,120 I come from Ukraine. 144 00:07:32,120 --> 00:07:34,370 And I'm interested in how microorganisms 145 00:07:34,370 --> 00:07:36,350 fight each other. 146 00:07:36,350 --> 00:07:37,630 Humans are very smart. 147 00:07:37,630 --> 00:07:41,320 They have found chemical ways to make new drugs. 148 00:07:41,320 --> 00:07:43,520 And so we spend a lot of time and money 149 00:07:43,520 --> 00:07:48,140 on making a new antibiotic, and then we put it into people 150 00:07:48,140 --> 00:07:51,090 and we hope for the best. 151 00:07:51,090 --> 00:07:53,390 Bacteria are also very, very smart. 152 00:07:53,390 --> 00:07:57,690 And they somehow learn to resist this new antibiotic 153 00:07:57,690 --> 00:07:59,740 that we just made. 154 00:07:59,740 --> 00:08:02,170 Bugs, on the other hand, have been fighting each other 155 00:08:02,170 --> 00:08:04,382 with the same molecules for thousands 156 00:08:04,382 --> 00:08:06,850 and thousands of years, and we see 157 00:08:06,850 --> 00:08:10,060 no resistance developing there. 158 00:08:10,060 --> 00:08:12,680 So we want to learn what it is about the antibiotics 159 00:08:12,680 --> 00:08:15,260 that bugs make to fight each other that 160 00:08:15,260 --> 00:08:18,710 makes them so difficult to develop resistance to. 161 00:08:18,710 --> 00:08:22,830 Normally, living organisms use 20 amino acids. 162 00:08:22,830 --> 00:08:25,080 But these bugs get very tricky in building 163 00:08:25,080 --> 00:08:29,410 their anti-other bugs molecules. 164 00:08:29,410 --> 00:08:32,049 So to add the functionality, bugs 165 00:08:32,049 --> 00:08:35,409 can do chemistry on the building blocks, 166 00:08:35,409 --> 00:08:37,630 the amino acids themselves. 167 00:08:37,630 --> 00:08:39,419 And so one of the things that they can do 168 00:08:39,419 --> 00:08:44,290 is chlorinate carbons, which activates these carbons 169 00:08:44,290 --> 00:08:46,360 for future chemistry. 170 00:08:46,360 --> 00:08:49,740 The protein I study, the halogenase SyrB2, 171 00:08:49,740 --> 00:08:53,450 takes chloride ion from the environment and a molecule 172 00:08:53,450 --> 00:08:56,270 of threonine, the amino acid, and puts those together 173 00:08:56,270 --> 00:08:59,380 forming a chlorine-carbon bond. 174 00:08:59,380 --> 00:09:02,150 Since chlorine is so electronegative, when bonded 175 00:09:02,150 --> 00:09:06,350 to carbon it pulls electrons away from the carbon atom. 176 00:09:06,350 --> 00:09:08,270 And so it makes the carbon to which 177 00:09:08,270 --> 00:09:10,340 it is attached much more reactive 178 00:09:10,340 --> 00:09:12,020 toward other molecules. 179 00:09:12,020 --> 00:09:14,190 And this increased reactivity, at least partially, 180 00:09:14,190 --> 00:09:18,470 accounts for the antibiotic effect of the molecule. 181 00:09:18,470 --> 00:09:22,370 I am hoping to figure out how SyrB2 positions all the atoms 182 00:09:22,370 --> 00:09:25,900 in such a way that they react in this very controlled, very 183 00:09:25,900 --> 00:09:28,740 appropriate manner, so that later we could maybe 184 00:09:28,740 --> 00:09:31,670 re-engineer this or other proteins to make 185 00:09:31,670 --> 00:09:34,550 them do chemistry that we want them to do. 186 00:09:34,550 --> 00:09:36,870 My hope is that we could understand SyrB2 187 00:09:36,870 --> 00:09:41,020 well enough that we'll be able to remake it 188 00:09:41,020 --> 00:09:42,840 into a protein that will actively 189 00:09:42,840 --> 00:09:45,426 participate in synthesis of new antibiotics. 190 00:09:48,305 --> 00:09:48,888 [END PLAYBACK] 191 00:09:48,888 --> 00:09:49,888 CATHERINE DRENNAN: Yeah. 192 00:09:49,888 --> 00:09:51,410 So that is Kateryna. 193 00:09:51,410 --> 00:09:54,570 And so how adding halogens, because they 194 00:09:54,570 --> 00:09:59,355 are so electronegative, is actually a very important area, 195 00:09:59,355 --> 00:10:01,700 and I'll give you a couple more examples. 196 00:10:01,700 --> 00:10:04,365 I also haven't watched that video in a little bit, 197 00:10:04,365 --> 00:10:06,060 and she said she was from the Ukraine. 198 00:10:06,060 --> 00:10:08,210 She's technically now part of Russia, 199 00:10:08,210 --> 00:10:11,070 I think, from part of Russia, so. 200 00:10:11,070 --> 00:10:13,470 [LAUGHTER] 201 00:10:13,470 --> 00:10:17,820 Anyway, that's a topic for a different day. 202 00:10:17,820 --> 00:10:22,340 So this is big business, actually, putting halogens 203 00:10:22,340 --> 00:10:23,390 on things. 204 00:10:23,390 --> 00:10:28,310 And if you become interested-- if you're taking an antibiotic 205 00:10:28,310 --> 00:10:31,300 or something, start looking at what the molecule is, 206 00:10:31,300 --> 00:10:34,920 start counting how many halogens are on that molecule, 207 00:10:34,920 --> 00:10:36,700 you will find a lot. 208 00:10:36,700 --> 00:10:40,470 So a lot of antibiotics have halogens, either chlorides 209 00:10:40,470 --> 00:10:43,920 or fluorine, as shown here. 210 00:10:43,920 --> 00:10:46,680 Also a very common antidepressant 211 00:10:46,680 --> 00:10:49,780 has it, another example of something 212 00:10:49,780 --> 00:10:51,820 that is an anti-diabetic. 213 00:10:51,820 --> 00:10:56,220 Huge numbers of molecules have halogens added to them. 214 00:10:56,220 --> 00:10:58,760 Some of these are derived from natural products. 215 00:10:58,760 --> 00:11:03,530 So nature was making these molecules to kill other bugs, 216 00:11:03,530 --> 00:11:05,480 as you heard about in the video. 217 00:11:05,480 --> 00:11:09,220 Other times, they came up with this molecule 218 00:11:09,220 --> 00:11:12,950 and they said, well, we need to make it a little bit different. 219 00:11:12,950 --> 00:11:14,947 It's being consumed too fast. 220 00:11:14,947 --> 00:11:16,530 It's broken down too fast in the body. 221 00:11:16,530 --> 00:11:17,890 Let's add some halogens. 222 00:11:17,890 --> 00:11:20,600 So sometimes it's sort of a man-made tailoring, 223 00:11:20,600 --> 00:11:23,740 but often it's a tailoring that nature came up with. 224 00:11:23,740 --> 00:11:26,140 So why all these halogens? 225 00:11:26,140 --> 00:11:30,710 What's the benefit of having a carbon-fluorine bond instead 226 00:11:30,710 --> 00:11:32,840 of carbon-hydrogen? 227 00:11:32,840 --> 00:11:37,870 And one reason is that having a fluorine, this really 228 00:11:37,870 --> 00:11:43,150 electronegative atom, on, say, an aryl ring like this one, 229 00:11:43,150 --> 00:11:47,610 it actually sucks the electrons out of the ring and makes it 230 00:11:47,610 --> 00:11:49,800 what we call, or organic chemists like to call, 231 00:11:49,800 --> 00:11:51,040 electron poor. 232 00:11:51,040 --> 00:11:54,560 So it just kind of hauls those electrons away. 233 00:11:54,560 --> 00:11:58,210 And when you make something electron poor, 234 00:11:58,210 --> 00:12:01,950 so by replacing C-H with C-F, that 235 00:12:01,950 --> 00:12:05,180 can make a potential drug molecule electron poor. 236 00:12:05,180 --> 00:12:08,000 And what this does often is make it 237 00:12:08,000 --> 00:12:11,060 harder to oxidize the molecule. 238 00:12:11,060 --> 00:12:14,370 So we're going to talk about oxidation-reduction much later 239 00:12:14,370 --> 00:12:17,160 in the semester, and we'll come back to this idea. 240 00:12:17,160 --> 00:12:19,600 But this turns out to be really important, 241 00:12:19,600 --> 00:12:23,660 because the way that the body metabolizes or breaks down 242 00:12:23,660 --> 00:12:26,530 these molecules is that it can oxidize it. 243 00:12:26,530 --> 00:12:29,120 And there are a number of enzymes in your liver 244 00:12:29,120 --> 00:12:32,780 which will oxidize and break apart these drugs. 245 00:12:32,780 --> 00:12:35,500 And so if you make it harder to oxidize, 246 00:12:35,500 --> 00:12:38,780 that makes the drug more stable in your body. 247 00:12:38,780 --> 00:12:40,290 So if you want to take a drug, you 248 00:12:40,290 --> 00:12:42,590 want it to last for a while. 249 00:12:42,590 --> 00:12:44,990 And especially if it's something, an antibiotic, 250 00:12:44,990 --> 00:12:48,050 you want it to last till it kills all of the bacteria, 251 00:12:48,050 --> 00:12:50,345 not just half of them. 252 00:12:50,345 --> 00:12:53,060 And most medicines, you need them to be around 253 00:12:53,060 --> 00:12:54,520 for them to have their effect. 254 00:12:54,520 --> 00:12:56,770 So you want to tailor those molecules 255 00:12:56,770 --> 00:12:59,850 so that they don't get broken down as easily in the body. 256 00:12:59,850 --> 00:13:02,360 And so this is one reason, and this is big business. 257 00:13:02,360 --> 00:13:06,530 And a lot of the times, adding those halogens actually 258 00:13:06,530 --> 00:13:09,330 involves pretty toxic chemicals. 259 00:13:09,330 --> 00:13:12,800 So some people, like Kateryna, are interested in using enzymes 260 00:13:12,800 --> 00:13:14,990 to do it instead. 261 00:13:14,990 --> 00:13:19,330 Some people who are still using organic synthesis-- example 262 00:13:19,330 --> 00:13:21,210 is Steve Buchwald's laboratory. 263 00:13:21,210 --> 00:13:23,960 In fact, if you go to almost any top chemistry department, 264 00:13:23,960 --> 00:13:25,460 I think there's someone who's trying 265 00:13:25,460 --> 00:13:30,112 to find out new methods of putting halogens on molecules. 266 00:13:30,112 --> 00:13:33,320 It's a very important area in designing new molecules 267 00:13:33,320 --> 00:13:35,240 and improving them. 268 00:13:35,240 --> 00:13:36,110 OK. 269 00:13:36,110 --> 00:13:38,770 So that is electronegativity. 270 00:13:38,770 --> 00:13:42,460 So one atom added to a big number of atoms 271 00:13:42,460 --> 00:13:44,160 can change the property of the molecule 272 00:13:44,160 --> 00:13:46,750 by sucking electrons away. 273 00:13:46,750 --> 00:13:51,540 And now we're going to talk about atomic and ionic radius 274 00:13:51,540 --> 00:13:54,280 and also isoelectric atoms. 275 00:13:54,280 --> 00:13:58,000 So these trends are pretty good. 276 00:13:58,000 --> 00:14:00,760 We're back to some pretty good-- fewer glitches. 277 00:14:00,760 --> 00:14:02,400 So what is the atomic radius? 278 00:14:02,400 --> 00:14:06,090 So here we have 2r, 2 times the atomic radius. 279 00:14:06,090 --> 00:14:09,630 So the atomic radius is defined by the value 280 00:14:09,630 --> 00:14:14,590 of r that has about 90% of the electron density. 281 00:14:14,590 --> 00:14:16,540 I mean, technically, an electron could 282 00:14:16,540 --> 00:14:19,520 be infinitely or close to infinitely far away, 283 00:14:19,520 --> 00:14:24,020 but pretty much most of them are going to be about within 90%. 284 00:14:24,020 --> 00:14:26,540 We call that the radius of the atom. 285 00:14:26,540 --> 00:14:28,730 So they're trends. 286 00:14:28,730 --> 00:14:30,580 Trends are pretty good. 287 00:14:30,580 --> 00:14:34,090 So across the periodic table, what matters 288 00:14:34,090 --> 00:14:35,620 is the Z effective. 289 00:14:35,620 --> 00:14:37,770 See, everything we've learned about it comes back. 290 00:14:37,770 --> 00:14:39,190 If you didn't learn it on exam 1, 291 00:14:39,190 --> 00:14:41,740 still learn it because we're going to use it again. 292 00:14:41,740 --> 00:14:44,310 Across the periodic table, Z effective matters. 293 00:14:44,310 --> 00:14:47,810 Down the periodic table, n, or the principle quantum number, 294 00:14:47,810 --> 00:14:48,650 matters. 295 00:14:48,650 --> 00:14:50,460 So how do these matter? 296 00:14:50,460 --> 00:14:54,800 So across the periodic table, Z effective is going to do what? 297 00:14:54,800 --> 00:14:56,613 Increase or decrease? 298 00:14:56,613 --> 00:14:58,277 AUDIENCE: Increase. 299 00:14:58,277 --> 00:15:00,110 CATHERINE DRENNAN: So it's going to increase 300 00:15:00,110 --> 00:15:02,640 across the periodic table, and this 301 00:15:02,640 --> 00:15:05,710 results in a decrease in the atomic radius. 302 00:15:05,710 --> 00:15:08,870 So again, it increases going across the periodic table 303 00:15:08,870 --> 00:15:11,970 because you're adding both electrons and protons. 304 00:15:11,970 --> 00:15:14,850 But the electrons are not giving you complete shielding, 305 00:15:14,850 --> 00:15:17,250 so you're not canceling out every proton 306 00:15:17,250 --> 00:15:18,520 with every electron. 307 00:15:18,520 --> 00:15:22,480 So overall, you get an increase in the Z effective. 308 00:15:22,480 --> 00:15:28,540 And because of that, when you have this increased Z, 309 00:15:28,540 --> 00:15:31,590 it's kind of pulling the electrons in. 310 00:15:31,590 --> 00:15:35,690 And then it isn't until you go down the periodic table 311 00:15:35,690 --> 00:15:40,690 when n increases that you start to see the radius increase. 312 00:15:40,690 --> 00:15:44,750 So there, the electrons are getting farther away, 313 00:15:44,750 --> 00:15:46,510 and so you are getting this bigger thing. 314 00:15:46,510 --> 00:15:48,968 So I like to think about it as sort of the mom at the park. 315 00:15:48,968 --> 00:15:52,990 There are some moms, their kids are sort of running everywhere, 316 00:15:52,990 --> 00:15:57,190 but other ones are sort of hauling their kids in. 317 00:15:57,190 --> 00:15:59,110 They have this positive force that 318 00:15:59,110 --> 00:16:02,240 seems to keep them all sort of in the general area. 319 00:16:02,240 --> 00:16:03,855 And so they're shrinking the size 320 00:16:03,855 --> 00:16:07,070 of their kids' play area with this force 321 00:16:07,070 --> 00:16:08,230 that they're exerting. 322 00:16:08,230 --> 00:16:09,790 But if the kids get too far away, 323 00:16:09,790 --> 00:16:11,600 it's like, yeah, they're not going to hear you call. 324 00:16:11,600 --> 00:16:12,900 They're not going to hear you jump up and down. 325 00:16:12,900 --> 00:16:14,190 They're not going to see you. 326 00:16:14,190 --> 00:16:16,160 And so they're just going to be out there, 327 00:16:16,160 --> 00:16:19,880 and the radius of your kids is going to be farther away. 328 00:16:19,880 --> 00:16:22,620 So we can look at these trends. 329 00:16:22,620 --> 00:16:24,800 If we're over here in the beginning, 330 00:16:24,800 --> 00:16:26,130 we're starting on this side. 331 00:16:26,130 --> 00:16:28,760 We're going across the periodic table. 332 00:16:28,760 --> 00:16:32,960 We go down, then we have a jump up when we increase n. 333 00:16:32,960 --> 00:16:35,610 And then we go down again, then we have a jump up 334 00:16:35,610 --> 00:16:36,900 when we increase n. 335 00:16:36,900 --> 00:16:41,010 We go down again, except over here there's a little glitch. 336 00:16:41,010 --> 00:16:43,007 Those d electrons, they're back. 337 00:16:43,007 --> 00:16:44,840 They're going to give you a couple glitches. 338 00:16:44,840 --> 00:16:45,890 I love d electrons. 339 00:16:45,890 --> 00:16:48,317 We're going to talk about them more around Thanksgiving. 340 00:16:48,317 --> 00:16:49,900 That's my favorite part of the course. 341 00:16:49,900 --> 00:16:51,990 Those d electrons are always causing trouble. 342 00:16:51,990 --> 00:16:52,489 OK. 343 00:16:52,489 --> 00:16:55,200 Then we go up again, and then we go down, and go up, 344 00:16:55,200 --> 00:16:55,840 and go down. 345 00:16:55,840 --> 00:16:56,756 Those are pretty good. 346 00:16:56,756 --> 00:16:58,820 Those are pretty good trends. 347 00:16:58,820 --> 00:16:59,760 All right. 348 00:16:59,760 --> 00:17:00,960 OK. 349 00:17:00,960 --> 00:17:05,040 So ions. 350 00:17:05,040 --> 00:17:08,030 Ions are different than their neutral parent, once again. 351 00:17:08,030 --> 00:17:12,670 So we saw this before, that when you start filling the 3d, 352 00:17:12,670 --> 00:17:14,710 the energy levels change. 353 00:17:14,710 --> 00:17:16,710 So ions can have different properties 354 00:17:16,710 --> 00:17:18,420 than their neutral parents. 355 00:17:18,420 --> 00:17:21,410 And so if we have two kinds of ions, 356 00:17:21,410 --> 00:17:24,670 we can have cations, which are positively charged. 357 00:17:24,670 --> 00:17:28,300 And so a positively charged ion will have lost an electron, 358 00:17:28,300 --> 00:17:31,520 and so it's going to be smaller than its parent. 359 00:17:31,520 --> 00:17:34,620 And so we can see here lithium. 360 00:17:34,620 --> 00:17:37,460 And then in the center, that's lithium plus. 361 00:17:37,460 --> 00:17:39,470 So when you lose the electron, the radius 362 00:17:39,470 --> 00:17:40,990 actually shrinks quite a bit. 363 00:17:40,990 --> 00:17:44,420 It's like that electron was just really kind 364 00:17:44,420 --> 00:17:46,190 of causing a bigger radius. 365 00:17:46,190 --> 00:17:48,300 And when it's finally gone, you're 366 00:17:48,300 --> 00:17:51,930 at a smaller size over there. 367 00:17:51,930 --> 00:17:54,480 Anions-- negatively charged ions. 368 00:17:54,480 --> 00:17:57,660 So they're gaining an electron, and their radius 369 00:17:57,660 --> 00:17:59,510 is larger than their parent. 370 00:17:59,510 --> 00:18:02,810 And so you can see over here, we have oxygen in the center. 371 00:18:02,810 --> 00:18:05,950 Oxygen minus 2 is much larger. 372 00:18:05,950 --> 00:18:08,090 And again, we can see some of the other trends. 373 00:18:08,090 --> 00:18:09,850 Some of them are the same. 374 00:18:09,850 --> 00:18:13,860 The ionic radius also will increase 375 00:18:13,860 --> 00:18:16,940 when we're going down a group, so when n is increasing, 376 00:18:16,940 --> 00:18:18,720 so from lithium to sodium. 377 00:18:18,720 --> 00:18:20,830 We have an increase from fluorine 378 00:18:20,830 --> 00:18:23,440 to the top of the periodic table to chlorine. 379 00:18:23,440 --> 00:18:26,990 So we still, as we increase n, increase in size. 380 00:18:26,990 --> 00:18:30,200 But you have to think about the ion-- did it lose an electron, 381 00:18:30,200 --> 00:18:32,220 or did it gain an electron-- to think 382 00:18:32,220 --> 00:18:38,430 about how its size changed with respect to its parent. 383 00:18:38,430 --> 00:18:42,380 So why does this matter? 384 00:18:42,380 --> 00:18:45,510 There's one example of-- if you're 385 00:18:45,510 --> 00:18:48,320 interested in biology or anything 386 00:18:48,320 --> 00:18:51,870 to do with the brain or neurons, then 387 00:18:51,870 --> 00:18:53,990 you should care about ion channels. 388 00:18:53,990 --> 00:18:57,600 So there are channels in membranes that bring ions in, 389 00:18:57,600 --> 00:19:00,440 and this is really important. 390 00:19:00,440 --> 00:19:03,890 So ion channels are in muscle cells and in neurons. 391 00:19:03,890 --> 00:19:08,020 So if you want to move or think, something that MIT students 392 00:19:08,020 --> 00:19:10,220 generally like to do both of those things, 393 00:19:10,220 --> 00:19:12,470 you need ion channels to do that. 394 00:19:12,470 --> 00:19:15,510 So ion channels should be important to you. 395 00:19:15,510 --> 00:19:17,120 And you want ion channels. 396 00:19:17,120 --> 00:19:20,680 They regulate the influx of ions into the cell 397 00:19:20,680 --> 00:19:24,140 and allow for really rapid responses, 398 00:19:24,140 --> 00:19:26,310 which is also really important. 399 00:19:26,310 --> 00:19:30,750 And amazingly, they're highly selective for certain ions. 400 00:19:30,750 --> 00:19:32,360 So this is important. 401 00:19:32,360 --> 00:19:35,120 It needs to be tightly regulated to say how much sodium you 402 00:19:35,120 --> 00:19:36,710 have in there or how much potassium 403 00:19:36,710 --> 00:19:38,080 that you have coming in. 404 00:19:38,080 --> 00:19:40,380 And if the ion channel took potassium 405 00:19:40,380 --> 00:19:43,580 when it was supposed to take sodium, that would not be good. 406 00:19:43,580 --> 00:19:46,990 So these channels are designed by nature 407 00:19:46,990 --> 00:19:49,380 to be highly selective, and so they 408 00:19:49,380 --> 00:19:51,960 care to be highly selective. 409 00:19:51,960 --> 00:19:55,280 And you're talking about a plus 1 maybe or another plus 1, 410 00:19:55,280 --> 00:19:57,400 they have to think about the radius. 411 00:19:57,400 --> 00:20:01,480 So why don't you tell me what the differential 412 00:20:01,480 --> 00:20:05,150 is in the radius from smallest to largest for these three 413 00:20:05,150 --> 00:20:06,030 different ones. 414 00:20:56,745 --> 00:20:57,870 All right, 10 more seconds. 415 00:21:13,400 --> 00:21:15,470 OK, great. 416 00:21:15,470 --> 00:21:19,080 So most people got that right. 417 00:21:19,080 --> 00:21:22,370 Let's just kind of take a look at that. 418 00:21:22,370 --> 00:21:27,400 So here we want to think about the neutral, 419 00:21:27,400 --> 00:21:29,690 and then this one has one less electron. 420 00:21:29,690 --> 00:21:31,380 So that's going to be smaller. 421 00:21:31,380 --> 00:21:34,530 And then when you compare potassium with sodium, 422 00:21:34,530 --> 00:21:36,180 you have to think about n. 423 00:21:36,180 --> 00:21:38,310 And so this is down farther, so that's 424 00:21:38,310 --> 00:21:40,170 going to be bigger than sodium. 425 00:21:40,170 --> 00:21:42,670 So here we're thinking about the difference 426 00:21:42,670 --> 00:21:48,490 in electron configuration, and here we're thinking about n. 427 00:21:48,490 --> 00:21:48,990 All right. 428 00:21:48,990 --> 00:21:53,130 So amazingly, these channels have it figured out, 429 00:21:53,130 --> 00:21:54,290 and so look at this. 430 00:21:54,290 --> 00:21:56,035 This is about significant figures. 431 00:21:56,035 --> 00:21:58,160 Too many people lost points on significant figures, 432 00:21:58,160 --> 00:22:01,370 I have to say, on the exam, so make sure you learn them. 433 00:22:01,370 --> 00:22:03,620 But if you had, say-- you say, oh, 434 00:22:03,620 --> 00:22:06,410 what's the difference between 1 to 3 significant figures? 435 00:22:06,410 --> 00:22:09,680 The difference is sort of the potassium versus the sodium 436 00:22:09,680 --> 00:22:10,330 radius. 437 00:22:10,330 --> 00:22:13,420 So 1.38 times 10 to the minus 10, 438 00:22:13,420 --> 00:22:16,450 1.02 times 10 to the minus 10-- those 439 00:22:16,450 --> 00:22:17,820 seem like pretty small numbers. 440 00:22:17,820 --> 00:22:20,880 Does it really matter if it's 0.2 versus 0.38? 441 00:22:20,880 --> 00:22:23,400 And the answer is, yes, you'd be dead 442 00:22:23,400 --> 00:22:26,930 if nature could not distinguish between these significant 443 00:22:26,930 --> 00:22:28,260 figures for you. 444 00:22:28,260 --> 00:22:30,680 So these channels are designed to be 445 00:22:30,680 --> 00:22:36,370 selective at that kind of atomic scale, and only let one ion in. 446 00:22:36,370 --> 00:22:38,720 And so Rod MacKinnon, who's a crystallographer, 447 00:22:38,720 --> 00:22:41,520 won a Nobel Prize for solving some of the structures 448 00:22:41,520 --> 00:22:42,950 of these ion channels. 449 00:22:42,950 --> 00:22:44,940 And this just shows a ribbon drawing, 450 00:22:44,940 --> 00:22:47,720 and this shows an all atom drawing of a channel, 451 00:22:47,720 --> 00:22:49,900 and there's an ion going through. 452 00:22:49,900 --> 00:22:54,600 That's its hole, and its radius is perfect for that ion. 453 00:22:54,600 --> 00:22:57,130 And the other one, even though it's not 454 00:22:57,130 --> 00:23:00,370 that many significant figures different, doesn't fit. 455 00:23:00,370 --> 00:23:01,610 And that's pretty amazing. 456 00:23:01,610 --> 00:23:03,320 Nature is truly amazing. 457 00:23:03,320 --> 00:23:04,110 That's the hole. 458 00:23:04,110 --> 00:23:08,530 It makes a perfect hole just for the one kind of ion 459 00:23:08,530 --> 00:23:11,160 that it's supposed to accept. 460 00:23:11,160 --> 00:23:11,750 All right. 461 00:23:11,750 --> 00:23:16,912 So now, there are one more definition 462 00:23:16,912 --> 00:23:17,870 that we're going to do. 463 00:23:17,870 --> 00:23:20,330 There are things that can have the same electron 464 00:23:20,330 --> 00:23:21,650 configuration. 465 00:23:21,650 --> 00:23:27,170 Those are called isoelectronic, and let's think about those. 466 00:23:27,170 --> 00:23:29,220 They don't necessarily have the same size, 467 00:23:29,220 --> 00:23:32,400 but they have the same electron configuration. 468 00:23:32,400 --> 00:23:34,370 And I'm just going to write these out. 469 00:23:34,370 --> 00:23:40,480 So when we think around other ones near neon, noble gas, that 470 00:23:40,480 --> 00:23:43,820 would have that exact configuration-- so how 471 00:23:43,820 --> 00:23:46,990 do we get flourine to have that configuration? 472 00:23:46,990 --> 00:23:50,380 What does it need to do-- gain or lose an electron, 473 00:23:50,380 --> 00:23:51,110 and how many? 474 00:23:51,110 --> 00:23:52,728 What would its state be? 475 00:23:58,500 --> 00:24:02,270 So what do I write-- what's the thing for flourine that 476 00:24:02,270 --> 00:24:05,480 is going to be the same electron configuration just in terms 477 00:24:05,480 --> 00:24:06,390 of its charge? 478 00:24:06,390 --> 00:24:07,870 I'd write F what? 479 00:24:07,870 --> 00:24:08,780 AUDIENCE: Minus. 480 00:24:08,780 --> 00:24:10,030 CATHERINE DRENNAN: Minus. 481 00:24:10,030 --> 00:24:11,480 For O, what am I going to write? 482 00:24:11,480 --> 00:24:13,021 For oxygen, what am I going to write? 483 00:24:13,021 --> 00:24:13,850 AUDIENCE: 2 minus. 484 00:24:13,850 --> 00:24:15,300 CATHERINE DRENNAN: 2 minus. 485 00:24:15,300 --> 00:24:16,590 And, say, nitrogen? 486 00:24:16,590 --> 00:24:17,620 We'll stop there. 487 00:24:17,620 --> 00:24:18,310 What's that? 488 00:24:18,310 --> 00:24:19,170 AUDIENCE: 3 minus. 489 00:24:19,170 --> 00:24:20,295 CATHERINE DRENNAN: 3 minus. 490 00:24:20,295 --> 00:24:20,960 Great. 491 00:24:20,960 --> 00:24:22,490 Let's go on the other side. 492 00:24:22,490 --> 00:24:23,900 What about for sodium? 493 00:24:23,900 --> 00:24:26,510 What does sodium have to do to have that configuration? 494 00:24:26,510 --> 00:24:27,440 AUDIENCE: Plus. 495 00:24:27,440 --> 00:24:28,440 CATHERINE DRENNAN: Plus. 496 00:24:31,560 --> 00:24:32,910 What about Mg? 497 00:24:32,910 --> 00:24:34,330 AUDIENCE: 2 plus. 498 00:24:34,330 --> 00:24:36,290 CATHERINE DRENNAN: 2 plus. 499 00:24:36,290 --> 00:24:37,780 Aluminum? 500 00:24:37,780 --> 00:24:39,419 AUDIENCE: 3 plus. 501 00:24:39,419 --> 00:24:40,335 CATHERINE DRENNAN: OK. 502 00:24:40,335 --> 00:24:42,850 And silicon-- 4 plus. 503 00:24:42,850 --> 00:24:46,050 So you get the idea. 504 00:24:46,050 --> 00:24:49,300 And now, we can think about which will have bigger 505 00:24:49,300 --> 00:24:53,700 and which will have smaller radii as well. 506 00:24:53,700 --> 00:24:59,287 So are these going to be bigger or smaller? 507 00:24:59,287 --> 00:25:00,162 AUDIENCE: [INAUDIBLE] 508 00:25:00,162 --> 00:25:01,203 CATHERINE DRENNAN: Right. 509 00:25:01,203 --> 00:25:06,400 So they're going to have larger radii than their parents 510 00:25:06,400 --> 00:25:08,510 because they've all gained. 511 00:25:08,510 --> 00:25:11,550 And then over here, these will be 512 00:25:11,550 --> 00:25:14,140 smaller since they've lost electrons 513 00:25:14,140 --> 00:25:17,270 compared to their parent ion. 514 00:25:17,270 --> 00:25:22,150 OK, so let's just do one, which should be very fast, 515 00:25:22,150 --> 00:25:23,130 clicker question. 516 00:25:23,130 --> 00:25:25,820 It is a clicker competition after all, 517 00:25:25,820 --> 00:25:28,363 so we've got to get in some extra clicker questions. 518 00:25:40,370 --> 00:25:42,290 And this should be very fast, I think. 519 00:25:56,160 --> 00:25:58,138 All right, let's just do 10 more seconds. 520 00:26:01,970 --> 00:26:04,345 You have a periodic table up here in case you need it. 521 00:26:13,130 --> 00:26:17,030 Yeah, OK, that's not going to distinguish the recitations 522 00:26:17,030 --> 00:26:18,000 very much. 523 00:26:18,000 --> 00:26:21,110 Yeah, so you just have to look at what is nearby 524 00:26:21,110 --> 00:26:23,760 and think about how many electrons it needs to gain 525 00:26:23,760 --> 00:26:27,220 or lose to have the same configuration. 526 00:26:27,220 --> 00:26:32,090 All right, bonds-- now, we're up to bonds. 527 00:26:32,090 --> 00:26:36,294 There are three types of bonds we will discuss today. 528 00:26:36,294 --> 00:26:37,960 We probably won't get to them all today. 529 00:26:37,960 --> 00:26:39,300 After all, the handout stuff. 530 00:26:39,300 --> 00:26:42,450 But anyway, then we will discuss in the class over time. 531 00:26:42,450 --> 00:26:45,600 Now, some of you have probably figured out-- almost everyone 532 00:26:45,600 --> 00:26:49,120 has probably figured out that one of the things I love to do 533 00:26:49,120 --> 00:26:50,940 is teach chemistry. 534 00:26:50,940 --> 00:26:53,860 I love to teach chemistry. 535 00:26:53,860 --> 00:26:56,530 Some of you have come to my office hours or pizza forums 536 00:26:56,530 --> 00:26:58,350 or even paid attention to some of my slides 537 00:26:58,350 --> 00:27:03,910 about office hours may realize that I love dogs. 538 00:27:03,910 --> 00:27:08,100 So I love teaching chemistry, and I love dogs. 539 00:27:08,100 --> 00:27:13,140 What is the most amazing thing that you can think of? 540 00:27:13,140 --> 00:27:15,026 Dogs teaching chemistry. 541 00:27:18,172 --> 00:27:18,838 [VIDEO PLAYBACK] 542 00:27:18,838 --> 00:27:21,303 CATHERINE DRENNAN: So now, I'm going to let dogs-- 543 00:27:21,303 --> 00:27:23,275 - Welcome to "Dogs Teaching Chemistry!" 544 00:27:23,275 --> 00:27:25,108 CATHERINE DRENNAN: --tell you about bonding. 545 00:27:25,108 --> 00:27:27,712 - --is chemical bonding. 546 00:27:27,712 --> 00:27:31,163 Chemical bonds are what holds atoms together. 547 00:27:31,163 --> 00:27:34,121 A chemical bond is an attraction between atoms 548 00:27:34,121 --> 00:27:37,079 that allows the formation of a chemical substance. 549 00:27:37,079 --> 00:27:40,530 The electrons that participate in a chemical bond 550 00:27:40,530 --> 00:27:42,009 are called valence electrons. 551 00:27:42,009 --> 00:27:44,967 These are electrons that are found in an atom's outermost 552 00:27:44,967 --> 00:27:47,450 shell. 553 00:27:47,450 --> 00:27:49,674 Let's take a look at the types of chemical bonds 554 00:27:49,674 --> 00:27:51,168 that can be formed between atoms. 555 00:27:56,646 --> 00:27:59,634 An ionic bond is formed when one of the atoms 556 00:27:59,634 --> 00:28:02,640 will lose its electron to the other atom. 557 00:28:02,640 --> 00:28:05,335 This results in a positively charged ion 558 00:28:05,335 --> 00:28:09,279 called a cation and a negatively charged ion called an anion. 559 00:28:12,250 --> 00:28:14,586 Positive and negative attract, and the result 560 00:28:14,586 --> 00:28:16,038 is an ionic bond. 561 00:28:22,330 --> 00:28:25,030 Covalent chemical bonds involve the sharing 562 00:28:25,030 --> 00:28:29,361 of a pair of valence electrons by two atoms. 563 00:28:29,361 --> 00:28:32,464 There is also what is called polar covalent bonds. 564 00:28:32,464 --> 00:28:34,844 These are covalent bonds in which 565 00:28:34,844 --> 00:28:38,180 the sharing of the electron pair is unequal. 566 00:28:38,180 --> 00:28:40,515 The result is a bond where the electron 567 00:28:40,515 --> 00:28:43,872 pair is displaced toward the more electronegative atom. 568 00:28:49,848 --> 00:28:52,836 Thanks for watching, and we'll see you guys next time. 569 00:29:02,796 --> 00:29:03,792 [END PLAYBACK] 570 00:29:07,029 --> 00:29:09,768 [APPLAUSE] 571 00:29:14,055 --> 00:29:16,180 CATHERINE DRENNAN: There's, I think, one other one, 572 00:29:16,180 --> 00:29:18,090 but that is totally the best one. 573 00:29:18,090 --> 00:29:20,860 And everything they say is exactly right, 574 00:29:20,860 --> 00:29:21,900 so it's really exciting. 575 00:29:21,900 --> 00:29:24,070 I even love it when they had the two-- 576 00:29:24,070 --> 00:29:26,750 they put two balls in there that they were sharing. 577 00:29:26,750 --> 00:29:29,220 It's just really very well done. 578 00:29:29,220 --> 00:29:33,050 OK, so now, you've probably filled in some of your notes 579 00:29:33,050 --> 00:29:34,950 here, but in case you missed some of them, 580 00:29:34,950 --> 00:29:38,260 I will tell you exactly what the dogs just told you. 581 00:29:38,260 --> 00:29:41,240 The dogs had it completely correct. 582 00:29:41,240 --> 00:29:47,170 So ionic bonds is the transfer of an electron, as you saw, 583 00:29:47,170 --> 00:29:51,220 and then the generation of a cation and an anion 584 00:29:51,220 --> 00:29:54,490 that are attracted to each other due to the charge. 585 00:29:54,490 --> 00:29:56,860 So the bonding comes from that attraction 586 00:29:56,860 --> 00:29:59,610 between the positively charged and the negatively charged 587 00:29:59,610 --> 00:30:04,380 atom, and an example that you're probably all familiar with is 588 00:30:04,380 --> 00:30:06,190 table salt, NaCL. 589 00:30:06,190 --> 00:30:09,490 And so you have Na plus and CL minus that are 590 00:30:09,490 --> 00:30:11,640 attracted to each other and form these bonds, 591 00:30:11,640 --> 00:30:16,110 which creates table salt. So let's see 592 00:30:16,110 --> 00:30:19,110 how far we can get in thinking about, really, 593 00:30:19,110 --> 00:30:20,990 this interaction between ionic bonds, 594 00:30:20,990 --> 00:30:22,990 and we'll see if we can get through ionic bonds. 595 00:30:22,990 --> 00:30:26,260 We might have to wait until covalent bonds until Monday, 596 00:30:26,260 --> 00:30:27,940 but let's see if we can finish this. 597 00:30:27,940 --> 00:30:35,160 So the formation of NaCL from neutral Na and neutral CL 598 00:30:35,160 --> 00:30:38,620 will first involve forming your cations and your anions. 599 00:30:38,620 --> 00:30:42,940 So you have Na going to Na plus plus an electron, 600 00:30:42,940 --> 00:30:45,690 and so here, you're talking about a process 601 00:30:45,690 --> 00:30:50,400 where the energy is going to be equal to the ionization energy 602 00:30:50,400 --> 00:30:50,900 again. 603 00:30:50,900 --> 00:30:53,580 So we're not moving far away from these terms 604 00:30:53,580 --> 00:30:58,580 because you're talking about ionizing a neutral atom to Na 605 00:30:58,580 --> 00:31:02,350 plus, and there's a value for that. 606 00:31:02,350 --> 00:31:06,610 And then we're talking about neutral CL, neutral chlorine, 607 00:31:06,610 --> 00:31:10,950 going to CL minus, and so it's gaining an electron. 608 00:31:10,950 --> 00:31:13,080 So here, the process you're talking about 609 00:31:13,080 --> 00:31:18,720 is the electron affinity, and so the energy change here 610 00:31:18,720 --> 00:31:22,960 is equal to the negative electron affinity, which 611 00:31:22,960 --> 00:31:26,520 is minus 349 in this case. 612 00:31:26,520 --> 00:31:30,320 So this is a favorable process here 613 00:31:30,320 --> 00:31:36,130 to gain this extra electron, and so overall, the Delta E here 614 00:31:36,130 --> 00:31:37,520 is negative. 615 00:31:37,520 --> 00:31:41,300 So now, if we're going to talk about this process here, 616 00:31:41,300 --> 00:31:42,800 we have two of these, so we're going 617 00:31:42,800 --> 00:31:45,050 to go-- we're going to put these guys together. 618 00:31:45,050 --> 00:31:47,070 So we need both of those to ionize, 619 00:31:47,070 --> 00:31:50,940 and so we can add up what energy difference 620 00:31:50,940 --> 00:31:54,890 we should expect to form Na plus and CL 621 00:31:54,890 --> 00:31:57,360 minus together from their parents. 622 00:31:57,360 --> 00:31:59,480 And so we have, now, a plus-- we've 623 00:31:59,480 --> 00:32:04,940 added these two together-- a plus 145 kilojoules per mol. 624 00:32:04,940 --> 00:32:06,700 So this seems weird. 625 00:32:06,700 --> 00:32:10,280 It's plus, and so now, we're seeing 626 00:32:10,280 --> 00:32:13,120 that the formation of these ions from their neutral atoms 627 00:32:13,120 --> 00:32:16,350 has this positive value, which means it requires energy. 628 00:32:16,350 --> 00:32:20,510 But we think about NaCL as being this natural table salt thing, 629 00:32:20,510 --> 00:32:27,510 so why is there so much table salt if this requires energy 630 00:32:27,510 --> 00:32:28,550 to do it? 631 00:32:28,550 --> 00:32:32,190 And the answer is that this is only part of the process. 632 00:32:32,190 --> 00:32:35,030 You need to form your cations and anions, 633 00:32:35,030 --> 00:32:38,200 but then you have energy of them coming together. 634 00:32:38,200 --> 00:32:39,850 So they're attracted to each other, 635 00:32:39,850 --> 00:32:42,980 and that's a really important part of forming the bond. 636 00:32:42,980 --> 00:32:46,540 And they're attracted by a simple coulombic relationship 637 00:32:46,540 --> 00:32:47,530 here. 638 00:32:47,530 --> 00:32:51,170 So the attraction between the positively charged 639 00:32:51,170 --> 00:32:53,830 and the negatively charged ion has 640 00:32:53,830 --> 00:32:59,100 an energy of minus 589 kilojoules per mol, 641 00:32:59,100 --> 00:33:05,520 so overall then, if you consider both forming Na plus and CL 642 00:33:05,520 --> 00:33:08,480 minus and the attraction between them, 643 00:33:08,480 --> 00:33:12,080 we have a negative Delta E 444. 644 00:33:12,080 --> 00:33:18,510 So the net energy here is in favor of forming NaCL. 645 00:33:18,510 --> 00:33:20,230 We have a decrease in energy. 646 00:33:20,230 --> 00:33:24,490 This is a stable compound, so let's look at where 647 00:33:24,490 --> 00:33:26,240 this number comes from. 648 00:33:26,240 --> 00:33:27,240 So we just put this out. 649 00:33:27,240 --> 00:33:29,680 That's the coulomb thing, but let's 650 00:33:29,680 --> 00:33:34,540 actually calculate this and see where that number comes from. 651 00:33:34,540 --> 00:33:37,220 So we're back to coulombic equations again. 652 00:33:37,220 --> 00:33:38,650 We never get very far away. 653 00:33:38,650 --> 00:33:41,880 They turn out to be very important in chemistry. 654 00:33:41,880 --> 00:33:43,690 So we have the coulombic potential. 655 00:33:43,690 --> 00:33:46,460 We have z's, our charge on our ions. 656 00:33:46,460 --> 00:33:48,590 We have the absolute value of the charge 657 00:33:48,590 --> 00:33:53,500 of an electron squared over 4 pi, our permittivity 658 00:33:53,500 --> 00:33:57,750 constant in r, our distance between those ions. 659 00:33:57,750 --> 00:34:00,620 And for any CL the bond length, or the distance 660 00:34:00,620 --> 00:34:06,310 between Na plus and CL minus, is 2.36 angstroms, 661 00:34:06,310 --> 00:34:09,900 so we can use that and just plug it into the equation. 662 00:34:09,900 --> 00:34:14,010 And we have plus 1 for the sodium, minus 1 for chloride, 663 00:34:14,010 --> 00:34:17,944 so overall, this will be a negative term. 664 00:34:17,944 --> 00:34:19,360 And if you work out the math, it's 665 00:34:19,360 --> 00:34:24,610 minus 9.774 times 10 to the minus 19th joules, 666 00:34:24,610 --> 00:34:26,730 and we have three significant figures. 667 00:34:26,730 --> 00:34:29,504 What's limiting our significant figures? 668 00:34:29,504 --> 00:34:30,452 AUDIENCE: [INAUDIBLE] 669 00:34:30,452 --> 00:34:32,493 CATHERINE DRENNAN: What-- yeah, the bottom length 670 00:34:32,493 --> 00:34:34,100 is limiting it here. 671 00:34:34,100 --> 00:34:36,270 I'm just going to talk about significant figures 672 00:34:36,270 --> 00:34:39,449 for a while, until Test 2 when you can demonstrate I 673 00:34:39,449 --> 00:34:41,860 can stop talking about significant figures. 674 00:34:41,860 --> 00:34:45,760 All right, so then we want to convert to kilojoules per mol 675 00:34:45,760 --> 00:34:47,810 because that was the number I gave you, 676 00:34:47,810 --> 00:34:51,750 was in kilojoules per mol, so we have our conversion factor 677 00:34:51,750 --> 00:34:53,136 between joules and kilojoules. 678 00:34:53,136 --> 00:34:54,719 And I guess I should mention up here-- 679 00:34:54,719 --> 00:34:57,820 coulombs cancel, and our meters cancel, 680 00:34:57,820 --> 00:34:59,810 so we're left in joules up there. 681 00:34:59,810 --> 00:35:01,980 Units are also important. 682 00:35:01,980 --> 00:35:03,690 Then we can use Avogadro's number, 683 00:35:03,690 --> 00:35:06,380 because we're given kilojoules per mol, 684 00:35:06,380 --> 00:35:08,690 and we can get out the number I gave you before. 685 00:35:08,690 --> 00:35:10,820 So this number really just comes right out 686 00:35:10,820 --> 00:35:16,250 of this equation minus 589 kilojoules per mol. 687 00:35:16,250 --> 00:35:20,620 So we have our ionization energy to tell us about forming ions. 688 00:35:20,620 --> 00:35:22,660 We have our electron affinity, and now, we 689 00:35:22,660 --> 00:35:26,120 have a coulombic relationship. 690 00:35:26,120 --> 00:35:28,250 So I said before-- this is what I showed you 691 00:35:28,250 --> 00:35:32,370 before-- that we have this attraction that's favorable. 692 00:35:32,370 --> 00:35:35,420 We have forming the ions, which had 693 00:35:35,420 --> 00:35:38,120 a positive energy associated with it, 694 00:35:38,120 --> 00:35:42,380 but overall, this process has a lower energy. 695 00:35:42,380 --> 00:35:43,710 It forms a bond. 696 00:35:43,710 --> 00:35:46,560 But this is just based on this calculation. 697 00:35:46,560 --> 00:35:50,880 So we can ask what is the experimental measurement 698 00:35:50,880 --> 00:35:53,480 for this interaction, and we note 699 00:35:53,480 --> 00:35:55,480 that it's somewhat different. 700 00:35:55,480 --> 00:35:58,155 So we have, instead of minus 444, 701 00:35:58,155 --> 00:36:02,510 we have minus 411 kilojoules per mol. 702 00:36:02,510 --> 00:36:06,000 So why the difference? 703 00:36:06,000 --> 00:36:09,330 So again, our ionic model, which just considers 704 00:36:09,330 --> 00:36:11,940 ionization energy, electron affinity, 705 00:36:11,940 --> 00:36:15,360 and that positive coulimbic interaction 706 00:36:15,360 --> 00:36:19,990 in our experimental result-- so problems with this model 707 00:36:19,990 --> 00:36:24,630 that we did up here include that we only thought 708 00:36:24,630 --> 00:36:26,710 about favorable interactions. 709 00:36:26,710 --> 00:36:30,510 There are also going to be some that are not favorable; 710 00:36:30,510 --> 00:36:33,900 protons against other protons, repulsion, electron electron 711 00:36:33,900 --> 00:36:35,000 repulsion. 712 00:36:35,000 --> 00:36:36,330 So there are some negatives. 713 00:36:36,330 --> 00:36:39,030 It's never all positive in any relationship, 714 00:36:39,030 --> 00:36:41,640 whether it's sodium chloride or anything else. 715 00:36:41,640 --> 00:36:44,710 Always some negatives, and we ignore those. 716 00:36:44,710 --> 00:36:46,360 And the result of this is that you're 717 00:36:46,360 --> 00:36:49,590 going to get a larger Delta E predicted 718 00:36:49,590 --> 00:36:51,310 than the experimental value. 719 00:36:51,310 --> 00:36:53,550 So it seems like this is more favorable, 720 00:36:53,550 --> 00:36:56,770 like that's a stronger bond, a stronger interaction; 721 00:36:56,770 --> 00:36:59,010 but really, if there's some repulsion, 722 00:36:59,010 --> 00:37:00,830 that's overestimated. 723 00:37:00,830 --> 00:37:02,800 It's really going to be a lower value, which is 724 00:37:02,800 --> 00:37:04,890 what you see in the experiment. 725 00:37:04,890 --> 00:37:10,450 Also, we just said that sodium plus was one point charge 726 00:37:10,450 --> 00:37:12,280 and CL minus was another. 727 00:37:12,280 --> 00:37:13,590 It's more complicated. 728 00:37:13,590 --> 00:37:16,460 Their interactions are more complicated than that. 729 00:37:16,460 --> 00:37:21,160 And we ignored quantum mechanics, but in doing that, 730 00:37:21,160 --> 00:37:22,220 we did pretty well. 731 00:37:22,220 --> 00:37:23,950 If we had one significant figure, 732 00:37:23,950 --> 00:37:28,460 we would have been perfect, so to one significant figure, 733 00:37:28,460 --> 00:37:31,380 these approximations work really well. 734 00:37:31,380 --> 00:37:33,900 OK, that's it for today, and I will see you Monday. 735 00:37:33,900 --> 00:37:34,800 Have a great weekend. 736 00:37:45,880 --> 00:37:48,435 All right, let's just take 10 more seconds 737 00:37:48,435 --> 00:37:49,434 on the clicker question. 738 00:38:04,750 --> 00:38:06,890 Great, so people are getting the hang of this. 739 00:38:06,890 --> 00:38:10,520 If you hadn't yet, there's still time. 740 00:38:10,520 --> 00:38:17,040 So as you're going across the periodic table, 741 00:38:17,040 --> 00:38:20,340 you are increasing z and increasing the z effective 742 00:38:20,340 --> 00:38:23,240 as well, because you don't have total shielding, 743 00:38:23,240 --> 00:38:25,370 so aluminum is the correct answer. 744 00:38:25,370 --> 00:38:27,930 It has a lower z effective, and so therefore, 745 00:38:27,930 --> 00:38:30,280 a smaller ionization energy. 746 00:38:30,280 --> 00:38:32,890 The electrons aren't held as tightly. 747 00:38:32,890 --> 00:38:35,790 All right, so we're talking about chemical bonds, 748 00:38:35,790 --> 00:38:37,860 and it seems like an appropriate topic 749 00:38:37,860 --> 00:38:40,340 to talk about when we're also talking about bonding 750 00:38:40,340 --> 00:38:43,930 as a community, so that seems like a good thing. 751 00:38:43,930 --> 00:38:47,690 So a chemical bond is an arrangement of atoms 752 00:38:47,690 --> 00:38:50,770 so that they come together in such a way 753 00:38:50,770 --> 00:38:53,290 that they're lower in energy than they 754 00:38:53,290 --> 00:38:54,720 were when they were apart. 755 00:38:54,720 --> 00:38:57,610 So they're more stable together than they were apart, 756 00:38:57,610 --> 00:38:58,910 and that's a chemical bond. 757 00:38:58,910 --> 00:39:02,380 So this is page five of the handout from last time. 758 00:39:02,380 --> 00:39:05,600 And excitingly, we have Lecture 10 handouts today, 759 00:39:05,600 --> 00:39:09,180 so there's lots of things working today. 760 00:39:09,180 --> 00:39:11,590 All right, so this is lower in energy, i.e. 761 00:39:11,590 --> 00:39:15,140 more negative, when these atoms come together. 762 00:39:15,140 --> 00:39:19,080 So a chemical bond-- as you saw last time 763 00:39:19,080 --> 00:39:22,260 with those wonderful dogs sharing a pull toy, 764 00:39:22,260 --> 00:39:25,490 a covalent bond is a bond where the electrons are shared 765 00:39:25,490 --> 00:39:28,860 between these two atoms, and each atom 766 00:39:28,860 --> 00:39:33,020 is giving up one bond to share. 767 00:39:33,020 --> 00:39:36,240 So we can think about this more graphically of what 768 00:39:36,240 --> 00:39:40,490 is happening, and we have this little plot on your notes, 769 00:39:40,490 --> 00:39:43,420 where you're going to be filling in a bunch of details. 770 00:39:43,420 --> 00:39:47,740 So we have the internuclear distance, r, the distance 771 00:39:47,740 --> 00:39:48,980 between the two nuclei. 772 00:39:48,980 --> 00:39:50,730 And we're back to hydrogen for the moment, 773 00:39:50,730 --> 00:39:54,990 so we're going to talk about a bond between two H atoms. 774 00:39:54,990 --> 00:39:58,640 And on the axis over here, we have energy. 775 00:39:58,640 --> 00:40:02,670 So we have energy versus the distance between these two 776 00:40:02,670 --> 00:40:04,180 hydrogens. 777 00:40:04,180 --> 00:40:08,340 So at 0 energy, we just have the hydrogens by themselves. 778 00:40:08,340 --> 00:40:11,050 They're not interacting with each other in any kind of way 779 00:40:11,050 --> 00:40:13,230 that lowers either one of their energies. 780 00:40:13,230 --> 00:40:17,350 There's no interaction, no energy change. 781 00:40:17,350 --> 00:40:19,510 They're not interacting. 782 00:40:19,510 --> 00:40:22,560 Down here at this dash line, we do have a bond, 783 00:40:22,560 --> 00:40:24,210 so we formed H2. 784 00:40:24,210 --> 00:40:27,090 The hydrogen atoms are interacting with each other, 785 00:40:27,090 --> 00:40:29,960 and this is lower in energy. 786 00:40:29,960 --> 00:40:34,150 So what does this plot look like then 787 00:40:34,150 --> 00:40:37,750 if you draw energy versus this distance? 788 00:40:37,750 --> 00:40:42,060 So up here, it's above and higher energy, above 0-- 789 00:40:42,060 --> 00:40:47,080 this is unfavorable-- going down to this dashed line 790 00:40:47,080 --> 00:40:49,740 and then going back up to 0. 791 00:40:49,740 --> 00:40:52,277 So let's think about what's happening here, 792 00:40:52,277 --> 00:40:53,860 and there's a bunch of different kinds 793 00:40:53,860 --> 00:40:56,610 of interactions you can have between those two hydrogen 794 00:40:56,610 --> 00:40:58,110 atoms. 795 00:40:58,110 --> 00:41:00,070 There are repulsive interactions, 796 00:41:00,070 --> 00:41:04,030 nuclear nuclear repulsion, electron electron repulsion; 797 00:41:04,030 --> 00:41:05,540 and there are positive interactions 798 00:41:05,540 --> 00:41:09,330 like the electron nuclear, the positive and negative. 799 00:41:09,330 --> 00:41:12,840 So up here, these atoms are very close together, 800 00:41:12,840 --> 00:41:16,720 and that is-- they're really too close, and that's unfavorable. 801 00:41:16,720 --> 00:41:19,970 Two objects trying to occupy the same space at the same time 802 00:41:19,970 --> 00:41:21,530 is unfavorable. 803 00:41:21,530 --> 00:41:24,990 But as you start separating out these atoms, 804 00:41:24,990 --> 00:41:27,700 then they become more comfortable, 805 00:41:27,700 --> 00:41:29,920 and you get to a distance where you 806 00:41:29,920 --> 00:41:31,800 have the sharing of electrons. 807 00:41:31,800 --> 00:41:33,020 They're next to each other. 808 00:41:33,020 --> 00:41:35,490 They're interacting in a positive way, 809 00:41:35,490 --> 00:41:37,390 and they're sharing. 810 00:41:37,390 --> 00:41:40,010 But then if you bring them too far apart, 811 00:41:40,010 --> 00:41:42,090 they're no longer communicating with each other. 812 00:41:42,090 --> 00:41:44,890 We don't even know where one of them is at this point. 813 00:41:44,890 --> 00:41:47,310 They're just kind of infinitely far apart. 814 00:41:47,310 --> 00:41:48,870 No interaction. 815 00:41:48,870 --> 00:41:55,180 So we go from too close to just right to too far away. 816 00:41:55,180 --> 00:42:02,770 So this distance here that has this minimum energy-- OK, 817 00:42:02,770 --> 00:42:05,980 so we have this minimum energy here, 818 00:42:05,980 --> 00:42:09,760 and that's the distance at which you have this really favorable 819 00:42:09,760 --> 00:42:11,770 interaction, this bond length. 820 00:42:11,770 --> 00:42:15,760 That's the bond length between those two hydrogens right here. 821 00:42:15,760 --> 00:42:18,750 Now, we can think about this energy difference over here, 822 00:42:18,750 --> 00:42:21,840 and it has a special name. 823 00:42:21,840 --> 00:42:28,040 So this is the dissociation energy, or Delta E sub d. 824 00:42:28,040 --> 00:42:32,160 Sometimes, it's just called big capital D in your textbook. 825 00:42:32,160 --> 00:42:33,940 So this is the energy that's needed 826 00:42:33,940 --> 00:42:36,060 to dissociate those atoms. 827 00:42:36,060 --> 00:42:39,310 So if the atoms come together, and they're lower in energy, 828 00:42:39,310 --> 00:42:41,350 if you then want to break them apart, 829 00:42:41,350 --> 00:42:45,250 you need to put energy in so that they can be broken apart. 830 00:42:45,250 --> 00:42:48,090 And that's called the dissociation energy. 831 00:42:48,090 --> 00:42:49,960 And if it's a really big number, that 832 00:42:49,960 --> 00:42:52,240 means it's very hard to dissociate them, 833 00:42:52,240 --> 00:42:54,990 and if it's a small number, it's pretty easy. 834 00:42:54,990 --> 00:42:58,030 So this is our dissociation energy here. 835 00:42:58,030 --> 00:43:04,430 So for hydrogen, this value is 424 kilojoules per mol. 836 00:43:04,430 --> 00:43:07,690 And if we were putting it on this axis here, 837 00:43:07,690 --> 00:43:10,520 it would be right down here, so it's 838 00:43:10,520 --> 00:43:13,130 going to be a negative value on this axis. 839 00:43:13,130 --> 00:43:17,250 We're below 0, so the negative of the dissociation energy 840 00:43:17,250 --> 00:43:21,410 is found down here, minus 424 kilojoules per mol. 841 00:43:21,410 --> 00:43:23,490 And if you wanted to break that bond, 842 00:43:23,490 --> 00:43:26,930 you would need to put in energy, so dissociation energy 843 00:43:26,930 --> 00:43:27,870 is positive. 844 00:43:27,870 --> 00:43:33,110 It's the energy you need to put in to break those bonds. 845 00:43:33,110 --> 00:43:38,480 So we can think about this plot now and consider looking 846 00:43:38,480 --> 00:43:43,230 at the plot and evaluating what you can and cannot say about 847 00:43:43,230 --> 00:43:45,610 different kinds of compounds, their bond lengths, 848 00:43:45,610 --> 00:43:48,990 and also their dissociation energies. 849 00:43:48,990 --> 00:43:50,350 So we good people? 850 00:43:50,350 --> 00:43:52,530 Mostly got this written down? 851 00:43:52,530 --> 00:43:56,440 And today, for some of the people who wanted to come late, 852 00:43:56,440 --> 00:43:59,540 we can post versions of this, too. 853 00:43:59,540 --> 00:44:02,020 All right, so now let's consider this plot 854 00:44:02,020 --> 00:44:04,830 and ask which bond is stronger? 855 00:44:04,830 --> 00:44:07,220 So we have hydrazine, and we also 856 00:44:07,220 --> 00:44:10,840 have molecular nitrogen, N2. 857 00:44:10,840 --> 00:44:14,930 And in your dashed line, you have nitrogen, N2, 858 00:44:14,930 --> 00:44:18,190 and in the solid line, you have hydrazine. 859 00:44:18,190 --> 00:44:20,960 So can you just look at this plot 860 00:44:20,960 --> 00:44:24,700 and tell me which is stronger? 861 00:44:24,700 --> 00:44:27,160 Is nitrogen or hydrazine stronger? 862 00:44:27,160 --> 00:44:29,225 And you can just yell out what you think. 863 00:44:29,225 --> 00:44:30,100 AUDIENCE: [INAUDIBLE] 864 00:44:30,100 --> 00:44:31,780 CATHERINE DRENNAN: Yeah. 865 00:44:31,780 --> 00:44:34,440 So nitrogen is going to be stronger here, 866 00:44:34,440 --> 00:44:37,430 and people knew that because it's a deeper well. 867 00:44:37,430 --> 00:44:41,870 So you go way farther down, there's more stabilization. 868 00:44:41,870 --> 00:44:46,580 It's a lower energy, a lower negative value of energy, 869 00:44:46,580 --> 00:44:50,060 so that means it's going to have a greater dissociation energy. 870 00:44:50,060 --> 00:44:54,230 You'd have to put in more energy to dissociate nitrogen nitrogen 871 00:44:54,230 --> 00:45:00,660 than these two submolecules here in hydrozine. 872 00:45:00,660 --> 00:45:02,720 All right, so we can also look at this plot 873 00:45:02,720 --> 00:45:07,091 and ask the question, which has a shorter bond? 874 00:45:07,091 --> 00:45:07,840 What do you think? 875 00:45:07,840 --> 00:45:09,050 Which bond is shorter? 876 00:45:09,050 --> 00:45:10,000 AUDIENCE: [INAUDIBLE] 877 00:45:10,000 --> 00:45:12,200 CATHERINE DRENNAN: Nitrogen as well, right. 878 00:45:12,200 --> 00:45:14,730 So nitrogen is also the shorter bond, 879 00:45:14,730 --> 00:45:18,630 and we know this because this is increasing distance. 880 00:45:18,630 --> 00:45:22,820 And this is closer to the axis, so this has a shorter distance. 881 00:45:22,820 --> 00:45:24,550 And later, we're going to be doing 882 00:45:24,550 --> 00:45:26,867 Lewis structures and other things of nitrogen 883 00:45:26,867 --> 00:45:29,200 and discover it has a triple bond, which you may already 884 00:45:29,200 --> 00:45:32,490 know, and we'll talk more about nitrogen's amazing triple bond 885 00:45:32,490 --> 00:45:34,230 as we go along. 886 00:45:34,230 --> 00:45:37,450 And so that's a very short bond and a very, very strong bond 887 00:45:37,450 --> 00:45:38,217 as well. 888 00:45:38,217 --> 00:45:40,050 So you should be able to look at these plots 889 00:45:40,050 --> 00:45:44,010 and evaluate what kind of dissociation energy 890 00:45:44,010 --> 00:45:46,250 it would have, is it bigger or smaller, 891 00:45:46,250 --> 00:45:49,620 and also what kind of distance you expect between them. 892 00:45:49,620 --> 00:45:52,460 And you should be able to draw these kinds of plots 893 00:45:52,460 --> 00:45:56,790 on the exam, if asked, in just kind of simple detail. 894 00:45:56,790 --> 00:45:59,630 Nothing too fancy. 895 00:45:59,630 --> 00:46:04,310 All right, so in terms of bond strengths, 896 00:46:04,310 --> 00:46:07,390 carbon monoxide has one of the strongest bonds, 897 00:46:07,390 --> 00:46:10,430 so it has a very large dissociation energy. 898 00:46:10,430 --> 00:46:13,387 And iodide, I2, has one of the weaker ones. 899 00:46:13,387 --> 00:46:14,970 And later in the semester, we're going 900 00:46:14,970 --> 00:46:19,260 to be doing a demo that shows why that's kind of cool, 901 00:46:19,260 --> 00:46:25,110 that weak bond leads to some cool, cool demos. 902 00:46:25,110 --> 00:46:29,020 OK, so those are covalent bonds. 903 00:46:29,020 --> 00:46:32,880 And, Ashley, could you just close that door, please? 904 00:46:32,880 --> 00:46:35,130 All right, so let's finish polar covalent, 905 00:46:35,130 --> 00:46:37,780 and then we'll have our moment of silence, 906 00:46:37,780 --> 00:46:40,180 and maybe it'll be silent in the hallway by then as well. 907 00:46:40,180 --> 00:46:42,500 All right, so polar covalent bonds-- 908 00:46:42,500 --> 00:46:45,250 so last time, the dog showed you that you 909 00:46:45,250 --> 00:46:48,414 can have equal sharing and unequal sharing. 910 00:46:48,414 --> 00:46:49,830 And of those of you who've watched 911 00:46:49,830 --> 00:46:52,790 dogs play with pully toys, most of the time, 912 00:46:52,790 --> 00:46:55,350 the sharing is pretty unequal, and so whenever 913 00:46:55,350 --> 00:46:57,769 you see that again, you can think polar covalent 914 00:46:57,769 --> 00:46:59,560 and tell your friends, and they'll be like, 915 00:46:59,560 --> 00:47:01,310 "I don't know what happened to you at MIT. 916 00:47:01,310 --> 00:47:02,630 Those are two dogs playing. 917 00:47:02,630 --> 00:47:04,630 What are you talking about?" 918 00:47:04,630 --> 00:47:10,280 So this is unequal sharing of electrons between two atoms, 919 00:47:10,280 --> 00:47:13,553 and this happens when those atoms have a very different 920 00:47:13,553 --> 00:47:14,303 electronegativity. 921 00:47:16,830 --> 00:47:21,140 So in general, a bond between two atoms 922 00:47:21,140 --> 00:47:24,600 is considered a polar covalent bond 923 00:47:24,600 --> 00:47:28,310 when the difference in electric negativity between the atoms 924 00:47:28,310 --> 00:47:32,670 is greater than 0.4 and less than 1.7, 925 00:47:32,670 --> 00:47:37,380 and that's Linus Pauling's scale and works quite well. 926 00:47:37,380 --> 00:47:39,200 So if we look at this little plot, 927 00:47:39,200 --> 00:47:42,760 we could see that carbon hydrogen bond only 928 00:47:42,760 --> 00:47:45,490 has a 0.4 difference, so that would not 929 00:47:45,490 --> 00:47:48,450 be considered a polar covalent bond. 930 00:47:48,450 --> 00:47:51,510 But nitrogen hydrogen is a difference 931 00:47:51,510 --> 00:47:55,740 that is greater than 0.4, so that would be a polar bond. 932 00:47:55,740 --> 00:47:58,250 And so you can use these values to think about 933 00:47:58,250 --> 00:48:00,880 whether you're going to have unequal sharing, 934 00:48:00,880 --> 00:48:03,280 and the more electronegative element 935 00:48:03,280 --> 00:48:07,780 is going to be pulling on those electrons. 936 00:48:07,780 --> 00:48:11,480 We also can use this to think about polar molecules, 937 00:48:11,480 --> 00:48:13,660 and this is kind of a little bit of a flash forward 938 00:48:13,660 --> 00:48:15,410 to Friday's lecture. 939 00:48:15,410 --> 00:48:18,010 We're going to talk about shapes of molecules 940 00:48:18,010 --> 00:48:21,750 because a polar molecule has to have polar bonds, 941 00:48:21,750 --> 00:48:25,230 but also has to have those bonds arranged in such a way 942 00:48:25,230 --> 00:48:27,950 that there's a net difference, there's a net dipole, 943 00:48:27,950 --> 00:48:31,000 there's a net kind of pulling of those electrons 944 00:48:31,000 --> 00:48:32,540 in a different way. 945 00:48:32,540 --> 00:48:34,690 So here is carbon dioxide, a molecule 946 00:48:34,690 --> 00:48:37,180 that causes global warming. 947 00:48:37,180 --> 00:48:40,090 Yes, I said it on the videotape. 948 00:48:40,090 --> 00:48:42,210 Yes, I believe that human beings are 949 00:48:42,210 --> 00:48:44,530 responsible for some of the global warming, 950 00:48:44,530 --> 00:48:47,220 and we should do something about it. 951 00:48:47,220 --> 00:48:50,340 This molecule does have polar bonds, 952 00:48:50,340 --> 00:48:53,150 so we have carbon in the middle and oxygen here. 953 00:48:53,150 --> 00:48:57,100 So carbon and oxygen have an electronegativity difference 954 00:48:57,100 --> 00:49:02,070 greater than 0.4, but it's not really a polar molecule. 955 00:49:02,070 --> 00:49:05,610 It's a non-polar molecule, and that's because of its shape. 956 00:49:05,610 --> 00:49:07,910 So shape matters. 957 00:49:07,910 --> 00:49:11,410 So we have pulling of electrons one way, 958 00:49:11,410 --> 00:49:14,149 but we have equal and opposite pulling of electrons other way. 959 00:49:14,149 --> 00:49:15,690 This would be really cool if the dogs 960 00:49:15,690 --> 00:49:17,180 could have done this as well. 961 00:49:17,180 --> 00:49:20,930 So in this case, we have a non-polar molecule 962 00:49:20,930 --> 00:49:22,670 that has polar bonds. 963 00:49:22,670 --> 00:49:25,800 Now, there's only very few cases where this would actually 964 00:49:25,800 --> 00:49:30,270 be true, and you really need to think about the shape. 965 00:49:30,270 --> 00:49:33,170 And so that's why we're going to talk about shape. 966 00:49:33,170 --> 00:49:40,380 Another molecule that also has polar bonds is water. 967 00:49:40,380 --> 00:49:44,600 So here, we have polar bonds between the oxygen 968 00:49:44,600 --> 00:49:49,090 and the hydrogen, so oxygen and hydrogen, greater than 0.4. 969 00:49:49,090 --> 00:49:51,880 But in this case, the shape of water 970 00:49:51,880 --> 00:49:55,740 is such that they don't cancel out, 971 00:49:55,740 --> 00:49:57,690 and you do have a net dipole. 972 00:49:57,690 --> 00:50:01,440 You do have a net charge on that molecule, 973 00:50:01,440 --> 00:50:04,890 makes it a polar molecule. 974 00:50:04,890 --> 00:50:07,530 So we need to know about the shape, 975 00:50:07,530 --> 00:50:11,150 and we need to know about electronegativity. 976 00:50:11,150 --> 00:50:14,370 So in large, organic molecules, sometimes we 977 00:50:14,370 --> 00:50:16,920 just talk about the number of polar bonds 978 00:50:16,920 --> 00:50:18,520 and then think about whether that's 979 00:50:18,520 --> 00:50:20,814 likely to be a polar molecule or not. 980 00:50:20,814 --> 00:50:23,230 You can't always think about the shape of something really 981 00:50:23,230 --> 00:50:27,660 complicated and what direction all of the pulling of electrons 982 00:50:27,660 --> 00:50:31,960 is going, but we can at least count polar bonds. 983 00:50:31,960 --> 00:50:36,960 So here are two vitamins, vitamin A and vitamin B-9, 984 00:50:36,960 --> 00:50:40,610 which I think is also B-10 and a number of other B's. 985 00:50:40,610 --> 00:50:42,960 Its name is folic acid. 986 00:50:42,960 --> 00:50:44,730 They kept finding it again and again, 987 00:50:44,730 --> 00:50:47,640 so there's a whole gap of b-vitamins where they're like, 988 00:50:47,640 --> 00:50:52,080 oh, B-9, folic acid; B-10, also folic acid; 989 00:50:52,080 --> 00:50:53,970 B-11, I think also folic acid. 990 00:50:53,970 --> 00:50:55,970 B-12 something different, though. 991 00:50:55,970 --> 00:50:59,114 So anyway, this is a very important B vitamin, 992 00:50:59,114 --> 00:51:01,530 and I'm actually going to come back to this molecule later 993 00:51:01,530 --> 00:51:02,900 in the course. 994 00:51:02,900 --> 00:51:06,434 But we can think about how many polar bonds it has, 995 00:51:06,434 --> 00:51:07,684 and that's a clicker question. 996 00:51:13,500 --> 00:51:16,020 And there, you have the molecules up here to look at. 997 00:51:26,520 --> 00:51:29,470 All right, let's just take 10 more seconds. 998 00:51:29,470 --> 00:51:31,440 It takes a while to count, but I think 999 00:51:31,440 --> 00:51:34,700 you probably can answer it without maybe fully counting 1000 00:51:34,700 --> 00:51:35,535 all of them. 1001 00:51:44,550 --> 00:51:49,840 All right, so over here now, the answer is the folic acid, 1002 00:51:49,840 --> 00:51:51,670 and I'll just highlight. 1003 00:51:51,670 --> 00:51:54,400 You might not have found all of these polar bonds, 1004 00:51:54,400 --> 00:51:56,650 but you should have at least seen that this one really 1005 00:51:56,650 --> 00:51:57,950 didn't have many. 1006 00:51:57,950 --> 00:52:00,450 Vitamin A only has one. 1007 00:52:00,450 --> 00:52:03,760 So we have polar bonds down here between carbon and oxygen 1008 00:52:03,760 --> 00:52:08,230 over here, carbon and oxygen, nitrogen, hydrogen, carbon 1009 00:52:08,230 --> 00:52:12,750 oxygen again, nitrogen with hydrogen, oxygen with hydrogen, 1010 00:52:12,750 --> 00:52:15,080 nitrogen with hydrogen over here. 1011 00:52:15,080 --> 00:52:19,950 So folic acid is quite polar, and if we're 1012 00:52:19,950 --> 00:52:22,860 going to think, now, about whether it is a water 1013 00:52:22,860 --> 00:52:26,510 soluble vitamin or a fat soluble vitamin-- which 1014 00:52:26,510 --> 00:52:29,350 is something that a lot of times your supplements 1015 00:52:29,350 --> 00:52:31,900 will tell you about. 1016 00:52:31,900 --> 00:52:34,630 If it's water soluble-- and we'll talk about this more 1017 00:52:34,630 --> 00:52:36,940 later-- like dissolves likes. 1018 00:52:36,940 --> 00:52:39,460 So water likes polar molecules, which 1019 00:52:39,460 --> 00:52:44,150 makes folic acid water soluble and makes vitamin 1020 00:52:44,150 --> 00:52:45,850 A fat soluble. 1021 00:52:45,850 --> 00:52:50,070 It's not very polar, doesn't dissolve very well in water. 1022 00:52:50,070 --> 00:52:53,100 And this kind of turns out to be important, 1023 00:52:53,100 --> 00:52:56,740 in that if you read your vitamin supplements, 1024 00:52:56,740 --> 00:52:59,640 if you take vitamin supplements, it will often tell you 1025 00:52:59,640 --> 00:53:02,950 interesting things like how many hundred times 1026 00:53:02,950 --> 00:53:05,890 over the daily recommended allowance 1027 00:53:05,890 --> 00:53:09,980 this vitamin tablet is, and if things are water soluble, 1028 00:53:09,980 --> 00:53:11,810 it doesn't matter so much that you're 1029 00:53:11,810 --> 00:53:15,290 taking way more than your body actually needs. 1030 00:53:15,290 --> 00:53:19,120 You just have a very expensive pee. 1031 00:53:19,120 --> 00:53:21,547 But if it's fat soluble, then it's 1032 00:53:21,547 --> 00:53:23,630 going to stay in your body, and you don't need it, 1033 00:53:23,630 --> 00:53:25,450 and it can be a little bit toxic. 1034 00:53:25,450 --> 00:53:27,930 So try to think about the vitamins. 1035 00:53:27,930 --> 00:53:30,850 Not everything-- even though vitamins are good for you, 1036 00:53:30,850 --> 00:53:34,150 they're not good in every kind of amount that you could take, 1037 00:53:34,150 --> 00:53:38,680 so you will now use your knowledge of polar bonds 1038 00:53:38,680 --> 00:53:41,510 to figure out whether you should be taking certain vitamin 1039 00:53:41,510 --> 00:53:43,060 supplements.