WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.780 Commons license. 00:00:03.780 --> 00:00:06.020 Your support will help MIT OpenCourseWare 00:00:06.020 --> 00:00:10.090 continue to offer high-quality educational resources for free. 00:00:10.090 --> 00:00:12.660 To make a donation or to view additional materials 00:00:12.660 --> 00:00:16.486 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.486 --> 00:00:17.110 at ocw.mit.edu. 00:00:21.330 --> 00:00:25.760 CATHERINE DRENNAN: So that is electron affinity. 00:00:25.760 --> 00:00:28.750 But honestly, chemists don't really talk so much 00:00:28.750 --> 00:00:31.000 about electron affinity. 00:00:31.000 --> 00:00:35.010 They prefer to talk about electronegativity. 00:00:35.010 --> 00:00:37.450 And these are highly related terms. 00:00:37.450 --> 00:00:42.440 So this was also re-copied, although completely identical, 00:00:42.440 --> 00:00:43.750 I think, between the handouts. 00:00:43.750 --> 00:00:46.300 I just thought it was weird to have re-copied this and not 00:00:46.300 --> 00:00:46.800 this. 00:00:46.800 --> 00:00:51.430 So your handout for today is perfect on this point. 00:00:51.430 --> 00:00:54.730 So electron negativity, the net ability 00:00:54.730 --> 00:00:59.610 of an atom to attract an electron from another atom. 00:00:59.610 --> 00:01:04.090 So you can see that electron affinity and electronegativity 00:01:04.090 --> 00:01:04.970 are very similar. 00:01:04.970 --> 00:01:08.230 In fact, all of these terms are highly related to each other. 00:01:08.230 --> 00:01:11.310 And this idea of electronegativity, 00:01:11.310 --> 00:01:14.400 of this as a term for a way of thinking about atoms 00:01:14.400 --> 00:01:16.460 initiated with Linus Pauling. 00:01:16.460 --> 00:01:18.860 But here I have up a different picture. 00:01:18.860 --> 00:01:20.790 I have Robert Millikan. 00:01:20.790 --> 00:01:22.610 And the reason why I picked this picture 00:01:22.610 --> 00:01:26.830 is because he helped, a few years after Linus Pauling came 00:01:26.830 --> 00:01:29.790 up with this idea, coming up with an equation 00:01:29.790 --> 00:01:33.030 that help people think better about what electronegativity 00:01:33.030 --> 00:01:33.760 is. 00:01:33.760 --> 00:01:36.360 And another reason why I picked his picture 00:01:36.360 --> 00:01:38.890 to put up here instead of Linus Pauling 00:01:38.890 --> 00:01:41.470 is that he was an MIT undergraduate, 00:01:41.470 --> 00:01:43.390 and he was a chemistry major. 00:01:43.390 --> 00:01:46.810 I'm not sure in 1917 or whenever-- 00:01:46.810 --> 00:01:50.530 that was the yearbook picture-- when he took-- I don't think 00:01:50.530 --> 00:01:52.380 it was 5.111 at that point. 00:01:52.380 --> 00:01:56.140 But at some point, he was here studying chemistry just 00:01:56.140 --> 00:01:57.140 like you. 00:01:57.140 --> 00:02:00.150 And then when he got a faculty position later on, 00:02:00.150 --> 00:02:04.070 he did some beautiful work that had to do with bonding that he 00:02:04.070 --> 00:02:05.360 got the Nobel Prize. 00:02:05.360 --> 00:02:09.539 So some of you may have a Nobel Prize in chemistry one day. 00:02:09.539 --> 00:02:12.620 And so you want to make sure your yearbook picture 00:02:12.620 --> 00:02:16.720 is at least as good as this one for other generations 00:02:16.720 --> 00:02:18.850 of professors to show your picture 00:02:18.850 --> 00:02:21.640 and describe the work that you did to contribute 00:02:21.640 --> 00:02:25.020 to the field of chemistry. 00:02:25.020 --> 00:02:27.110 Oh, and he was born in Massachusetts too, 00:02:27.110 --> 00:02:31.730 so he is a native to this area in more than one way. 00:02:31.730 --> 00:02:32.230 All right. 00:02:32.230 --> 00:02:35.410 So the way that he-- and this is a little bit more 00:02:35.410 --> 00:02:37.740 of a squishy definition. 00:02:37.740 --> 00:02:42.460 So electron negativity is proportional to a 1/2-- 00:02:42.460 --> 00:02:44.334 and IE stands for what? 00:02:44.334 --> 00:02:45.250 AUDIENCE: [INAUDIBLE]. 00:02:45.250 --> 00:02:46.860 CATHERINE DRENNAN: Ionization energy. 00:02:46.860 --> 00:02:51.330 And EA-- our electron affinity that we just discussed. 00:02:51.330 --> 00:02:51.830 All right. 00:02:51.830 --> 00:02:54.390 So it's related to these other terms 00:02:54.390 --> 00:02:57.080 that we have already talked about. 00:02:57.080 --> 00:03:01.460 So let's think about then what this means. 00:03:01.460 --> 00:03:05.950 So we can consider an atom with high electronegativity 00:03:05.950 --> 00:03:08.482 and an atom with low electronegativity. 00:03:08.482 --> 00:03:10.550 And we want to think about whether an atom 00:03:10.550 --> 00:03:13.730 with high electronegativity is going to be an electron 00:03:13.730 --> 00:03:16.080 acceptor or an electron donor. 00:03:16.080 --> 00:03:19.060 And that you can tell me, and that is 00:03:19.060 --> 00:03:21.690 going to be a clicker question. 00:03:21.690 --> 00:03:24.090 So you can try to grab your handout while clicking 00:03:24.090 --> 00:03:24.835 at the same time. 00:04:10.080 --> 00:04:10.580 All right. 00:04:10.580 --> 00:04:11.850 So we'll take 10 more seconds. 00:04:27.650 --> 00:04:28.900 Yes, 88%. 00:04:28.900 --> 00:04:29.970 That's great. 00:04:29.970 --> 00:04:32.570 Of course, if you looked and if you didn't believe 00:04:32.570 --> 00:04:35.420 it could be a donor, then that ruled out three of the four, 00:04:35.420 --> 00:04:36.670 but that's OK. 00:04:36.670 --> 00:04:38.710 Those are good things. 00:04:38.710 --> 00:04:41.990 So yes, if it has high electronegativity, 00:04:41.990 --> 00:04:44.730 it's going to be an electron acceptor. 00:04:44.730 --> 00:04:46.600 And part of the reason for that is 00:04:46.600 --> 00:04:49.400 that it has a high affinity for electrons. 00:04:49.400 --> 00:04:51.420 And another part of the reason for that 00:04:51.420 --> 00:04:53.320 is that if you look at the equation, when 00:04:53.320 --> 00:04:55.260 you have a high ionization energy, 00:04:55.260 --> 00:05:00.320 something that has a high ionization energy 00:05:00.320 --> 00:05:01.970 is not going to be a good donor. 00:05:01.970 --> 00:05:03.920 So that wouldn't make sense. 00:05:03.920 --> 00:05:07.830 So both of those terms having high in both categories 00:05:07.830 --> 00:05:11.050 is consistent then with this trend. 00:05:11.050 --> 00:05:14.060 So let's take a little bit more of a look at that 00:05:14.060 --> 00:05:15.640 and why this is true. 00:05:15.640 --> 00:05:18.070 So high electronegativity, an atom 00:05:18.070 --> 00:05:21.570 with high electronegativity is an electron acceptor, 00:05:21.570 --> 00:05:24.510 and then low would be a donor. 00:05:24.510 --> 00:05:28.012 And so if we think about this-- and this is our periodic table. 00:05:28.012 --> 00:05:29.720 And again, it's not going to be including 00:05:29.720 --> 00:05:32.100 our noble gases, which really don't want to be accepting 00:05:32.100 --> 00:05:33.770 or donating anything. 00:05:33.770 --> 00:05:38.070 So in this corner then we had our high ionization energy, 00:05:38.070 --> 00:05:41.670 and we had a high electron affinity. 00:05:41.670 --> 00:05:44.650 And we saw last class we had high ionization energy. 00:05:44.650 --> 00:05:48.160 So it doesn't want to give up an electron, 00:05:48.160 --> 00:05:49.910 but it does want to accept one. 00:05:49.910 --> 00:05:53.070 So we have things that are going to be good acceptors. 00:05:53.070 --> 00:05:56.560 And down here, we have low ionization energy, 00:05:56.560 --> 00:05:59.855 so it's easy to donate an electron. 00:05:59.855 --> 00:06:01.570 Oh, let me just put these up, sorry. 00:06:01.570 --> 00:06:04.940 So we have then if you're high and high up here, 00:06:04.940 --> 00:06:07.080 you have something that's a good acceptor, 00:06:07.080 --> 00:06:09.950 and it's going to have a high electronegativity. 00:06:09.950 --> 00:06:12.980 So high high means high over there. 00:06:12.980 --> 00:06:15.330 And then down here, we have low low, 00:06:15.330 --> 00:06:18.310 which means we have low electronegativity. 00:06:18.310 --> 00:06:21.270 Low ionization energy-- it's easy to give something up. 00:06:21.270 --> 00:06:24.310 Low electron affinity-- it doesn't want electrons. 00:06:24.310 --> 00:06:26.180 It's happy to give up electrons. 00:06:26.180 --> 00:06:28.220 And if it gives up electrons, then you 00:06:28.220 --> 00:06:30.390 can get a complete octet. 00:06:30.390 --> 00:06:32.191 It can have a noble gas configuration. 00:06:32.191 --> 00:06:33.690 So on this side, you need electrons. 00:06:33.690 --> 00:06:36.920 This side, it's happy to give them up. 00:06:36.920 --> 00:06:41.240 So if we look then just at a periodic table again, 00:06:41.240 --> 00:06:42.220 this makes sense. 00:06:42.220 --> 00:06:44.140 We gain an electron over here. 00:06:44.140 --> 00:06:47.300 We get our happy noble gas configuration. 00:06:47.300 --> 00:06:48.970 We lose electrons over here. 00:06:48.970 --> 00:06:50.480 We do the same thing. 00:06:50.480 --> 00:06:53.491 So that's a way to think about electronegativity. 00:06:53.491 --> 00:06:53.990 All right. 00:06:53.990 --> 00:06:57.350 So why should we care about electronegativity? 00:06:57.350 --> 00:07:02.120 And that's because a lot of atoms that are electronegative 00:07:02.120 --> 00:07:05.620 are used in pharmaceutical molecules, 00:07:05.620 --> 00:07:08.170 and that this gives them special properties. 00:07:08.170 --> 00:07:10.860 So to hear in their own words, we're 00:07:10.860 --> 00:07:14.910 going to hear from a former uropper, Kateryna, talking 00:07:14.910 --> 00:07:19.187 about why you should care about electronegativity. 00:07:26.988 --> 00:07:28.420 [VIDEO PLAYBACK] 00:07:28.420 --> 00:07:30.550 - My name is Kateryna Kozyrytska. 00:07:30.550 --> 00:07:32.120 I come from Ukraine. 00:07:32.120 --> 00:07:34.370 And I'm interested in how microorganisms 00:07:34.370 --> 00:07:36.350 fight each other. 00:07:36.350 --> 00:07:37.630 Humans are very smart. 00:07:37.630 --> 00:07:41.320 They have found chemical ways to make new drugs. 00:07:41.320 --> 00:07:43.520 And so we spend a lot of time and money 00:07:43.520 --> 00:07:48.140 on making a new antibiotic, and then we put it into people 00:07:48.140 --> 00:07:51.090 and we hope for the best. 00:07:51.090 --> 00:07:53.390 Bacteria are also very, very smart. 00:07:53.390 --> 00:07:57.690 And they somehow learn to resist this new antibiotic 00:07:57.690 --> 00:07:59.740 that we just made. 00:07:59.740 --> 00:08:02.170 Bugs, on the other hand, have been fighting each other 00:08:02.170 --> 00:08:04.382 with the same molecules for thousands 00:08:04.382 --> 00:08:06.850 and thousands of years, and we see 00:08:06.850 --> 00:08:10.060 no resistance developing there. 00:08:10.060 --> 00:08:12.680 So we want to learn what it is about the antibiotics 00:08:12.680 --> 00:08:15.260 that bugs make to fight each other that 00:08:15.260 --> 00:08:18.710 makes them so difficult to develop resistance to. 00:08:18.710 --> 00:08:22.830 Normally, living organisms use 20 amino acids. 00:08:22.830 --> 00:08:25.080 But these bugs get very tricky in building 00:08:25.080 --> 00:08:29.410 their anti-other bugs molecules. 00:08:29.410 --> 00:08:32.049 So to add the functionality, bugs 00:08:32.049 --> 00:08:35.409 can do chemistry on the building blocks, 00:08:35.409 --> 00:08:37.630 the amino acids themselves. 00:08:37.630 --> 00:08:39.419 And so one of the things that they can do 00:08:39.419 --> 00:08:44.290 is chlorinate carbons, which activates these carbons 00:08:44.290 --> 00:08:46.360 for future chemistry. 00:08:46.360 --> 00:08:49.740 The protein I study, the halogenase SyrB2, 00:08:49.740 --> 00:08:53.450 takes chloride ion from the environment and a molecule 00:08:53.450 --> 00:08:56.270 of threonine, the amino acid, and puts those together 00:08:56.270 --> 00:08:59.380 forming a chlorine-carbon bond. 00:08:59.380 --> 00:09:02.150 Since chlorine is so electronegative, when bonded 00:09:02.150 --> 00:09:06.350 to carbon it pulls electrons away from the carbon atom. 00:09:06.350 --> 00:09:08.270 And so it makes the carbon to which 00:09:08.270 --> 00:09:10.340 it is attached much more reactive 00:09:10.340 --> 00:09:12.020 toward other molecules. 00:09:12.020 --> 00:09:14.190 And this increased reactivity, at least partially, 00:09:14.190 --> 00:09:18.470 accounts for the antibiotic effect of the molecule. 00:09:18.470 --> 00:09:22.370 I am hoping to figure out how SyrB2 positions all the atoms 00:09:22.370 --> 00:09:25.900 in such a way that they react in this very controlled, very 00:09:25.900 --> 00:09:28.740 appropriate manner, so that later we could maybe 00:09:28.740 --> 00:09:31.670 re-engineer this or other proteins to make 00:09:31.670 --> 00:09:34.550 them do chemistry that we want them to do. 00:09:34.550 --> 00:09:36.870 My hope is that we could understand SyrB2 00:09:36.870 --> 00:09:41.020 well enough that we'll be able to remake it 00:09:41.020 --> 00:09:42.840 into a protein that will actively 00:09:42.840 --> 00:09:45.426 participate in synthesis of new antibiotics. 00:09:48.305 --> 00:09:48.888 [END PLAYBACK] 00:09:48.888 --> 00:09:49.888 CATHERINE DRENNAN: Yeah. 00:09:49.888 --> 00:09:51.410 So that is Kateryna. 00:09:51.410 --> 00:09:54.570 And so how adding halogens, because they 00:09:54.570 --> 00:09:59.355 are so electronegative, is actually a very important area, 00:09:59.355 --> 00:10:01.700 and I'll give you a couple more examples. 00:10:01.700 --> 00:10:04.365 I also haven't watched that video in a little bit, 00:10:04.365 --> 00:10:06.060 and she said she was from the Ukraine. 00:10:06.060 --> 00:10:08.210 She's technically now part of Russia, 00:10:08.210 --> 00:10:11.070 I think, from part of Russia, so. 00:10:11.070 --> 00:10:13.470 [LAUGHTER] 00:10:13.470 --> 00:10:17.820 Anyway, that's a topic for a different day. 00:10:17.820 --> 00:10:22.340 So this is big business, actually, putting halogens 00:10:22.340 --> 00:10:23.390 on things. 00:10:23.390 --> 00:10:28.310 And if you become interested-- if you're taking an antibiotic 00:10:28.310 --> 00:10:31.300 or something, start looking at what the molecule is, 00:10:31.300 --> 00:10:34.920 start counting how many halogens are on that molecule, 00:10:34.920 --> 00:10:36.700 you will find a lot. 00:10:36.700 --> 00:10:40.470 So a lot of antibiotics have halogens, either chlorides 00:10:40.470 --> 00:10:43.920 or fluorine, as shown here. 00:10:43.920 --> 00:10:46.680 Also a very common antidepressant 00:10:46.680 --> 00:10:49.780 has it, another example of something 00:10:49.780 --> 00:10:51.820 that is an anti-diabetic. 00:10:51.820 --> 00:10:56.220 Huge numbers of molecules have halogens added to them. 00:10:56.220 --> 00:10:58.760 Some of these are derived from natural products. 00:10:58.760 --> 00:11:03.530 So nature was making these molecules to kill other bugs, 00:11:03.530 --> 00:11:05.480 as you heard about in the video. 00:11:05.480 --> 00:11:09.220 Other times, they came up with this molecule 00:11:09.220 --> 00:11:12.950 and they said, well, we need to make it a little bit different. 00:11:12.950 --> 00:11:14.947 It's being consumed too fast. 00:11:14.947 --> 00:11:16.530 It's broken down too fast in the body. 00:11:16.530 --> 00:11:17.890 Let's add some halogens. 00:11:17.890 --> 00:11:20.600 So sometimes it's sort of a man-made tailoring, 00:11:20.600 --> 00:11:23.740 but often it's a tailoring that nature came up with. 00:11:23.740 --> 00:11:26.140 So why all these halogens? 00:11:26.140 --> 00:11:30.710 What's the benefit of having a carbon-fluorine bond instead 00:11:30.710 --> 00:11:32.840 of carbon-hydrogen? 00:11:32.840 --> 00:11:37.870 And one reason is that having a fluorine, this really 00:11:37.870 --> 00:11:43.150 electronegative atom, on, say, an aryl ring like this one, 00:11:43.150 --> 00:11:47.610 it actually sucks the electrons out of the ring and makes it 00:11:47.610 --> 00:11:49.800 what we call, or organic chemists like to call, 00:11:49.800 --> 00:11:51.040 electron poor. 00:11:51.040 --> 00:11:54.560 So it just kind of hauls those electrons away. 00:11:54.560 --> 00:11:58.210 And when you make something electron poor, 00:11:58.210 --> 00:12:01.950 so by replacing C-H with C-F, that 00:12:01.950 --> 00:12:05.180 can make a potential drug molecule electron poor. 00:12:05.180 --> 00:12:08.000 And what this does often is make it 00:12:08.000 --> 00:12:11.060 harder to oxidize the molecule. 00:12:11.060 --> 00:12:14.370 So we're going to talk about oxidation-reduction much later 00:12:14.370 --> 00:12:17.160 in the semester, and we'll come back to this idea. 00:12:17.160 --> 00:12:19.600 But this turns out to be really important, 00:12:19.600 --> 00:12:23.660 because the way that the body metabolizes or breaks down 00:12:23.660 --> 00:12:26.530 these molecules is that it can oxidize it. 00:12:26.530 --> 00:12:29.120 And there are a number of enzymes in your liver 00:12:29.120 --> 00:12:32.780 which will oxidize and break apart these drugs. 00:12:32.780 --> 00:12:35.500 And so if you make it harder to oxidize, 00:12:35.500 --> 00:12:38.780 that makes the drug more stable in your body. 00:12:38.780 --> 00:12:40.290 So if you want to take a drug, you 00:12:40.290 --> 00:12:42.590 want it to last for a while. 00:12:42.590 --> 00:12:44.990 And especially if it's something, an antibiotic, 00:12:44.990 --> 00:12:48.050 you want it to last till it kills all of the bacteria, 00:12:48.050 --> 00:12:50.345 not just half of them. 00:12:50.345 --> 00:12:53.060 And most medicines, you need them to be around 00:12:53.060 --> 00:12:54.520 for them to have their effect. 00:12:54.520 --> 00:12:56.770 So you want to tailor those molecules 00:12:56.770 --> 00:12:59.850 so that they don't get broken down as easily in the body. 00:12:59.850 --> 00:13:02.360 And so this is one reason, and this is big business. 00:13:02.360 --> 00:13:06.530 And a lot of the times, adding those halogens actually 00:13:06.530 --> 00:13:09.330 involves pretty toxic chemicals. 00:13:09.330 --> 00:13:12.800 So some people, like Kateryna, are interested in using enzymes 00:13:12.800 --> 00:13:14.990 to do it instead. 00:13:14.990 --> 00:13:19.330 Some people who are still using organic synthesis-- example 00:13:19.330 --> 00:13:21.210 is Steve Buchwald's laboratory. 00:13:21.210 --> 00:13:23.960 In fact, if you go to almost any top chemistry department, 00:13:23.960 --> 00:13:25.460 I think there's someone who's trying 00:13:25.460 --> 00:13:30.112 to find out new methods of putting halogens on molecules. 00:13:30.112 --> 00:13:33.320 It's a very important area in designing new molecules 00:13:33.320 --> 00:13:35.240 and improving them. 00:13:35.240 --> 00:13:36.110 OK. 00:13:36.110 --> 00:13:38.770 So that is electronegativity. 00:13:38.770 --> 00:13:42.460 So one atom added to a big number of atoms 00:13:42.460 --> 00:13:44.160 can change the property of the molecule 00:13:44.160 --> 00:13:46.750 by sucking electrons away. 00:13:46.750 --> 00:13:51.540 And now we're going to talk about atomic and ionic radius 00:13:51.540 --> 00:13:54.280 and also isoelectric atoms. 00:13:54.280 --> 00:13:58.000 So these trends are pretty good. 00:13:58.000 --> 00:14:00.760 We're back to some pretty good-- fewer glitches. 00:14:00.760 --> 00:14:02.400 So what is the atomic radius? 00:14:02.400 --> 00:14:06.090 So here we have 2r, 2 times the atomic radius. 00:14:06.090 --> 00:14:09.630 So the atomic radius is defined by the value 00:14:09.630 --> 00:14:14.590 of r that has about 90% of the electron density. 00:14:14.590 --> 00:14:16.540 I mean, technically, an electron could 00:14:16.540 --> 00:14:19.520 be infinitely or close to infinitely far away, 00:14:19.520 --> 00:14:24.020 but pretty much most of them are going to be about within 90%. 00:14:24.020 --> 00:14:26.540 We call that the radius of the atom. 00:14:26.540 --> 00:14:28.730 So they're trends. 00:14:28.730 --> 00:14:30.580 Trends are pretty good. 00:14:30.580 --> 00:14:34.090 So across the periodic table, what matters 00:14:34.090 --> 00:14:35.620 is the Z effective. 00:14:35.620 --> 00:14:37.770 See, everything we've learned about it comes back. 00:14:37.770 --> 00:14:39.190 If you didn't learn it on exam 1, 00:14:39.190 --> 00:14:41.740 still learn it because we're going to use it again. 00:14:41.740 --> 00:14:44.310 Across the periodic table, Z effective matters. 00:14:44.310 --> 00:14:47.810 Down the periodic table, n, or the principle quantum number, 00:14:47.810 --> 00:14:48.650 matters. 00:14:48.650 --> 00:14:50.460 So how do these matter? 00:14:50.460 --> 00:14:54.800 So across the periodic table, Z effective is going to do what? 00:14:54.800 --> 00:14:56.613 Increase or decrease? 00:14:56.613 --> 00:14:58.277 AUDIENCE: Increase. 00:14:58.277 --> 00:15:00.110 CATHERINE DRENNAN: So it's going to increase 00:15:00.110 --> 00:15:02.640 across the periodic table, and this 00:15:02.640 --> 00:15:05.710 results in a decrease in the atomic radius. 00:15:05.710 --> 00:15:08.870 So again, it increases going across the periodic table 00:15:08.870 --> 00:15:11.970 because you're adding both electrons and protons. 00:15:11.970 --> 00:15:14.850 But the electrons are not giving you complete shielding, 00:15:14.850 --> 00:15:17.250 so you're not canceling out every proton 00:15:17.250 --> 00:15:18.520 with every electron. 00:15:18.520 --> 00:15:22.480 So overall, you get an increase in the Z effective. 00:15:22.480 --> 00:15:28.540 And because of that, when you have this increased Z, 00:15:28.540 --> 00:15:31.590 it's kind of pulling the electrons in. 00:15:31.590 --> 00:15:35.690 And then it isn't until you go down the periodic table 00:15:35.690 --> 00:15:40.690 when n increases that you start to see the radius increase. 00:15:40.690 --> 00:15:44.750 So there, the electrons are getting farther away, 00:15:44.750 --> 00:15:46.510 and so you are getting this bigger thing. 00:15:46.510 --> 00:15:48.968 So I like to think about it as sort of the mom at the park. 00:15:48.968 --> 00:15:52.990 There are some moms, their kids are sort of running everywhere, 00:15:52.990 --> 00:15:57.190 but other ones are sort of hauling their kids in. 00:15:57.190 --> 00:15:59.110 They have this positive force that 00:15:59.110 --> 00:16:02.240 seems to keep them all sort of in the general area. 00:16:02.240 --> 00:16:03.855 And so they're shrinking the size 00:16:03.855 --> 00:16:07.070 of their kids' play area with this force 00:16:07.070 --> 00:16:08.230 that they're exerting. 00:16:08.230 --> 00:16:09.790 But if the kids get too far away, 00:16:09.790 --> 00:16:11.600 it's like, yeah, they're not going to hear you call. 00:16:11.600 --> 00:16:12.900 They're not going to hear you jump up and down. 00:16:12.900 --> 00:16:14.190 They're not going to see you. 00:16:14.190 --> 00:16:16.160 And so they're just going to be out there, 00:16:16.160 --> 00:16:19.880 and the radius of your kids is going to be farther away. 00:16:19.880 --> 00:16:22.620 So we can look at these trends. 00:16:22.620 --> 00:16:24.800 If we're over here in the beginning, 00:16:24.800 --> 00:16:26.130 we're starting on this side. 00:16:26.130 --> 00:16:28.760 We're going across the periodic table. 00:16:28.760 --> 00:16:32.960 We go down, then we have a jump up when we increase n. 00:16:32.960 --> 00:16:35.610 And then we go down again, then we have a jump up 00:16:35.610 --> 00:16:36.900 when we increase n. 00:16:36.900 --> 00:16:41.010 We go down again, except over here there's a little glitch. 00:16:41.010 --> 00:16:43.007 Those d electrons, they're back. 00:16:43.007 --> 00:16:44.840 They're going to give you a couple glitches. 00:16:44.840 --> 00:16:45.890 I love d electrons. 00:16:45.890 --> 00:16:48.317 We're going to talk about them more around Thanksgiving. 00:16:48.317 --> 00:16:49.900 That's my favorite part of the course. 00:16:49.900 --> 00:16:51.990 Those d electrons are always causing trouble. 00:16:51.990 --> 00:16:52.489 OK. 00:16:52.489 --> 00:16:55.200 Then we go up again, and then we go down, and go up, 00:16:55.200 --> 00:16:55.840 and go down. 00:16:55.840 --> 00:16:56.756 Those are pretty good. 00:16:56.756 --> 00:16:58.820 Those are pretty good trends. 00:16:58.820 --> 00:16:59.760 All right. 00:16:59.760 --> 00:17:00.960 OK. 00:17:00.960 --> 00:17:05.040 So ions. 00:17:05.040 --> 00:17:08.030 Ions are different than their neutral parent, once again. 00:17:08.030 --> 00:17:12.670 So we saw this before, that when you start filling the 3d, 00:17:12.670 --> 00:17:14.710 the energy levels change. 00:17:14.710 --> 00:17:16.710 So ions can have different properties 00:17:16.710 --> 00:17:18.420 than their neutral parents. 00:17:18.420 --> 00:17:21.410 And so if we have two kinds of ions, 00:17:21.410 --> 00:17:24.670 we can have cations, which are positively charged. 00:17:24.670 --> 00:17:28.300 And so a positively charged ion will have lost an electron, 00:17:28.300 --> 00:17:31.520 and so it's going to be smaller than its parent. 00:17:31.520 --> 00:17:34.620 And so we can see here lithium. 00:17:34.620 --> 00:17:37.460 And then in the center, that's lithium plus. 00:17:37.460 --> 00:17:39.470 So when you lose the electron, the radius 00:17:39.470 --> 00:17:40.990 actually shrinks quite a bit. 00:17:40.990 --> 00:17:44.420 It's like that electron was just really kind 00:17:44.420 --> 00:17:46.190 of causing a bigger radius. 00:17:46.190 --> 00:17:48.300 And when it's finally gone, you're 00:17:48.300 --> 00:17:51.930 at a smaller size over there. 00:17:51.930 --> 00:17:54.480 Anions-- negatively charged ions. 00:17:54.480 --> 00:17:57.660 So they're gaining an electron, and their radius 00:17:57.660 --> 00:17:59.510 is larger than their parent. 00:17:59.510 --> 00:18:02.810 And so you can see over here, we have oxygen in the center. 00:18:02.810 --> 00:18:05.950 Oxygen minus 2 is much larger. 00:18:05.950 --> 00:18:08.090 And again, we can see some of the other trends. 00:18:08.090 --> 00:18:09.850 Some of them are the same. 00:18:09.850 --> 00:18:13.860 The ionic radius also will increase 00:18:13.860 --> 00:18:16.940 when we're going down a group, so when n is increasing, 00:18:16.940 --> 00:18:18.720 so from lithium to sodium. 00:18:18.720 --> 00:18:20.830 We have an increase from fluorine 00:18:20.830 --> 00:18:23.440 to the top of the periodic table to chlorine. 00:18:23.440 --> 00:18:26.990 So we still, as we increase n, increase in size. 00:18:26.990 --> 00:18:30.200 But you have to think about the ion-- did it lose an electron, 00:18:30.200 --> 00:18:32.220 or did it gain an electron-- to think 00:18:32.220 --> 00:18:38.430 about how its size changed with respect to its parent. 00:18:38.430 --> 00:18:42.380 So why does this matter? 00:18:42.380 --> 00:18:45.510 There's one example of-- if you're 00:18:45.510 --> 00:18:48.320 interested in biology or anything 00:18:48.320 --> 00:18:51.870 to do with the brain or neurons, then 00:18:51.870 --> 00:18:53.990 you should care about ion channels. 00:18:53.990 --> 00:18:57.600 So there are channels in membranes that bring ions in, 00:18:57.600 --> 00:19:00.440 and this is really important. 00:19:00.440 --> 00:19:03.890 So ion channels are in muscle cells and in neurons. 00:19:03.890 --> 00:19:08.020 So if you want to move or think, something that MIT students 00:19:08.020 --> 00:19:10.220 generally like to do both of those things, 00:19:10.220 --> 00:19:12.470 you need ion channels to do that. 00:19:12.470 --> 00:19:15.510 So ion channels should be important to you. 00:19:15.510 --> 00:19:17.120 And you want ion channels. 00:19:17.120 --> 00:19:20.680 They regulate the influx of ions into the cell 00:19:20.680 --> 00:19:24.140 and allow for really rapid responses, 00:19:24.140 --> 00:19:26.310 which is also really important. 00:19:26.310 --> 00:19:30.750 And amazingly, they're highly selective for certain ions. 00:19:30.750 --> 00:19:32.360 So this is important. 00:19:32.360 --> 00:19:35.120 It needs to be tightly regulated to say how much sodium you 00:19:35.120 --> 00:19:36.710 have in there or how much potassium 00:19:36.710 --> 00:19:38.080 that you have coming in. 00:19:38.080 --> 00:19:40.380 And if the ion channel took potassium 00:19:40.380 --> 00:19:43.580 when it was supposed to take sodium, that would not be good. 00:19:43.580 --> 00:19:46.990 So these channels are designed by nature 00:19:46.990 --> 00:19:49.380 to be highly selective, and so they 00:19:49.380 --> 00:19:51.960 care to be highly selective. 00:19:51.960 --> 00:19:55.280 And you're talking about a plus 1 maybe or another plus 1, 00:19:55.280 --> 00:19:57.400 they have to think about the radius. 00:19:57.400 --> 00:20:01.480 So why don't you tell me what the differential 00:20:01.480 --> 00:20:05.150 is in the radius from smallest to largest for these three 00:20:05.150 --> 00:20:06.030 different ones. 00:20:56.745 --> 00:20:57.870 All right, 10 more seconds. 00:21:13.400 --> 00:21:15.470 OK, great. 00:21:15.470 --> 00:21:19.080 So most people got that right. 00:21:19.080 --> 00:21:22.370 Let's just kind of take a look at that. 00:21:22.370 --> 00:21:27.400 So here we want to think about the neutral, 00:21:27.400 --> 00:21:29.690 and then this one has one less electron. 00:21:29.690 --> 00:21:31.380 So that's going to be smaller. 00:21:31.380 --> 00:21:34.530 And then when you compare potassium with sodium, 00:21:34.530 --> 00:21:36.180 you have to think about n. 00:21:36.180 --> 00:21:38.310 And so this is down farther, so that's 00:21:38.310 --> 00:21:40.170 going to be bigger than sodium. 00:21:40.170 --> 00:21:42.670 So here we're thinking about the difference 00:21:42.670 --> 00:21:48.490 in electron configuration, and here we're thinking about n. 00:21:48.490 --> 00:21:48.990 All right. 00:21:48.990 --> 00:21:53.130 So amazingly, these channels have it figured out, 00:21:53.130 --> 00:21:54.290 and so look at this. 00:21:54.290 --> 00:21:56.035 This is about significant figures. 00:21:56.035 --> 00:21:58.160 Too many people lost points on significant figures, 00:21:58.160 --> 00:22:01.370 I have to say, on the exam, so make sure you learn them. 00:22:01.370 --> 00:22:03.620 But if you had, say-- you say, oh, 00:22:03.620 --> 00:22:06.410 what's the difference between 1 to 3 significant figures? 00:22:06.410 --> 00:22:09.680 The difference is sort of the potassium versus the sodium 00:22:09.680 --> 00:22:10.330 radius. 00:22:10.330 --> 00:22:13.420 So 1.38 times 10 to the minus 10, 00:22:13.420 --> 00:22:16.450 1.02 times 10 to the minus 10-- those 00:22:16.450 --> 00:22:17.820 seem like pretty small numbers. 00:22:17.820 --> 00:22:20.880 Does it really matter if it's 0.2 versus 0.38? 00:22:20.880 --> 00:22:23.400 And the answer is, yes, you'd be dead 00:22:23.400 --> 00:22:26.930 if nature could not distinguish between these significant 00:22:26.930 --> 00:22:28.260 figures for you. 00:22:28.260 --> 00:22:30.680 So these channels are designed to be 00:22:30.680 --> 00:22:36.370 selective at that kind of atomic scale, and only let one ion in. 00:22:36.370 --> 00:22:38.720 And so Rod MacKinnon, who's a crystallographer, 00:22:38.720 --> 00:22:41.520 won a Nobel Prize for solving some of the structures 00:22:41.520 --> 00:22:42.950 of these ion channels. 00:22:42.950 --> 00:22:44.940 And this just shows a ribbon drawing, 00:22:44.940 --> 00:22:47.720 and this shows an all atom drawing of a channel, 00:22:47.720 --> 00:22:49.900 and there's an ion going through. 00:22:49.900 --> 00:22:54.600 That's its hole, and its radius is perfect for that ion. 00:22:54.600 --> 00:22:57.130 And the other one, even though it's not 00:22:57.130 --> 00:23:00.370 that many significant figures different, doesn't fit. 00:23:00.370 --> 00:23:01.610 And that's pretty amazing. 00:23:01.610 --> 00:23:03.320 Nature is truly amazing. 00:23:03.320 --> 00:23:04.110 That's the hole. 00:23:04.110 --> 00:23:08.530 It makes a perfect hole just for the one kind of ion 00:23:08.530 --> 00:23:11.160 that it's supposed to accept. 00:23:11.160 --> 00:23:11.750 All right. 00:23:11.750 --> 00:23:16.912 So now, there are one more definition 00:23:16.912 --> 00:23:17.870 that we're going to do. 00:23:17.870 --> 00:23:20.330 There are things that can have the same electron 00:23:20.330 --> 00:23:21.650 configuration. 00:23:21.650 --> 00:23:27.170 Those are called isoelectronic, and let's think about those. 00:23:27.170 --> 00:23:29.220 They don't necessarily have the same size, 00:23:29.220 --> 00:23:32.400 but they have the same electron configuration. 00:23:32.400 --> 00:23:34.370 And I'm just going to write these out. 00:23:34.370 --> 00:23:40.480 So when we think around other ones near neon, noble gas, that 00:23:40.480 --> 00:23:43.820 would have that exact configuration-- so how 00:23:43.820 --> 00:23:46.990 do we get flourine to have that configuration? 00:23:46.990 --> 00:23:50.380 What does it need to do-- gain or lose an electron, 00:23:50.380 --> 00:23:51.110 and how many? 00:23:51.110 --> 00:23:52.728 What would its state be? 00:23:58.500 --> 00:24:02.270 So what do I write-- what's the thing for flourine that 00:24:02.270 --> 00:24:05.480 is going to be the same electron configuration just in terms 00:24:05.480 --> 00:24:06.390 of its charge? 00:24:06.390 --> 00:24:07.870 I'd write F what? 00:24:07.870 --> 00:24:08.780 AUDIENCE: Minus. 00:24:08.780 --> 00:24:10.030 CATHERINE DRENNAN: Minus. 00:24:10.030 --> 00:24:11.480 For O, what am I going to write? 00:24:11.480 --> 00:24:13.021 For oxygen, what am I going to write? 00:24:13.021 --> 00:24:13.850 AUDIENCE: 2 minus. 00:24:13.850 --> 00:24:15.300 CATHERINE DRENNAN: 2 minus. 00:24:15.300 --> 00:24:16.590 And, say, nitrogen? 00:24:16.590 --> 00:24:17.620 We'll stop there. 00:24:17.620 --> 00:24:18.310 What's that? 00:24:18.310 --> 00:24:19.170 AUDIENCE: 3 minus. 00:24:19.170 --> 00:24:20.295 CATHERINE DRENNAN: 3 minus. 00:24:20.295 --> 00:24:20.960 Great. 00:24:20.960 --> 00:24:22.490 Let's go on the other side. 00:24:22.490 --> 00:24:23.900 What about for sodium? 00:24:23.900 --> 00:24:26.510 What does sodium have to do to have that configuration? 00:24:26.510 --> 00:24:27.440 AUDIENCE: Plus. 00:24:27.440 --> 00:24:28.440 CATHERINE DRENNAN: Plus. 00:24:31.560 --> 00:24:32.910 What about Mg? 00:24:32.910 --> 00:24:34.330 AUDIENCE: 2 plus. 00:24:34.330 --> 00:24:36.290 CATHERINE DRENNAN: 2 plus. 00:24:36.290 --> 00:24:37.780 Aluminum? 00:24:37.780 --> 00:24:39.419 AUDIENCE: 3 plus. 00:24:39.419 --> 00:24:40.335 CATHERINE DRENNAN: OK. 00:24:40.335 --> 00:24:42.850 And silicon-- 4 plus. 00:24:42.850 --> 00:24:46.050 So you get the idea. 00:24:46.050 --> 00:24:49.300 And now, we can think about which will have bigger 00:24:49.300 --> 00:24:53.700 and which will have smaller radii as well. 00:24:53.700 --> 00:24:59.287 So are these going to be bigger or smaller? 00:24:59.287 --> 00:25:00.162 AUDIENCE: [INAUDIBLE] 00:25:00.162 --> 00:25:01.203 CATHERINE DRENNAN: Right. 00:25:01.203 --> 00:25:06.400 So they're going to have larger radii than their parents 00:25:06.400 --> 00:25:08.510 because they've all gained. 00:25:08.510 --> 00:25:11.550 And then over here, these will be 00:25:11.550 --> 00:25:14.140 smaller since they've lost electrons 00:25:14.140 --> 00:25:17.270 compared to their parent ion. 00:25:17.270 --> 00:25:22.150 OK, so let's just do one, which should be very fast, 00:25:22.150 --> 00:25:23.130 clicker question. 00:25:23.130 --> 00:25:25.820 It is a clicker competition after all, 00:25:25.820 --> 00:25:28.363 so we've got to get in some extra clicker questions. 00:25:40.370 --> 00:25:42.290 And this should be very fast, I think. 00:25:56.160 --> 00:25:58.138 All right, let's just do 10 more seconds. 00:26:01.970 --> 00:26:04.345 You have a periodic table up here in case you need it. 00:26:13.130 --> 00:26:17.030 Yeah, OK, that's not going to distinguish the recitations 00:26:17.030 --> 00:26:18.000 very much. 00:26:18.000 --> 00:26:21.110 Yeah, so you just have to look at what is nearby 00:26:21.110 --> 00:26:23.760 and think about how many electrons it needs to gain 00:26:23.760 --> 00:26:27.220 or lose to have the same configuration. 00:26:27.220 --> 00:26:32.090 All right, bonds-- now, we're up to bonds. 00:26:32.090 --> 00:26:36.294 There are three types of bonds we will discuss today. 00:26:36.294 --> 00:26:37.960 We probably won't get to them all today. 00:26:37.960 --> 00:26:39.300 After all, the handout stuff. 00:26:39.300 --> 00:26:42.450 But anyway, then we will discuss in the class over time. 00:26:42.450 --> 00:26:45.600 Now, some of you have probably figured out-- almost everyone 00:26:45.600 --> 00:26:49.120 has probably figured out that one of the things I love to do 00:26:49.120 --> 00:26:50.940 is teach chemistry. 00:26:50.940 --> 00:26:53.860 I love to teach chemistry. 00:26:53.860 --> 00:26:56.530 Some of you have come to my office hours or pizza forums 00:26:56.530 --> 00:26:58.350 or even paid attention to some of my slides 00:26:58.350 --> 00:27:03.910 about office hours may realize that I love dogs. 00:27:03.910 --> 00:27:08.100 So I love teaching chemistry, and I love dogs. 00:27:08.100 --> 00:27:13.140 What is the most amazing thing that you can think of? 00:27:13.140 --> 00:27:15.026 Dogs teaching chemistry. 00:27:18.172 --> 00:27:18.838 [VIDEO PLAYBACK] 00:27:18.838 --> 00:27:21.303 CATHERINE DRENNAN: So now, I'm going to let dogs-- 00:27:21.303 --> 00:27:23.275 - Welcome to "Dogs Teaching Chemistry!" 00:27:23.275 --> 00:27:25.108 CATHERINE DRENNAN: --tell you about bonding. 00:27:25.108 --> 00:27:27.712 - --is chemical bonding. 00:27:27.712 --> 00:27:31.163 Chemical bonds are what holds atoms together. 00:27:31.163 --> 00:27:34.121 A chemical bond is an attraction between atoms 00:27:34.121 --> 00:27:37.079 that allows the formation of a chemical substance. 00:27:37.079 --> 00:27:40.530 The electrons that participate in a chemical bond 00:27:40.530 --> 00:27:42.009 are called valence electrons. 00:27:42.009 --> 00:27:44.967 These are electrons that are found in an atom's outermost 00:27:44.967 --> 00:27:47.450 shell. 00:27:47.450 --> 00:27:49.674 Let's take a look at the types of chemical bonds 00:27:49.674 --> 00:27:51.168 that can be formed between atoms. 00:27:56.646 --> 00:27:59.634 An ionic bond is formed when one of the atoms 00:27:59.634 --> 00:28:02.640 will lose its electron to the other atom. 00:28:02.640 --> 00:28:05.335 This results in a positively charged ion 00:28:05.335 --> 00:28:09.279 called a cation and a negatively charged ion called an anion. 00:28:12.250 --> 00:28:14.586 Positive and negative attract, and the result 00:28:14.586 --> 00:28:16.038 is an ionic bond. 00:28:22.330 --> 00:28:25.030 Covalent chemical bonds involve the sharing 00:28:25.030 --> 00:28:29.361 of a pair of valence electrons by two atoms. 00:28:29.361 --> 00:28:32.464 There is also what is called polar covalent bonds. 00:28:32.464 --> 00:28:34.844 These are covalent bonds in which 00:28:34.844 --> 00:28:38.180 the sharing of the electron pair is unequal. 00:28:38.180 --> 00:28:40.515 The result is a bond where the electron 00:28:40.515 --> 00:28:43.872 pair is displaced toward the more electronegative atom. 00:28:49.848 --> 00:28:52.836 Thanks for watching, and we'll see you guys next time. 00:29:02.796 --> 00:29:03.792 [END PLAYBACK] 00:29:07.029 --> 00:29:09.768 [APPLAUSE] 00:29:14.055 --> 00:29:16.180 CATHERINE DRENNAN: There's, I think, one other one, 00:29:16.180 --> 00:29:18.090 but that is totally the best one. 00:29:18.090 --> 00:29:20.860 And everything they say is exactly right, 00:29:20.860 --> 00:29:21.900 so it's really exciting. 00:29:21.900 --> 00:29:24.070 I even love it when they had the two-- 00:29:24.070 --> 00:29:26.750 they put two balls in there that they were sharing. 00:29:26.750 --> 00:29:29.220 It's just really very well done. 00:29:29.220 --> 00:29:33.050 OK, so now, you've probably filled in some of your notes 00:29:33.050 --> 00:29:34.950 here, but in case you missed some of them, 00:29:34.950 --> 00:29:38.260 I will tell you exactly what the dogs just told you. 00:29:38.260 --> 00:29:41.240 The dogs had it completely correct. 00:29:41.240 --> 00:29:47.170 So ionic bonds is the transfer of an electron, as you saw, 00:29:47.170 --> 00:29:51.220 and then the generation of a cation and an anion 00:29:51.220 --> 00:29:54.490 that are attracted to each other due to the charge. 00:29:54.490 --> 00:29:56.860 So the bonding comes from that attraction 00:29:56.860 --> 00:29:59.610 between the positively charged and the negatively charged 00:29:59.610 --> 00:30:04.380 atom, and an example that you're probably all familiar with is 00:30:04.380 --> 00:30:06.190 table salt, NaCL. 00:30:06.190 --> 00:30:09.490 And so you have Na plus and CL minus that are 00:30:09.490 --> 00:30:11.640 attracted to each other and form these bonds, 00:30:11.640 --> 00:30:16.110 which creates table salt. So let's see 00:30:16.110 --> 00:30:19.110 how far we can get in thinking about, really, 00:30:19.110 --> 00:30:20.990 this interaction between ionic bonds, 00:30:20.990 --> 00:30:22.990 and we'll see if we can get through ionic bonds. 00:30:22.990 --> 00:30:26.260 We might have to wait until covalent bonds until Monday, 00:30:26.260 --> 00:30:27.940 but let's see if we can finish this. 00:30:27.940 --> 00:30:35.160 So the formation of NaCL from neutral Na and neutral CL 00:30:35.160 --> 00:30:38.620 will first involve forming your cations and your anions. 00:30:38.620 --> 00:30:42.940 So you have Na going to Na plus plus an electron, 00:30:42.940 --> 00:30:45.690 and so here, you're talking about a process 00:30:45.690 --> 00:30:50.400 where the energy is going to be equal to the ionization energy 00:30:50.400 --> 00:30:50.900 again. 00:30:50.900 --> 00:30:53.580 So we're not moving far away from these terms 00:30:53.580 --> 00:30:58.580 because you're talking about ionizing a neutral atom to Na 00:30:58.580 --> 00:31:02.350 plus, and there's a value for that. 00:31:02.350 --> 00:31:06.610 And then we're talking about neutral CL, neutral chlorine, 00:31:06.610 --> 00:31:10.950 going to CL minus, and so it's gaining an electron. 00:31:10.950 --> 00:31:13.080 So here, the process you're talking about 00:31:13.080 --> 00:31:18.720 is the electron affinity, and so the energy change here 00:31:18.720 --> 00:31:22.960 is equal to the negative electron affinity, which 00:31:22.960 --> 00:31:26.520 is minus 349 in this case. 00:31:26.520 --> 00:31:30.320 So this is a favorable process here 00:31:30.320 --> 00:31:36.130 to gain this extra electron, and so overall, the Delta E here 00:31:36.130 --> 00:31:37.520 is negative. 00:31:37.520 --> 00:31:41.300 So now, if we're going to talk about this process here, 00:31:41.300 --> 00:31:42.800 we have two of these, so we're going 00:31:42.800 --> 00:31:45.050 to go-- we're going to put these guys together. 00:31:45.050 --> 00:31:47.070 So we need both of those to ionize, 00:31:47.070 --> 00:31:50.940 and so we can add up what energy difference 00:31:50.940 --> 00:31:54.890 we should expect to form Na plus and CL 00:31:54.890 --> 00:31:57.360 minus together from their parents. 00:31:57.360 --> 00:31:59.480 And so we have, now, a plus-- we've 00:31:59.480 --> 00:32:04.940 added these two together-- a plus 145 kilojoules per mol. 00:32:04.940 --> 00:32:06.700 So this seems weird. 00:32:06.700 --> 00:32:10.280 It's plus, and so now, we're seeing 00:32:10.280 --> 00:32:13.120 that the formation of these ions from their neutral atoms 00:32:13.120 --> 00:32:16.350 has this positive value, which means it requires energy. 00:32:16.350 --> 00:32:20.510 But we think about NaCL as being this natural table salt thing, 00:32:20.510 --> 00:32:27.510 so why is there so much table salt if this requires energy 00:32:27.510 --> 00:32:28.550 to do it? 00:32:28.550 --> 00:32:32.190 And the answer is that this is only part of the process. 00:32:32.190 --> 00:32:35.030 You need to form your cations and anions, 00:32:35.030 --> 00:32:38.200 but then you have energy of them coming together. 00:32:38.200 --> 00:32:39.850 So they're attracted to each other, 00:32:39.850 --> 00:32:42.980 and that's a really important part of forming the bond. 00:32:42.980 --> 00:32:46.540 And they're attracted by a simple coulombic relationship 00:32:46.540 --> 00:32:47.530 here. 00:32:47.530 --> 00:32:51.170 So the attraction between the positively charged 00:32:51.170 --> 00:32:53.830 and the negatively charged ion has 00:32:53.830 --> 00:32:59.100 an energy of minus 589 kilojoules per mol, 00:32:59.100 --> 00:33:05.520 so overall then, if you consider both forming Na plus and CL 00:33:05.520 --> 00:33:08.480 minus and the attraction between them, 00:33:08.480 --> 00:33:12.080 we have a negative Delta E 444. 00:33:12.080 --> 00:33:18.510 So the net energy here is in favor of forming NaCL. 00:33:18.510 --> 00:33:20.230 We have a decrease in energy. 00:33:20.230 --> 00:33:24.490 This is a stable compound, so let's look at where 00:33:24.490 --> 00:33:26.240 this number comes from. 00:33:26.240 --> 00:33:27.240 So we just put this out. 00:33:27.240 --> 00:33:29.680 That's the coulomb thing, but let's 00:33:29.680 --> 00:33:34.540 actually calculate this and see where that number comes from. 00:33:34.540 --> 00:33:37.220 So we're back to coulombic equations again. 00:33:37.220 --> 00:33:38.650 We never get very far away. 00:33:38.650 --> 00:33:41.880 They turn out to be very important in chemistry. 00:33:41.880 --> 00:33:43.690 So we have the coulombic potential. 00:33:43.690 --> 00:33:46.460 We have z's, our charge on our ions. 00:33:46.460 --> 00:33:48.590 We have the absolute value of the charge 00:33:48.590 --> 00:33:53.500 of an electron squared over 4 pi, our permittivity 00:33:53.500 --> 00:33:57.750 constant in r, our distance between those ions. 00:33:57.750 --> 00:34:00.620 And for any CL the bond length, or the distance 00:34:00.620 --> 00:34:06.310 between Na plus and CL minus, is 2.36 angstroms, 00:34:06.310 --> 00:34:09.900 so we can use that and just plug it into the equation. 00:34:09.900 --> 00:34:14.010 And we have plus 1 for the sodium, minus 1 for chloride, 00:34:14.010 --> 00:34:17.944 so overall, this will be a negative term. 00:34:17.944 --> 00:34:19.360 And if you work out the math, it's 00:34:19.360 --> 00:34:24.610 minus 9.774 times 10 to the minus 19th joules, 00:34:24.610 --> 00:34:26.730 and we have three significant figures. 00:34:26.730 --> 00:34:29.504 What's limiting our significant figures? 00:34:29.504 --> 00:34:30.452 AUDIENCE: [INAUDIBLE] 00:34:30.452 --> 00:34:32.493 CATHERINE DRENNAN: What-- yeah, the bottom length 00:34:32.493 --> 00:34:34.100 is limiting it here. 00:34:34.100 --> 00:34:36.270 I'm just going to talk about significant figures 00:34:36.270 --> 00:34:39.449 for a while, until Test 2 when you can demonstrate I 00:34:39.449 --> 00:34:41.860 can stop talking about significant figures. 00:34:41.860 --> 00:34:45.760 All right, so then we want to convert to kilojoules per mol 00:34:45.760 --> 00:34:47.810 because that was the number I gave you, 00:34:47.810 --> 00:34:51.750 was in kilojoules per mol, so we have our conversion factor 00:34:51.750 --> 00:34:53.136 between joules and kilojoules. 00:34:53.136 --> 00:34:54.719 And I guess I should mention up here-- 00:34:54.719 --> 00:34:57.820 coulombs cancel, and our meters cancel, 00:34:57.820 --> 00:34:59.810 so we're left in joules up there. 00:34:59.810 --> 00:35:01.980 Units are also important. 00:35:01.980 --> 00:35:03.690 Then we can use Avogadro's number, 00:35:03.690 --> 00:35:06.380 because we're given kilojoules per mol, 00:35:06.380 --> 00:35:08.690 and we can get out the number I gave you before. 00:35:08.690 --> 00:35:10.820 So this number really just comes right out 00:35:10.820 --> 00:35:16.250 of this equation minus 589 kilojoules per mol. 00:35:16.250 --> 00:35:20.620 So we have our ionization energy to tell us about forming ions. 00:35:20.620 --> 00:35:22.660 We have our electron affinity, and now, we 00:35:22.660 --> 00:35:26.120 have a coulombic relationship. 00:35:26.120 --> 00:35:28.250 So I said before-- this is what I showed you 00:35:28.250 --> 00:35:32.370 before-- that we have this attraction that's favorable. 00:35:32.370 --> 00:35:35.420 We have forming the ions, which had 00:35:35.420 --> 00:35:38.120 a positive energy associated with it, 00:35:38.120 --> 00:35:42.380 but overall, this process has a lower energy. 00:35:42.380 --> 00:35:43.710 It forms a bond. 00:35:43.710 --> 00:35:46.560 But this is just based on this calculation. 00:35:46.560 --> 00:35:50.880 So we can ask what is the experimental measurement 00:35:50.880 --> 00:35:53.480 for this interaction, and we note 00:35:53.480 --> 00:35:55.480 that it's somewhat different. 00:35:55.480 --> 00:35:58.155 So we have, instead of minus 444, 00:35:58.155 --> 00:36:02.510 we have minus 411 kilojoules per mol. 00:36:02.510 --> 00:36:06.000 So why the difference? 00:36:06.000 --> 00:36:09.330 So again, our ionic model, which just considers 00:36:09.330 --> 00:36:11.940 ionization energy, electron affinity, 00:36:11.940 --> 00:36:15.360 and that positive coulimbic interaction 00:36:15.360 --> 00:36:19.990 in our experimental result-- so problems with this model 00:36:19.990 --> 00:36:24.630 that we did up here include that we only thought 00:36:24.630 --> 00:36:26.710 about favorable interactions. 00:36:26.710 --> 00:36:30.510 There are also going to be some that are not favorable; 00:36:30.510 --> 00:36:33.900 protons against other protons, repulsion, electron electron 00:36:33.900 --> 00:36:35.000 repulsion. 00:36:35.000 --> 00:36:36.330 So there are some negatives. 00:36:36.330 --> 00:36:39.030 It's never all positive in any relationship, 00:36:39.030 --> 00:36:41.640 whether it's sodium chloride or anything else. 00:36:41.640 --> 00:36:44.710 Always some negatives, and we ignore those. 00:36:44.710 --> 00:36:46.360 And the result of this is that you're 00:36:46.360 --> 00:36:49.590 going to get a larger Delta E predicted 00:36:49.590 --> 00:36:51.310 than the experimental value. 00:36:51.310 --> 00:36:53.550 So it seems like this is more favorable, 00:36:53.550 --> 00:36:56.770 like that's a stronger bond, a stronger interaction; 00:36:56.770 --> 00:36:59.010 but really, if there's some repulsion, 00:36:59.010 --> 00:37:00.830 that's overestimated. 00:37:00.830 --> 00:37:02.800 It's really going to be a lower value, which is 00:37:02.800 --> 00:37:04.890 what you see in the experiment. 00:37:04.890 --> 00:37:10.450 Also, we just said that sodium plus was one point charge 00:37:10.450 --> 00:37:12.280 and CL minus was another. 00:37:12.280 --> 00:37:13.590 It's more complicated. 00:37:13.590 --> 00:37:16.460 Their interactions are more complicated than that. 00:37:16.460 --> 00:37:21.160 And we ignored quantum mechanics, but in doing that, 00:37:21.160 --> 00:37:22.220 we did pretty well. 00:37:22.220 --> 00:37:23.950 If we had one significant figure, 00:37:23.950 --> 00:37:28.460 we would have been perfect, so to one significant figure, 00:37:28.460 --> 00:37:31.380 these approximations work really well. 00:37:31.380 --> 00:37:33.900 OK, that's it for today, and I will see you Monday. 00:37:33.900 --> 00:37:34.800 Have a great weekend. 00:37:45.880 --> 00:37:48.435 All right, let's just take 10 more seconds 00:37:48.435 --> 00:37:49.434 on the clicker question. 00:38:04.750 --> 00:38:06.890 Great, so people are getting the hang of this. 00:38:06.890 --> 00:38:10.520 If you hadn't yet, there's still time. 00:38:10.520 --> 00:38:17.040 So as you're going across the periodic table, 00:38:17.040 --> 00:38:20.340 you are increasing z and increasing the z effective 00:38:20.340 --> 00:38:23.240 as well, because you don't have total shielding, 00:38:23.240 --> 00:38:25.370 so aluminum is the correct answer. 00:38:25.370 --> 00:38:27.930 It has a lower z effective, and so therefore, 00:38:27.930 --> 00:38:30.280 a smaller ionization energy. 00:38:30.280 --> 00:38:32.890 The electrons aren't held as tightly. 00:38:32.890 --> 00:38:35.790 All right, so we're talking about chemical bonds, 00:38:35.790 --> 00:38:37.860 and it seems like an appropriate topic 00:38:37.860 --> 00:38:40.340 to talk about when we're also talking about bonding 00:38:40.340 --> 00:38:43.930 as a community, so that seems like a good thing. 00:38:43.930 --> 00:38:47.690 So a chemical bond is an arrangement of atoms 00:38:47.690 --> 00:38:50.770 so that they come together in such a way 00:38:50.770 --> 00:38:53.290 that they're lower in energy than they 00:38:53.290 --> 00:38:54.720 were when they were apart. 00:38:54.720 --> 00:38:57.610 So they're more stable together than they were apart, 00:38:57.610 --> 00:38:58.910 and that's a chemical bond. 00:38:58.910 --> 00:39:02.380 So this is page five of the handout from last time. 00:39:02.380 --> 00:39:05.600 And excitingly, we have Lecture 10 handouts today, 00:39:05.600 --> 00:39:09.180 so there's lots of things working today. 00:39:09.180 --> 00:39:11.590 All right, so this is lower in energy, i.e. 00:39:11.590 --> 00:39:15.140 more negative, when these atoms come together. 00:39:15.140 --> 00:39:19.080 So a chemical bond-- as you saw last time 00:39:19.080 --> 00:39:22.260 with those wonderful dogs sharing a pull toy, 00:39:22.260 --> 00:39:25.490 a covalent bond is a bond where the electrons are shared 00:39:25.490 --> 00:39:28.860 between these two atoms, and each atom 00:39:28.860 --> 00:39:33.020 is giving up one bond to share. 00:39:33.020 --> 00:39:36.240 So we can think about this more graphically of what 00:39:36.240 --> 00:39:40.490 is happening, and we have this little plot on your notes, 00:39:40.490 --> 00:39:43.420 where you're going to be filling in a bunch of details. 00:39:43.420 --> 00:39:47.740 So we have the internuclear distance, r, the distance 00:39:47.740 --> 00:39:48.980 between the two nuclei. 00:39:48.980 --> 00:39:50.730 And we're back to hydrogen for the moment, 00:39:50.730 --> 00:39:54.990 so we're going to talk about a bond between two H atoms. 00:39:54.990 --> 00:39:58.640 And on the axis over here, we have energy. 00:39:58.640 --> 00:40:02.670 So we have energy versus the distance between these two 00:40:02.670 --> 00:40:04.180 hydrogens. 00:40:04.180 --> 00:40:08.340 So at 0 energy, we just have the hydrogens by themselves. 00:40:08.340 --> 00:40:11.050 They're not interacting with each other in any kind of way 00:40:11.050 --> 00:40:13.230 that lowers either one of their energies. 00:40:13.230 --> 00:40:17.350 There's no interaction, no energy change. 00:40:17.350 --> 00:40:19.510 They're not interacting. 00:40:19.510 --> 00:40:22.560 Down here at this dash line, we do have a bond, 00:40:22.560 --> 00:40:24.210 so we formed H2. 00:40:24.210 --> 00:40:27.090 The hydrogen atoms are interacting with each other, 00:40:27.090 --> 00:40:29.960 and this is lower in energy. 00:40:29.960 --> 00:40:34.150 So what does this plot look like then 00:40:34.150 --> 00:40:37.750 if you draw energy versus this distance? 00:40:37.750 --> 00:40:42.060 So up here, it's above and higher energy, above 0-- 00:40:42.060 --> 00:40:47.080 this is unfavorable-- going down to this dashed line 00:40:47.080 --> 00:40:49.740 and then going back up to 0. 00:40:49.740 --> 00:40:52.277 So let's think about what's happening here, 00:40:52.277 --> 00:40:53.860 and there's a bunch of different kinds 00:40:53.860 --> 00:40:56.610 of interactions you can have between those two hydrogen 00:40:56.610 --> 00:40:58.110 atoms. 00:40:58.110 --> 00:41:00.070 There are repulsive interactions, 00:41:00.070 --> 00:41:04.030 nuclear nuclear repulsion, electron electron repulsion; 00:41:04.030 --> 00:41:05.540 and there are positive interactions 00:41:05.540 --> 00:41:09.330 like the electron nuclear, the positive and negative. 00:41:09.330 --> 00:41:12.840 So up here, these atoms are very close together, 00:41:12.840 --> 00:41:16.720 and that is-- they're really too close, and that's unfavorable. 00:41:16.720 --> 00:41:19.970 Two objects trying to occupy the same space at the same time 00:41:19.970 --> 00:41:21.530 is unfavorable. 00:41:21.530 --> 00:41:24.990 But as you start separating out these atoms, 00:41:24.990 --> 00:41:27.700 then they become more comfortable, 00:41:27.700 --> 00:41:29.920 and you get to a distance where you 00:41:29.920 --> 00:41:31.800 have the sharing of electrons. 00:41:31.800 --> 00:41:33.020 They're next to each other. 00:41:33.020 --> 00:41:35.490 They're interacting in a positive way, 00:41:35.490 --> 00:41:37.390 and they're sharing. 00:41:37.390 --> 00:41:40.010 But then if you bring them too far apart, 00:41:40.010 --> 00:41:42.090 they're no longer communicating with each other. 00:41:42.090 --> 00:41:44.890 We don't even know where one of them is at this point. 00:41:44.890 --> 00:41:47.310 They're just kind of infinitely far apart. 00:41:47.310 --> 00:41:48.870 No interaction. 00:41:48.870 --> 00:41:55.180 So we go from too close to just right to too far away. 00:41:55.180 --> 00:42:02.770 So this distance here that has this minimum energy-- OK, 00:42:02.770 --> 00:42:05.980 so we have this minimum energy here, 00:42:05.980 --> 00:42:09.760 and that's the distance at which you have this really favorable 00:42:09.760 --> 00:42:11.770 interaction, this bond length. 00:42:11.770 --> 00:42:15.760 That's the bond length between those two hydrogens right here. 00:42:15.760 --> 00:42:18.750 Now, we can think about this energy difference over here, 00:42:18.750 --> 00:42:21.840 and it has a special name. 00:42:21.840 --> 00:42:28.040 So this is the dissociation energy, or Delta E sub d. 00:42:28.040 --> 00:42:32.160 Sometimes, it's just called big capital D in your textbook. 00:42:32.160 --> 00:42:33.940 So this is the energy that's needed 00:42:33.940 --> 00:42:36.060 to dissociate those atoms. 00:42:36.060 --> 00:42:39.310 So if the atoms come together, and they're lower in energy, 00:42:39.310 --> 00:42:41.350 if you then want to break them apart, 00:42:41.350 --> 00:42:45.250 you need to put energy in so that they can be broken apart. 00:42:45.250 --> 00:42:48.090 And that's called the dissociation energy. 00:42:48.090 --> 00:42:49.960 And if it's a really big number, that 00:42:49.960 --> 00:42:52.240 means it's very hard to dissociate them, 00:42:52.240 --> 00:42:54.990 and if it's a small number, it's pretty easy. 00:42:54.990 --> 00:42:58.030 So this is our dissociation energy here. 00:42:58.030 --> 00:43:04.430 So for hydrogen, this value is 424 kilojoules per mol. 00:43:04.430 --> 00:43:07.690 And if we were putting it on this axis here, 00:43:07.690 --> 00:43:10.520 it would be right down here, so it's 00:43:10.520 --> 00:43:13.130 going to be a negative value on this axis. 00:43:13.130 --> 00:43:17.250 We're below 0, so the negative of the dissociation energy 00:43:17.250 --> 00:43:21.410 is found down here, minus 424 kilojoules per mol. 00:43:21.410 --> 00:43:23.490 And if you wanted to break that bond, 00:43:23.490 --> 00:43:26.930 you would need to put in energy, so dissociation energy 00:43:26.930 --> 00:43:27.870 is positive. 00:43:27.870 --> 00:43:33.110 It's the energy you need to put in to break those bonds. 00:43:33.110 --> 00:43:38.480 So we can think about this plot now and consider looking 00:43:38.480 --> 00:43:43.230 at the plot and evaluating what you can and cannot say about 00:43:43.230 --> 00:43:45.610 different kinds of compounds, their bond lengths, 00:43:45.610 --> 00:43:48.990 and also their dissociation energies. 00:43:48.990 --> 00:43:50.350 So we good people? 00:43:50.350 --> 00:43:52.530 Mostly got this written down? 00:43:52.530 --> 00:43:56.440 And today, for some of the people who wanted to come late, 00:43:56.440 --> 00:43:59.540 we can post versions of this, too. 00:43:59.540 --> 00:44:02.020 All right, so now let's consider this plot 00:44:02.020 --> 00:44:04.830 and ask which bond is stronger? 00:44:04.830 --> 00:44:07.220 So we have hydrazine, and we also 00:44:07.220 --> 00:44:10.840 have molecular nitrogen, N2. 00:44:10.840 --> 00:44:14.930 And in your dashed line, you have nitrogen, N2, 00:44:14.930 --> 00:44:18.190 and in the solid line, you have hydrazine. 00:44:18.190 --> 00:44:20.960 So can you just look at this plot 00:44:20.960 --> 00:44:24.700 and tell me which is stronger? 00:44:24.700 --> 00:44:27.160 Is nitrogen or hydrazine stronger? 00:44:27.160 --> 00:44:29.225 And you can just yell out what you think. 00:44:29.225 --> 00:44:30.100 AUDIENCE: [INAUDIBLE] 00:44:30.100 --> 00:44:31.780 CATHERINE DRENNAN: Yeah. 00:44:31.780 --> 00:44:34.440 So nitrogen is going to be stronger here, 00:44:34.440 --> 00:44:37.430 and people knew that because it's a deeper well. 00:44:37.430 --> 00:44:41.870 So you go way farther down, there's more stabilization. 00:44:41.870 --> 00:44:46.580 It's a lower energy, a lower negative value of energy, 00:44:46.580 --> 00:44:50.060 so that means it's going to have a greater dissociation energy. 00:44:50.060 --> 00:44:54.230 You'd have to put in more energy to dissociate nitrogen nitrogen 00:44:54.230 --> 00:45:00.660 than these two submolecules here in hydrozine. 00:45:00.660 --> 00:45:02.720 All right, so we can also look at this plot 00:45:02.720 --> 00:45:07.091 and ask the question, which has a shorter bond? 00:45:07.091 --> 00:45:07.840 What do you think? 00:45:07.840 --> 00:45:09.050 Which bond is shorter? 00:45:09.050 --> 00:45:10.000 AUDIENCE: [INAUDIBLE] 00:45:10.000 --> 00:45:12.200 CATHERINE DRENNAN: Nitrogen as well, right. 00:45:12.200 --> 00:45:14.730 So nitrogen is also the shorter bond, 00:45:14.730 --> 00:45:18.630 and we know this because this is increasing distance. 00:45:18.630 --> 00:45:22.820 And this is closer to the axis, so this has a shorter distance. 00:45:22.820 --> 00:45:24.550 And later, we're going to be doing 00:45:24.550 --> 00:45:26.867 Lewis structures and other things of nitrogen 00:45:26.867 --> 00:45:29.200 and discover it has a triple bond, which you may already 00:45:29.200 --> 00:45:32.490 know, and we'll talk more about nitrogen's amazing triple bond 00:45:32.490 --> 00:45:34.230 as we go along. 00:45:34.230 --> 00:45:37.450 And so that's a very short bond and a very, very strong bond 00:45:37.450 --> 00:45:38.217 as well. 00:45:38.217 --> 00:45:40.050 So you should be able to look at these plots 00:45:40.050 --> 00:45:44.010 and evaluate what kind of dissociation energy 00:45:44.010 --> 00:45:46.250 it would have, is it bigger or smaller, 00:45:46.250 --> 00:45:49.620 and also what kind of distance you expect between them. 00:45:49.620 --> 00:45:52.460 And you should be able to draw these kinds of plots 00:45:52.460 --> 00:45:56.790 on the exam, if asked, in just kind of simple detail. 00:45:56.790 --> 00:45:59.630 Nothing too fancy. 00:45:59.630 --> 00:46:04.310 All right, so in terms of bond strengths, 00:46:04.310 --> 00:46:07.390 carbon monoxide has one of the strongest bonds, 00:46:07.390 --> 00:46:10.430 so it has a very large dissociation energy. 00:46:10.430 --> 00:46:13.387 And iodide, I2, has one of the weaker ones. 00:46:13.387 --> 00:46:14.970 And later in the semester, we're going 00:46:14.970 --> 00:46:19.260 to be doing a demo that shows why that's kind of cool, 00:46:19.260 --> 00:46:25.110 that weak bond leads to some cool, cool demos. 00:46:25.110 --> 00:46:29.020 OK, so those are covalent bonds. 00:46:29.020 --> 00:46:32.880 And, Ashley, could you just close that door, please? 00:46:32.880 --> 00:46:35.130 All right, so let's finish polar covalent, 00:46:35.130 --> 00:46:37.780 and then we'll have our moment of silence, 00:46:37.780 --> 00:46:40.180 and maybe it'll be silent in the hallway by then as well. 00:46:40.180 --> 00:46:42.500 All right, so polar covalent bonds-- 00:46:42.500 --> 00:46:45.250 so last time, the dog showed you that you 00:46:45.250 --> 00:46:48.414 can have equal sharing and unequal sharing. 00:46:48.414 --> 00:46:49.830 And of those of you who've watched 00:46:49.830 --> 00:46:52.790 dogs play with pully toys, most of the time, 00:46:52.790 --> 00:46:55.350 the sharing is pretty unequal, and so whenever 00:46:55.350 --> 00:46:57.769 you see that again, you can think polar covalent 00:46:57.769 --> 00:46:59.560 and tell your friends, and they'll be like, 00:46:59.560 --> 00:47:01.310 "I don't know what happened to you at MIT. 00:47:01.310 --> 00:47:02.630 Those are two dogs playing. 00:47:02.630 --> 00:47:04.630 What are you talking about?" 00:47:04.630 --> 00:47:10.280 So this is unequal sharing of electrons between two atoms, 00:47:10.280 --> 00:47:13.553 and this happens when those atoms have a very different 00:47:13.553 --> 00:47:14.303 electronegativity. 00:47:16.830 --> 00:47:21.140 So in general, a bond between two atoms 00:47:21.140 --> 00:47:24.600 is considered a polar covalent bond 00:47:24.600 --> 00:47:28.310 when the difference in electric negativity between the atoms 00:47:28.310 --> 00:47:32.670 is greater than 0.4 and less than 1.7, 00:47:32.670 --> 00:47:37.380 and that's Linus Pauling's scale and works quite well. 00:47:37.380 --> 00:47:39.200 So if we look at this little plot, 00:47:39.200 --> 00:47:42.760 we could see that carbon hydrogen bond only 00:47:42.760 --> 00:47:45.490 has a 0.4 difference, so that would not 00:47:45.490 --> 00:47:48.450 be considered a polar covalent bond. 00:47:48.450 --> 00:47:51.510 But nitrogen hydrogen is a difference 00:47:51.510 --> 00:47:55.740 that is greater than 0.4, so that would be a polar bond. 00:47:55.740 --> 00:47:58.250 And so you can use these values to think about 00:47:58.250 --> 00:48:00.880 whether you're going to have unequal sharing, 00:48:00.880 --> 00:48:03.280 and the more electronegative element 00:48:03.280 --> 00:48:07.780 is going to be pulling on those electrons. 00:48:07.780 --> 00:48:11.480 We also can use this to think about polar molecules, 00:48:11.480 --> 00:48:13.660 and this is kind of a little bit of a flash forward 00:48:13.660 --> 00:48:15.410 to Friday's lecture. 00:48:15.410 --> 00:48:18.010 We're going to talk about shapes of molecules 00:48:18.010 --> 00:48:21.750 because a polar molecule has to have polar bonds, 00:48:21.750 --> 00:48:25.230 but also has to have those bonds arranged in such a way 00:48:25.230 --> 00:48:27.950 that there's a net difference, there's a net dipole, 00:48:27.950 --> 00:48:31.000 there's a net kind of pulling of those electrons 00:48:31.000 --> 00:48:32.540 in a different way. 00:48:32.540 --> 00:48:34.690 So here is carbon dioxide, a molecule 00:48:34.690 --> 00:48:37.180 that causes global warming. 00:48:37.180 --> 00:48:40.090 Yes, I said it on the videotape. 00:48:40.090 --> 00:48:42.210 Yes, I believe that human beings are 00:48:42.210 --> 00:48:44.530 responsible for some of the global warming, 00:48:44.530 --> 00:48:47.220 and we should do something about it. 00:48:47.220 --> 00:48:50.340 This molecule does have polar bonds, 00:48:50.340 --> 00:48:53.150 so we have carbon in the middle and oxygen here. 00:48:53.150 --> 00:48:57.100 So carbon and oxygen have an electronegativity difference 00:48:57.100 --> 00:49:02.070 greater than 0.4, but it's not really a polar molecule. 00:49:02.070 --> 00:49:05.610 It's a non-polar molecule, and that's because of its shape. 00:49:05.610 --> 00:49:07.910 So shape matters. 00:49:07.910 --> 00:49:11.410 So we have pulling of electrons one way, 00:49:11.410 --> 00:49:14.149 but we have equal and opposite pulling of electrons other way. 00:49:14.149 --> 00:49:15.690 This would be really cool if the dogs 00:49:15.690 --> 00:49:17.180 could have done this as well. 00:49:17.180 --> 00:49:20.930 So in this case, we have a non-polar molecule 00:49:20.930 --> 00:49:22.670 that has polar bonds. 00:49:22.670 --> 00:49:25.800 Now, there's only very few cases where this would actually 00:49:25.800 --> 00:49:30.270 be true, and you really need to think about the shape. 00:49:30.270 --> 00:49:33.170 And so that's why we're going to talk about shape. 00:49:33.170 --> 00:49:40.380 Another molecule that also has polar bonds is water. 00:49:40.380 --> 00:49:44.600 So here, we have polar bonds between the oxygen 00:49:44.600 --> 00:49:49.090 and the hydrogen, so oxygen and hydrogen, greater than 0.4. 00:49:49.090 --> 00:49:51.880 But in this case, the shape of water 00:49:51.880 --> 00:49:55.740 is such that they don't cancel out, 00:49:55.740 --> 00:49:57.690 and you do have a net dipole. 00:49:57.690 --> 00:50:01.440 You do have a net charge on that molecule, 00:50:01.440 --> 00:50:04.890 makes it a polar molecule. 00:50:04.890 --> 00:50:07.530 So we need to know about the shape, 00:50:07.530 --> 00:50:11.150 and we need to know about electronegativity. 00:50:11.150 --> 00:50:14.370 So in large, organic molecules, sometimes we 00:50:14.370 --> 00:50:16.920 just talk about the number of polar bonds 00:50:16.920 --> 00:50:18.520 and then think about whether that's 00:50:18.520 --> 00:50:20.814 likely to be a polar molecule or not. 00:50:20.814 --> 00:50:23.230 You can't always think about the shape of something really 00:50:23.230 --> 00:50:27.660 complicated and what direction all of the pulling of electrons 00:50:27.660 --> 00:50:31.960 is going, but we can at least count polar bonds. 00:50:31.960 --> 00:50:36.960 So here are two vitamins, vitamin A and vitamin B-9, 00:50:36.960 --> 00:50:40.610 which I think is also B-10 and a number of other B's. 00:50:40.610 --> 00:50:42.960 Its name is folic acid. 00:50:42.960 --> 00:50:44.730 They kept finding it again and again, 00:50:44.730 --> 00:50:47.640 so there's a whole gap of b-vitamins where they're like, 00:50:47.640 --> 00:50:52.080 oh, B-9, folic acid; B-10, also folic acid; 00:50:52.080 --> 00:50:53.970 B-11, I think also folic acid. 00:50:53.970 --> 00:50:55.970 B-12 something different, though. 00:50:55.970 --> 00:50:59.114 So anyway, this is a very important B vitamin, 00:50:59.114 --> 00:51:01.530 and I'm actually going to come back to this molecule later 00:51:01.530 --> 00:51:02.900 in the course. 00:51:02.900 --> 00:51:06.434 But we can think about how many polar bonds it has, 00:51:06.434 --> 00:51:07.684 and that's a clicker question. 00:51:13.500 --> 00:51:16.020 And there, you have the molecules up here to look at. 00:51:26.520 --> 00:51:29.470 All right, let's just take 10 more seconds. 00:51:29.470 --> 00:51:31.440 It takes a while to count, but I think 00:51:31.440 --> 00:51:34.700 you probably can answer it without maybe fully counting 00:51:34.700 --> 00:51:35.535 all of them. 00:51:44.550 --> 00:51:49.840 All right, so over here now, the answer is the folic acid, 00:51:49.840 --> 00:51:51.670 and I'll just highlight. 00:51:51.670 --> 00:51:54.400 You might not have found all of these polar bonds, 00:51:54.400 --> 00:51:56.650 but you should have at least seen that this one really 00:51:56.650 --> 00:51:57.950 didn't have many. 00:51:57.950 --> 00:52:00.450 Vitamin A only has one. 00:52:00.450 --> 00:52:03.760 So we have polar bonds down here between carbon and oxygen 00:52:03.760 --> 00:52:08.230 over here, carbon and oxygen, nitrogen, hydrogen, carbon 00:52:08.230 --> 00:52:12.750 oxygen again, nitrogen with hydrogen, oxygen with hydrogen, 00:52:12.750 --> 00:52:15.080 nitrogen with hydrogen over here. 00:52:15.080 --> 00:52:19.950 So folic acid is quite polar, and if we're 00:52:19.950 --> 00:52:22.860 going to think, now, about whether it is a water 00:52:22.860 --> 00:52:26.510 soluble vitamin or a fat soluble vitamin-- which 00:52:26.510 --> 00:52:29.350 is something that a lot of times your supplements 00:52:29.350 --> 00:52:31.900 will tell you about. 00:52:31.900 --> 00:52:34.630 If it's water soluble-- and we'll talk about this more 00:52:34.630 --> 00:52:36.940 later-- like dissolves likes. 00:52:36.940 --> 00:52:39.460 So water likes polar molecules, which 00:52:39.460 --> 00:52:44.150 makes folic acid water soluble and makes vitamin 00:52:44.150 --> 00:52:45.850 A fat soluble. 00:52:45.850 --> 00:52:50.070 It's not very polar, doesn't dissolve very well in water. 00:52:50.070 --> 00:52:53.100 And this kind of turns out to be important, 00:52:53.100 --> 00:52:56.740 in that if you read your vitamin supplements, 00:52:56.740 --> 00:52:59.640 if you take vitamin supplements, it will often tell you 00:52:59.640 --> 00:53:02.950 interesting things like how many hundred times 00:53:02.950 --> 00:53:05.890 over the daily recommended allowance 00:53:05.890 --> 00:53:09.980 this vitamin tablet is, and if things are water soluble, 00:53:09.980 --> 00:53:11.810 it doesn't matter so much that you're 00:53:11.810 --> 00:53:15.290 taking way more than your body actually needs. 00:53:15.290 --> 00:53:19.120 You just have a very expensive pee. 00:53:19.120 --> 00:53:21.547 But if it's fat soluble, then it's 00:53:21.547 --> 00:53:23.630 going to stay in your body, and you don't need it, 00:53:23.630 --> 00:53:25.450 and it can be a little bit toxic. 00:53:25.450 --> 00:53:27.930 So try to think about the vitamins. 00:53:27.930 --> 00:53:30.850 Not everything-- even though vitamins are good for you, 00:53:30.850 --> 00:53:34.150 they're not good in every kind of amount that you could take, 00:53:34.150 --> 00:53:38.680 so you will now use your knowledge of polar bonds 00:53:38.680 --> 00:53:41.510 to figure out whether you should be taking certain vitamin 00:53:41.510 --> 00:53:43.060 supplements.