WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.830 Commons license. 00:00:03.830 --> 00:00:06.850 Your support will help MIT OpenCourseWare continue to 00:00:06.850 --> 00:00:10.510 offer high quality educational resources for free. 00:00:10.510 --> 00:00:13.390 To make a donation or view additional materials from 00:00:13.390 --> 00:00:17.490 hundreds of MIT courses, visit MIT OpenCourseWare at 00:00:17.490 --> 00:00:18.740 ocw.mit.edu. 00:00:21.050 --> 00:00:22.700 PROFESSOR: Settle down. 00:00:22.700 --> 00:00:25.160 Settle down. 00:00:25.160 --> 00:00:27.780 Settle down. 00:00:27.780 --> 00:00:31.890 All right, first announcement is the 00:00:31.890 --> 00:00:32.960 celebration of learning. 00:00:32.960 --> 00:00:36.460 Reminding you that the celebration of learning is a 00:00:36.460 --> 00:00:37.710 week from today. 00:00:40.190 --> 00:00:47.340 Go to your assigned rooms. A through Ha will write in here. 00:00:47.340 --> 00:00:49.530 This group will go into 26-100. 00:00:49.530 --> 00:00:54.060 And the last group over to 4-270. 00:00:54.060 --> 00:00:56.810 And you will take with you your periodic table, table of 00:00:56.810 --> 00:01:02.720 constants, aid sheet, something to write with. 00:01:02.720 --> 00:01:04.430 And we'll give you paper. 00:01:04.430 --> 00:01:07.760 You'll write on the exam paper itself. 00:01:07.760 --> 00:01:12.840 And I'll say more about test taking strategies on Monday. 00:01:12.840 --> 00:01:15.330 Coverage will be right up through Monday. 00:01:15.330 --> 00:01:18.810 With emphasis obviously on the material that you've had some 00:01:18.810 --> 00:01:24.790 time to digest. And there will be no weekly quiz next week. 00:01:24.790 --> 00:01:26.510 So let's get right into the lesson. 00:01:26.510 --> 00:01:30.440 Last day we looked at Louis, who gave us the notion of 00:01:30.440 --> 00:01:34.210 achieving octet stability by electron sharing. 00:01:34.210 --> 00:01:39.790 And that led to the concept of covalent bonds. 00:01:39.790 --> 00:01:42.860 And then Pauling helped us understand the energetics of 00:01:42.860 --> 00:01:47.680 covalent bonding by putting forth the idea in a 00:01:47.680 --> 00:01:49.640 heteronuclear molecule there's unequal 00:01:49.640 --> 00:01:51.330 sharing of the electrons. 00:01:51.330 --> 00:01:55.180 And out of that emerged the concept of polar covalency and 00:01:55.180 --> 00:01:56.930 the definition of electronegativity. 00:01:56.930 --> 00:02:03.180 And on the slide you see how electronegativity varies 00:02:03.180 --> 00:02:04.840 across the periodic table. 00:02:04.840 --> 00:02:08.470 Electronegativity being a measure of the pull an atom 00:02:08.470 --> 00:02:11.840 has for electrons within a covalent bond. 00:02:11.840 --> 00:02:15.000 And as you would expect, the non metals which are good 00:02:15.000 --> 00:02:18.960 electronic receptors are also the ones that have the highest 00:02:18.960 --> 00:02:19.960 electronegativity. 00:02:19.960 --> 00:02:23.300 And the metals, over here, which are good electron donors 00:02:23.300 --> 00:02:26.280 are those that have the the lowest value of 00:02:26.280 --> 00:02:27.430 electronegativity. 00:02:27.430 --> 00:02:30.860 And Pauling was quantitative in his formulation and gave us 00:02:30.860 --> 00:02:35.280 this equation that tells us how to measure the energy of 00:02:35.280 --> 00:02:37.490 the x-y covalent bond. 00:02:37.490 --> 00:02:40.580 If x is not equal to y, then axiomatically this is going to 00:02:40.580 --> 00:02:43.100 be a polar covalent bond. 00:02:43.100 --> 00:02:45.760 And you take the geometric mean of the 00:02:45.760 --> 00:02:46.980 homonuclear bond energies. 00:02:46.980 --> 00:02:50.110 This is x-x bond energy in x two. 00:02:50.110 --> 00:02:52.460 This is the y-y bond energy in y two. 00:02:52.460 --> 00:02:54.840 You take the product square root of which. 00:02:54.840 --> 00:02:58.610 And then the partial ionic character, this is the 00:02:58.610 --> 00:03:00.840 contribution to unequal sharing of the electrons. 00:03:00.840 --> 00:03:03.080 You take the square of the difference in the 00:03:03.080 --> 00:03:05.100 electonegativities of the two elements. 00:03:05.100 --> 00:03:09.350 And the 96.3 is a factor that allows you to get the overall 00:03:09.350 --> 00:03:11.700 quantity in kilojoules per mole. 00:03:11.700 --> 00:03:14.480 So you need these numbers in kilojoules per mole. 00:03:14.480 --> 00:03:18.750 And we further had a formula for the percent ionic 00:03:18.750 --> 00:03:22.370 character, which you can get by looking at the difference 00:03:22.370 --> 00:03:24.030 in electronegativity. 00:03:24.030 --> 00:03:28.690 And then through this formula, you end up with a scale that 00:03:28.690 --> 00:03:32.180 runs from 0 to 100%. 00:03:32.180 --> 00:03:37.090 And just by way of example, we had a look at H-F. 00:03:37.090 --> 00:03:38.670 We spent a fair bit of time on that. 00:03:38.670 --> 00:03:41.240 Obviously, it's a heteronuclear molecule. 00:03:41.240 --> 00:03:43.770 We calculated its bond energy and so on. 00:03:43.770 --> 00:03:46.640 And then to indicate polar covalency, we 00:03:46.640 --> 00:03:48.470 use the dipole notation. 00:03:48.470 --> 00:03:50.160 The dipole shown here. 00:03:50.160 --> 00:03:51.150 It's just an oval. 00:03:51.150 --> 00:03:52.400 It's net neutral. 00:03:52.400 --> 00:03:56.020 But the charge is not uniformly distributed. 00:03:56.020 --> 00:03:58.220 You see one end is a little more negative. 00:03:58.220 --> 00:03:59.920 The other end is a little more positive. 00:03:59.920 --> 00:04:02.890 Sometimes people write lowercase Greek delta, 00:04:02.890 --> 00:04:07.530 indicating little bit negative here, a little bit positive 00:04:07.530 --> 00:04:08.940 here, net neutral. 00:04:08.940 --> 00:04:12.200 There's the arrow indicating the dipole. 00:04:12.200 --> 00:04:17.880 And furthermore, we argue that this is a polar bond. 00:04:20.610 --> 00:04:25.620 And since this is just a molecule with the two atoms, 00:04:25.620 --> 00:04:29.370 then this is also a polar molecule. 00:04:29.370 --> 00:04:33.805 Which means it has a net dipole moment. 00:04:33.805 --> 00:04:37.260 And I made some observations about dipole moments and the 00:04:37.260 --> 00:04:39.790 ability to store energy capacitively. 00:04:39.790 --> 00:04:43.940 And how you'd go about finding a really good capacitor. 00:04:43.940 --> 00:04:48.870 And so that all came out of the desire to find something 00:04:48.870 --> 00:04:50.540 that has a net dipole moment. 00:04:50.540 --> 00:04:53.600 And I want to continue that conversation. 00:04:53.600 --> 00:04:58.570 And so I want to look at some other elements. 00:04:58.570 --> 00:05:02.130 And what I want to look at in particular is 00:05:02.130 --> 00:05:04.150 the compound methane. 00:05:04.150 --> 00:05:08.760 Let's look at methane from this new found appreciation of 00:05:08.760 --> 00:05:09.780 polar covalency. 00:05:09.780 --> 00:05:12.870 So first thing I want to do is to put it's structure up. 00:05:12.870 --> 00:05:15.770 We've gone through the Louis notation and the structure. 00:05:15.770 --> 00:05:17.980 It forms sp3 hybrids. 00:05:17.980 --> 00:05:19.950 And you end up with a structure 00:05:19.950 --> 00:05:20.930 that looks like this. 00:05:20.930 --> 00:05:24.700 These are all at a 109 degrees, symmetrically 00:05:24.700 --> 00:05:26.520 disposed in space. 00:05:26.520 --> 00:05:30.790 And we know since we have a heteronuclear system here, 00:05:30.790 --> 00:05:33.290 we're going to have some polar covalency. 00:05:33.290 --> 00:05:37.360 You can look up that the electronegativity of carbon is 00:05:37.360 --> 00:05:42.945 2.55 electronegativity of hydrogen is less than that 00:05:42.945 --> 00:05:44.810 from its position in the periodic table. 00:05:44.810 --> 00:05:47.140 But to be quantitative it's 2.2. 00:05:47.140 --> 00:05:52.000 So that means if I look at the carbon hydrogen bond, carbon 00:05:52.000 --> 00:05:53.930 has the higher electronegativity. 00:05:53.930 --> 00:05:55.490 So it's going to pull the electrons. 00:05:55.490 --> 00:05:58.180 So that means that the carbon end is going to be a little 00:05:58.180 --> 00:05:59.140 bit negative. 00:05:59.140 --> 00:06:02.540 And the hydrogen end is going to be a little bit positive. 00:06:02.540 --> 00:06:05.500 So that means I've got a dipole moment here. 00:06:05.500 --> 00:06:08.510 Polar bond. 00:06:08.510 --> 00:06:11.110 Everything is the same as above here. 00:06:11.110 --> 00:06:12.200 Polar bond. 00:06:12.200 --> 00:06:15.900 But, I want to address the question, is 00:06:15.900 --> 00:06:17.680 the molecule polar? 00:06:17.680 --> 00:06:23.130 So the way I interpret the question, is it a polar 00:06:23.130 --> 00:06:27.060 molecule, I ask is there a net charge displacement? 00:06:27.060 --> 00:06:29.970 Well, we're off to a good start here. 00:06:29.970 --> 00:06:32.860 We see that we have charge displacement on the bonds. 00:06:32.860 --> 00:06:35.580 But, is there a net dipole for the molecule. 00:06:35.580 --> 00:06:39.090 So the way I think about that is to say, where is the center 00:06:39.090 --> 00:06:41.010 of positive charge for the molecule? 00:06:41.010 --> 00:06:42.500 Where's the center of negative charge? 00:06:42.500 --> 00:06:46.430 So I know all the hydrogens are a little bit positive. 00:06:46.430 --> 00:06:49.480 And they're all the same distance from the nucleus. 00:06:49.480 --> 00:06:52.140 And I can draw a circle that will 00:06:52.140 --> 00:06:54.860 capture all four hydrogens. 00:06:54.860 --> 00:06:57.750 Or more appropriately, it's a sphere, right? 00:06:57.750 --> 00:07:00.300 These three on the bottom don't lie in the plane. 00:07:00.300 --> 00:07:03.880 So the corners of the tetrahedron lie on a sphere. 00:07:03.880 --> 00:07:06.920 So where is the center of positive charge? 00:07:06.920 --> 00:07:09.870 The center positive charge is right here at the center of 00:07:09.870 --> 00:07:11.130 the molecule. 00:07:11.130 --> 00:07:13.900 And where's the center of local negative charge? 00:07:13.900 --> 00:07:17.630 It's on the carbon because the carbon is the negative end of 00:07:17.630 --> 00:07:18.420 all the bonds. 00:07:18.420 --> 00:07:22.290 So the centers of positive and negative charge for the entire 00:07:22.290 --> 00:07:26.560 molecule are colocated at the center of the molecule. 00:07:26.560 --> 00:07:31.750 So this means no net dipole moments. 00:07:31.750 --> 00:07:35.625 No net dipole moment for the molecule. 00:07:39.890 --> 00:07:42.740 So this is a non polar molecule. 00:07:42.740 --> 00:07:47.950 It's a non polar molecule consisting of polar bonds. 00:07:47.950 --> 00:07:50.670 Non polar molecule. 00:07:50.670 --> 00:07:53.770 So there's two ways that I can get a non polar molecule. 00:07:53.770 --> 00:07:57.340 One way is to have a homonuclear molecule, right? 00:07:57.340 --> 00:07:59.890 So, homonuclear molecule. 00:08:02.860 --> 00:08:04.100 Homonuclear molecules 00:08:04.100 --> 00:08:09.250 axiomatically must be non polar. 00:08:09.250 --> 00:08:10.500 Because they have equal sharing. 00:08:13.190 --> 00:08:15.580 So if I give you anything that's homonuclear trivially, 00:08:15.580 --> 00:08:16.830 it's non polar. 00:08:16.830 --> 00:08:24.240 So you can look at things like H2, P4, S8. 00:08:24.240 --> 00:08:25.570 These things are all non polar. 00:08:25.570 --> 00:08:26.730 I don't care what their structures 00:08:26.730 --> 00:08:29.090 are, it doesn't matter. 00:08:29.090 --> 00:08:31.640 This one here is definitely polar. 00:08:31.640 --> 00:08:37.330 And then the last one here that we're looking at, the the 00:08:37.330 --> 00:08:42.800 methane, is non polar because we have spatially symmetric 00:08:42.800 --> 00:08:46.360 disposition of identical polar bonds. 00:08:46.360 --> 00:08:53.210 So spatially symmetric, that means there's three 00:08:53.210 --> 00:08:54.260 dimensional symmetry. 00:08:54.260 --> 00:09:00.670 Spatially symmetric disposition of identical polar 00:09:00.670 --> 00:09:14.450 bonds leads to non polar molecule. 00:09:14.450 --> 00:09:18.340 Because the centers of positive and negative charge 00:09:18.340 --> 00:09:21.290 are colocated. 00:09:21.290 --> 00:09:24.990 So, now it's time to move on. 00:09:24.990 --> 00:09:27.900 Now I want to look at covalent bonding from an energetic 00:09:27.900 --> 00:09:28.430 standpoint. 00:09:28.430 --> 00:09:33.120 I want to go back to energy level diagrams. We've looked 00:09:33.120 --> 00:09:37.240 at energy level diagrams in the past for atoms, single 00:09:37.240 --> 00:09:40.500 atoms. But now, what I want to do is build energy level 00:09:40.500 --> 00:09:42.150 diagrams for molecules. 00:09:42.150 --> 00:09:45.590 So, for that we're going to go back to 00:09:45.590 --> 00:09:47.610 the Schrodinger equation. 00:09:47.610 --> 00:09:53.370 I want energy level diagrams for molecules. 00:09:57.750 --> 00:10:04.660 And for that I'm going to call upon the Schrodinger equation. 00:10:04.660 --> 00:10:07.640 And we're not going to go through the quantum mechanics 00:10:07.640 --> 00:10:09.360 in mathematical detail. 00:10:09.360 --> 00:10:12.490 We're going to do quantum mechanics pictorally. 00:10:12.490 --> 00:10:14.160 We've got a long way. 00:10:14.160 --> 00:10:17.620 And in particular, what I want to do with regard to the 00:10:17.620 --> 00:10:20.890 Schrodinger equation is to recognize that we can move 00:10:20.890 --> 00:10:25.560 from atomic orbitals to molecular orbitals by 00:10:25.560 --> 00:10:29.550 exploiting the fact that the Schrodinger equation is a 00:10:29.550 --> 00:10:31.980 linear equation. 00:10:31.980 --> 00:10:42.160 Using the linearity property of Schrodinger equation. 00:10:42.160 --> 00:10:44.390 All right, what do I mean by linearity? 00:10:44.390 --> 00:10:51.170 Well, I'm going to do just a little bit of math. 00:10:51.170 --> 00:10:55.480 Just enough to make you sit up in your math classes and find 00:10:55.480 --> 00:10:57.660 out that there's some utility there. 00:10:57.660 --> 00:11:00.280 Here's what I mean. 00:11:00.280 --> 00:11:02.670 Let me talk about what the linearity principal is. 00:11:02.670 --> 00:11:07.860 So if I have some equation, f of x, y, and z. 00:11:07.860 --> 00:11:09.790 It's a three variable equation. 00:11:09.790 --> 00:11:13.730 And the equation is f of x, y, z is k1. 00:11:13.730 --> 00:11:16.670 k1 could be a constant, it could be a function, it could 00:11:16.670 --> 00:11:17.630 be anything. 00:11:17.630 --> 00:11:21.040 And let's say that it has as its solution, 00:11:21.040 --> 00:11:23.360 the solution is s1. 00:11:23.360 --> 00:11:28.160 And then I've got a second variant of this x, y, z. 00:11:28.160 --> 00:11:31.140 And it equals k2. 00:11:31.140 --> 00:11:33.450 So it's the same function but it equals k2 here. 00:11:33.450 --> 00:11:35.350 It could be a constant, it could be something else. 00:11:35.350 --> 00:11:37.910 And it has as its solution, s2. 00:11:40.700 --> 00:11:46.230 If this f of x, y, z is a linear equation, then if I'd 00:11:46.230 --> 00:11:53.050 give you f of x, y, z equals k1 plus k2, you don't have to 00:11:53.050 --> 00:11:55.320 go and solve the equation with impunity. 00:11:55.320 --> 00:11:57.920 You can write that the solution is 00:11:57.920 --> 00:12:00.330 equal to s1 plus s2. 00:12:00.330 --> 00:12:03.730 And that doesn't hold if it's a non linear equation. 00:12:03.730 --> 00:12:06.150 Just put y equals x squared. 00:12:06.150 --> 00:12:10.300 And if I tell you y equals one, y equals two, and then I 00:12:10.300 --> 00:12:15.180 give you y equals 1 plus 2, it doesn't equal the sum of the 00:12:15.180 --> 00:12:16.185 solution to that. 00:12:16.185 --> 00:12:17.560 You can prove it to yourself. 00:12:17.560 --> 00:12:22.560 So this is the fact that superposition holds. 00:12:22.560 --> 00:12:26.860 You can superpose solutions and build a library. 00:12:26.860 --> 00:12:31.470 So superposition holds when you have a linear equation. 00:12:31.470 --> 00:12:35.320 And that's all we're going to use in order to make equations 00:12:35.320 --> 00:12:37.810 for the molecular orbital. 00:12:37.810 --> 00:12:41.490 So that way I can write that the wave function of a 00:12:41.490 --> 00:12:44.600 molecular orbital then is a linear combination of the 00:12:44.600 --> 00:12:45.450 atomic orbitals. 00:12:45.450 --> 00:12:46.630 That's all this is doing. 00:12:46.630 --> 00:12:49.670 This is setting the stage for, if you take quantum mechanics 00:12:49.670 --> 00:12:51.440 later, you'll go through this in gory detail. 00:12:51.440 --> 00:12:54.290 But I'm saying that you know enough now to appreciate that 00:12:54.290 --> 00:12:57.350 we can do what we're about to do so. 00:12:57.350 --> 00:13:02.220 That means I'm just going to sum the wave functions of the 00:13:02.220 --> 00:13:05.440 atomic orbitals. 00:13:05.440 --> 00:13:08.250 In this case, i goes from 1 to 2 because there's only two 00:13:08.250 --> 00:13:09.930 orbitals in a bond. 00:13:09.930 --> 00:13:15.350 And there will be some pre factor here, a sub i times ci. 00:13:15.350 --> 00:13:20.000 And this whole business is called linear combination of 00:13:20.000 --> 00:13:22.990 atomic orbitals into molecular orbitals. 00:13:22.990 --> 00:13:26.010 So it's an SLI, it's a six letter initialization. 00:13:26.010 --> 00:13:28.960 You know like FBI is a TLI, it's a three letter 00:13:28.960 --> 00:13:30.350 initialization. 00:13:30.350 --> 00:13:33.400 This is an SLI, in 3L91 we go big. 00:13:33.400 --> 00:13:36.360 This is an SLI, six letter initialization. 00:13:36.360 --> 00:13:38.770 So we're going to do some examples here. 00:13:38.770 --> 00:13:42.590 And all you need to do in order to run the examples is 00:13:42.590 --> 00:13:46.310 use two ideas in LCAO-MO. 00:13:46.310 --> 00:13:49.130 First of all, conservation of states. 00:13:49.130 --> 00:13:52.805 Conservation of orbital states. 00:13:55.540 --> 00:14:01.040 And the second one is, we're going to fill the newly 00:14:01.040 --> 00:14:02.940 created molecular orbitals according 00:14:02.940 --> 00:14:04.730 to the Aufbau principle. 00:14:04.730 --> 00:14:10.620 Fill MOs by Aufbau. 00:14:10.620 --> 00:14:13.230 If we do that, we're in good shape. 00:14:13.230 --> 00:14:14.020 So let's take a look. 00:14:14.020 --> 00:14:15.920 So first think I want to do is just 00:14:15.920 --> 00:14:17.840 rationalize this one here. 00:14:17.840 --> 00:14:21.330 H plus H goes to H2. 00:14:21.330 --> 00:14:23.050 I want to demonstrate that there's a 00:14:23.050 --> 00:14:24.280 rational basis for this. 00:14:24.280 --> 00:14:27.620 So what I'm going to do is make energy level diagrams. So 00:14:27.620 --> 00:14:30.100 there's an energy coordinate here. 00:14:30.100 --> 00:14:32.900 Energy goes up in the vertical direction. 00:14:32.900 --> 00:14:36.770 And out here is zero. 00:14:36.770 --> 00:14:40.460 And what I'm going to draw for you is energy level diagrams 00:14:40.460 --> 00:14:44.120 for two atomic hydrogen gas atoms. 00:14:44.120 --> 00:14:46.330 So if this is the zero, we know that somewhere 00:14:46.330 --> 00:14:49.300 down here is the 1s. 00:14:49.300 --> 00:14:51.820 And I'm going to be complete in my label. 00:14:51.820 --> 00:14:54.430 This is the 1s atomic orbital of hydrogen. 00:14:54.430 --> 00:14:58.570 And over here there's a 1s atomic 00:14:58.570 --> 00:15:02.890 orbital of atomic hydrogen. 00:15:02.890 --> 00:15:08.570 And furthermore, I've got an electron sitting in 1s. 00:15:08.570 --> 00:15:10.850 Now these are at infinite separation. 00:15:10.850 --> 00:15:13.560 These are far apart. 00:15:13.560 --> 00:15:14.840 Far, you know what that means. 00:15:14.840 --> 00:15:17.910 Far with quotation marks means that they're separate quantum 00:15:17.910 --> 00:15:21.050 systems. So I'm not violating the poly exclusion principal 00:15:21.050 --> 00:15:23.840 by having both of these electrons sitting in the 00:15:23.840 --> 00:15:24.530 ground state. 00:15:24.530 --> 00:15:26.540 So they both have the same set of quantum numbers. 00:15:26.540 --> 00:15:28.830 1, 0, 0, 1/2. 00:15:28.830 --> 00:15:29.680 Both of them. 00:15:29.680 --> 00:15:30.720 Same thing. 00:15:30.720 --> 00:15:33.290 Let's put it over here, 1, 0, 0, 1/2. 00:15:33.290 --> 00:15:35.120 So they're very, very far apart. 00:15:35.120 --> 00:15:38.290 Now if I bring them close enough together to make the 00:15:38.290 --> 00:15:43.160 molecule H2, what happens is I'm going to violate the poly 00:15:43.160 --> 00:15:47.410 exclusion principal if I have both of these atomic orbitals 00:15:47.410 --> 00:15:48.250 at the same level. 00:15:48.250 --> 00:15:50.590 Because I start filling them according to Aufbau principal 00:15:50.590 --> 00:15:53.300 I'm going to end up with more electrons than 00:15:53.300 --> 00:15:54.510 two at the same state. 00:15:54.510 --> 00:15:57.960 So what happens is this splits. 00:15:57.960 --> 00:16:01.630 One orbitals ends up at a lower energy and one orbital 00:16:01.630 --> 00:16:05.170 ends up at a higher energy than the ground state energy 00:16:05.170 --> 00:16:07.990 in the atom itself. 00:16:07.990 --> 00:16:12.730 And so now this is called the sigma 1s molecular orbital. 00:16:15.640 --> 00:16:20.300 And the one above it is called sigma star 00:16:20.300 --> 00:16:23.040 1s molecular orbital. 00:16:23.040 --> 00:16:25.110 And sigma is at a lower energy. 00:16:25.110 --> 00:16:28.820 So if electrons populate this orbital, the system's energy 00:16:28.820 --> 00:16:31.370 will decrease and a bond will form. 00:16:31.370 --> 00:16:35.690 So this is called a bonding orbital. 00:16:35.690 --> 00:16:39.390 If electrons populate the upper orbital, they will raise 00:16:39.390 --> 00:16:41.910 the energy of the system, destabilizing it. 00:16:41.910 --> 00:16:45.000 And so this orbital denoted with the star is called 00:16:45.000 --> 00:16:46.500 antibonding. 00:16:46.500 --> 00:16:49.670 It's an antibonding orbital. 00:16:49.670 --> 00:16:52.000 So now I've got the energy level diagram 00:16:52.000 --> 00:16:55.460 for molecular hydrogen. 00:16:55.460 --> 00:16:57.500 So now let's go and populate according 00:16:57.500 --> 00:16:58.520 to the Aufbau principal. 00:16:58.520 --> 00:17:03.280 I've got two electrons and they go in spin up spin down. 00:17:03.280 --> 00:17:07.260 And now you can see that this occupancy put the electrons at 00:17:07.260 --> 00:17:12.060 lower energy than they were by being in the energy state of 00:17:12.060 --> 00:17:13.490 the atoms. 00:17:13.490 --> 00:17:16.460 And so I can argue that by this diagram, I haven't 00:17:16.460 --> 00:17:19.050 predicted anything, but I can use this diagram to 00:17:19.050 --> 00:17:23.980 rationalize that for this reaction delta E is negative. 00:17:23.980 --> 00:17:25.330 And we know how negative it is. 00:17:25.330 --> 00:17:29.260 It's minus 435 kilojoules per mole. 00:17:29.260 --> 00:17:31.620 It's hugely negative. 00:17:31.620 --> 00:17:36.970 Minus 435 kilojoules per mole. 00:17:36.970 --> 00:17:38.150 All right. 00:17:38.150 --> 00:17:40.590 So, that's good. 00:17:40.590 --> 00:17:43.930 In fact I think I've got some pictures. 00:17:43.930 --> 00:17:45.940 You can go through the whole quantum mechanics. 00:17:45.940 --> 00:17:53.950 And just as is the case for a single atoms, you can use the 00:17:53.950 --> 00:17:57.920 product of the wave function and it's complex conjugate and 00:17:57.920 --> 00:18:01.050 so on and make plots. 00:18:01.050 --> 00:18:03.940 Oh by the way, this was just making the point that you can 00:18:03.940 --> 00:18:07.100 pull out the electronegativity off of the periodic table. 00:18:07.100 --> 00:18:08.210 It's given here. 00:18:08.210 --> 00:18:10.540 And the periodic table is pretty good. 00:18:10.540 --> 00:18:13.480 But obviously somebody got a little bit ahead of himself. 00:18:13.480 --> 00:18:16.490 They called this the first ionization potential, which is 00:18:16.490 --> 00:18:18.970 the potential you'd put across the plates in a 00:18:18.970 --> 00:18:20.610 gas discharge tube. 00:18:20.610 --> 00:18:23.390 But the unit is not the electronvolt. 00:18:23.390 --> 00:18:24.900 If it's a potential, it's a volt. 00:18:24.900 --> 00:18:27.470 Somebody here that put this together seemed to think the 00:18:27.470 --> 00:18:29.320 electronvolt is a unit of potential. 00:18:29.320 --> 00:18:30.880 And I want to make sure that nobody in this 00:18:30.880 --> 00:18:34.020 class believes that. 00:18:34.020 --> 00:18:37.260 You got to get a little bit of nail polish or something and 00:18:37.260 --> 00:18:39.790 cover up the little e there. 00:18:39.790 --> 00:18:43.660 Or take the nail polish, paint over potential and write first 00:18:43.660 --> 00:18:44.990 ionization energy. 00:18:44.990 --> 00:18:48.980 One or the other, but not this. 00:18:48.980 --> 00:18:51.230 This was a plot if percent ionic characters. 00:18:51.230 --> 00:18:55.830 You can see that the strongly covalent compounds down here 00:18:55.830 --> 00:18:58.200 have very very low electronegativity differences 00:18:58.200 --> 00:19:00.270 and therefore very low ionic character. 00:19:00.270 --> 00:19:02.470 And way up here are the ionics. 00:19:02.470 --> 00:19:03.970 And in between is HF. 00:19:03.970 --> 00:19:05.860 It's almost at the cusp. 00:19:05.860 --> 00:19:07.980 But, can you see that there must be a 00:19:07.980 --> 00:19:09.160 mistake on this diagram? 00:19:09.160 --> 00:19:11.690 Because this thing has got a greater electronegativity 00:19:11.690 --> 00:19:14.350 difference than lithium iodide and yet lithium iodide has a 00:19:14.350 --> 00:19:16.450 higher percent ionic character. 00:19:16.450 --> 00:19:19.110 How can this function zigzag like that? 00:19:19.110 --> 00:19:20.870 Something's wrong. 00:19:20.870 --> 00:19:24.470 So when you look at something, you go wait a minute, that 00:19:24.470 --> 00:19:25.080 doesn't make sense. 00:19:25.080 --> 00:19:28.930 So I go back and I say, do I trust anything here? 00:19:28.930 --> 00:19:30.250 Trust, but verify. 00:19:30.250 --> 00:19:31.950 Read critically. 00:19:31.950 --> 00:19:32.700 It's a good book. 00:19:32.700 --> 00:19:34.500 But, it's a big book and there's 00:19:34.500 --> 00:19:35.710 going to be some mistakes. 00:19:35.710 --> 00:19:38.130 All right, so here's some pictoral stuff of what we were 00:19:38.130 --> 00:19:39.060 just doing over here. 00:19:39.060 --> 00:19:42.730 So here are the two spherical 1s orbitals and now they come 00:19:42.730 --> 00:19:45.210 closer and closer together and they overlap. 00:19:45.210 --> 00:19:48.970 And this is what the shape of the sigma 1s 00:19:48.970 --> 00:19:50.765 orbital looks like. 00:19:50.765 --> 00:19:54.670 It's like an oval with the two nuclei inside. 00:19:54.670 --> 00:19:56.690 Here's taken from a different text book. 00:19:56.690 --> 00:20:00.560 The overlap of atomic 1s atomic and now here's the 00:20:00.560 --> 00:20:02.240 molecular orbital. 00:20:02.240 --> 00:20:06.660 Now you can also look at what the shape of the antibonding 00:20:06.660 --> 00:20:07.350 orbital would be. 00:20:07.350 --> 00:20:10.030 This is what the shape of the antibonding orbital would be. 00:20:10.030 --> 00:20:12.580 It would have two lobes with a nodal plane in between. 00:20:16.400 --> 00:20:19.510 I think this is taken from yet another book. 00:20:19.510 --> 00:20:20.960 Oh this is our book. 00:20:20.960 --> 00:20:21.340 There we go. 00:20:21.340 --> 00:20:23.850 1s, 1s, as there we go. 00:20:23.850 --> 00:20:25.380 Hydrogen molecular orbitals. 00:20:25.380 --> 00:20:28.160 Now look, suppose we do the same thing for helium. 00:20:28.160 --> 00:20:31.310 If we do the same thing for helium, helium starts with two 00:20:31.310 --> 00:20:32.620 electrons in 1s. 00:20:32.620 --> 00:20:34.610 And now it's going to have four electrons. 00:20:34.610 --> 00:20:37.240 Two will go in the bonding and two will go in the 00:20:37.240 --> 00:20:38.560 antibonding. 00:20:38.560 --> 00:20:41.740 And the two that go in the antibonding raise the energy 00:20:41.740 --> 00:20:45.430 of the system more than the two that go into bonding 00:20:45.430 --> 00:20:46.980 decrease the energy of the system. 00:20:46.980 --> 00:20:48.470 There's a net increase in energy. 00:20:48.470 --> 00:20:52.190 And this is the way you could rationalize that helium exists 00:20:52.190 --> 00:20:55.130 as the atom in the gas phase. 00:20:55.130 --> 00:20:57.860 You don't see He2 gas molecules. 00:20:57.860 --> 00:21:01.230 So using this energy level diagram you can go through an 00:21:01.230 --> 00:21:01.940 rationalize. 00:21:01.940 --> 00:21:03.820 I would never ask you to predict. 00:21:03.820 --> 00:21:11.100 I would say, fact, helium is found as the atomic species in 00:21:11.100 --> 00:21:13.160 the gas phase. 00:21:13.160 --> 00:21:17.710 With the use of energy level diagrams, rationalize. 00:21:17.710 --> 00:21:19.450 And that's what I would expect you to do. 00:21:19.450 --> 00:21:23.930 Take this and show that the two are 00:21:23.930 --> 00:21:26.950 of different stability. 00:21:26.950 --> 00:21:32.160 One other point to to make, the level of 1s. 00:21:32.160 --> 00:21:35.270 If I wanted to put helium on this board here. 00:21:35.270 --> 00:21:37.560 Not the scale, but just roughly. 00:21:37.560 --> 00:21:42.330 Where would helium 1s be relative the hydrogen 1s. 00:21:42.330 --> 00:21:43.760 We got three choices. 00:21:43.760 --> 00:21:51.480 Same level, closer to zero energy, or more negative. 00:21:51.480 --> 00:21:52.880 How do you think about the problem? 00:21:52.880 --> 00:21:56.280 What determines what this energy is? 00:21:56.280 --> 00:21:58.820 It's the electrostatic force of attraction. 00:21:58.820 --> 00:22:01.640 Is the electrostatic force of attraction on the 1s electron 00:22:01.640 --> 00:22:07.490 in helium greater than or less than it is in hydrogen? 00:22:07.490 --> 00:22:08.230 It's greater. 00:22:08.230 --> 00:22:11.570 So that means that the energy is going to be more negative. 00:22:11.570 --> 00:22:15.170 And so if I were to put over here helium, I'd put down here 00:22:15.170 --> 00:22:19.510 this would be 1s atomic orbital for helium and then we 00:22:19.510 --> 00:22:21.090 go through the analysis. 00:22:21.090 --> 00:22:22.410 And why does that come into play? 00:22:22.410 --> 00:22:25.450 Well you might want to do a heteronuclear molecule. 00:22:25.450 --> 00:22:28.030 Suppose you wanted to do the bonding diagram 00:22:28.030 --> 00:22:29.530 for hydrogen fluoride. 00:22:29.530 --> 00:22:32.390 So we'd have hydrogen here and the fluorine 00:22:32.390 --> 00:22:33.820 and would be here. 00:22:33.820 --> 00:22:36.410 And all the fluorine orbitals would be much lower and then 00:22:36.410 --> 00:22:39.760 they'd combine to make the molecular orbitals. 00:22:39.760 --> 00:22:42.710 And I think there's something opportunities to practice that 00:22:42.710 --> 00:22:44.980 in the homework. 00:22:44.980 --> 00:22:46.410 OK let's do one more. 00:22:46.410 --> 00:22:47.280 Let's do one more. 00:22:47.280 --> 00:22:49.940 How about lithium. 00:22:49.940 --> 00:22:50.800 Let's do lithium. 00:22:50.800 --> 00:22:54.985 I want to ask, is dilithium stable? 00:22:58.220 --> 00:22:58.780 Li2. 00:22:58.780 --> 00:23:01.040 And this is all gas phase. 00:23:01.040 --> 00:23:02.210 Is dilithium stable? 00:23:02.210 --> 00:23:05.982 So I'll start off with here's the zeroes. 00:23:05.982 --> 00:23:10.360 The zero of energy for infinite separation. 00:23:10.360 --> 00:23:15.330 So this'll be a lithium gas atom. 00:23:15.330 --> 00:23:18.610 And this is the putative dilithium gas atom. 00:23:18.610 --> 00:23:20.280 We're going to figure out if it's stable or not. 00:23:20.280 --> 00:23:23.870 So it's going to have bonding and antibonding orbitals. 00:23:23.870 --> 00:23:29.690 And then this is the 1s, 2s. 00:23:29.690 --> 00:23:37.130 And this will be sigma 1s, sigma star 1s, and so on. 00:23:37.130 --> 00:23:44.660 And then we'll have 2s atomic orbital splitting into sigma 00:23:44.660 --> 00:23:48.030 and sigma star of 2s. 00:23:48.030 --> 00:23:52.000 And now lithium is 1s2, 2s1. 00:23:52.000 --> 00:23:54.640 So there's one, two, three. 00:23:54.640 --> 00:23:57.270 And one, two, three. 00:23:57.270 --> 00:23:59.260 And so now let's use the Hund rule. 00:23:59.260 --> 00:24:02.900 So I've got four electrons in the n equals one shell, and 00:24:02.900 --> 00:24:05.000 they populate in this manner. 00:24:05.000 --> 00:24:08.600 And then I've got two electrons that go only into 00:24:08.600 --> 00:24:09.620 the bonding orbital. 00:24:09.620 --> 00:24:13.700 And according to this it appears that lithium 2 is 00:24:13.700 --> 00:24:16.020 favored over atomic lithium. 00:24:16.020 --> 00:24:17.930 And that in fact is the case. 00:24:17.930 --> 00:24:21.960 That in fact is the case and so that applies to all of the 00:24:21.960 --> 00:24:25.080 all of the alkalide metals. 00:24:25.080 --> 00:24:29.010 Because they all have the ns1 configuration. 00:24:29.010 --> 00:24:32.780 So when you're driving down the highway and you see those 00:24:32.780 --> 00:24:36.390 orangey yellow low pressure sodium vapor lamps. 00:24:36.390 --> 00:24:39.070 What you're looking at is emission not from atomic 00:24:39.070 --> 00:24:42.560 sodium but from Na2 vapor. 00:24:42.560 --> 00:24:45.210 And you've got the energy level diagram to convince 00:24:45.210 --> 00:24:46.720 yourself of that. 00:24:46.720 --> 00:24:50.680 So next time you see those, just smile, knowing that 00:24:50.680 --> 00:24:54.850 you're looking at disodium, not sodium. 00:24:54.850 --> 00:24:57.290 OK, well so far we've only looked at single bonds. 00:24:57.290 --> 00:24:59.820 Now I want to look at multiple bonds. 00:24:59.820 --> 00:25:00.820 Double and triple bonds. 00:25:00.820 --> 00:25:03.210 So let's look at nitrogen now. 00:25:03.210 --> 00:25:03.730 N2. 00:25:03.730 --> 00:25:06.440 Remember that gave us the the triple bond? 00:25:06.440 --> 00:25:10.340 We had nitrogen, one, two, three, four, five. 00:25:10.340 --> 00:25:13.890 Second nitrogen, one, two, three, four, five. 00:25:13.890 --> 00:25:16.880 So in order to get octet stability, we had three pairs 00:25:16.880 --> 00:25:17.990 of electronic sharing. 00:25:17.990 --> 00:25:21.430 Which then give us a triple bond here. 00:25:21.430 --> 00:25:25.680 And so what's that going to look like? 00:25:25.680 --> 00:25:29.670 what's that going to look like pictorally? 00:25:29.670 --> 00:25:32.600 And the way to think about that is first of all these are 00:25:32.600 --> 00:25:34.940 all p orbitals. 00:25:34.940 --> 00:25:40.520 If we look at, this is 2s2, 2p3. 00:25:40.520 --> 00:25:44.420 So 2s and these are the 2d orbitals. 00:25:44.420 --> 00:25:48.740 So I've got s is filled first and then I've got the lone 00:25:48.740 --> 00:25:50.930 electrons in each of the p orbitals. 00:25:50.930 --> 00:25:54.050 And these things are shaped like figure eights. 00:25:57.410 --> 00:26:02.630 And the p orbital, this is the p atomic orbital. 00:26:02.630 --> 00:26:04.830 It has two lobes. 00:26:04.830 --> 00:26:07.020 Each of these zones are just called lobes. 00:26:07.020 --> 00:26:09.070 Same word is earlobe. 00:26:09.070 --> 00:26:14.460 And this zone in the middle, this one point in the middle, 00:26:14.460 --> 00:26:15.710 is called a node. 00:26:18.650 --> 00:26:21.990 And that's a sight of zero electron density. 00:26:21.990 --> 00:26:24.980 Zero electronic density. 00:26:24.980 --> 00:26:27.290 And remember the electron, even if it only has one 00:26:27.290 --> 00:26:30.100 electron here, the electron can be in either a lobe. 00:26:30.100 --> 00:26:32.990 It can move from lobe to lobe even though it can never be at 00:26:32.990 --> 00:26:33.660 the nucleus. 00:26:33.660 --> 00:26:36.850 And how does it get from one lobe to the other while never 00:26:36.850 --> 00:26:39.110 crossing the nucleus because it's never supposed to be in 00:26:39.110 --> 00:26:40.380 the nucleus? 00:26:40.380 --> 00:26:43.630 By behaving as a wave. The same way that you can have a 00:26:43.630 --> 00:26:47.420 jump rope and you can have a fixed point of zero motion, 00:26:47.420 --> 00:26:52.530 yet you can transmit energy down the rope passed the node. 00:26:52.530 --> 00:26:54.730 So, let's draw these things. 00:26:58.150 --> 00:27:00.570 A word about how you draw, you have to use 00:27:00.570 --> 00:27:02.140 the right hand rule. 00:27:02.140 --> 00:27:03.320 Have to use the right hand rule. 00:27:03.320 --> 00:27:06.830 So when I put the coordinate system up, it's going to have 00:27:06.830 --> 00:27:10.630 to conform so that the thumb is x and they go y and z. 00:27:10.630 --> 00:27:13.000 And if you don't use the right hand rule later on if you get 00:27:13.000 --> 00:27:16.740 into electromagnetics, you start looking at forces and 00:27:16.740 --> 00:27:18.920 vectors, you're going to end up with things moving in the 00:27:18.920 --> 00:27:19.650 wrong direction. 00:27:19.650 --> 00:27:21.540 So we conform to the right hand rule. 00:27:21.540 --> 00:27:22.690 And the other thing, and it's kind of 00:27:22.690 --> 00:27:24.260 nice, it's not mandatory. 00:27:24.260 --> 00:27:28.150 But chemists generally like to have atoms 00:27:28.150 --> 00:27:30.110 bond along the z-axis. 00:27:30.110 --> 00:27:33.425 So that means we start with the z-axis here, if I'm going 00:27:33.425 --> 00:27:36.650 to put my second nitrogen, have them bond on the z-axis. 00:27:36.650 --> 00:27:40.580 And so that means if z is in the plane of the board 00:27:40.580 --> 00:27:44.340 pointing to the right, then pointing up must be y, and 00:27:44.340 --> 00:27:46.830 then pointing into the board must be x. 00:27:46.830 --> 00:27:51.930 So I'll have, here's my px orbital, then the py orbital, 00:27:51.930 --> 00:27:53.710 and the pz orbital. 00:27:53.710 --> 00:27:57.740 And the three of these are all symmetrically disposed around 00:27:57.740 --> 00:27:59.990 the nucleus. 00:27:59.990 --> 00:28:02.540 And next to it at the same kind of floral arrangement. 00:28:02.540 --> 00:28:09.070 I'll start with px, py, and pz. 00:28:09.070 --> 00:28:10.450 And now these are going to come close 00:28:10.450 --> 00:28:12.360 together and overlap. 00:28:12.360 --> 00:28:14.590 So I want to figure out what those orbitals are 00:28:14.590 --> 00:28:15.670 going to look like. 00:28:15.670 --> 00:28:17.970 So let's start along the z-axis. 00:28:17.970 --> 00:28:19.910 The z-axis is the easy one. 00:28:19.910 --> 00:28:23.130 So I've got two of these things lying on their sides 00:28:23.130 --> 00:28:24.740 like infinity signs. 00:28:24.740 --> 00:28:26.970 We're going to do quantum mechanics pictorally. 00:28:26.970 --> 00:28:27.700 Why? 00:28:27.700 --> 00:28:30.420 Because it's a linear equation. 00:28:30.420 --> 00:28:33.300 So I can add pictural. 00:28:33.300 --> 00:28:36.140 So this is 2pz of one of them. 00:28:36.140 --> 00:28:38.340 And a 2pz of the other. 00:28:38.340 --> 00:28:42.410 And this is the nitrogen atomic orbital. 00:28:45.190 --> 00:28:46.780 And I'm going to smear these things and what are they going 00:28:46.780 --> 00:28:47.930 to look like? 00:28:47.930 --> 00:28:51.510 The nucleus is here, the nucleus is where I'm 00:28:51.510 --> 00:28:52.790 indicating the dot. 00:28:52.790 --> 00:28:55.580 These two combine, very simply, it's going 00:28:55.580 --> 00:28:56.830 to look like this. 00:28:59.650 --> 00:29:01.360 So there's one nitrogen. 00:29:01.360 --> 00:29:03.010 Here's the other nitrogen. 00:29:03.010 --> 00:29:04.780 And what's this thing? 00:29:04.780 --> 00:29:06.570 This is electronic density. 00:29:06.570 --> 00:29:11.550 And so this is our sigma bond. 00:29:11.550 --> 00:29:15.820 Looks a little bit different from the case of of hydrogen, 00:29:15.820 --> 00:29:19.330 because hydrogen was the blending of two s orbitals and 00:29:19.330 --> 00:29:21.280 s orbitals are spherically symmetric. 00:29:21.280 --> 00:29:23.620 In this case, we've got two lobes. 00:29:23.620 --> 00:29:27.500 But what's characteristic about this one and hydrogen, 00:29:27.500 --> 00:29:29.930 hydrogen looked like this, remember? 00:29:29.930 --> 00:29:31.020 This was hydrogen. 00:29:31.020 --> 00:29:32.570 This was H2. 00:29:32.570 --> 00:29:35.080 And this was a sigma bond. 00:29:35.080 --> 00:29:39.780 The characteristic of a sigma bond is that when you start 00:29:39.780 --> 00:29:44.020 from one nucleus and you go to the other nucleus, you move 00:29:44.020 --> 00:29:47.240 through unbroken electron density. 00:29:47.240 --> 00:29:51.420 So there are no nodes, no holidays, between the nitrogen 00:29:51.420 --> 00:29:54.940 nucleus on the left and the nitrogen nucleus on the right. 00:29:54.940 --> 00:29:57.460 There is a node here, but that's different. 00:29:57.460 --> 00:30:00.240 I can go from one nitrogen to the other with unbroken 00:30:00.240 --> 00:30:01.170 electron density. 00:30:01.170 --> 00:30:02.680 That's what makes this a sigma bond. 00:30:08.790 --> 00:30:09.760 So that's good. 00:30:09.760 --> 00:30:12.390 So this thing here is going to be called stigma 00:30:12.390 --> 00:30:15.710 2p molecular orbital. 00:30:15.710 --> 00:30:21.370 Sigma 2p molecular orbital And I think I've got the slide 00:30:21.370 --> 00:30:22.230 that shows this. 00:30:22.230 --> 00:30:23.780 There's some art work. 00:30:23.780 --> 00:30:27.710 People really get excited about this. 00:30:27.710 --> 00:30:28.960 OK there's dilithium. 00:30:28.960 --> 00:30:31.060 Or dipotasium, disodium. 00:30:31.060 --> 00:30:31.920 OK so here we are. 00:30:31.920 --> 00:30:37.790 This is the smearing of two pz atomic orbitals to make the 00:30:37.790 --> 00:30:41.080 sigma 2p bonding orbital. 00:30:41.080 --> 00:30:43.020 And there just for grins and chuckles is what the 00:30:43.020 --> 00:30:44.200 antibonding would look like. 00:30:44.200 --> 00:30:48.640 But we don't care, because it doesn't form. 00:30:48.640 --> 00:30:49.980 Well this is the book. 00:30:49.980 --> 00:30:50.980 And you know what I'm going to say. 00:30:50.980 --> 00:30:52.350 I've got my little hobby horse here. 00:30:52.350 --> 00:30:55.810 I don't know why they change color on the lobes. 00:30:55.810 --> 00:30:58.680 Because when I look at that, it starts conjuring up to me 00:30:58.680 --> 00:31:01.900 the image that one electron stays in the blue lobe and one 00:31:01.900 --> 00:31:03.730 electron stays in the yellow lobe. 00:31:03.730 --> 00:31:06.190 Besides, I've seen them and they're not different colors. 00:31:06.190 --> 00:31:07.440 They're the same color. 00:31:13.830 --> 00:31:17.800 So now let's look at what happens when we blend off of 00:31:17.800 --> 00:31:18.420 the z-axis. 00:31:18.420 --> 00:31:22.300 So let's blend the two py orbitals. 00:31:22.300 --> 00:31:24.660 See what that goes like. 00:31:24.660 --> 00:31:26.460 OK so let's do that one. 00:31:26.460 --> 00:31:28.450 And that one is going to look like this. 00:31:28.450 --> 00:31:32.650 We're to start with, again figure eights. 00:31:32.650 --> 00:31:35.140 But now they're their side by side, they're lateral. 00:31:35.140 --> 00:31:38.300 So this is 2py atomic orbital. 00:31:38.300 --> 00:31:40.190 2py atomic orbital. 00:31:40.190 --> 00:31:45.350 And then we're going to blend them along the z-axis to give 00:31:45.350 --> 00:31:48.680 us, and I'm going to do this stylized, OK? 00:31:48.680 --> 00:31:52.590 So there's the the two nitrogen nuclei. 00:31:52.590 --> 00:31:54.340 So I put the nuclei up. 00:31:54.340 --> 00:31:56.880 And I'm going to smear the upper lobes. 00:31:56.880 --> 00:31:58.850 So these two lobes are going to smear. 00:31:58.850 --> 00:32:00.690 And I'm going to get really stylized. 00:32:00.690 --> 00:32:04.990 I feel like it's France and it's the late 1800s. 00:32:04.990 --> 00:32:06.430 So there it is. 00:32:06.430 --> 00:32:08.720 And I'm going to smear the two bottom ones. 00:32:08.720 --> 00:32:12.120 And it's going to look like this. 00:32:12.120 --> 00:32:13.820 So what do I have here? 00:32:13.820 --> 00:32:20.660 Now I have two lobes as before. 00:32:20.660 --> 00:32:25.340 But if I look at the second nucleus from the first 00:32:25.340 --> 00:32:30.480 nucleus, not only do I fail to have unbroken electron 00:32:30.480 --> 00:32:32.990 density, I have zero electron density. 00:32:32.990 --> 00:32:35.880 See, this was a nodal point in the atom. 00:32:35.880 --> 00:32:39.170 With the two atoms together, the plane orthogonal to the 00:32:39.170 --> 00:32:40.740 board is a nodal plane. 00:32:40.740 --> 00:32:43.370 There's no electron density in the plane 00:32:43.370 --> 00:32:45.800 orthogonal to the board. 00:32:45.800 --> 00:32:47.500 Nodal plane. 00:32:47.500 --> 00:32:52.630 So this is definitely not a sigma bond, this is a pi bond. 00:32:52.630 --> 00:32:53.860 This is a pi bond. 00:32:53.860 --> 00:32:57.160 And it's characterized by smearing of atomic orbitals, 00:32:57.160 --> 00:32:58.740 just as the sigma bond is. 00:32:58.740 --> 00:33:02.290 But it has a nodal plane that separates the two lobes. 00:33:06.080 --> 00:33:10.590 I think I've got some cartoon illustrations from other books 00:33:10.590 --> 00:33:13.440 Here OK this is from one book. 00:33:13.440 --> 00:33:15.030 This is good. 00:33:15.030 --> 00:33:17.460 They call the px, I call it py. 00:33:17.460 --> 00:33:20.270 There it is. 00:33:20.270 --> 00:33:23.200 And I'd go further and I'd say that if I were to slice this 00:33:23.200 --> 00:33:28.610 and look at it from angle, if we were to cut this and look 00:33:28.610 --> 00:33:31.470 from here, I'd venture that you'd see something that's 00:33:31.470 --> 00:33:32.720 sort of figure eightish. 00:33:35.730 --> 00:33:40.870 Oh here's our book, bless them with two colors. 00:33:40.870 --> 00:33:43.370 But anyway, there's what it looks like. 00:33:43.370 --> 00:33:44.620 That's the pie. 00:33:47.630 --> 00:33:50.040 And then the same thing happens with the x. 00:33:50.040 --> 00:33:53.426 So this is this is going to be pi 2py. 00:33:56.040 --> 00:33:58.830 This is pi 2py. 00:33:58.830 --> 00:34:00.480 And it's a molecular orbital. 00:34:00.480 --> 00:34:02.000 And there's going to be a pi 2px. 00:34:02.000 --> 00:34:05.120 And it's going to blend front and back. 00:34:05.120 --> 00:34:07.320 So we can make a catalog. 00:34:07.320 --> 00:34:09.710 So we're going to do quantum mechanics in pictures here. 00:34:09.710 --> 00:34:15.510 So I know that s plus s must always make a sigma bond. 00:34:15.510 --> 00:34:16.370 There's no other way. 00:34:16.370 --> 00:34:19.570 Because I've got electron density all around. 00:34:19.570 --> 00:34:20.820 Let's do it pictorally. 00:34:26.230 --> 00:34:27.020 That's easy. 00:34:27.020 --> 00:34:28.610 And this is sigma. 00:34:28.610 --> 00:34:30.650 We know this has to be sigma. 00:34:30.650 --> 00:34:37.290 What about something like HF where the H is an s and the F 00:34:37.290 --> 00:34:39.660 is going to have the one last electron 00:34:39.660 --> 00:34:41.040 missing in the p orbital. 00:34:41.040 --> 00:34:45.270 S plus p must give sigma always. 00:34:45.270 --> 00:34:48.720 Because that's this cartoon. 00:34:48.720 --> 00:34:52.080 See there's no way that when this smears with this, there's 00:34:52.080 --> 00:34:56.360 going to be zero electron holidays from the hydrogen 00:34:56.360 --> 00:34:59.060 nucleus to the fluorine nucleus. 00:34:59.060 --> 00:35:00.060 So you're going to end up with something 00:35:00.060 --> 00:35:01.290 that looks like this. 00:35:01.290 --> 00:35:03.860 It starts around and gives you something that's going to be a 00:35:03.860 --> 00:35:06.070 little bit asymmetric. 00:35:06.070 --> 00:35:07.810 So this will also be a sigma. 00:35:07.810 --> 00:35:11.970 So I'll just put HF here as sort of prototypical of that. 00:35:11.970 --> 00:35:22.350 And then if I take p plus p axially, on axis, that also 00:35:22.350 --> 00:35:23.600 gives us a sigma. 00:35:26.520 --> 00:35:29.790 Because that's this one, the infinity signs. 00:35:29.790 --> 00:35:31.630 Two infinity signs give us this. 00:35:35.110 --> 00:35:41.335 And then finally, if we get p plus p longitudinally. 00:35:45.580 --> 00:35:50.710 So that will give us a pi bond and that's the 88. 00:35:50.710 --> 00:35:55.730 8 plus 8 gives me, and this is pi. 00:35:55.730 --> 00:35:57.730 So that's quantum mechanics. 00:35:57.730 --> 00:36:00.790 The math will follow. 00:36:00.790 --> 00:36:04.600 So now what I want to do is go back to this energy level 00:36:04.600 --> 00:36:06.530 diagram and show how these energy level 00:36:06.530 --> 00:36:09.290 diagrams can work. 00:36:09.290 --> 00:36:12.810 So here's the energy level diagram for nitrogen, N2. 00:36:12.810 --> 00:36:17.760 There's two f's, two p, and the scaffolding is in place. 00:36:17.760 --> 00:36:19.120 There's the energy levels. 00:36:19.120 --> 00:36:24.110 Now here's the molecular orbitals and here are the 00:36:24.110 --> 00:36:25.830 atomic orbitals. 00:36:25.830 --> 00:36:27.390 And now here they are occupied. 00:36:27.390 --> 00:36:30.560 So nitrogen has one, two, three, four, five according to 00:36:30.560 --> 00:36:31.590 the Hund rule. 00:36:31.590 --> 00:36:35.100 And here is the set up for the N2 molecules. 00:36:35.100 --> 00:36:39.200 So the 2s's and the 2s's go bonding antibonding. 00:36:39.200 --> 00:36:41.420 Now I've got three plus three is six. 00:36:41.420 --> 00:36:43.650 Two, two, two. 00:36:43.650 --> 00:36:45.230 Everything's paired. 00:36:45.230 --> 00:36:49.480 So we get the triple bond, 946 kilojoules per mole. 00:36:49.480 --> 00:36:51.730 Enormous energy in nitrogen. 00:36:51.730 --> 00:36:54.120 Enormous energy in nitrogen. 00:36:54.120 --> 00:36:55.890 Now, let's keep going. 00:36:55.890 --> 00:36:57.270 Now let's look at oxygen. 00:36:57.270 --> 00:36:59.820 This is the scaffolding for oxygen and fluorine. 00:36:59.820 --> 00:37:01.700 And there's a little change here. 00:37:01.700 --> 00:37:03.780 A little change, I'm going to draw your attention to it. 00:37:03.780 --> 00:37:05.050 And you can't predict this. 00:37:05.050 --> 00:37:06.030 We would give you this. 00:37:06.030 --> 00:37:10.220 I would tell you what the energy sequence is of the 00:37:10.220 --> 00:37:11.050 energy levels. 00:37:11.050 --> 00:37:16.260 But look here carefully, you see in the case all nitrogen, 00:37:16.260 --> 00:37:18.540 the pi's lie below the sigma. 00:37:18.540 --> 00:37:21.580 In the case of oxygen and fluorine the sigma 00:37:21.580 --> 00:37:22.730 lies below the pi's. 00:37:22.730 --> 00:37:24.850 These are tiny, tiny differences. 00:37:24.850 --> 00:37:28.220 But, they are measurable. 00:37:28.220 --> 00:37:29.270 That's the little difference. 00:37:29.270 --> 00:37:30.520 Now let's see what happens. 00:37:30.520 --> 00:37:33.690 Actually here's from out text book and it shows the stigma 00:37:33.690 --> 00:37:39.380 2pix slowly meandering down, down, down, down, down. 00:37:39.380 --> 00:37:44.540 And somewhere between nitrogen and oxygen it crisscrosses. 00:37:44.540 --> 00:37:46.600 All right, so now let's fill oxygen. 00:37:46.600 --> 00:37:50.460 So oxygen is two, four, five, six. 00:37:50.460 --> 00:37:53.730 So two plus two is four. 00:37:53.730 --> 00:37:56.540 There's the two, two, two. 00:37:56.540 --> 00:37:59.790 And then these last ones go up into the antibonding. 00:37:59.790 --> 00:38:02.220 But look at the antibonding. 00:38:02.220 --> 00:38:06.510 We have, according to the Hund rule, not two electrons in the 00:38:06.510 --> 00:38:09.330 first orbital, but one and one. 00:38:09.330 --> 00:38:12.070 So we end up with unpaired electrons in 00:38:12.070 --> 00:38:13.790 the antibonding orbital. 00:38:13.790 --> 00:38:17.570 So these offset the three pairs here, and so we end up 00:38:17.570 --> 00:38:18.910 with a double bond. 00:38:18.910 --> 00:38:22.210 And its energy us 498 kilojoules per mole. 00:38:22.210 --> 00:38:25.460 Substantially less then nitrogen. 00:38:25.460 --> 00:38:30.190 And this energy level diagram rationalizes that. 00:38:30.190 --> 00:38:31.440 And then here's for fluorine. 00:38:31.440 --> 00:38:33.960 If you go to fluorine it's the same scaffolding only there's 00:38:33.960 --> 00:38:34.890 two more electrons. 00:38:34.890 --> 00:38:37.070 These are paired and you have a single bond. 00:38:37.070 --> 00:38:39.960 160 kilojoules per mole. 00:38:39.960 --> 00:38:43.470 Now there's a property that we get from the fact that we have 00:38:43.470 --> 00:38:44.700 these unpaired electrons. 00:38:44.700 --> 00:38:46.490 We're going back oxygen now. 00:38:46.490 --> 00:38:48.950 We have unpaired electrons in the antibonding orbitals. 00:38:48.950 --> 00:38:50.770 We saw unpaired electrons. 00:38:50.770 --> 00:38:53.350 What did they do in the Stern-Gerlach experiment? 00:38:53.350 --> 00:38:55.960 Changed the magnetics substantially, right? 00:38:55.960 --> 00:39:01.370 We ended up with the splitting of the silver bean. 00:39:01.370 --> 00:39:03.610 So this will also have an impact. 00:39:03.610 --> 00:39:06.190 An impact known as a paramagnetism. 00:39:06.190 --> 00:39:07.320 What's paramagnetism? 00:39:07.320 --> 00:39:09.110 It's shown in this little cartoon. 00:39:09.110 --> 00:39:13.950 If you have a substance here that's balanced and it is 00:39:13.950 --> 00:39:18.380 paramagnetic, if you engage the magnetic field, the 00:39:18.380 --> 00:39:22.480 magnetic field will pull on a substance that's paramagnetic. 00:39:22.480 --> 00:39:24.920 And oxygen is paramagnetic. 00:39:24.920 --> 00:39:28.410 Not only in a gas state, but paramagnetic as liquid. 00:39:28.410 --> 00:39:30.520 And here's an illustration from your text book. 00:39:30.520 --> 00:39:32.870 You may have seen this and said, yeah a guy is pouring 00:39:32.870 --> 00:39:35.170 liquid oxygen, wow. 00:39:35.170 --> 00:39:37.720 Look carefully now. 00:39:37.720 --> 00:39:40.410 The boiling point of oxygen at one atmosphere 00:39:40.410 --> 00:39:42.550 pressure is 90 Kelvin. 00:39:42.550 --> 00:39:45.030 So room temperature is substantially higher. 00:39:45.030 --> 00:39:49.930 This is the equivalent of taking water and putting it in 00:39:49.930 --> 00:39:53.750 an oven at about 300 degrees celsius and watching it pour. 00:39:53.750 --> 00:39:55.770 It would still be liquid, but it would be 00:39:55.770 --> 00:39:57.020 liquid trying to boil. 00:39:57.020 --> 00:39:58.910 Why doesn't it all turned to gas? 00:39:58.910 --> 00:40:02.220 Because there's a time to heat everything. 00:40:02.220 --> 00:40:06.180 So this is sitting at 90 Kelvin, he pouring it down, or 00:40:06.180 --> 00:40:09.220 she's pouring it down, and these are the jaw of a 00:40:09.220 --> 00:40:10.870 permanent magnet. 00:40:10.870 --> 00:40:12.920 And it's in a gravity field, it doesn't 00:40:12.920 --> 00:40:14.940 keep falling, it stops. 00:40:14.940 --> 00:40:18.400 And it continues to boil away and as fast as you can pour 00:40:18.400 --> 00:40:21.310 it, it sits between the jaws of the magnet. 00:40:21.310 --> 00:40:22.660 Because it's paramagnetic. 00:40:22.660 --> 00:40:24.300 You can think of this as the liquid 00:40:24.300 --> 00:40:27.090 equivalent of iron filings. 00:40:27.090 --> 00:40:30.020 If I told you that were these were iron filings you'd say, 00:40:30.020 --> 00:40:32.400 yeah the iron filings go and they stick to the magnet, 00:40:32.400 --> 00:40:34.670 that's what magnets do to iron filings. 00:40:34.670 --> 00:40:36.250 They do the same thing to liquid oxygen. 00:40:39.520 --> 00:40:40.290 And why? 00:40:40.290 --> 00:40:41.700 Because of this. 00:40:41.700 --> 00:40:44.640 This explains this. 00:40:44.640 --> 00:40:50.870 So we can do a lot with these primitive little diagrams. 00:40:50.870 --> 00:40:51.950 Let's do one more. 00:40:51.950 --> 00:40:54.436 I want to do one more of these things. 00:40:54.436 --> 00:40:58.010 I want to go to hybridized systems. I want to go to a 00:40:58.010 --> 00:40:59.010 hybridized system. 00:40:59.010 --> 00:41:03.560 And I want to look at ethylene. 00:41:03.560 --> 00:41:08.740 C2H4 is ethylene. 00:41:08.740 --> 00:41:12.250 OK, so if I told you give me the Louis structure of 00:41:12.250 --> 00:41:15.500 ethylene, you just start going according to the rule. 00:41:15.500 --> 00:41:22.090 So I'm going to have carbon, carbon, hydrogen, hydrogen. 00:41:22.090 --> 00:41:27.680 And carbon has one, two, three, four electrons. 00:41:27.680 --> 00:41:30.610 The hydrogen has one electron. 00:41:30.610 --> 00:41:32.890 I'll put the one electron from hydrogen. 00:41:32.890 --> 00:41:34.870 And then the other carbon over here, I'm 00:41:34.870 --> 00:41:36.630 going to give it dots. 00:41:36.630 --> 00:41:38.470 One, two, three, four. 00:41:38.470 --> 00:41:41.000 And then these hydrogen each have one. 00:41:41.000 --> 00:41:42.770 And now let's see what I have here. 00:41:42.770 --> 00:41:46.230 These hydrogens are all isoelectronic with helium, so 00:41:46.230 --> 00:41:47.210 they're happy. 00:41:47.210 --> 00:41:52.340 And the carbon now has two, let's see, what does he got? 00:41:52.340 --> 00:41:53.410 Yeah, carbon is happy. 00:41:53.410 --> 00:41:55.190 Two, four, six, OK good. 00:41:55.190 --> 00:41:56.070 So now carbon is. 00:41:56.070 --> 00:41:57.350 Happy as well. 00:41:57.350 --> 00:41:58.580 So what do we have here? 00:41:58.580 --> 00:42:02.906 This is the equivalent to a carbon carbon double bond. 00:42:06.320 --> 00:42:10.510 So what have I learned with the nitrogen example? 00:42:10.510 --> 00:42:13.900 What I learned with the nitrogen example was, that if 00:42:13.900 --> 00:42:17.650 I want to make more than one bond, I have to have a 00:42:17.650 --> 00:42:20.210 combination of a stigma and a pi. 00:42:20.210 --> 00:42:23.210 You can only make one sigma bond because there's no room. 00:42:23.210 --> 00:42:27.050 Once you've got zero electron density between the two nuclei 00:42:27.050 --> 00:42:29.430 you're going to violate the Pauli exclusion principle if 00:42:29.430 --> 00:42:31.140 you have another orbital cutting 00:42:31.140 --> 00:42:32.800 through that same domain. 00:42:32.800 --> 00:42:39.200 So this means that multiple bonds require a mix 00:42:39.200 --> 00:42:42.080 of sigma and pi. 00:42:42.080 --> 00:42:53.770 Multiple bonds require a mix of sigma and pi. 00:42:53.770 --> 00:42:58.390 If you have a single bond, all you need is the sigma. 00:42:58.390 --> 00:43:04.050 So let's go back to how we got the original hybridization. 00:43:04.050 --> 00:43:08.020 Remember, we started with carbon, with the box notation 00:43:08.020 --> 00:43:09.330 looking like this. 00:43:09.330 --> 00:43:15.190 Here's the 2s and the 2p's and we started with just native 00:43:15.190 --> 00:43:17.240 carbon off the periodic table. 00:43:17.240 --> 00:43:18.470 And we have this. 00:43:18.470 --> 00:43:22.590 Just if we go according to the 2s2, 2p2. 00:43:22.590 --> 00:43:26.640 And then in order to get hybridization to rationalize 00:43:26.640 --> 00:43:31.070 methane, where we've got four equivalent bonds. 00:43:31.070 --> 00:43:35.810 In that case, we hybridize the s and all of the p's. 00:43:35.810 --> 00:43:39.500 We took the s and all three of the p's to make 00:43:39.500 --> 00:43:43.000 the sp3 hybrid orbitals. 00:43:43.000 --> 00:43:45.800 And then we were able to take these electrons and put them 00:43:45.800 --> 00:43:47.820 in one at a time. 00:43:47.820 --> 00:43:50.620 And then bring in the four hydrogens and we end up with 00:43:50.620 --> 00:43:52.480 something that is 00:43:52.480 --> 00:43:54.250 symmetrically disposed in space. 00:43:54.250 --> 00:43:57.700 The carbon, hydrogen and so. 00:43:57.700 --> 00:44:01.290 So now I give to you ethylene I say, well 00:44:01.290 --> 00:44:02.500 how do I make ethylene. 00:44:02.500 --> 00:44:08.010 Well, if I started with this sp3 thing maybe I could bring 00:44:08.010 --> 00:44:10.620 another carbon over here. 00:44:10.620 --> 00:44:14.820 And I'd have three hydrogen sticking out. 00:44:14.820 --> 00:44:16.250 So there's my sigma bond. 00:44:16.250 --> 00:44:17.670 So I'm off to the races. 00:44:17.670 --> 00:44:18.180 This is good. 00:44:18.180 --> 00:44:20.390 But now I need to build a second bond. 00:44:20.390 --> 00:44:22.760 I'm going to throw away a couple of hydrogen. 00:44:22.760 --> 00:44:25.490 So i'll throw this hydrogen away and this hydrogen away. 00:44:25.490 --> 00:44:28.290 So I'm going to have now C2H4. 00:44:28.290 --> 00:44:29.590 I'm almost there. 00:44:29.590 --> 00:44:32.890 The only problem is, this orbital is sticking out this 00:44:32.890 --> 00:44:34.810 way, this orbital is sticking out this way. 00:44:34.810 --> 00:44:38.240 And I don't have the license to bend these orbitals. 00:44:38.240 --> 00:44:39.920 And build a double bond. 00:44:39.920 --> 00:44:43.460 Because they're 109 degrees apart and they're inflexible 00:44:43.460 --> 00:44:45.010 and they won't bend. 00:44:45.010 --> 00:44:50.460 So this hybridization technique will not work as 00:44:50.460 --> 00:44:56.210 such to give me what I need to build ethylene. 00:44:56.210 --> 00:44:57.180 What do I need? 00:44:57.180 --> 00:45:00.380 If I'm going to build a sigma and a pi bond, I'm going to 00:45:00.380 --> 00:45:04.440 need to preserve one of the p orbitals so that it can still 00:45:04.440 --> 00:45:06.850 be available for lateral smearing 00:45:06.850 --> 00:45:08.900 Because how do I make a pi orbital? 00:45:08.900 --> 00:45:11.900 I make a pi orbital middle with an 88. 00:45:11.900 --> 00:45:17.820 So I need to preserve a p orbital so that in both of the 00:45:17.820 --> 00:45:22.590 carbons I still have this pi bonding capability. 00:45:22.590 --> 00:45:26.680 So what I'm going do is instead of taking the s and 00:45:26.680 --> 00:45:30.610 all three of the p's I'm going to take the s and only two of 00:45:30.610 --> 00:45:34.410 the p' s and reserve one of the p' s to be sitting there 00:45:34.410 --> 00:45:35.580 for lateral smearing. 00:45:35.580 --> 00:45:39.300 So instead what I'm going to do is this. 00:45:39.300 --> 00:45:43.720 So this is going to be an unmixed p and this is going to 00:45:43.720 --> 00:45:48.430 take the s and not three p' s but only two p 's. 00:45:48.430 --> 00:45:49.680 And this would be called sp2. 00:45:52.040 --> 00:45:53.090 And now what do I have? 00:45:53.090 --> 00:45:55.280 I've one, two, three. 00:45:55.280 --> 00:45:57.630 And how do I put three of these in space? 00:45:57.630 --> 00:46:03.330 They lie symmetrically in a plane at 120 degrees. 00:46:03.330 --> 00:46:06.830 And then this thing is normal to the plane of the board. 00:46:06.830 --> 00:46:07.540 It's sticking out. 00:46:07.540 --> 00:46:09.780 Actually, I should've maybe drawn it in 00:46:09.780 --> 00:46:13.400 perspective like this. 00:46:13.400 --> 00:46:15.990 And so now I've got the ability to put 00:46:15.990 --> 00:46:17.290 two of these together. 00:46:17.290 --> 00:46:19.500 This'll giving me my sigma. 00:46:19.500 --> 00:46:23.100 And then these two things lying on their side will smear 00:46:23.100 --> 00:46:25.600 with the lobes to give me the pi. 00:46:25.600 --> 00:46:28.020 Now that's what gives me the double bond. 00:46:28.020 --> 00:46:30.680 And here are the cartoons that show this. 00:46:33.480 --> 00:46:36.040 Oh here, I took this from an old text book, it's text book 00:46:36.040 --> 00:46:37.920 I had when I was your age. 00:46:40.455 --> 00:46:41.590 The wrote great books. 00:46:41.590 --> 00:46:43.710 And then I flipped this around see. 00:46:43.710 --> 00:46:44.820 I did all this just for you. 00:46:44.820 --> 00:46:46.980 So I took this one, I flipped the image around. 00:46:46.980 --> 00:46:49.850 So there are the sp2's these are the unmixed p's. 00:46:49.850 --> 00:46:51.510 And you bring them close together and bingo. 00:46:51.510 --> 00:46:52.870 There's the sigma. 00:46:52.870 --> 00:46:55.550 That's this one, p plus p axially, that gives me the 00:46:55.550 --> 00:46:56.570 sigma bond. 00:46:56.570 --> 00:46:59.540 And then I smear those two and that gives me the pi. 00:46:59.540 --> 00:47:01.190 And there's ethylene. 00:47:01.190 --> 00:47:03.710 You put the hydrogens here, one, two, three, four. 00:47:03.710 --> 00:47:05.040 Two carbons. 00:47:05.040 --> 00:47:06.440 Tada. 00:47:06.440 --> 00:47:08.640 Isn't that cool? 00:47:08.640 --> 00:47:10.740 Here's from another book. 00:47:10.740 --> 00:47:14.580 It's looks like a Boston traffic map, doesn't it? 00:47:14.580 --> 00:47:17.410 Just crazy. 00:47:17.410 --> 00:47:18.850 This is from our book. 00:47:18.850 --> 00:47:25.050 So they're showing you there's the sigmas and the pi's. 00:47:25.050 --> 00:47:26.300 There we go. 00:47:31.360 --> 00:47:32.610 More pictures. 00:47:35.670 --> 00:47:38.860 OK, I'm going to take three minutes at the end here and 00:47:38.860 --> 00:47:39.960 show you an example of 00:47:39.960 --> 00:47:42.000 electronegativity at the extreme. 00:47:42.000 --> 00:47:44.990 So if you take a look at the periodic table and look at a 00:47:44.990 --> 00:47:47.050 compound like sodium iodide. 00:47:47.050 --> 00:47:50.240 If I just told you sodium iodide, covalent or ionic? 00:47:50.240 --> 00:47:52.150 You'd say, ionic. 00:47:52.150 --> 00:47:54.850 Because you got something from the the most metallic of the 00:47:54.850 --> 00:47:56.870 metals and something from the most non 00:47:56.870 --> 00:47:58.500 metallic of the non metals. 00:47:58.500 --> 00:47:59.180 And you're right. 00:47:59.180 --> 00:48:01.520 And the delta chi is 1.73. 00:48:01.520 --> 00:48:06.560 And the covalent character is high. 00:48:06.560 --> 00:48:09.280 And you end up with something that's very polar and so on. 00:48:09.280 --> 00:48:12.640 Now here's a very interesting one, if you compare cesium and 00:48:12.640 --> 00:48:14.870 gold, you get 1.75. 00:48:14.870 --> 00:48:18.670 Which is greater than what it was for sodium and iodine. 00:48:18.670 --> 00:48:21.700 Now no one would argue that sodium and iodine is covalent, 00:48:21.700 --> 00:48:22.950 you'd argue that's ionic. 00:48:22.950 --> 00:48:26.340 Well by the same metrics, cesium and gold is as ionic. 00:48:29.470 --> 00:48:30.970 The plot thickens. 00:48:30.970 --> 00:48:35.090 Cesium, if you melt it it's a metal, liquid metal. 00:48:35.090 --> 00:48:37.720 If you melt gold, it's a liquid metal. 00:48:37.720 --> 00:48:41.020 But if you mix them in equal number. 00:48:41.020 --> 00:48:47.200 So you make a alloy of 50 mole percent cesium and 50 mole 00:48:47.200 --> 00:48:51.700 percent gold and you've got a delta chi 1.75, you've 00:48:51.700 --> 00:48:55.360 essentially made a cocktail with equal numbers of really 00:48:55.360 --> 00:48:57.340 good electron donors and really 00:48:57.340 --> 00:48:58.690 good electron acceptors. 00:48:58.690 --> 00:49:01.660 And guess what happens, electron transfer. 00:49:01.660 --> 00:49:05.990 And the melt is not metallic, it turns clear and colorless 00:49:05.990 --> 00:49:08.620 just as molten sodium iodide would be. 00:49:08.620 --> 00:49:10.810 It turns into a molten salt. 00:49:10.810 --> 00:49:12.960 So cesium gives its electron to gold. 00:49:12.960 --> 00:49:16.730 And gold becomes the negative gold ion. 00:49:16.730 --> 00:49:19.310 And what color is every ion? 00:49:19.310 --> 00:49:21.550 It's got stable octet configurations. 00:49:21.550 --> 00:49:24.860 It's got be the same color as neon, argon, and helium. 00:49:24.860 --> 00:49:26.110 They are clear and colorless. 00:49:29.080 --> 00:49:32.190 Big drop in electrical conductivity and a shift from 00:49:32.190 --> 00:49:33.910 electronic to ionic conduction. 00:49:33.910 --> 00:49:37.710 Metals have electronic conductivity, ions, what do 00:49:37.710 --> 00:49:38.800 ionic liquids have? 00:49:38.800 --> 00:49:40.110 They have like ionic conductivity. 00:49:40.110 --> 00:49:41.940 Here's some data from the literature. 00:49:41.940 --> 00:49:44.890 This is the log on the connductivity as a function of 00:49:44.890 --> 00:49:45.700 concentration. 00:49:45.700 --> 00:49:47.950 So here's pure cesium over here, here's 00:49:47.950 --> 00:49:48.900 pure gold over here. 00:49:48.900 --> 00:49:50.230 This is 600 degrees C. 00:49:50.230 --> 00:49:54.460 So the line stops here because gold melts at about 1060. 00:49:54.460 --> 00:49:57.760 So gold is a solid beyond this alloy concentration. 00:49:57.760 --> 00:50:01.040 But you can see this is roughly what you'd get. 00:50:01.040 --> 00:50:05.320 So electronic conductivity up here at about 10 to the 4 00:50:05.320 --> 00:50:08.870 siemens per, this is reciprocal ohms, but siemens 00:50:08.870 --> 00:50:09.790 per centimeter. 00:50:09.790 --> 00:50:13.000 And down here, this is very low value ionic conductivity. 00:50:13.000 --> 00:50:14.200 So this is a liquid metal. 00:50:14.200 --> 00:50:15.680 And this is a molten solid. 00:50:15.680 --> 00:50:17.170 And it all happens just when you get very 00:50:17.170 --> 00:50:20.260 very close to 50/50. 00:50:20.260 --> 00:50:24.230 So you end up with something called cesium oride. 00:50:24.230 --> 00:50:25.540 And it's sorcery. 00:50:25.540 --> 00:50:28.700 You have one vial of liquid metal. 00:50:28.700 --> 00:50:30.320 You have a second vial of liquid metal. 00:50:30.320 --> 00:50:33.500 You pour then and it turns clear and colorless. 00:50:33.500 --> 00:50:37.010 And the conductivity drops three orders of magnitude all 00:50:37.010 --> 00:50:41.240 because of electron transfer due to this electronegativity 00:50:41.240 --> 00:50:42.860 difference. 00:50:42.860 --> 00:50:44.870 That's so cool. 00:50:44.870 --> 00:50:47.090 That is so cool. 00:50:47.090 --> 00:50:48.680 OK, with that I'll let you go. 00:50:48.680 --> 00:50:50.000 We'll see you on Friday.