1 00:00:00,030 --> 00:00:02,670 The following content is provided under a Creative 2 00:00:02,670 --> 00:00:03,830 Commons license. 3 00:00:03,830 --> 00:00:06,850 Your support will help MIT OpenCourseWare continue to 4 00:00:06,850 --> 00:00:10,550 offer high-quality educational resources for free. 5 00:00:10,550 --> 00:00:13,390 To make a donation or view additional materials from 6 00:00:13,390 --> 00:00:17,490 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:17,490 --> 00:00:18,740 ocw.mit.edu. 8 00:00:20,950 --> 00:00:21,755 Let's go to the lesson. 9 00:00:21,755 --> 00:00:27,380 So the last day we studied VSEPR, which allowed us to 10 00:00:27,380 --> 00:00:30,410 infer the shapes of molecules, covalent molecules. 11 00:00:30,410 --> 00:00:33,640 Today I want to talk about the state of aggregation. 12 00:00:33,640 --> 00:00:35,710 What do I mean by state of aggregation? 13 00:00:35,710 --> 00:00:39,680 Is something a solid, a liquid or a gas at a particular 14 00:00:39,680 --> 00:00:40,240 temperature? 15 00:00:40,240 --> 00:00:43,500 So 3.091 is introduction to solid state chemistry. 16 00:00:43,500 --> 00:00:47,010 One of the things we need to know is, under what conditions 17 00:00:47,010 --> 00:00:49,680 is the solid state stable? 18 00:00:49,680 --> 00:00:52,950 And the state of aggregation, when it applies to covalent 19 00:00:52,950 --> 00:00:55,750 molecules, is going to lead us to a discussion 20 00:00:55,750 --> 00:00:56,760 of secondary bonding. 21 00:00:56,760 --> 00:00:59,730 So today's lecture is state of aggregation 22 00:00:59,730 --> 00:01:01,270 or secondary bonding. 23 00:01:01,270 --> 00:01:03,630 Now let's just reflect upon what we've 24 00:01:03,630 --> 00:01:04,520 learned up until now. 25 00:01:04,520 --> 00:01:07,660 We studied ironic bonding and we knew that ionic bonding 26 00:01:07,660 --> 00:01:11,930 necessarily leads to crystal formation because we have 27 00:01:11,930 --> 00:01:13,750 unsaturated bonds. 28 00:01:13,750 --> 00:01:17,620 And that leads to an ion array of unlimited size until you 29 00:01:17,620 --> 00:01:18,860 run out of reagent. 30 00:01:18,860 --> 00:01:22,210 And something honking big made of ions is going to be a solid 31 00:01:22,210 --> 00:01:24,450 at room temperature. 32 00:01:24,450 --> 00:01:27,190 Last day we appreciated with covalent 33 00:01:27,190 --> 00:01:29,590 bonding we have two options. 34 00:01:29,590 --> 00:01:34,970 We can either make discrete molecules such as HCl. 35 00:01:34,970 --> 00:01:38,860 And HCl as a discrete molecule, depending on a 36 00:01:38,860 --> 00:01:41,010 number of factors that I'm going to show you today, could 37 00:01:41,010 --> 00:01:44,390 be a solid, liquid or gas. 38 00:01:44,390 --> 00:01:47,790 And we're going to understand more deeply how to sort out 39 00:01:47,790 --> 00:01:49,060 between the three of them. 40 00:01:49,060 --> 00:01:52,630 Or, I showed you at the end of the last lecture, you can make 41 00:01:52,630 --> 00:01:55,630 a three-dimensional network and diamond was one example. 42 00:01:55,630 --> 00:01:59,200 If it's a three-dimensional network that is a large array 43 00:01:59,200 --> 00:02:02,920 of solid then that means that you're going to end up with a 44 00:02:02,920 --> 00:02:05,670 formation of a crystal, which is going to give you a solid. 45 00:02:05,670 --> 00:02:08,040 Diamond is solid at room temperature, graphite is solid 46 00:02:08,040 --> 00:02:09,730 at room temperature. 47 00:02:09,730 --> 00:02:14,190 So let's now go to the one case that we haven't dealt 48 00:02:14,190 --> 00:02:18,570 with, and that's the formation of discrete molecules. 49 00:02:18,570 --> 00:02:20,920 So let's look at discrete molecules. 50 00:02:20,920 --> 00:02:23,270 And what we want to understand is whether they're going to be 51 00:02:23,270 --> 00:02:27,000 solid, liquid or gas at room temperature and what's the 52 00:02:27,000 --> 00:02:31,280 relevant physics here in order to make that determination? 53 00:02:31,280 --> 00:02:35,220 The relevant physics is the following. 54 00:02:35,220 --> 00:02:36,980 We're going to compare two forces. 55 00:02:36,980 --> 00:02:42,420 We're going to compare the cohesive force between 56 00:02:42,420 --> 00:02:46,920 molecules versus the disruptive force. 57 00:02:46,920 --> 00:02:50,120 And the disruptive force in 3.091 is 58 00:02:50,120 --> 00:02:52,360 always going to be thermal. 59 00:02:52,360 --> 00:02:53,530 It's thermal energy. 60 00:02:53,530 --> 00:02:54,060 We know this. 61 00:02:54,060 --> 00:02:57,190 As we heat things they go from solid to liquid to vapor as 62 00:02:57,190 --> 00:02:58,500 temperature increases. 63 00:02:58,500 --> 00:03:04,650 So thermal energy plays the role of the destructive force 64 00:03:04,650 --> 00:03:07,800 whereas the bonding is something that is 65 00:03:07,800 --> 00:03:09,230 the cohesive force. 66 00:03:09,230 --> 00:03:11,010 So let's go back to HCL. 67 00:03:11,010 --> 00:03:13,510 Last day we looked at HCl. 68 00:03:13,510 --> 00:03:16,840 So here's one HCl molecule. 69 00:03:16,840 --> 00:03:18,230 We have a covalent bond. 70 00:03:21,160 --> 00:03:24,690 It's a covalent bond within the molecule. 71 00:03:24,690 --> 00:03:29,510 We know this is a polar covalent molecule, with the 72 00:03:29,510 --> 00:03:32,690 chlorine having greater electronegativity and pulling 73 00:03:32,690 --> 00:03:35,250 the electrons towards its end. 74 00:03:35,250 --> 00:03:40,460 And furthermore, this bond I want to categorize as within 75 00:03:40,460 --> 00:03:44,290 the molecule so I'm going to call it intramolecular. 76 00:03:44,290 --> 00:03:46,510 Intramolecular. 77 00:03:46,510 --> 00:03:49,610 Now if I want to answer the question, is hydrogen chloride 78 00:03:49,610 --> 00:03:52,290 going to be a solid, liquid or gas at room temperature, I 79 00:03:52,290 --> 00:03:54,720 don't have the information on the board. 80 00:03:54,720 --> 00:03:57,890 The only way I can answer that question is to put another 81 00:03:57,890 --> 00:03:59,380 hydrogen chloride. 82 00:03:59,380 --> 00:04:02,470 This simply forms the bond between hydrogen and chlorine. 83 00:04:02,470 --> 00:04:05,760 To determine whether hydrogen chloride is a solid, liquid or 84 00:04:05,760 --> 00:04:10,290 gas, I need to look at how one hydrogen chloride molecule 85 00:04:10,290 --> 00:04:13,970 bonds to another hydrogen chloride molecule. 86 00:04:13,970 --> 00:04:19,260 So now this end is the delta plus this end is delta minus. 87 00:04:19,260 --> 00:04:21,300 And we have again an intramolecular bond. 88 00:04:21,300 --> 00:04:24,580 So the question is, how does one hydrogen chloride bond to 89 00:04:24,580 --> 00:04:25,970 another hydrogen chloride? 90 00:04:25,970 --> 00:04:29,080 This is the intermolecular bond. 91 00:04:29,080 --> 00:04:30,330 Intermolecular bond. 92 00:04:32,100 --> 00:04:37,210 So intermolecular bonds govern the state of aggregation. 93 00:04:37,210 --> 00:04:38,880 And I'm going to show you the different types of 94 00:04:38,880 --> 00:04:40,310 intermolecular bonds. 95 00:04:40,310 --> 00:04:42,680 This intermolecular bond isn't the primary bond. 96 00:04:42,680 --> 00:04:46,740 The primary bond is here inside the molecule. 97 00:04:46,740 --> 00:04:51,840 So the intermolecular bond is known as a secondary bond. 98 00:04:51,840 --> 00:04:53,900 And there three types of secondary bonds. 99 00:04:53,900 --> 00:04:55,750 We're going to look at them in turn. 100 00:04:55,750 --> 00:04:59,340 So the first one is depicted here on the board. 101 00:04:59,340 --> 00:05:01,120 And it's called dipole-dipole interactions. 102 00:05:07,060 --> 00:05:12,370 And these obviously occur only between polar molecules. 103 00:05:12,370 --> 00:05:16,400 Operative and polar molecules. 104 00:05:16,400 --> 00:05:17,160 Not in. 105 00:05:17,160 --> 00:05:18,220 Between. 106 00:05:18,220 --> 00:05:19,700 Take out in. 107 00:05:19,700 --> 00:05:26,760 Operative between polar molecules. 108 00:05:26,760 --> 00:05:29,430 So we've made something covalent and now how does one 109 00:05:29,430 --> 00:05:30,920 stick to the other? 110 00:05:30,920 --> 00:05:35,990 Alright and I think I've got a cartoon here that shows this. 111 00:05:35,990 --> 00:05:36,460 There we go. 112 00:05:36,460 --> 00:05:39,260 That's taken from an old book that I had. 113 00:05:39,260 --> 00:05:43,280 So you see the H and the Cl and we have a dipole moment 114 00:05:43,280 --> 00:05:46,490 and the negative end of one dipole attracted to the 115 00:05:46,490 --> 00:05:49,480 positive end of another dipole. 116 00:05:49,480 --> 00:05:54,990 So we have a net dipole moment here and we can measure the 117 00:05:54,990 --> 00:05:58,375 distance between the center of the dipole-- 118 00:05:58,375 --> 00:06:01,190 center to center spacing and call it r. 119 00:06:01,190 --> 00:06:04,450 Center to center dipole spacing. 120 00:06:04,450 --> 00:06:09,250 And say that the energy in dipole-dipole interaction-- 121 00:06:09,250 --> 00:06:11,690 I don't expect you to know this by heart and do anything 122 00:06:11,690 --> 00:06:13,550 with it, but I just want you to realize 123 00:06:13,550 --> 00:06:15,460 that it's a weak force. 124 00:06:15,460 --> 00:06:17,680 This is very weak. 125 00:06:17,680 --> 00:06:20,270 It's proportional to the magnitude 126 00:06:20,270 --> 00:06:21,430 of the dipole moment. 127 00:06:21,430 --> 00:06:22,390 You'd expect that. 128 00:06:22,390 --> 00:06:24,460 Weak dipole moment, weak energy. 129 00:06:24,460 --> 00:06:26,350 Strong dipole moment, strong energy. 130 00:06:26,350 --> 00:06:28,800 Turns out it goes as the fourth order 131 00:06:28,800 --> 00:06:30,060 of the dipole moment. 132 00:06:30,060 --> 00:06:31,740 And it's inversely proportional to the 133 00:06:31,740 --> 00:06:32,990 separation. 134 00:06:32,990 --> 00:06:36,470 Only it's not coulombic, it goes as 1 over r cubed. 135 00:06:36,470 --> 00:06:38,770 And there's a temperature factor. 136 00:06:38,770 --> 00:06:39,850 1 over t. 137 00:06:39,850 --> 00:06:43,100 As temperature goes up, this energy goes down. 138 00:06:43,100 --> 00:06:46,780 And these are very weak forces on the order of about 5 139 00:06:46,780 --> 00:06:50,190 kilojoules per mole. 140 00:06:50,190 --> 00:06:52,130 You remember what the crystallization energy of 141 00:06:52,130 --> 00:06:53,700 sodium chloride was? 142 00:06:53,700 --> 00:06:56,310 787 kilojoules per mole. 143 00:06:56,310 --> 00:06:58,030 So this is very, very weak. 144 00:06:58,030 --> 00:07:02,140 It's operative at, between, at low temperatures. 145 00:07:02,140 --> 00:07:03,380 At very low temperatures. 146 00:07:03,380 --> 00:07:07,550 So for example, hydrogen chloride, in the case of 147 00:07:07,550 --> 00:07:12,170 hydrogen chloride, the melting point of hydrogen chloride is 148 00:07:12,170 --> 00:07:16,370 minus 115 degrees C and the boiling point is 149 00:07:16,370 --> 00:07:18,840 minus 85 degrees C. 150 00:07:18,840 --> 00:07:22,260 You can see at very low temperatures we already have 151 00:07:22,260 --> 00:07:24,880 enough thermal energy to disrupt. 152 00:07:24,880 --> 00:07:28,370 So if we have solid hydrogen chloride, the forces between 153 00:07:28,370 --> 00:07:31,480 the hydrogen chloride molecules in the solid are 154 00:07:31,480 --> 00:07:34,810 these very weak dipole-dipole interactions. 155 00:07:34,810 --> 00:07:37,180 And I think there's a couple more cartoons here. 156 00:07:37,180 --> 00:07:41,160 This is taken from your text. 157 00:07:41,160 --> 00:07:43,350 So there we go. 158 00:07:43,350 --> 00:07:43,770 There it is. 159 00:07:43,770 --> 00:07:47,380 That's the soup that might be HCl liquid. 160 00:07:47,380 --> 00:07:51,340 OK so that's the first type of secondary bond. 161 00:07:51,340 --> 00:07:53,910 Dipole-dipole interaction. 162 00:07:53,910 --> 00:07:55,050 Let's look at the second one. 163 00:07:55,050 --> 00:07:59,200 The second one is called induced dipole-induced dipole. 164 00:08:07,730 --> 00:08:11,850 And it's operative in non-polar species. 165 00:08:11,850 --> 00:08:18,170 Dominant in non-polar species. 166 00:08:18,170 --> 00:08:19,820 Why am I using the word, species? 167 00:08:19,820 --> 00:08:23,040 I'm trying to be a pedant and use fancy words? 168 00:08:23,040 --> 00:08:23,180 No. 169 00:08:23,180 --> 00:08:24,460 Because I'm going to make it generic. 170 00:08:24,460 --> 00:08:27,275 I'm going to show it works in atoms and in compounds. 171 00:08:30,110 --> 00:08:34,040 So as examples, what are some non-polar species? 172 00:08:34,040 --> 00:08:37,500 Well how about something like argon? 173 00:08:37,500 --> 00:08:40,450 Argon if you look on the periodic table it'll show you 174 00:08:40,450 --> 00:08:46,960 that it has a melting point of 84 kelvin and a boiling point 175 00:08:46,960 --> 00:08:49,600 of 87 kelvin. 176 00:08:49,600 --> 00:08:54,890 So if I cool argon to below 84 kelvin, I get argon ice. 177 00:08:54,890 --> 00:08:57,790 So what are the bonds between one argon atom and another 178 00:08:57,790 --> 00:08:58,430 argon atom? 179 00:08:58,430 --> 00:08:59,900 It can't be this. 180 00:08:59,900 --> 00:09:00,970 There's no net charge. 181 00:09:00,970 --> 00:09:02,460 It can't be ionic. 182 00:09:02,460 --> 00:09:05,590 It doesn't form a covalent network the way diamond does. 183 00:09:05,590 --> 00:09:09,150 How do you justify the existence of solid argon? 184 00:09:09,150 --> 00:09:11,590 It's just so cold that it just sits there and freezes? 185 00:09:11,590 --> 00:09:13,190 I mean, how does it bond? 186 00:09:13,190 --> 00:09:15,150 There needs to be some kind of bonding. 187 00:09:15,150 --> 00:09:16,700 And other non-polar species. 188 00:09:16,700 --> 00:09:19,540 So for example, the molecule iodine. 189 00:09:19,540 --> 00:09:23,320 Iodine melting point is above room temperature. 190 00:09:23,320 --> 00:09:26,330 It's a solid crystal at room temperature. 191 00:09:26,330 --> 00:09:29,700 Well there's a strong covalent bond inside iodine. 192 00:09:29,700 --> 00:09:32,635 But how does one iodine bond to another and to another and 193 00:09:32,635 --> 00:09:33,330 to another? 194 00:09:33,330 --> 00:09:35,040 And we can even go to polyatomic 195 00:09:35,040 --> 00:09:38,930 species, such as methane. 196 00:09:38,930 --> 00:09:41,610 You know there was data from the Cassini Probe. 197 00:09:41,610 --> 00:09:42,710 Look at this. 198 00:09:42,710 --> 00:09:45,110 This is an image from the Cassini Probe. 199 00:09:45,110 --> 00:09:47,270 This is an island group. 200 00:09:47,270 --> 00:09:48,770 The yellow is an island group. 201 00:09:48,770 --> 00:09:51,270 The blue is a methane sea. 202 00:09:51,270 --> 00:09:56,530 A sea of liquid methane on the moon of Saturn, Titan. 203 00:09:56,530 --> 00:09:58,700 So there's liquid methane. 204 00:09:58,700 --> 00:10:00,730 How does liquid methane form? 205 00:10:00,730 --> 00:10:05,830 What causes one methane to bond with another methane? 206 00:10:05,830 --> 00:10:07,630 That's the question that we're wrestling with. 207 00:10:07,630 --> 00:10:10,580 So let's look at it first in a simpler context. 208 00:10:10,580 --> 00:10:14,950 Let's look at it with argon. 209 00:10:14,950 --> 00:10:16,900 So I put argon here. 210 00:10:16,900 --> 00:10:20,480 It's a spherical atom. 211 00:10:20,480 --> 00:10:24,180 And the question we had before says, how does one argon bond 212 00:10:24,180 --> 00:10:28,300 to another argon, as has to be the case in the solid or 213 00:10:28,300 --> 00:10:29,680 liquid argon. 214 00:10:29,680 --> 00:10:35,490 So we know that this has a lot of electrons and the atom is 215 00:10:35,490 --> 00:10:36,380 net neutral. 216 00:10:36,380 --> 00:10:38,410 But the electrons are in motion. 217 00:10:38,410 --> 00:10:41,460 If I had an attosecond camera and I went in here and I went, 218 00:10:41,460 --> 00:10:46,760 snap, I could catch a freeze frame where the electrons 219 00:10:46,760 --> 00:10:48,100 aren't symmetrically 220 00:10:48,100 --> 00:10:51,010 distributed around the nucleus. 221 00:10:51,010 --> 00:10:55,220 This nucleus, the atom is in constant fluctuation. 222 00:10:55,220 --> 00:10:56,060 But net neutral. 223 00:10:56,060 --> 00:10:57,960 Time average it's symmetrical. 224 00:10:57,960 --> 00:11:01,500 So at some moment I might have a preponderance of electrons 225 00:11:01,500 --> 00:11:03,630 over here at around three o'clock. 226 00:11:03,630 --> 00:11:08,050 So this end of the argon is a little bit delta minus. 227 00:11:08,050 --> 00:11:11,240 Which means the other end is delta plus. 228 00:11:11,240 --> 00:11:13,060 Now what happens if this is delta minus, 229 00:11:13,060 --> 00:11:14,290 this is delta plus? 230 00:11:14,290 --> 00:11:19,000 Can you see that the positive end of this argon atom will 231 00:11:19,000 --> 00:11:21,730 then pull on the electron distribution of the adjacent 232 00:11:21,730 --> 00:11:25,280 argon atom rendering it delta minus at three o'clock and 233 00:11:25,280 --> 00:11:26,730 delta plus? 234 00:11:26,730 --> 00:11:34,350 So this is the dipole in one, induces a dipole in another. 235 00:11:34,350 --> 00:11:36,460 And why does it occur in the first place? 236 00:11:36,460 --> 00:11:41,080 It occurs because the electrons are in motion. 237 00:11:41,080 --> 00:11:46,100 Electrons in motion lead to time fluctuating dipoles. 238 00:11:46,100 --> 00:11:47,350 They're time fluctuating. 239 00:11:49,630 --> 00:11:51,800 Time average, there's no net dipole moment. 240 00:11:51,800 --> 00:11:53,700 I'm not going back on what I said before. 241 00:11:53,700 --> 00:11:56,000 There's no net dipole moment here, but this is time 242 00:11:56,000 --> 00:11:59,090 fluctuating dipole. 243 00:11:59,090 --> 00:12:00,340 Time fluctuating dipole. 244 00:12:06,560 --> 00:12:08,850 So who was the first to explain this? 245 00:12:08,850 --> 00:12:16,366 The first to explain this was Fritz London in 1930. 246 00:12:16,366 --> 00:12:20,530 In 1930 Fritz London gave us the explanation because this 247 00:12:20,530 --> 00:12:22,630 troubled people for a long time. 248 00:12:22,630 --> 00:12:26,350 And he did so in a quantitative manner. 249 00:12:26,350 --> 00:12:30,730 And the mathematics of the treatment are identical to the 250 00:12:30,730 --> 00:12:34,140 mathematics for the dispersion of light under certain 251 00:12:34,140 --> 00:12:35,170 conditions. 252 00:12:35,170 --> 00:12:37,660 This has nothing to do with the dispersion of light. 253 00:12:37,660 --> 00:12:41,240 The mathematical formulation imitates the formulation of 254 00:12:41,240 --> 00:12:44,450 the dispersion of light and hence the force here is known 255 00:12:44,450 --> 00:12:47,200 as the London Dispersion Force. 256 00:12:49,810 --> 00:12:53,540 So people will say that solid argon is held together by 257 00:12:53,540 --> 00:12:55,560 London Dispersion Forces. 258 00:12:55,560 --> 00:13:01,160 Or the bond is known by the name van der Waals. 259 00:13:01,160 --> 00:13:04,290 We either call them van der Waals bonds or London 260 00:13:04,290 --> 00:13:05,640 Dispersion Forces. 261 00:13:05,640 --> 00:13:07,110 Van der Waals was Dutch. 262 00:13:07,110 --> 00:13:08,490 I know how to spell the word wall. 263 00:13:08,490 --> 00:13:09,690 This is the dutch spelling. 264 00:13:09,690 --> 00:13:12,191 W A A L S. 265 00:13:12,191 --> 00:13:14,900 Van der Waals. 266 00:13:14,900 --> 00:13:21,100 So in this system here, the London Dispersion Force or the 267 00:13:21,100 --> 00:13:24,270 van der Waals bond is in fact not the secondary bond. 268 00:13:24,270 --> 00:13:26,270 It's the primary bond, isn't it? 269 00:13:26,270 --> 00:13:27,340 It's the only bond. 270 00:13:27,340 --> 00:13:31,480 So by definition it must be the primary bond. 271 00:13:31,480 --> 00:13:35,110 But can you see that in every compound, including diamond, 272 00:13:35,110 --> 00:13:38,900 we have time fluctuating dipoles in all of the atoms? 273 00:13:38,900 --> 00:13:41,770 It's not just argon that has time fluctuating dipoles. 274 00:13:41,770 --> 00:13:45,390 Every atom in you and me has time fluctuating dipoles. 275 00:13:45,390 --> 00:13:48,110 But we're held together by much greater forces. 276 00:13:48,110 --> 00:13:51,730 So that's why I say that in this case this is 277 00:13:51,730 --> 00:13:55,540 the dominant force. 278 00:13:55,540 --> 00:13:57,590 But it's operative in everything. 279 00:13:57,590 --> 00:14:01,240 Because wherever we have electrons, they're in motion. 280 00:14:01,240 --> 00:14:04,840 So this is the dominant one. 281 00:14:04,840 --> 00:14:08,360 Alright so we can look at it in a few other instances. 282 00:14:08,360 --> 00:14:12,260 So for example we can look at it in the case of iodine. 283 00:14:12,260 --> 00:14:14,200 We can look at it in the case of methane. 284 00:14:14,200 --> 00:14:15,080 All the same. 285 00:14:15,080 --> 00:14:16,680 Time fluctuating dipoles. 286 00:14:16,680 --> 00:14:18,860 And this is a very, very weak force. 287 00:14:18,860 --> 00:14:23,450 So the energy in London Dispersion Forces, or van der 288 00:14:23,450 --> 00:14:32,660 Waals bonds, is proportional to a quantity called the 289 00:14:32,660 --> 00:14:33,910 polarizability. 290 00:14:36,480 --> 00:14:39,620 The polarizability is-- 291 00:14:39,620 --> 00:14:42,950 and this was defined by London-- 292 00:14:42,950 --> 00:14:47,440 polarizability is the measure of how easy it is to induce 293 00:14:47,440 --> 00:14:49,440 this dipole. 294 00:14:49,440 --> 00:14:52,130 A measure of the ease of electron 295 00:14:52,130 --> 00:14:54,470 displacement within an atom. 296 00:15:08,190 --> 00:15:15,660 And it depends on, it's influenced by the size and 297 00:15:15,660 --> 00:15:21,620 it's influenced by the number of electrons in the atom. 298 00:15:21,620 --> 00:15:23,070 So what do I mean by that? 299 00:15:23,070 --> 00:15:30,310 Well let's take two systems. Suppose I've got argon and 300 00:15:30,310 --> 00:15:33,490 I'll go up two members in the same series 301 00:15:33,490 --> 00:15:34,740 and I've got helium. 302 00:15:37,160 --> 00:15:39,270 So what are the forces that hold helium together? 303 00:15:39,270 --> 00:15:40,480 Same thing. 304 00:15:40,480 --> 00:15:45,100 But the boiling point here is 87 kelvin. 305 00:15:45,100 --> 00:15:49,190 The boiling point here is 4.2 kelvin. 306 00:15:49,190 --> 00:15:51,590 Why does this have such a low boiling point? 307 00:15:51,590 --> 00:15:53,660 Well, polarizability. 308 00:15:53,660 --> 00:15:55,040 Size-- 309 00:15:55,040 --> 00:15:56,240 helium is smaller. 310 00:15:56,240 --> 00:16:00,490 So the degree, the corral in which the electrons can roam 311 00:16:00,490 --> 00:16:01,810 is smaller. 312 00:16:01,810 --> 00:16:04,960 So the extent of electron displacement is smaller. 313 00:16:04,960 --> 00:16:07,830 And secondly there are only two electrons. 314 00:16:07,830 --> 00:16:10,490 And they're in 1s, so they're tightly held. 315 00:16:10,490 --> 00:16:16,150 And so the delta minus delta plus capable of being created 316 00:16:16,150 --> 00:16:20,150 in helium is tiny compared to the delta minus delta plus 317 00:16:20,150 --> 00:16:22,570 that can be created in argon. 318 00:16:22,570 --> 00:16:28,120 So the energy goes as polarizability squared and 319 00:16:28,120 --> 00:16:32,140 divided by r to the sixth, where this is the separation. 320 00:16:32,140 --> 00:16:36,300 Not the radius but this is separation. 321 00:16:36,300 --> 00:16:37,430 Dipole separation. 322 00:16:37,430 --> 00:16:39,440 And this is a font issue. 323 00:16:39,440 --> 00:16:41,510 This is proportional to alpha. 324 00:16:41,510 --> 00:16:43,400 So you can tell the difference. 325 00:16:43,400 --> 00:16:44,870 This is a proportional that's to alpha. 326 00:16:44,870 --> 00:16:46,940 Of course you can't tell the difference. 327 00:16:46,940 --> 00:16:48,790 This is contextual. 328 00:16:48,790 --> 00:16:50,880 If I just wrote this by itself, you don't 329 00:16:50,880 --> 00:16:54,310 know what it is. 330 00:16:54,310 --> 00:16:56,150 My goodness you're nervous. 331 00:16:56,150 --> 00:16:57,590 So nervous. 332 00:16:57,590 --> 00:16:59,590 All right let's take a look at-- 333 00:16:59,590 --> 00:17:00,680 here's the cartoons. 334 00:17:00,680 --> 00:17:01,640 Induced dipole. 335 00:17:01,640 --> 00:17:04,160 And here's London's paper. 336 00:17:04,160 --> 00:17:07,710 Here's London's paper as it first appeared in 1930. 337 00:17:07,710 --> 00:17:10,190 Theory and System of Molecular Forces. 338 00:17:10,190 --> 00:17:12,930 And here's some cartoons from your book. 339 00:17:12,930 --> 00:17:15,630 This is helium, showing helium. 340 00:17:15,630 --> 00:17:17,680 It's kind of funny how the artist shows. 341 00:17:17,680 --> 00:17:20,150 Like there's these two helium atoms and they're absolutely 342 00:17:20,150 --> 00:17:23,160 immobile and all of a sudden one of the helium atom starts 343 00:17:23,160 --> 00:17:25,700 jiggling and then it induces a dipole moment 344 00:17:25,700 --> 00:17:26,750 in the other one. 345 00:17:26,750 --> 00:17:30,740 That's not how it happens, but anyway. 346 00:17:30,740 --> 00:17:32,560 Alright this is hydrogen. 347 00:17:32,560 --> 00:17:34,150 Same thing as iodine. 348 00:17:34,150 --> 00:17:37,000 All right so there's delta plus, delta minus. 349 00:17:37,000 --> 00:17:39,000 Because the electrons are moving even though there's a 350 00:17:39,000 --> 00:17:40,210 strong covalent bond inside. 351 00:17:40,210 --> 00:17:43,310 So let me just make the point, I want to make sure people are 352 00:17:43,310 --> 00:17:45,950 very clear about primary versus secondary bonding. 353 00:17:45,950 --> 00:17:47,610 So if I look at iodine. 354 00:17:47,610 --> 00:17:48,660 I2. 355 00:17:48,660 --> 00:17:50,080 This is a covalent bond. 356 00:17:50,080 --> 00:17:51,360 It's homonuclear. 357 00:17:51,360 --> 00:17:54,480 This covalent bond, there's no net dipole moment. 358 00:17:54,480 --> 00:17:56,840 Here's another iodine. 359 00:17:56,840 --> 00:17:59,000 But then we have time fluctuating dipoles. 360 00:17:59,000 --> 00:18:02,690 So this delta minus isn't because this is the bond here. 361 00:18:02,690 --> 00:18:05,020 This is the primary bond. 362 00:18:05,020 --> 00:18:07,660 This is the primary bond and it's covalent. 363 00:18:07,660 --> 00:18:10,110 And this is the secondary bond. 364 00:18:10,110 --> 00:18:14,073 And the secondary bond is London Dispersion Force or van 365 00:18:14,073 --> 00:18:17,360 der Waals bond because this is induced dipole. 366 00:18:22,350 --> 00:18:25,500 So now let's look at-- oh here's another one. 367 00:18:25,500 --> 00:18:27,970 This one gets on my nerves. 368 00:18:27,970 --> 00:18:29,460 Oh actually this is good. 369 00:18:29,460 --> 00:18:31,140 This is this the bad one. 370 00:18:31,140 --> 00:18:33,300 All right so I'm going to show you polarizability. 371 00:18:33,300 --> 00:18:38,510 So here's a series of sp3 hybridized chains. 372 00:18:38,510 --> 00:18:40,580 Propane, octane, icosane. 373 00:18:40,580 --> 00:18:41,630 They're all the same. 374 00:18:41,630 --> 00:18:43,330 They look like this. 375 00:18:43,330 --> 00:18:46,000 Ch is sp3 hybridized. 376 00:18:46,000 --> 00:18:47,650 And so you just have-- 377 00:18:47,650 --> 00:18:50,240 so what is that, C 3 H H. 378 00:18:50,240 --> 00:18:51,650 So it's just-- 379 00:18:51,650 --> 00:18:54,320 so you have 1, 2, 3 4. so there's hydrogen, hydrogen, 380 00:18:54,320 --> 00:18:56,780 hydrogen, and on the fourth one I'm going to make it flat 381 00:18:56,780 --> 00:18:59,130 instead of trying to make a tetrahedron. 382 00:18:59,130 --> 00:19:00,980 So 1, 2, 3 hydrogens. 383 00:19:00,980 --> 00:19:02,420 The fourth one is carbon. 384 00:19:02,420 --> 00:19:05,610 1, 2 hydrogens, carbon carbon. 385 00:19:05,610 --> 00:19:08,220 1, 2 3 hydrogens. 386 00:19:08,220 --> 00:19:09,740 So this is C3H8. 387 00:19:09,740 --> 00:19:12,620 And compare that to the longer one. 388 00:19:12,620 --> 00:19:16,170 Which is octane, which is C8H18. 389 00:19:16,170 --> 00:19:21,790 So it's 1, 2, 3, 4, 5, 6, 7, 8. 390 00:19:21,790 --> 00:19:24,970 And all you have got to do is, 1, 2, 3, 4, 5, 6, 7, 8. 391 00:19:24,970 --> 00:19:28,930 You put four sticks off of every carbon. 392 00:19:28,930 --> 00:19:29,900 You can't go wrong. 393 00:19:29,900 --> 00:19:30,540 You count them up. 394 00:19:30,540 --> 00:19:33,510 You got C8H18. 395 00:19:33,510 --> 00:19:34,500 Look at the structures. 396 00:19:34,500 --> 00:19:35,570 They are the same. 397 00:19:35,570 --> 00:19:41,220 So how come propane is a gas at room temperature? 398 00:19:41,220 --> 00:19:43,940 Whereas octane, which is the principal constituent of 399 00:19:43,940 --> 00:19:47,780 gasoline, is a liquid at room temperature? 400 00:19:47,780 --> 00:19:50,310 It all comes down to polarizability. 401 00:19:50,310 --> 00:19:52,060 This one's got a longer corral. 402 00:19:52,060 --> 00:19:56,320 So if you like the delta minus versus the delta plus, it's 403 00:19:56,320 --> 00:20:00,540 basically the same here except that separation is bigger. 404 00:20:00,540 --> 00:20:03,590 All right, so that's pretty good. 405 00:20:03,590 --> 00:20:05,380 This is the one that gets on my nerves. 406 00:20:05,380 --> 00:20:06,110 You see this? 407 00:20:06,110 --> 00:20:08,150 It's exactly what I just showed you. 408 00:20:08,150 --> 00:20:10,010 So here's methane. 409 00:20:10,010 --> 00:20:11,460 And there's the propane. 410 00:20:11,460 --> 00:20:15,120 Here's butane and somewhere between butane and pentane we 411 00:20:15,120 --> 00:20:17,250 cross the the line at room temperature. 412 00:20:17,250 --> 00:20:20,960 So pentane is liquid, butane is gas. 413 00:20:20,960 --> 00:20:24,220 And you keep going up, up, up, and finally if you get to C20 414 00:20:24,220 --> 00:20:27,420 you go from liquid to solid because now the van der Waals 415 00:20:27,420 --> 00:20:30,180 forces are strong enough that even at room temperature you 416 00:20:30,180 --> 00:20:30,810 make a solid. 417 00:20:30,810 --> 00:20:32,710 It looks like paraffin. 418 00:20:32,710 --> 00:20:35,450 That's why you can melt paraffin, because it's got 419 00:20:35,450 --> 00:20:37,280 weak van der Waals forces. 420 00:20:37,280 --> 00:20:40,420 So temperature disrupts and it reforms. If you try to break a 421 00:20:40,420 --> 00:20:42,980 covalent bond, you pyrolyze the thing and you 422 00:20:42,980 --> 00:20:44,320 don't get it back. 423 00:20:44,320 --> 00:20:47,540 Secondary bonds allow for ready processing. 424 00:20:47,540 --> 00:20:49,400 Here's what bothers me about this. 425 00:20:49,400 --> 00:20:52,310 Instead of talking about polarizability, which is a 426 00:20:52,310 --> 00:20:54,426 physical quantity that means something. 427 00:20:54,426 --> 00:20:55,480 They say molecular weight. 428 00:20:55,480 --> 00:20:58,340 Well it's true that these scales, these are heavier and 429 00:20:58,340 --> 00:20:59,460 it's monotonic. 430 00:20:59,460 --> 00:21:04,340 But to me what's the relevant physics between the ability to 431 00:21:04,340 --> 00:21:07,120 form van der Waals bonds and the mass. 432 00:21:07,120 --> 00:21:08,350 It's just a dumb thing. 433 00:21:08,350 --> 00:21:10,030 It's not a gravitational effect. 434 00:21:10,030 --> 00:21:11,760 So that's why this thing is stupid. 435 00:21:11,760 --> 00:21:14,800 And you see it all over in chemistry books. 436 00:21:14,800 --> 00:21:16,630 And it's just dumb. 437 00:21:16,630 --> 00:21:19,040 And if you put that on my exam you're not going to 438 00:21:19,040 --> 00:21:20,770 get points for it. 439 00:21:20,770 --> 00:21:22,020 Because that's dumb. 440 00:21:24,540 --> 00:21:25,810 OK let's go to the next one. 441 00:21:25,810 --> 00:21:29,270 There's a third type of bonding. 442 00:21:29,270 --> 00:21:31,950 There are certain things I feel strongly about and that's 443 00:21:31,950 --> 00:21:33,170 one of them. 444 00:21:33,170 --> 00:21:35,680 OK so let's look at the third type of secondary bonding. 445 00:21:35,680 --> 00:21:37,600 The third type of secondary bonding is 446 00:21:37,600 --> 00:21:40,120 called hydrogen bonding. 447 00:21:40,120 --> 00:21:43,010 Hydrogen bonding. 448 00:21:43,010 --> 00:21:45,560 It's a type of secondary bonding. 449 00:21:45,560 --> 00:21:54,300 And it occurs between hydrogen and the most electronegative 450 00:21:54,300 --> 00:21:57,760 elements, fluorine, oxygen, nitrogen. 451 00:22:00,760 --> 00:22:01,410 Why these? 452 00:22:01,410 --> 00:22:03,700 Because they have very, very high average 453 00:22:03,700 --> 00:22:05,000 valance electron energy. 454 00:22:05,000 --> 00:22:07,960 So the average valance electron energy I'm quoting in 455 00:22:07,960 --> 00:22:11,530 that second column, not megajoules per mole, but in 456 00:22:11,530 --> 00:22:14,190 sensible units like electron volts. 457 00:22:14,190 --> 00:22:18,440 And so you can see, when you get up around 18, 19 electron 458 00:22:18,440 --> 00:22:20,560 volts you cross a threshold. 459 00:22:20,560 --> 00:22:23,320 And that's the electronegativity as 460 00:22:23,320 --> 00:22:27,320 represented by average valance electron energy. 461 00:22:27,320 --> 00:22:30,660 So you can see that as the electronegativity gets beyond 462 00:22:30,660 --> 00:22:34,330 some threshold value, roughly 3, then you can 463 00:22:34,330 --> 00:22:36,160 form hydrogen bonds. 464 00:22:36,160 --> 00:22:44,280 So this is owing to high average valance electron 465 00:22:44,280 --> 00:22:47,600 energy, or if you like electronegativity. 466 00:22:47,600 --> 00:22:55,110 Which means very strong polarity in the covalent bond. 467 00:22:59,290 --> 00:23:02,420 So you say, well there's polar and there's even more polar. 468 00:23:02,420 --> 00:23:03,960 So let's see what happens. 469 00:23:03,960 --> 00:23:06,380 So I'm going to use a prototypical value here. 470 00:23:06,380 --> 00:23:07,860 I'm going to look at HF. 471 00:23:07,860 --> 00:23:11,250 So if I look at HF let's go through the Lewis structure. 472 00:23:11,250 --> 00:23:13,180 There's H with it's 1 electron. 473 00:23:13,180 --> 00:23:14,620 And F with the 7. 474 00:23:14,620 --> 00:23:17,770 1, 2, 3, 4, 5, 6, 7. 475 00:23:17,770 --> 00:23:19,570 And so we have a covalent bond here. 476 00:23:19,570 --> 00:23:22,440 We know fluorine is the most electronegative so we have a 477 00:23:22,440 --> 00:23:24,120 dipole moment here. 478 00:23:24,120 --> 00:23:29,060 Now the dipole moment is very strong here. 479 00:23:29,060 --> 00:23:31,450 This is an accounting procedure to put the two 480 00:23:31,450 --> 00:23:34,510 electrons, but it no way represents the physical 481 00:23:34,510 --> 00:23:35,810 position of these electrons. 482 00:23:35,810 --> 00:23:39,360 They're brought in very close to the fluorine. 483 00:23:39,360 --> 00:23:42,840 So it's not some symmetrically disposed between H and the F. 484 00:23:42,840 --> 00:23:45,500 So I can't tell anything about whether HF is a 485 00:23:45,500 --> 00:23:46,720 solid, liquid or gas. 486 00:23:46,720 --> 00:23:47,570 Why? 487 00:23:47,570 --> 00:23:48,890 Because there's only one sitting here. 488 00:23:48,890 --> 00:23:50,410 I have got to put at least one more. 489 00:23:50,410 --> 00:23:53,440 Because this is a primary bond. 490 00:23:53,440 --> 00:23:55,500 And it tells me how H bonds to F. 491 00:23:55,500 --> 00:23:58,570 It doesn't tell me how one HF bonds to another HF. 492 00:23:58,570 --> 00:24:01,980 So I'm going to put another HF over here. 493 00:24:01,980 --> 00:24:03,360 So here's another HF. 494 00:24:03,360 --> 00:24:06,380 And it's also dipole. 495 00:24:06,380 --> 00:24:08,910 But there's something special about that. 496 00:24:08,910 --> 00:24:11,640 Already there's a dipole-dipole interaction. 497 00:24:11,640 --> 00:24:14,090 But the hydrogen bond is much stronger. 498 00:24:14,090 --> 00:24:15,290 It's much stronger. 499 00:24:15,290 --> 00:24:17,390 And why is it stronger? 500 00:24:17,390 --> 00:24:21,960 The electron in this hydrogen on the right is pulled towards 501 00:24:21,960 --> 00:24:27,140 the fluorine to such an extent that this hydrogen is so 502 00:24:27,140 --> 00:24:31,950 denuded of its electrons that it's acting as proton-like. 503 00:24:31,950 --> 00:24:32,800 It's proton-like. 504 00:24:32,800 --> 00:24:35,890 Now don't tell people that Professor Sadoway said that 505 00:24:35,890 --> 00:24:38,370 hydrogen inside an HF is a proton. 506 00:24:38,370 --> 00:24:41,600 It's not a proton but it's starting to get more nearly 507 00:24:41,600 --> 00:24:42,435 like a proton. 508 00:24:42,435 --> 00:24:44,680 Now what do we know about a proton? 509 00:24:44,680 --> 00:24:46,030 Positive charge. 510 00:24:46,030 --> 00:24:48,100 Tiny high-charge density. 511 00:24:48,100 --> 00:24:54,820 So this hydrogen's looking forlornly over at its electron 512 00:24:54,820 --> 00:24:57,220 that's being hogged by the fluorine to a right. 513 00:24:57,220 --> 00:24:58,720 And what do we know about these? 514 00:24:58,720 --> 00:25:00,990 Oh it's time for colored chalk. 515 00:25:00,990 --> 00:25:02,100 It's time for colored chalk! 516 00:25:02,100 --> 00:25:03,790 What color are those? 517 00:25:03,790 --> 00:25:07,730 They're red because they're non-bonding. 518 00:25:07,730 --> 00:25:10,270 And the bonding are the blue in-between. 519 00:25:10,270 --> 00:25:13,040 And what do we know about the volume occupied by a 520 00:25:13,040 --> 00:25:16,010 non-bonding pair versus a bonding pair? 521 00:25:16,010 --> 00:25:17,090 It's larger. 522 00:25:17,090 --> 00:25:18,630 Because they're not constrained. 523 00:25:18,630 --> 00:25:20,990 So not only do we have a non-bonding pair, 524 00:25:20,990 --> 00:25:22,160 they're not just here. 525 00:25:22,160 --> 00:25:24,200 They're sort of flopping around. 526 00:25:24,200 --> 00:25:27,840 Hanging way out and there's this thing here that's almost 527 00:25:27,840 --> 00:25:30,830 denuded of its electrons, so it starts looking over here 528 00:25:30,830 --> 00:25:32,910 saying, if I can't get any action over 529 00:25:32,910 --> 00:25:34,160 here, what about here? 530 00:25:36,940 --> 00:25:41,645 So that proton starts establishing contact with the 531 00:25:41,645 --> 00:25:47,100 non-bonding pair of electrons on the adjacent fluorine And 532 00:25:47,100 --> 00:25:49,560 this is the hydrogen bond. 533 00:25:49,560 --> 00:25:52,320 The hydrogen bond is here. 534 00:25:52,320 --> 00:25:54,220 The hydrogen bond is not here. 535 00:25:54,220 --> 00:25:56,590 If you write this I will give you a 0. 536 00:25:56,590 --> 00:25:58,880 I'll give you a 0 with a circle around it. 537 00:25:58,880 --> 00:26:00,260 It's called the doughnut. 538 00:26:00,260 --> 00:26:02,860 That's what you get when you write something so stupid. 539 00:26:02,860 --> 00:26:05,200 This is not the hydrogen bond. 540 00:26:05,200 --> 00:26:06,870 This is the hydrogen bond. 541 00:26:06,870 --> 00:26:09,350 OK? 542 00:26:09,350 --> 00:26:10,620 So it works. 543 00:26:10,620 --> 00:26:12,390 It works. 544 00:26:12,390 --> 00:26:14,180 Now let's see the effect of it. 545 00:26:14,180 --> 00:26:15,320 Let's see the effect. 546 00:26:15,320 --> 00:26:22,190 Alright so here's a cartoon showing that in water the 547 00:26:22,190 --> 00:26:24,970 hydrogen-oxygen spacing is about 1 angstrom. 548 00:26:24,970 --> 00:26:29,360 And the hydrogen-oxygen spacing in adjacent water 549 00:26:29,360 --> 00:26:32,600 molecules can be less than 2 angstroms. So there's 550 00:26:32,600 --> 00:26:33,660 certainly a difference. 551 00:26:33,660 --> 00:26:34,610 I mean it can't be the same. 552 00:26:34,610 --> 00:26:37,120 If it were the same it would be a covalent primary bond. 553 00:26:37,120 --> 00:26:39,870 You'd have a network. 554 00:26:39,870 --> 00:26:44,530 Now this is interesting here because this shows the values 555 00:26:44,530 --> 00:26:46,890 that are given on your periodic table, which were 556 00:26:46,890 --> 00:26:48,730 obtained without the use the average 557 00:26:48,730 --> 00:26:50,250 valance electron energies. 558 00:26:50,250 --> 00:26:51,830 These trends are correct. 559 00:26:51,830 --> 00:26:52,560 But look at this one. 560 00:26:52,560 --> 00:26:54,920 They've got chlorine up at 3.16. 561 00:26:54,920 --> 00:26:56,660 And all of these have been revised. 562 00:26:56,660 --> 00:26:59,880 Now on a test, just use what you've been given on the 563 00:26:59,880 --> 00:27:00,740 periodic table. 564 00:27:00,740 --> 00:27:03,790 But I want you to understand how this is rationalized with 565 00:27:03,790 --> 00:27:07,400 better data coming from photoelectron spectroscopy. 566 00:27:07,400 --> 00:27:08,620 So now I want to show you the 567 00:27:08,620 --> 00:27:11,050 implications of hydrogen bonding. 568 00:27:11,050 --> 00:27:12,230 The implications. 569 00:27:12,230 --> 00:27:14,580 So I've got 4 homologous series. 570 00:27:14,580 --> 00:27:18,130 So they're all element plus hydrogen. 571 00:27:18,130 --> 00:27:20,930 So let's start with the group 14. 572 00:27:20,930 --> 00:27:22,180 That's shown here in purple. 573 00:27:24,820 --> 00:27:27,120 And what do we have? 574 00:27:27,120 --> 00:27:31,870 All of these, we'll start with methane. 575 00:27:31,870 --> 00:27:35,440 CH4, so that's a central atom. 576 00:27:35,440 --> 00:27:36,480 1, 2, 3 4. 577 00:27:36,480 --> 00:27:37,910 They're all tetrahedral. 578 00:27:37,910 --> 00:27:39,500 Hydrogens at the corners. 579 00:27:39,500 --> 00:27:41,160 Non-polar. 580 00:27:41,160 --> 00:27:43,440 And no hydrogen bonding capability. 581 00:27:43,440 --> 00:27:48,190 So one methane bonds to another methane by weak van 582 00:27:48,190 --> 00:27:49,890 der Waals forces. 583 00:27:49,890 --> 00:27:50,770 That's how it does it. 584 00:27:50,770 --> 00:27:53,910 It doesn't matter if I substitute the carbon with 585 00:27:53,910 --> 00:27:56,580 silicon or germanium or tin. 586 00:27:56,580 --> 00:27:57,840 I can put SnH4. 587 00:27:57,840 --> 00:28:02,020 And how does SnH4 bond to another SnH4? 588 00:28:02,020 --> 00:28:05,350 It's just by London Dispersion Forces. 589 00:28:05,350 --> 00:28:07,120 That's all that's operative here. 590 00:28:07,120 --> 00:28:10,250 And so we have, what makes sense here, is that the 591 00:28:10,250 --> 00:28:13,170 heavier, more massive-- 592 00:28:13,170 --> 00:28:17,040 no, the ones that have greater polarizability have a higher 593 00:28:17,040 --> 00:28:18,130 boiling point. 594 00:28:18,130 --> 00:28:20,660 Because their van der Waals forces are stronger. 595 00:28:20,660 --> 00:28:23,350 That means you have to go to a higher temperature to achieve 596 00:28:23,350 --> 00:28:24,600 disruption. 597 00:28:24,600 --> 00:28:28,540 Same temperature, weak van der Waals force: gas. 598 00:28:28,540 --> 00:28:30,540 Same temperature, strong van der Waals 599 00:28:30,540 --> 00:28:32,210 force: liquid or solid. 600 00:28:32,210 --> 00:28:34,870 And you see the boiling point here. 601 00:28:34,870 --> 00:28:38,080 Monotonic from the lightest to the heaviest. Now let's go to 602 00:28:38,080 --> 00:28:38,550 the next one. 603 00:28:38,550 --> 00:28:40,820 Let's go to the green line, group 15. 604 00:28:40,820 --> 00:28:42,540 Well group 15, what's that look like? 605 00:28:42,540 --> 00:28:44,690 Let's look at the structure of group 15. 606 00:28:44,690 --> 00:28:46,250 Group 15 is-- 607 00:28:46,250 --> 00:28:47,920 ammonia is one of them. 608 00:28:47,920 --> 00:28:50,230 So we can look at the structure of ammonia. 609 00:28:50,230 --> 00:28:52,690 And if we use VSEPR we'll end up with something 610 00:28:52,690 --> 00:28:54,370 that looks like this. 611 00:28:54,370 --> 00:28:55,350 I'll go through the whole thing. 612 00:28:55,350 --> 00:28:57,110 You're going to end up with three bonds like this. 613 00:28:57,110 --> 00:29:00,350 It's a tetrahedral skeleton with a lone pair. 614 00:29:00,350 --> 00:29:01,420 Ah, colored chalk. 615 00:29:01,420 --> 00:29:03,610 So now, what happens? 616 00:29:03,610 --> 00:29:05,180 I can't say anything about this. 617 00:29:05,180 --> 00:29:07,470 Why can't I say anything about whether this is a solid, 618 00:29:07,470 --> 00:29:08,730 liquid or a gas? 619 00:29:08,730 --> 00:29:10,030 It's the only one there. 620 00:29:10,030 --> 00:29:11,860 I have got to put another one. 621 00:29:11,860 --> 00:29:14,220 So put another one up here. 622 00:29:14,220 --> 00:29:15,470 N. 623 00:29:15,470 --> 00:29:16,120 H. 624 00:29:16,120 --> 00:29:16,690 H. 625 00:29:16,690 --> 00:29:19,210 H. 626 00:29:19,210 --> 00:29:21,990 Same gambit with the hydrogen fluoride. 627 00:29:21,990 --> 00:29:25,520 This hydrogen sees this lone pair and establishes a 628 00:29:25,520 --> 00:29:26,720 hydrogen bond. 629 00:29:26,720 --> 00:29:29,500 And that adds to what otherwise would have been a 630 00:29:29,500 --> 00:29:30,390 dipole-dipole. 631 00:29:30,390 --> 00:29:32,500 This has a net dipole along moment, agreed? 632 00:29:32,500 --> 00:29:34,320 There's a dipole-dipole moment here. 633 00:29:34,320 --> 00:29:36,230 But this bond is even stronger. 634 00:29:36,230 --> 00:29:38,750 The hydrogen bond is even stronger than dipole-dipole 635 00:29:38,750 --> 00:29:39,990 interactions. 636 00:29:39,990 --> 00:29:43,330 So now let's look at the graph up here. 637 00:29:43,330 --> 00:29:47,110 So what about in phosphene, PH3? 638 00:29:47,110 --> 00:29:49,410 No phosphorous isn't electronegative enough. 639 00:29:49,410 --> 00:29:54,760 So in phosphene, in arsine and in stibine we don't have 640 00:29:54,760 --> 00:29:56,000 hydrogen bonding. 641 00:29:56,000 --> 00:29:59,760 So you see this series in the group 4 here? 642 00:29:59,760 --> 00:30:04,740 It goes monotonic from the heaviest element down to the 643 00:30:04,740 --> 00:30:05,540 lightest element. 644 00:30:05,540 --> 00:30:08,800 But here, heaviest, less heavy, less heavy, and the 645 00:30:08,800 --> 00:30:11,900 lightest element that should be down here is up here. 646 00:30:11,900 --> 00:30:14,710 Why is the lightest compound not down here? 647 00:30:14,710 --> 00:30:17,070 Because of the addition of hydrogen bond. 648 00:30:17,070 --> 00:30:18,980 So ammonia is off the line. 649 00:30:18,980 --> 00:30:22,050 This line shows the trend based on dipole-dipole 650 00:30:22,050 --> 00:30:25,210 interactions only. 651 00:30:25,210 --> 00:30:26,280 And you can see the difference. 652 00:30:26,280 --> 00:30:30,260 See if you have van der Waals forces alone, versus 653 00:30:30,260 --> 00:30:32,470 dipole-dipole, dipole-dipole are stronger. 654 00:30:32,470 --> 00:30:37,760 So SnH4 has a lower boiling point than SbH3. 655 00:30:37,760 --> 00:30:40,420 I can't predict this but I could ask you, if I gave you 656 00:30:40,420 --> 00:30:45,430 this data, I'd say, can you explain this to me? 657 00:30:45,430 --> 00:30:46,390 Let's keep going. 658 00:30:46,390 --> 00:30:48,420 Let's go to group 16. 659 00:30:48,420 --> 00:30:51,720 So, telluride, selenide, suphide, 660 00:30:51,720 --> 00:30:53,890 the oxide, H2O, water. 661 00:30:53,890 --> 00:30:58,580 It should have a boiling point of minus 100 centigrade were 662 00:30:58,580 --> 00:31:00,970 it not for hydrogen bonding. 663 00:31:00,970 --> 00:31:02,470 How does water work? 664 00:31:02,470 --> 00:31:05,340 Again, SP3 hybridization. 665 00:31:05,340 --> 00:31:11,000 Oxygen, 1, 2, 3, 4. 666 00:31:11,000 --> 00:31:12,570 Two lone pairs. 667 00:31:12,570 --> 00:31:16,440 1, 2, H, H. 668 00:31:16,440 --> 00:31:17,300 And now what happens? 669 00:31:17,300 --> 00:31:22,710 I bring another water molecule over here 670 00:31:22,710 --> 00:31:27,080 and hydrogen bonding. 671 00:31:27,080 --> 00:31:31,380 And that hydrogen bond raises the temperature, raises the 672 00:31:31,380 --> 00:31:33,920 requirement for thermal disruption and moves the 673 00:31:33,920 --> 00:31:37,250 boiling point of water up to 100 degrees celsius. 674 00:31:37,250 --> 00:31:39,090 It it weren't for this we wouldn't have this 675 00:31:39,090 --> 00:31:41,020 conversation. 676 00:31:41,020 --> 00:31:43,580 Because we wouldn't have evolved as a species capable 677 00:31:43,580 --> 00:31:48,010 of conducting business at room temperature if water boiled at 678 00:31:48,010 --> 00:31:49,750 minus 100 celsius. 679 00:31:49,750 --> 00:31:52,166 Hydrogen bonding is critical. 680 00:31:52,166 --> 00:31:56,040 It's absolutely critical. 681 00:31:56,040 --> 00:31:58,150 So now you know. 682 00:31:58,150 --> 00:31:59,990 And this isn't just some little bit of 683 00:31:59,990 --> 00:32:01,730 pedantry for a professor. 684 00:32:01,730 --> 00:32:03,080 This is very important. 685 00:32:03,080 --> 00:32:07,830 Because we're going to learn later that, when it comes to 686 00:32:07,830 --> 00:32:14,010 biochemistry, we will appreciate that most 687 00:32:14,010 --> 00:32:17,920 biochemicals are made of carbon, oxygen, 688 00:32:17,920 --> 00:32:20,670 nitrogen and hydrogen. 689 00:32:20,670 --> 00:32:23,400 And that means you can have hydrogen bonds here. 690 00:32:23,400 --> 00:32:24,990 And you can have hydrogen bonds here. 691 00:32:24,990 --> 00:32:28,160 Hydrogen bonding is critical to life. 692 00:32:28,160 --> 00:32:32,060 So this is a very important thing to know about. 693 00:32:32,060 --> 00:32:34,210 OK we have a minute or two. 694 00:32:34,210 --> 00:32:37,120 So you can see polarizability increases here but hydrogen 695 00:32:37,120 --> 00:32:38,690 bonding operative here. 696 00:32:38,690 --> 00:32:40,810 That's explains the mystery. 697 00:32:40,810 --> 00:32:42,390 All right we're going to jump over this because 698 00:32:42,390 --> 00:32:43,950 we are out of time. 699 00:32:43,950 --> 00:32:48,030 And so I will simply show you a few pictures. 700 00:32:48,030 --> 00:32:49,490 Pictures! 701 00:32:49,490 --> 00:32:53,770 So this is a conference in Copenhagen in June of 1936. 702 00:32:53,770 --> 00:32:56,160 And there's Fritz London. 703 00:32:56,160 --> 00:32:59,490 But look at who else is at the conference. 704 00:32:59,490 --> 00:33:05,810 Niels Bohr, Wolfgang Pauli, Werner Heisenberg, Max Born. 705 00:33:05,810 --> 00:33:07,430 Remember the Born Exponent? 706 00:33:07,430 --> 00:33:09,140 You better remember it for Wednesday. 707 00:33:09,140 --> 00:33:11,620 This is Lise Meitner, you haven't met her yet. 708 00:33:11,620 --> 00:33:12,880 We'll get to her. 709 00:33:12,880 --> 00:33:14,050 This is Walter Stern. 710 00:33:14,050 --> 00:33:15,510 Stern-Gerlach. 711 00:33:15,510 --> 00:33:17,470 And this is James Franck. 712 00:33:17,470 --> 00:33:20,440 That's just the first two rows! 713 00:33:20,440 --> 00:33:22,930 That's quite a conference. 714 00:33:22,930 --> 00:33:26,750 Here's a picture of Fritz London sitting on a bench in 715 00:33:26,750 --> 00:33:29,640 Berlin with Erwin Schrodinger. 716 00:33:29,640 --> 00:33:32,850 Schrodinger is brooding; he's thinking. 717 00:33:32,850 --> 00:33:34,270 Fritz London is smiling. 718 00:33:34,270 --> 00:33:34,980 You know why? 719 00:33:34,980 --> 00:33:36,330 Because he's figured it out. 720 00:33:36,330 --> 00:33:37,370 He's figured out. 721 00:33:37,370 --> 00:33:39,780 It's time-fluctuating dipole, but he's not going to tell 722 00:33:39,780 --> 00:33:40,570 Schrodinger. 723 00:33:40,570 --> 00:33:42,810 He's going to say, you have to read about it. 724 00:33:42,810 --> 00:33:44,340 But you have to go to the library. 725 00:33:44,340 --> 00:33:46,670 Because if you don't read the primary sources, if you go to 726 00:33:46,670 --> 00:33:48,560 Wikipedia, you won't find this. 727 00:33:48,560 --> 00:33:52,520 Because this is not going to be in Wikipedia in 1913. 728 00:33:52,520 --> 00:33:54,570 OK, last thing-- hold on, hold on, hey wait a minute! 729 00:33:54,570 --> 00:33:55,990 Where are you going? 730 00:33:55,990 --> 00:33:57,250 We're not done yet. 731 00:33:57,250 --> 00:33:59,710 Did the professor say class is dismissed? 732 00:33:59,710 --> 00:34:01,360 No. 733 00:34:01,360 --> 00:34:05,720 So here's a biography of Fritz London, which I would 734 00:34:05,720 --> 00:34:07,940 recommend if you have a few minutes and 735 00:34:07,940 --> 00:34:09,020 you'd like to unwind. 736 00:34:09,020 --> 00:34:11,060 Go out, sit in the sun and read something like this. 737 00:34:11,060 --> 00:34:14,130 It tells the story of how he came up with these ideas. 738 00:34:14,130 --> 00:34:16,480 The rise of fascism in the '30s. 739 00:34:16,480 --> 00:34:19,180 He comes to the United States, takes a teaching position at 740 00:34:19,180 --> 00:34:20,510 Duke University. 741 00:34:20,510 --> 00:34:23,020 All of the people that he met along the way. 742 00:34:23,020 --> 00:34:25,450 All these people that we study, all this stuff, it's 743 00:34:25,450 --> 00:34:27,360 all there from his perspective. 744 00:34:27,360 --> 00:34:29,480 And the perspective of his biographer. 745 00:34:29,480 --> 00:34:30,670 Plus the pictures. 746 00:34:30,670 --> 00:34:33,460 So anyway, primary sources. 747 00:34:33,460 --> 00:34:35,970 Go read the primary sources. 748 00:34:35,970 --> 00:34:38,010 All right, class dismissed.