1 00:00:07,270 --> 00:00:09,497 What is the shape of a molecule? 2 00:00:09,497 --> 00:00:12,167 Well, a molecule is mostly empty space. 3 00:00:12,167 --> 00:00:14,001 Almost all of its mass is concentrated 4 00:00:14,001 --> 00:00:17,368 in the extremely dense nuclei of its atoms. 5 00:00:17,368 --> 00:00:18,307 And its electrons, 6 00:00:18,307 --> 00:00:19,790 which determine how the atoms 7 00:00:19,790 --> 00:00:20,784 are bonded to each other, 8 00:00:20,784 --> 00:00:23,307 are more like clouds of negative charge 9 00:00:23,307 --> 00:00:25,584 than individual, discrete particles. 10 00:00:25,584 --> 00:00:27,394 So, a molecule doesn't have a shape 11 00:00:27,394 --> 00:00:29,139 in the same way that, for example, 12 00:00:29,139 --> 00:00:31,006 a statue has a shape. 13 00:00:31,006 --> 00:00:32,081 But for every molecule, 14 00:00:32,081 --> 00:00:33,456 there's at least one way 15 00:00:33,456 --> 00:00:35,511 to arrange the nuclei and electrons 16 00:00:35,511 --> 00:00:37,712 so as to maximize the attraction 17 00:00:37,712 --> 00:00:38,850 of opposite charges 18 00:00:38,850 --> 00:00:40,386 and minimize the repulsion 19 00:00:40,386 --> 00:00:42,599 of like charges. 20 00:00:42,599 --> 00:00:44,347 Now, let's assume that the only electrons 21 00:00:44,347 --> 00:00:45,891 that matter to a molecule's shape 22 00:00:45,891 --> 00:00:48,808 are the outermost ones from each participating atom. 23 00:00:49,454 --> 00:00:50,504 And let's also assume 24 00:00:50,504 --> 00:00:52,893 that the electron clouds in between atoms, 25 00:00:52,893 --> 00:00:54,561 in other words, a molecule's bonds, 26 00:00:54,561 --> 00:00:57,469 are shaped kind of like sausages. 27 00:00:57,469 --> 00:01:00,289 Remember that nuclei are positively charged 28 00:01:00,289 --> 00:01:02,245 and electrons are negatively charged, 29 00:01:02,245 --> 00:01:03,731 and if all of a molecule's nuclei 30 00:01:03,731 --> 00:01:04,821 were bunched up together 31 00:01:04,821 --> 00:01:06,989 or all of its electrons were bunched up together, 32 00:01:06,989 --> 00:01:09,233 they would just repel each other and fly apart, 33 00:01:09,233 --> 00:01:11,005 and that doesn't help anyone. 34 00:01:11,451 --> 00:01:14,103 In 1776, Alessandro Volta, 35 00:01:14,103 --> 00:01:16,599 decades before he would eventually invent batteries, 36 00:01:16,599 --> 00:01:18,263 discovered methane. 37 00:01:18,263 --> 00:01:22,133 Now, the chemical formula of methane is CH4. 38 00:01:22,133 --> 00:01:23,126 And this formula tells us 39 00:01:23,126 --> 00:01:24,800 that every molecule of methane 40 00:01:24,800 --> 00:01:28,442 is made up of one carbon and four hydrogen atoms, 41 00:01:28,442 --> 00:01:31,139 but it doesn't tell us what's bonded to what 42 00:01:31,139 --> 00:01:34,532 or how they atoms are arranged in 3D space. 43 00:01:34,532 --> 00:01:36,212 From their electron configurations, 44 00:01:36,212 --> 00:01:37,706 we know that carbon can bond 45 00:01:37,706 --> 00:01:39,531 with up to four other atoms 46 00:01:39,531 --> 00:01:41,576 and that each hydrogen can only bond 47 00:01:41,576 --> 00:01:43,034 with one other atom. 48 00:01:43,034 --> 00:01:44,402 So, we can guess 49 00:01:44,402 --> 00:01:46,288 that the carbon should be the central atom 50 00:01:46,288 --> 00:01:48,906 bonded to all the hydrogens. 51 00:01:48,906 --> 00:01:50,040 Now, each bond represents 52 00:01:50,040 --> 00:01:51,658 the sharing of two electrons 53 00:01:51,658 --> 00:01:54,570 and we draw each shared pair of electrons as a line. 54 00:01:54,570 --> 00:01:56,801 So, now we have a flat representation 55 00:01:56,801 --> 00:01:58,264 of this molecule, 56 00:01:58,264 --> 00:02:00,826 but how would it look in three dimensions? 57 00:02:00,826 --> 00:02:01,807 We can reasonably say 58 00:02:01,807 --> 00:02:03,269 that because each of these bonds 59 00:02:03,269 --> 00:02:05,595 is a region of negative electric charge 60 00:02:05,595 --> 00:02:07,403 and like charges repel each other, 61 00:02:07,403 --> 00:02:09,569 the most favorable configuration of atoms 62 00:02:09,569 --> 00:02:12,330 would maximize the distance between bonds. 63 00:02:12,330 --> 00:02:13,743 And to get all the bonds 64 00:02:13,743 --> 00:02:16,071 as far away from each other as possible, 65 00:02:16,071 --> 00:02:18,512 the optimal shape is this. 66 00:02:18,512 --> 00:02:20,858 This is called a tetrahedron. 67 00:02:20,858 --> 00:02:22,901 Now, depending on the different atoms involved, 68 00:02:22,901 --> 00:02:25,323 you can actually get lots of different shapes. 69 00:02:25,323 --> 00:02:28,299 Ammonia, or NH3, is shaped like a pyramid. 70 00:02:28,299 --> 00:02:31,122 Carbon dioxide, or CO2, is a straight line. 71 00:02:31,122 --> 00:02:34,548 Water, H2O, is bent like your elbow would be bent. 72 00:02:34,548 --> 00:02:37,129 And chlorine trifluoride, or ClF3, 73 00:02:37,129 --> 00:02:39,215 is shaped like the letter T. 74 00:02:39,215 --> 00:02:40,909 Remember that what we've been doing here 75 00:02:40,909 --> 00:02:43,561 is expanding on our model of atoms and electrons 76 00:02:43,561 --> 00:02:45,843 to build up to 3D shapes. 77 00:02:45,843 --> 00:02:46,937 We'd have to do experiments 78 00:02:46,937 --> 00:02:48,138 to figure out if these molecules 79 00:02:48,138 --> 00:02:50,489 actually do have the shapes we predict. 80 00:02:50,489 --> 00:02:51,362 Spoiler alert: 81 00:02:51,362 --> 00:02:53,554 most of the do, but some of them don't. 82 00:02:53,554 --> 00:02:54,938 Now, shapes get more complicated 83 00:02:54,938 --> 00:02:56,937 as you increase the number of atoms. 84 00:02:56,937 --> 00:02:58,574 All the examples we just talked about 85 00:02:58,574 --> 00:03:01,071 had one obviously central atom, 86 00:03:01,071 --> 00:03:02,325 but most molecules, 87 00:03:02,325 --> 00:03:03,948 from relatively small pharmaceuticals 88 00:03:03,948 --> 00:03:05,374 all the way up to long polymers 89 00:03:05,374 --> 00:03:07,743 like DNA or proteins, don't. 90 00:03:07,743 --> 00:03:08,808 The key thing to remember 91 00:03:08,808 --> 00:03:10,879 is that bonded atoms will arrange themselves 92 00:03:10,879 --> 00:03:13,612 to maximize the attraction between opposite charges 93 00:03:13,612 --> 00:03:16,717 and minimize the repulsion between like charges. 94 00:03:16,717 --> 00:03:18,969 Some molecules even have two or more 95 00:03:18,969 --> 00:03:20,508 stable arrangements of atoms, 96 00:03:20,508 --> 00:03:22,470 and we can actually get really cool chemistry 97 00:03:22,470 --> 00:03:25,190 from the switches between those configurations, 98 00:03:25,190 --> 00:03:27,306 even when the composition of that molecule, 99 00:03:27,306 --> 00:03:29,894 that's to say the number and identity of its atoms, 100 00:03:29,894 --> 00:03:32,185 has not changed at all.