WEBVTT 00:00:07.270 --> 00:00:09.497 What is the shape of a molecule? 00:00:09.497 --> 00:00:12.167 Well, a molecule is mostly empty space. 00:00:12.167 --> 00:00:14.001 Almost all of its mass is concentrated 00:00:14.001 --> 00:00:17.368 in the extremely dense nuclei of its atoms. 00:00:17.368 --> 00:00:18.307 And its electrons, 00:00:18.307 --> 00:00:19.790 which determine how the atoms 00:00:19.790 --> 00:00:20.784 are bonded to each other, 00:00:20.784 --> 00:00:23.307 are more like clouds of negative charge 00:00:23.307 --> 00:00:25.584 than individual, discrete particles. 00:00:25.584 --> 00:00:27.394 So, a molecule doesn't have a shape 00:00:27.394 --> 00:00:29.139 in the same way that, for example, 00:00:29.139 --> 00:00:31.006 a statue has a shape. 00:00:31.006 --> 00:00:32.081 But for every molecule, 00:00:32.081 --> 00:00:33.456 there's at least one way 00:00:33.456 --> 00:00:35.511 to arrange the nuclei and electrons 00:00:35.511 --> 00:00:37.712 so as to maximize the attraction 00:00:37.712 --> 00:00:38.850 of opposite charges 00:00:38.850 --> 00:00:40.386 and minimize the repulsion 00:00:40.386 --> 00:00:42.599 of like charges. 00:00:42.599 --> 00:00:44.347 Now, let's assume that the only electrons 00:00:44.347 --> 00:00:45.891 that matter to a molecule's shape 00:00:45.891 --> 00:00:48.808 are the outermost ones from each participating atom. 00:00:49.454 --> 00:00:50.504 And let's also assume 00:00:50.504 --> 00:00:52.893 that the electron clouds in between atoms, 00:00:52.893 --> 00:00:54.561 in other words, a molecule's bonds, 00:00:54.561 --> 00:00:57.469 are shaped kind of like sausages. 00:00:57.469 --> 00:01:00.289 Remember that nuclei are positively charged 00:01:00.289 --> 00:01:02.245 and electrons are negatively charged, 00:01:02.245 --> 00:01:03.731 and if all of a molecule's nuclei 00:01:03.731 --> 00:01:04.821 were bunched up together 00:01:04.821 --> 00:01:06.989 or all of its electrons were bunched up together, 00:01:06.989 --> 00:01:09.233 they would just repel each other and fly apart, 00:01:09.233 --> 00:01:11.005 and that doesn't help anyone. 00:01:11.451 --> 00:01:14.103 In 1776, Alessandro Volta, 00:01:14.103 --> 00:01:16.599 decades before he would eventually invent batteries, 00:01:16.599 --> 00:01:18.263 discovered methane. 00:01:18.263 --> 00:01:22.133 Now, the chemical formula of methane is CH4. 00:01:22.133 --> 00:01:23.126 And this formula tells us 00:01:23.126 --> 00:01:24.800 that every molecule of methane 00:01:24.800 --> 00:01:28.442 is made up of one carbon and four hydrogen atoms, 00:01:28.442 --> 00:01:31.139 but it doesn't tell us what's bonded to what 00:01:31.139 --> 00:01:34.532 or how they atoms are arranged in 3D space. 00:01:34.532 --> 00:01:36.212 From their electron configurations, 00:01:36.212 --> 00:01:37.706 we know that carbon can bond 00:01:37.706 --> 00:01:39.531 with up to four other atoms 00:01:39.531 --> 00:01:41.576 and that each hydrogen can only bond 00:01:41.576 --> 00:01:43.034 with one other atom. 00:01:43.034 --> 00:01:44.402 So, we can guess 00:01:44.402 --> 00:01:46.288 that the carbon should be the central atom 00:01:46.288 --> 00:01:48.906 bonded to all the hydrogens. 00:01:48.906 --> 00:01:50.040 Now, each bond represents 00:01:50.040 --> 00:01:51.658 the sharing of two electrons 00:01:51.658 --> 00:01:54.570 and we draw each shared pair of electrons as a line. 00:01:54.570 --> 00:01:56.801 So, now we have a flat representation 00:01:56.801 --> 00:01:58.264 of this molecule, 00:01:58.264 --> 00:02:00.826 but how would it look in three dimensions? 00:02:00.826 --> 00:02:01.807 We can reasonably say 00:02:01.807 --> 00:02:03.269 that because each of these bonds 00:02:03.269 --> 00:02:05.595 is a region of negative electric charge 00:02:05.595 --> 00:02:07.403 and like charges repel each other, 00:02:07.403 --> 00:02:09.569 the most favorable configuration of atoms 00:02:09.569 --> 00:02:12.330 would maximize the distance between bonds. 00:02:12.330 --> 00:02:13.743 And to get all the bonds 00:02:13.743 --> 00:02:16.071 as far away from each other as possible, 00:02:16.071 --> 00:02:18.512 the optimal shape is this. 00:02:18.512 --> 00:02:20.858 This is called a tetrahedron. 00:02:20.858 --> 00:02:22.901 Now, depending on the different atoms involved, 00:02:22.901 --> 00:02:25.323 you can actually get lots of different shapes. 00:02:25.323 --> 00:02:28.299 Ammonia, or NH3, is shaped like a pyramid. 00:02:28.299 --> 00:02:31.122 Carbon dioxide, or CO2, is a straight line. 00:02:31.122 --> 00:02:34.548 Water, H2O, is bent like your elbow would be bent. 00:02:34.548 --> 00:02:37.129 And chlorine trifluoride, or ClF3, 00:02:37.129 --> 00:02:39.215 is shaped like the letter T. 00:02:39.215 --> 00:02:40.909 Remember that what we've been doing here 00:02:40.909 --> 00:02:43.561 is expanding on our model of atoms and electrons 00:02:43.561 --> 00:02:45.843 to build up to 3D shapes. 00:02:45.843 --> 00:02:46.937 We'd have to do experiments 00:02:46.937 --> 00:02:48.138 to figure out if these molecules 00:02:48.138 --> 00:02:50.489 actually do have the shapes we predict. 00:02:50.489 --> 00:02:51.362 Spoiler alert: 00:02:51.362 --> 00:02:53.554 most of the do, but some of them don't. 00:02:53.554 --> 00:02:54.938 Now, shapes get more complicated 00:02:54.938 --> 00:02:56.937 as you increase the number of atoms. 00:02:56.937 --> 00:02:58.574 All the examples we just talked about 00:02:58.574 --> 00:03:01.071 had one obviously central atom, 00:03:01.071 --> 00:03:02.325 but most molecules, 00:03:02.325 --> 00:03:03.948 from relatively small pharmaceuticals 00:03:03.948 --> 00:03:05.374 all the way up to long polymers 00:03:05.374 --> 00:03:07.743 like DNA or proteins, don't. 00:03:07.743 --> 00:03:08.808 The key thing to remember 00:03:08.808 --> 00:03:10.879 is that bonded atoms will arrange themselves 00:03:10.879 --> 00:03:13.612 to maximize the attraction between opposite charges 00:03:13.612 --> 00:03:16.717 and minimize the repulsion between like charges. 00:03:16.717 --> 00:03:18.969 Some molecules even have two or more 00:03:18.969 --> 00:03:20.508 stable arrangements of atoms, 00:03:20.508 --> 00:03:22.470 and we can actually get really cool chemistry 00:03:22.470 --> 00:03:25.190 from the switches between those configurations, 00:03:25.190 --> 00:03:27.306 even when the composition of that molecule, 00:03:27.306 --> 00:03:29.894 that's to say the number and identity of its atoms, 00:03:29.894 --> 00:03:32.185 has not changed at all.