WEBVTT 00:00:06.989 --> 00:00:10.266 You probably know that all stuff is made up of atoms 00:00:10.266 --> 00:00:11.247 and that an atom 00:00:11.247 --> 00:00:15.190 is a really, really, really, really tiny particle. 00:00:15.190 --> 00:00:16.628 Every atom has a core, 00:00:16.628 --> 00:00:18.030 which is made up of at least one 00:00:18.030 --> 00:00:19.542 positively charged particle 00:00:19.542 --> 00:00:21.167 called a proton, 00:00:21.167 --> 00:00:22.421 and in most cases, 00:00:22.421 --> 00:00:25.592 some number of neutral particles called neutrons. 00:00:25.592 --> 00:00:26.966 That core is surrounded 00:00:26.966 --> 00:00:30.148 by negatively charged particles called electrons. 00:00:30.794 --> 00:00:32.638 The identity of an atom is determined 00:00:32.638 --> 00:00:36.922 only by the number of protons in its nucleus. 00:00:36.922 --> 00:00:39.698 Hydrogen is hydrogen because it has just one proton, 00:00:39.698 --> 00:00:41.308 carbon is carbon because it has six, 00:00:41.308 --> 00:00:43.423 gold is gold because it has 79, 00:00:43.423 --> 00:00:44.680 and so on. 00:00:45.140 --> 00:00:47.150 Indulge me in a momentary tangent. 00:00:47.150 --> 00:00:49.516 How do we know about atomic structure? 00:00:49.516 --> 00:00:52.723 We can't see protons, neutrons, or electrons. 00:00:53.814 --> 00:00:55.199 So, we do a bunch of experiments 00:00:55.199 --> 00:00:58.282 and develop a model for what we think is there. 00:00:58.282 --> 00:00:59.655 Then we do some more experiments 00:00:59.655 --> 00:01:01.421 and see if they agree with the model. 00:01:01.421 --> 00:01:02.737 If they do, great. 00:01:02.737 --> 00:01:05.702 If they don't, it might be time for new model. 00:01:05.702 --> 00:01:08.369 We've had lots of very different models for atoms 00:01:08.369 --> 00:01:10.510 since Democritus in 400 B.C. 00:01:10.510 --> 00:01:11.718 and there will almost certainly 00:01:11.718 --> 00:01:13.600 be many more to come. 00:01:13.600 --> 00:01:15.471 Okay, tangent over. 00:01:15.471 --> 00:01:18.232 The cores of atoms tend to stick together, 00:01:18.232 --> 00:01:20.017 but electrons are free to move, 00:01:20.017 --> 00:01:22.795 and this is why chemists love electrons. 00:01:22.795 --> 00:01:24.233 If we could marry them, 00:01:24.233 --> 00:01:25.661 we probably would. 00:01:25.661 --> 00:01:26.981 But electrons are weird. 00:01:26.981 --> 00:01:29.550 They appear to behave either as particles, 00:01:29.550 --> 00:01:31.132 like little baseballs, 00:01:31.132 --> 00:01:33.322 or as waves, like water waves, 00:01:33.322 --> 00:01:36.523 depending on the experiment that we perform. 00:01:36.523 --> 00:01:38.483 One of the weirdest things about electrons 00:01:38.483 --> 00:01:40.697 is that we can't exactly say where they are. 00:01:40.697 --> 00:01:42.739 It's not that we don't have the equipment, 00:01:42.739 --> 00:01:44.141 it's that this uncertainty 00:01:44.141 --> 00:01:46.498 is part of our model of the electron. 00:01:46.498 --> 00:01:49.060 So, we can't pinpoint them, fine. 00:01:49.060 --> 00:01:51.755 But we can say there's a certain probability 00:01:51.755 --> 00:01:53.740 of finding an electron in a given space 00:01:53.740 --> 00:01:55.705 around the nucleus. 00:01:55.705 --> 00:01:58.585 And that means that we can ask the following question: 00:01:58.585 --> 00:02:00.423 if we drew a shape around the nucleus 00:02:00.423 --> 00:02:02.487 such that we would be 95% sure 00:02:02.487 --> 00:02:05.520 of finding a given electron within that shape, 00:02:05.520 --> 00:02:07.067 what would it look like? 00:02:08.590 --> 00:02:10.821 Here are a few of these shapes. 00:02:10.821 --> 00:02:12.569 Chemists call them orbitals, 00:02:12.569 --> 00:02:13.849 and what each one looks like 00:02:13.849 --> 00:02:15.868 depends on, among other things, 00:02:15.868 --> 00:02:17.715 how much energy it has. 00:02:17.715 --> 00:02:19.365 The more energy an orbital has, 00:02:19.365 --> 00:02:21.042 the farther most of its density is 00:02:21.042 --> 00:02:22.294 from the nucleus. 00:02:22.294 --> 00:02:25.275 By they way, why did we pick 95% 00:02:25.275 --> 00:02:26.911 and not 100%? 00:02:26.911 --> 00:02:28.215 Well, that's another quirk 00:02:28.215 --> 00:02:30.003 of our model of the electron. 00:02:30.003 --> 00:02:32.228 Past a certain distance from the nucleus, 00:02:32.228 --> 00:02:34.073 the probability of finding an electron 00:02:34.073 --> 00:02:35.624 starts to decrease 00:02:35.624 --> 00:02:37.936 more or less exponentially, 00:02:37.936 --> 00:02:40.155 which means that while it will approach zero, 00:02:40.155 --> 00:02:42.582 it'll never actually hit zero. 00:02:43.136 --> 00:02:44.298 So, in every atom, 00:02:44.298 --> 00:02:47.438 there is some small, but non-zero, probability 00:02:47.438 --> 00:02:50.549 that for a very, very short period of time, 00:02:50.549 --> 00:02:51.632 one of its electrons 00:02:51.632 --> 00:02:54.973 is at the other end of the known universe. 00:02:55.973 --> 00:02:58.717 But mostly electrons stay close to their nucleus 00:02:58.717 --> 00:03:00.564 as clouds of negative-charged density 00:03:00.564 --> 00:03:02.684 that shift and move with time. 00:03:02.684 --> 00:03:04.415 How electrons from one atom 00:03:04.415 --> 00:03:06.438 interact with electrons from another 00:03:06.438 --> 00:03:08.785 determines almost everything. 00:03:08.785 --> 00:03:10.571 Atoms can give up their electrons, 00:03:10.571 --> 00:03:12.240 surrendering them to other atoms, 00:03:12.240 --> 00:03:14.624 or they can share electrons. 00:03:15.254 --> 00:03:17.084 And the dynamics of this social network 00:03:17.084 --> 00:03:19.551 are what make chemistry interesting. 00:03:19.551 --> 00:03:20.414 From plain old rocks 00:03:20.414 --> 00:03:22.422 to the beautiful complexity of life, 00:03:22.422 --> 00:03:24.384 the nature of everything we see, 00:03:24.384 --> 00:03:25.092 hear, 00:03:25.092 --> 00:03:25.842 smell, 00:03:25.842 --> 00:03:26.380 taste, 00:03:26.380 --> 00:03:27.139 touch, 00:03:27.139 --> 00:03:28.136 and even feel 00:03:28.136 --> 00:03:30.971 is determined at the atomic level.