1 00:00:06,989 --> 00:00:10,266 You probably know that all stuff is made up of atoms 2 00:00:10,266 --> 00:00:11,247 and that an atom 3 00:00:11,247 --> 00:00:15,190 is a really, really, really, really tiny particle. 4 00:00:15,190 --> 00:00:16,628 Every atom has a core, 5 00:00:16,628 --> 00:00:18,030 which is made up of at least one 6 00:00:18,030 --> 00:00:19,542 positively charged particle 7 00:00:19,542 --> 00:00:21,167 called a proton, 8 00:00:21,167 --> 00:00:22,421 and in most cases, 9 00:00:22,421 --> 00:00:25,592 some number of neutral particles called neutrons. 10 00:00:25,592 --> 00:00:26,966 That core is surrounded 11 00:00:26,966 --> 00:00:30,148 by negatively charged particles called electrons. 12 00:00:30,794 --> 00:00:32,638 The identity of an atom is determined 13 00:00:32,638 --> 00:00:36,922 only by the number of protons in its nucleus. 14 00:00:36,922 --> 00:00:39,698 Hydrogen is hydrogen because it has just one proton, 15 00:00:39,698 --> 00:00:41,308 carbon is carbon because it has six, 16 00:00:41,308 --> 00:00:43,423 gold is gold because it has 79, 17 00:00:43,423 --> 00:00:44,680 and so on. 18 00:00:45,140 --> 00:00:47,150 Indulge me in a momentary tangent. 19 00:00:47,150 --> 00:00:49,516 How do we know about atomic structure? 20 00:00:49,516 --> 00:00:52,723 We can't see protons, neutrons, or electrons. 21 00:00:53,814 --> 00:00:55,199 So, we do a bunch of experiments 22 00:00:55,199 --> 00:00:58,282 and develop a model for what we think is there. 23 00:00:58,282 --> 00:00:59,655 Then we do some more experiments 24 00:00:59,655 --> 00:01:01,421 and see if they agree with the model. 25 00:01:01,421 --> 00:01:02,737 If they do, great. 26 00:01:02,737 --> 00:01:05,702 If they don't, it might be time for a new model. 27 00:01:05,702 --> 00:01:08,369 We've had lots of very different models for atoms 28 00:01:08,369 --> 00:01:10,510 since Democritus in 400 BC, 29 00:01:10,510 --> 00:01:11,718 and there will almost certainly 30 00:01:11,718 --> 00:01:13,600 be many more to come. 31 00:01:13,600 --> 00:01:15,471 Okay, tangent over. 32 00:01:15,471 --> 00:01:18,232 The cores of atoms tend to stick together, 33 00:01:18,232 --> 00:01:20,017 but electrons are free to move, 34 00:01:20,017 --> 00:01:22,795 and this is why chemists love electrons. 35 00:01:22,795 --> 00:01:24,233 If we could marry them, 36 00:01:24,233 --> 00:01:25,661 we probably would. 37 00:01:25,661 --> 00:01:26,981 But electrons are weird. 38 00:01:26,981 --> 00:01:29,550 They appear to behave either as particles, 39 00:01:29,550 --> 00:01:31,132 like little baseballs, 40 00:01:31,132 --> 00:01:33,322 or as waves, like water waves, 41 00:01:33,322 --> 00:01:36,523 depending on the experiment that we perform. 42 00:01:36,523 --> 00:01:38,483 One of the weirdest things about electrons 43 00:01:38,483 --> 00:01:40,697 is that we can't exactly say where they are. 44 00:01:40,697 --> 00:01:42,739 It's not that we don't have the equipment, 45 00:01:42,739 --> 00:01:44,141 it's that this uncertainty 46 00:01:44,141 --> 00:01:46,498 is part of our model of the electron. 47 00:01:46,498 --> 00:01:49,060 So, we can't pinpoint them, fine. 48 00:01:49,060 --> 00:01:51,755 But we can say there's a certain probability 49 00:01:51,755 --> 00:01:53,740 of finding an electron in a given space 50 00:01:53,740 --> 00:01:55,705 around the nucleus. 51 00:01:55,705 --> 00:01:58,585 And that means that we can ask the following question: 52 00:01:58,585 --> 00:02:00,423 If we drew a shape around the nucleus 53 00:02:00,423 --> 00:02:02,487 such that we would be 95% sure 54 00:02:02,487 --> 00:02:05,520 of finding a given electron within that shape, 55 00:02:05,520 --> 00:02:07,067 what would it look like? 56 00:02:08,590 --> 00:02:10,821 Here are a few of these shapes. 57 00:02:10,821 --> 00:02:12,569 Chemists call them orbitals, 58 00:02:12,569 --> 00:02:13,849 and what each one looks like 59 00:02:13,849 --> 00:02:15,868 depends on, among other things, 60 00:02:15,868 --> 00:02:17,715 how much energy it has. 61 00:02:17,715 --> 00:02:19,365 The more energy an orbital has, 62 00:02:19,365 --> 00:02:21,042 the farther most of its density is 63 00:02:21,042 --> 00:02:22,294 from the nucleus. 64 00:02:22,294 --> 00:02:25,275 By they way, why did we pick 95% 65 00:02:25,275 --> 00:02:26,911 and not 100%? 66 00:02:26,911 --> 00:02:28,215 Well, that's another quirk 67 00:02:28,215 --> 00:02:30,003 of our model of the electron. 68 00:02:30,003 --> 00:02:32,228 Past a certain distance from the nucleus, 69 00:02:32,228 --> 00:02:34,073 the probability of finding an electron 70 00:02:34,073 --> 00:02:35,624 starts to decrease 71 00:02:35,624 --> 00:02:37,936 more or less exponentially, 72 00:02:37,936 --> 00:02:40,155 which means that while it will approach zero, 73 00:02:40,155 --> 00:02:42,582 it'll never actually hit zero. 74 00:02:43,136 --> 00:02:44,298 So, in every atom, 75 00:02:44,298 --> 00:02:47,438 there is some small, but non-zero, probability 76 00:02:47,438 --> 00:02:50,549 that for a very, very short period of time, 77 00:02:50,549 --> 00:02:51,632 one of its electrons 78 00:02:51,632 --> 00:02:54,973 is at the other end of the known universe. 79 00:02:55,973 --> 00:02:58,717 But mostly electrons stay close to their nucleus 80 00:02:58,717 --> 00:03:00,564 as clouds of negative charged density 81 00:03:00,564 --> 00:03:02,684 that shift and move with time. 82 00:03:02,684 --> 00:03:04,415 How electrons from one atom 83 00:03:04,415 --> 00:03:06,438 interact with electrons from another 84 00:03:06,438 --> 00:03:08,785 determines almost everything. 85 00:03:08,785 --> 00:03:10,571 Atoms can give up their electrons, 86 00:03:10,571 --> 00:03:12,240 surrendering them to other atoms, 87 00:03:12,240 --> 00:03:14,624 or they can share electrons. 88 00:03:15,254 --> 00:03:17,084 And the dynamics of this social network 89 00:03:17,084 --> 00:03:19,551 are what make chemistry interesting. 90 00:03:19,551 --> 00:03:20,414 From plain old rocks 91 00:03:20,414 --> 00:03:22,422 to the beautiful complexity of life, 92 00:03:22,422 --> 00:03:24,384 the nature of everything we see, 93 00:03:24,384 --> 00:03:25,920 hear, 94 00:03:25,920 --> 00:03:25,842 smell, 95 00:03:25,842 --> 00:03:26,380 taste, 96 00:03:26,380 --> 00:03:27,139 touch, 97 00:03:27,139 --> 00:03:28,136 and even feel 98 00:03:28,136 --> 00:03:30,971 is determined at the atomic level.