1 00:00:16,566 --> 00:00:20,736 Welcome to the third lecture of 3091. 2 00:00:20,736 --> 00:00:22,204 Let's get started. 3 00:00:22,204 --> 00:00:24,941 Today is a very exciting day, because we 4 00:00:24,941 --> 00:00:29,545 are going to discover the electron together. 5 00:00:29,545 --> 00:00:30,546 That's our goal today. 6 00:00:30,546 --> 00:00:34,016 It's right there, discovery of the electron. 7 00:00:34,016 --> 00:00:36,652 And we're going to start talking about models of the atom. 8 00:00:36,652 --> 00:00:39,322 Before we do that, I want to count. 9 00:00:39,322 --> 00:00:42,858 I want to count, because that's what we did on Friday. 10 00:00:42,858 --> 00:00:48,097 And we'll use this problem actually twice today. 11 00:00:48,097 --> 00:00:51,167 Either way, like if I do a problem in class, 12 00:00:51,167 --> 00:00:54,170 and it's in a goody bag and stuff, I don't know, 13 00:00:54,170 --> 00:00:56,739 it might be the kind of thing you might be asked on a quiz. 14 00:00:56,739 --> 00:00:58,941 I'm just saying. 15 00:00:58,941 --> 00:01:01,644 So that's good to know. 16 00:01:01,644 --> 00:01:04,980 If I ask you this question, because we counted on Friday, 17 00:01:04,980 --> 00:01:06,248 we learned how to count. 18 00:01:06,248 --> 00:01:08,718 And we learned about the mole and Avogadro's number 19 00:01:08,718 --> 00:01:13,990 and how that gets us into and out of the atomic world, right? 20 00:01:13,990 --> 00:01:15,658 So if I say, how many gallium atoms 21 00:01:15,658 --> 00:01:18,094 are this strip of gallium? 22 00:01:18,094 --> 00:01:19,495 Well, we can do that now, right? 23 00:01:19,495 --> 00:01:23,633 Because well, how do you start? 24 00:01:23,633 --> 00:01:26,769 This is where you start. 25 00:01:26,769 --> 00:01:29,138 And you are all taking it with you everywhere now. 26 00:01:29,138 --> 00:01:31,073 Remember on Friday, we talked about how 27 00:01:31,073 --> 00:01:36,212 you got to bring it with you everywhere and tweet about it 28 00:01:36,212 --> 00:01:40,116 and send me pictures and all that. 29 00:01:40,116 --> 00:01:42,585 I need the density, right? 30 00:01:42,585 --> 00:01:43,619 I need the density. 31 00:01:43,619 --> 00:01:44,854 Density in the periodic table. 32 00:01:44,854 --> 00:01:46,088 Let's start with the density. 33 00:01:46,088 --> 00:01:50,292 So if I have the density of gallium 34 00:01:50,292 --> 00:01:56,032 and I looked that up as 5.9 grams per centimeter cubed. 35 00:01:56,032 --> 00:01:57,500 OK, that's good. 36 00:01:57,500 --> 00:02:00,069 Well, you know, from the strip I can measure it 37 00:02:00,069 --> 00:02:02,905 with the super most accurate ruler in the world 38 00:02:02,905 --> 00:02:05,741 that you have now in your hands from the goody bag. 39 00:02:05,741 --> 00:02:09,145 And you see those are the measurements. 40 00:02:09,145 --> 00:02:14,083 And so if you measure it, you have a volume. 41 00:02:14,083 --> 00:02:16,719 OK, it's those three numbers multiplied. 42 00:02:16,719 --> 00:02:17,720 I won't write it out. 43 00:02:17,720 --> 00:02:21,590 It's 0.5 centimeters cubed. 44 00:02:21,590 --> 00:02:24,226 And so now with the density and the volume, 45 00:02:24,226 --> 00:02:27,530 I get that that strip has 2.95 grams. 46 00:02:27,530 --> 00:02:28,798 OK? 47 00:02:28,798 --> 00:02:30,332 Now that sounds pretty good. 48 00:02:30,332 --> 00:02:34,837 It's promising, but I want the number of atoms, right? 49 00:02:34,837 --> 00:02:36,072 I want the number of atoms. 50 00:02:36,072 --> 00:02:43,112 And that's where, again, you go to here, because in there 51 00:02:43,112 --> 00:02:47,716 is the grams per mole and also the atomic mass units, which, 52 00:02:47,716 --> 00:02:50,418 as we talked about Friday, are the same thing. 53 00:02:50,418 --> 00:02:51,387 Right? 54 00:02:51,387 --> 00:02:58,160 And so if I look that up, I have that for gallium, 55 00:02:58,160 --> 00:03:08,571 we have 69.723 grams per mole, right? 56 00:03:08,571 --> 00:03:16,579 And so that means that in 2.95 grams, 57 00:03:16,579 --> 00:03:28,023 I have 2.95 divided by that number, 69.723-- 58 00:03:28,023 --> 00:03:30,059 let's raise this up a little-- 59 00:03:30,059 --> 00:03:37,031 which equals 0.0423 moles. 60 00:03:37,031 --> 00:03:41,137 I still have not answered the question because the question 61 00:03:41,137 --> 00:03:42,838 says, how many atoms? 62 00:03:42,838 --> 00:03:43,339 Right? 63 00:03:43,339 --> 00:03:50,212 But that's where n sub a, or the mole, or this dozen, right? 64 00:03:50,212 --> 00:03:51,180 But it's not a dozen. 65 00:03:51,180 --> 00:03:52,281 It's a much bigger number. 66 00:03:52,281 --> 00:03:55,484 But this number, Avogadro's number, comes in. 67 00:03:55,484 --> 00:03:58,387 And so in that strip of gallium-- 68 00:03:58,387 --> 00:03:59,288 ah! 69 00:03:59,288 --> 00:04:00,623 No. 70 00:04:00,623 --> 00:04:07,930 In that strip of gallium, you've got, in the GA strip, 71 00:04:07,930 --> 00:04:19,774 you've got 0.0423 times na atoms. 72 00:04:19,774 --> 00:04:26,549 That's something like, you know, 2.5 times 10 to the 22nd atoms. 73 00:04:26,549 --> 00:04:29,785 That's a lot of atoms in that strip, but I was able to do. 74 00:04:29,785 --> 00:04:32,021 But the secret is the-- 75 00:04:32,021 --> 00:04:33,255 right, remember? 76 00:04:33,255 --> 00:04:40,262 One atomic mass unit gets you back and forth 77 00:04:40,262 --> 00:04:43,532 between one gram per mole. 78 00:04:43,532 --> 00:04:46,468 And by the way, the atomic mass unit is nothing. 79 00:04:46,468 --> 00:04:49,071 It's just a mass unit. 80 00:04:49,071 --> 00:04:52,608 I mean, one AMU happens to be, by the way, 81 00:04:52,608 --> 00:05:02,351 oh, roughly 1.66 times 10 to the minus 27th kilograms. 82 00:05:02,351 --> 00:05:03,385 Huh! 83 00:05:03,385 --> 00:05:06,822 That works because now you can multiply this by a whole mole, 84 00:05:06,822 --> 00:05:09,325 and it's a gram. 85 00:05:09,325 --> 00:05:11,660 Back and forth, OK? 86 00:05:11,660 --> 00:05:13,329 Good counting. 87 00:05:13,329 --> 00:05:14,430 Starting with counting. 88 00:05:14,430 --> 00:05:16,098 We're going to end with counting. 89 00:05:16,098 --> 00:05:19,668 But in between, we're going to discover something beautiful, 90 00:05:19,668 --> 00:05:22,371 because last week-- 91 00:05:22,371 --> 00:05:24,206 OK, the thesis of this whole class, 92 00:05:24,206 --> 00:05:26,075 remember, is electronic structure 93 00:05:26,075 --> 00:05:27,476 holds the key to everything. 94 00:05:27,476 --> 00:05:30,613 Everything in your life can be understood 95 00:05:30,613 --> 00:05:35,117 if you understood the electronic structure of the elements. 96 00:05:35,117 --> 00:05:38,120 Week one, last week, we talked about this. 97 00:05:38,120 --> 00:05:39,855 We built it on Friday. 98 00:05:39,855 --> 00:05:41,223 We talked about elements. 99 00:05:41,223 --> 00:05:42,424 We did a few reactions. 100 00:05:42,424 --> 00:05:44,058 We talked about balancing. 101 00:05:44,058 --> 00:05:45,728 We made sure we have enough nitrogen 102 00:05:45,728 --> 00:05:49,198 to grow plants for 100 million more years, eliminate reagents. 103 00:05:49,198 --> 00:05:52,668 This week, we're going to answer the question, 104 00:05:52,668 --> 00:05:55,905 why are these things different? 105 00:05:55,905 --> 00:05:58,040 We're switching gears, right? 106 00:05:58,040 --> 00:06:00,576 We know that there are these indivisible atoms now. 107 00:06:00,576 --> 00:06:02,811 And we built a whole table of them, right? 108 00:06:02,811 --> 00:06:03,445 Mendeleev. 109 00:06:03,445 --> 00:06:05,080 But now we want to know why they're 110 00:06:05,080 --> 00:06:06,415 different from one another. 111 00:06:06,415 --> 00:06:07,783 What causes that difference? 112 00:06:07,783 --> 00:06:12,554 And that starts with the discovery of the electron. 113 00:06:12,554 --> 00:06:14,957 And really this is such-- 114 00:06:14,957 --> 00:06:17,693 so let's write this here. 115 00:06:17,693 --> 00:06:20,296 We'll have this board. 116 00:06:20,296 --> 00:06:23,465 This is such a great story. 117 00:06:23,465 --> 00:06:30,306 This is a detective story that starts with an atom looking-- 118 00:06:30,306 --> 00:06:38,614 so these are atom models, OK? 119 00:06:38,614 --> 00:06:42,051 So back in, oh, let's say, 18-- 120 00:06:42,051 --> 00:06:43,686 ah, OK, 460. 121 00:06:43,686 --> 00:06:46,855 If it's Democritus, it's 460 B.C. 122 00:06:46,855 --> 00:06:50,826 And then remember, we sort of went all the way to Dalton, 123 00:06:50,826 --> 00:06:57,499 who, in 1803, he said, yup, Democritus, 124 00:06:57,499 --> 00:06:59,268 we're going with that. 125 00:06:59,268 --> 00:07:00,369 We're going with that. 126 00:07:00,369 --> 00:07:02,705 And oh, there are these indivisible 127 00:07:02,705 --> 00:07:05,007 things called atoms. 128 00:07:05,007 --> 00:07:07,009 So that was 1803. 129 00:07:07,009 --> 00:07:11,780 But we need to understand them in order to explain this. 130 00:07:11,780 --> 00:07:12,881 We need to understand them. 131 00:07:12,881 --> 00:07:15,084 And so this is a detective story. 132 00:07:15,084 --> 00:07:19,288 It's one of the greatest ones ever, ever, ever 133 00:07:19,288 --> 00:07:21,123 played out in science. 134 00:07:21,123 --> 00:07:25,494 It led to nothing less than the understanding of all of nature. 135 00:07:25,494 --> 00:07:27,463 OK, now, how did it start? 136 00:07:27,463 --> 00:07:31,600 Well, it started with J.J. Thomson. 137 00:07:31,600 --> 00:07:35,104 Now, so J.J. Thomson-- 138 00:07:35,104 --> 00:07:38,707 by the way, he won the Nobel Prize for this discovery. 139 00:07:38,707 --> 00:07:41,043 He's credited with discovering the electron. 140 00:07:41,043 --> 00:07:45,214 Also seven of his students won the Nobel Prize. 141 00:07:45,214 --> 00:07:46,148 That's cool! 142 00:07:46,148 --> 00:07:49,618 And I remind my graduate students of that 143 00:07:49,618 --> 00:07:54,390 all the time, without trying to put any pressure on them. 144 00:07:54,390 --> 00:07:55,591 How did he do it? 145 00:07:55,591 --> 00:07:58,594 Well, you see, he had these things. 146 00:07:58,594 --> 00:07:59,628 There is his lab. 147 00:07:59,628 --> 00:08:01,630 There's Thomson, there's his lab. 148 00:08:01,630 --> 00:08:03,532 And these things were around. 149 00:08:03,532 --> 00:08:05,567 They're called cathode ray tubes. 150 00:08:05,567 --> 00:08:06,535 Cathode ray tubes. 151 00:08:06,535 --> 00:08:07,870 What do they do? 152 00:08:07,870 --> 00:08:08,370 All right? 153 00:08:08,370 --> 00:08:09,038 What do they do? 154 00:08:09,038 --> 00:08:11,940 Well, your grandparents would know something about them. 155 00:08:11,940 --> 00:08:13,776 I'll talk about that in a few minutes. 156 00:08:13,776 --> 00:08:20,282 But Thomson, Thomson was interested in seeing 157 00:08:20,282 --> 00:08:21,683 what he saw on this thing here. 158 00:08:21,683 --> 00:08:23,285 And that's the photographic plate. 159 00:08:23,285 --> 00:08:24,887 So what we're going to do is we're just 160 00:08:24,887 --> 00:08:26,755 going to draw the cathode ray tube, 161 00:08:26,755 --> 00:08:30,626 draw it as best I can here. 162 00:08:30,626 --> 00:08:36,365 So there is a cathode ray tube, and over here we've 163 00:08:36,365 --> 00:08:43,539 got a cathode, and over here we've got an anode. 164 00:08:43,539 --> 00:08:50,813 So this would be a cathode, and this here is an anode. 165 00:08:50,813 --> 00:08:52,281 OK, good. 166 00:08:52,281 --> 00:08:55,517 Now, that means that if I hook the other two pieces of metal, 167 00:08:55,517 --> 00:08:57,719 two pieces of metal, and if I hook them up 168 00:08:57,719 --> 00:09:04,460 to a voltage supply, then I can get charge on them. 169 00:09:04,460 --> 00:09:06,562 We know that already. 170 00:09:06,562 --> 00:09:10,265 But what they didn't understand is what happened next. 171 00:09:10,265 --> 00:09:13,535 You see, because if you crank the voltage up really high 172 00:09:13,535 --> 00:09:16,672 and you put a phosphor screen here-- 173 00:09:16,672 --> 00:09:18,240 so this was a phosphor screen-- 174 00:09:22,344 --> 00:09:24,813 and if you crank the voltage up high enough, 175 00:09:24,813 --> 00:09:28,717 and this is what Thomson did, you pump the gas out of it. 176 00:09:28,717 --> 00:09:29,351 Right? 177 00:09:29,351 --> 00:09:32,387 So you get it so there's not too much stuff in there. 178 00:09:32,387 --> 00:09:36,725 Then what happens is something shot across 179 00:09:36,725 --> 00:09:38,627 and lit up the screen. 180 00:09:38,627 --> 00:09:40,629 Something. 181 00:09:40,629 --> 00:09:42,364 What? 182 00:09:42,364 --> 00:09:44,967 They really didn't know, and when you don't know, 183 00:09:44,967 --> 00:09:47,903 you experiment and you apply the scientific method. 184 00:09:47,903 --> 00:09:50,038 And one of the things that Thomson did 185 00:09:50,038 --> 00:09:54,409 is he said, well, what if I put some charge plates here, 186 00:09:54,409 --> 00:09:56,945 like this and this? 187 00:09:56,945 --> 00:09:58,614 So I have a [? set, ?] right? 188 00:09:58,614 --> 00:10:01,083 So it is said, this is going to have charge that flies off. 189 00:10:01,083 --> 00:10:03,719 We know that, because we kind of know what happens. 190 00:10:03,719 --> 00:10:05,354 But he didn't know what was happening. 191 00:10:05,354 --> 00:10:07,756 He's just cranking the voltage up, and all of a sudden the 192 00:10:07,756 --> 00:10:08,891 screen lights up. 193 00:10:08,891 --> 00:10:12,494 So he's like, well, let me put some charge plates here and see 194 00:10:12,494 --> 00:10:13,695 what happens. 195 00:10:13,695 --> 00:10:18,934 And what happened there is it went like this, right? 196 00:10:18,934 --> 00:10:22,571 And it struck the phosphor screen there, 197 00:10:22,571 --> 00:10:24,339 and it glowed there. 198 00:10:24,339 --> 00:10:30,913 So that's with charge, with charge plates. 199 00:10:30,913 --> 00:10:32,314 Right? 200 00:10:32,314 --> 00:10:37,386 No charge plates, just to be perfectly clear. 201 00:10:37,386 --> 00:10:39,388 And he could play with this, right? 202 00:10:39,388 --> 00:10:42,057 And people do play with this, to my great surprise, 203 00:10:42,057 --> 00:10:47,196 if you go onto the internet and you Google this. 204 00:10:47,196 --> 00:10:50,465 You find people make these and play with them, 205 00:10:50,465 --> 00:10:51,433 and that's a magnet. 206 00:10:51,433 --> 00:10:54,002 This is what it looks like, right? 207 00:10:54,002 --> 00:10:58,240 But we didn't know what this was, right? 208 00:10:58,240 --> 00:11:00,642 So that's a magnet, and this person 209 00:11:00,642 --> 00:11:03,845 is just turning over whether which part of the magnet, where 210 00:11:03,845 --> 00:11:05,147 the magnetic field is oriented. 211 00:11:05,147 --> 00:11:08,217 And you can see, you can really mess with this. 212 00:11:08,217 --> 00:11:10,819 What could it be? 213 00:11:10,819 --> 00:11:13,422 It's responding to an external field. 214 00:11:13,422 --> 00:11:21,396 It's responding to an external field, so we know right away, 215 00:11:21,396 --> 00:11:32,674 this tells us that atoms are not the ultimate form of matter. 216 00:11:39,448 --> 00:11:42,751 Atoms were not the ultimate form of matter. 217 00:11:42,751 --> 00:11:48,624 That was a big deal, because the atoms here were neutral. 218 00:11:48,624 --> 00:11:52,094 They didn't respond, so this must be something else that's 219 00:11:52,094 --> 00:11:55,430 flying off and responding, that I can see visually 220 00:11:55,430 --> 00:11:57,332 on this photographic plate. 221 00:11:57,332 --> 00:11:59,268 It must be something else. 222 00:11:59,268 --> 00:12:01,336 Well, he was also-- 223 00:12:01,336 --> 00:12:05,874 Thomson was-- oh, ha! 224 00:12:05,874 --> 00:12:09,211 Going down here, I'm going to use this 225 00:12:09,211 --> 00:12:12,781 so it's right next to it. 226 00:12:12,781 --> 00:12:17,552 He was not able to measure the mass of these things, 227 00:12:17,552 --> 00:12:19,888 but he was able, using a combination 228 00:12:19,888 --> 00:12:24,760 of magnetic and electric fields and the little bit of Maxwell, 229 00:12:24,760 --> 00:12:29,665 some ENM, he was able to deduce the charge to mass ratio. 230 00:12:29,665 --> 00:12:30,832 So this falls over here. 231 00:12:30,832 --> 00:12:35,304 I'll put it here, from Thomson. 232 00:12:35,304 --> 00:12:41,943 So he was able to get the charge to mass ratio. 233 00:12:44,513 --> 00:12:52,187 And he was able to measure that at minus 1.76 times 10 234 00:12:52,187 --> 00:12:56,290 to the 11th coulombs per kilogram. 235 00:12:56,290 --> 00:13:00,562 Now, he knew it was negatively charged because of this, right? 236 00:13:00,562 --> 00:13:02,464 He put the plates on, and it wanted 237 00:13:02,464 --> 00:13:04,900 to avoid the negative plate. 238 00:13:04,900 --> 00:13:07,703 So they knew their ENM at the time. 239 00:13:07,703 --> 00:13:10,772 They knew that the negative charges would repel. 240 00:13:10,772 --> 00:13:12,708 So he knew that it was a negative charge. 241 00:13:12,708 --> 00:13:17,579 Now, the last thing, oh, is this one important for chemistry! 242 00:13:17,579 --> 00:13:20,515 He would swap in different materials, different metals 243 00:13:20,515 --> 00:13:21,750 for the cathode and anode. 244 00:13:21,750 --> 00:13:23,118 For the cathode especially. 245 00:13:23,118 --> 00:13:25,921 And the results were the same. 246 00:13:25,921 --> 00:13:27,723 The results were the same. 247 00:13:27,723 --> 00:13:29,257 Oh! 248 00:13:29,257 --> 00:13:42,671 Independent, independent of metal. 249 00:13:42,671 --> 00:13:44,106 Independent of the metal. 250 00:13:44,106 --> 00:13:48,377 That meant this thing that he was 251 00:13:48,377 --> 00:13:50,579 observing for the first time was fundamental. 252 00:13:50,579 --> 00:13:53,415 There's something fundamental about it. 253 00:13:53,415 --> 00:13:54,549 Fundamental! 254 00:14:03,759 --> 00:14:05,360 This was a really big deal. 255 00:14:05,360 --> 00:14:07,329 This was a really big deal. 256 00:14:07,329 --> 00:14:12,567 And it really opened up the idea that the atom 257 00:14:12,567 --> 00:14:14,936 had something else in it. 258 00:14:14,936 --> 00:14:26,448 Now, see, Thomson knew that the atom was neutral, like I said. 259 00:14:26,448 --> 00:14:27,849 So you've got these charged things 260 00:14:27,849 --> 00:14:31,019 coming off of these atoms, which are neutral. 261 00:14:31,019 --> 00:14:33,655 That means that it must consist of these charged particles. 262 00:14:33,655 --> 00:14:35,891 But if it's neutral, it's got to have the other charge 263 00:14:35,891 --> 00:14:38,427 of particle in it as well. 264 00:14:38,427 --> 00:14:41,196 And so the detective story goes on. 265 00:14:41,196 --> 00:14:45,700 So this would be Thomson around 18-- 266 00:14:45,700 --> 00:14:47,836 oh, things really picked up. 267 00:14:47,836 --> 00:14:49,738 1807. 268 00:14:49,738 --> 00:14:53,775 And he said, well, OK, these electrons 269 00:14:53,775 --> 00:14:56,077 must be inside of the atom. 270 00:14:56,077 --> 00:14:58,246 That's how I'm liberating them. 271 00:14:58,246 --> 00:15:00,649 Crank up the voltage, and they must be in there. 272 00:15:00,649 --> 00:15:03,018 But so must positive charges. 273 00:15:03,018 --> 00:15:05,620 So must positive charges, OK? 274 00:15:05,620 --> 00:15:07,022 And that was Thomson's model. 275 00:15:07,022 --> 00:15:10,926 So at that time, you could really 276 00:15:10,926 --> 00:15:13,128 have this picture of stuff inside the atom. 277 00:15:13,128 --> 00:15:17,732 Now, like I said, he couldn't measure-- 278 00:15:17,732 --> 00:15:20,936 they didn't have a scale good enough 279 00:15:20,936 --> 00:15:22,437 to measure the mass of the electron. 280 00:15:22,437 --> 00:15:25,006 He could get it out, all right? 281 00:15:25,006 --> 00:15:31,012 But it was Millikan and his very famous experiments 282 00:15:31,012 --> 00:15:35,317 called the oil drop experiments that gave us the actual charge. 283 00:15:35,317 --> 00:15:37,919 And then from this ratio, we can get 284 00:15:37,919 --> 00:15:41,356 the mass of this mystery thing that 285 00:15:41,356 --> 00:15:43,258 was coming out of atoms, OK? 286 00:15:43,258 --> 00:15:49,931 And instead of trying to draw the Millikan experiment, 287 00:15:49,931 --> 00:15:53,602 I found a wonderful, short, like one-ish minute video, 288 00:15:53,602 --> 00:15:55,370 which I'll play, because it shows how 289 00:15:55,370 --> 00:15:57,172 the Millikan experiment worked. 290 00:15:57,172 --> 00:15:58,206 It's really cool. 291 00:15:58,206 --> 00:15:58,707 Oh! 292 00:15:58,707 --> 00:15:59,207 [VIDEO PLAYBACK] 293 00:15:59,207 --> 00:16:01,676 - Robert Millikan, working at the University of Chicago, 294 00:16:01,676 --> 00:16:04,179 succeeded in measuring the charge on the electron. 295 00:16:04,179 --> 00:16:05,046 That's not happening. 296 00:16:05,046 --> 00:16:08,383 - He allowed the fine spray of oil to settle through a hole, 297 00:16:08,383 --> 00:16:11,152 into a chamber where he could observe their fall. 298 00:16:11,152 --> 00:16:13,088 The top and bottom of the chamber 299 00:16:13,088 --> 00:16:15,524 consisted of electrically charged plates. 300 00:16:15,524 --> 00:16:18,193 He introduced a source of x-rays which 301 00:16:18,193 --> 00:16:21,363 can cause creation of charges when they strike matter. 302 00:16:21,363 --> 00:16:25,233 Charges reduced by the x-rays attached to an oil droplet, 303 00:16:25,233 --> 00:16:29,170 and producing one or more charges on the droplet. 304 00:16:29,170 --> 00:16:32,741 When there is no voltage of light, the fall of the droplets 305 00:16:32,741 --> 00:16:34,943 is determined by the mass and the viscosity of air 306 00:16:34,943 --> 00:16:36,845 through which they fall. 307 00:16:36,845 --> 00:16:39,214 When a voltage is applied, the droplets 308 00:16:39,214 --> 00:16:42,183 that have a negative charge will fall more slowly, 309 00:16:42,183 --> 00:16:45,620 stop falling, or even rise, depending on the number 310 00:16:45,620 --> 00:16:47,188 of charges on them. 311 00:16:47,188 --> 00:16:50,525 By adjusting the applied voltage and observing the droplets, 312 00:16:50,525 --> 00:16:52,994 both with voltage off and voltage 313 00:16:52,994 --> 00:16:55,497 on, Millikan was able to determine 314 00:16:55,497 --> 00:16:57,566 that the charges on the droplets were 315 00:16:57,566 --> 00:17:01,636 all multiples on a smallest value, 1.6 times 316 00:17:01,636 --> 00:17:04,940 10 to the minus 19th Coulombs. 317 00:17:04,940 --> 00:17:08,009 He took this to be the charge on a single electron. 318 00:17:08,009 --> 00:17:08,810 [END PLAYBACK] 319 00:17:08,810 --> 00:17:11,112 All right, so now you see that that would have been-- 320 00:17:11,112 --> 00:17:12,347 I hope that was-- 321 00:17:12,347 --> 00:17:14,082 could you hear that in the back? 322 00:17:14,082 --> 00:17:15,016 Sort of, OK. 323 00:17:15,016 --> 00:17:15,517 All right. 324 00:17:15,517 --> 00:17:17,319 Oh, I got this. 325 00:17:19,788 --> 00:17:21,222 You know, that would have been hard 326 00:17:21,222 --> 00:17:22,991 for me to draw the animation, but what 327 00:17:22,991 --> 00:17:24,224 a brilliant experiment, right? 328 00:17:24,224 --> 00:17:27,762 So spray some oil in a container, OK? 329 00:17:27,762 --> 00:17:28,930 Try to get the drop small. 330 00:17:28,930 --> 00:17:30,799 You don't know how small they're going to be. 331 00:17:30,799 --> 00:17:31,800 And then as they fall-- 332 00:17:31,800 --> 00:17:34,502 OK, have an electric field inside there. 333 00:17:34,502 --> 00:17:38,907 And then as they fall, zip them, zap them to charge them. 334 00:17:38,907 --> 00:17:40,942 You also don't know how much charge 335 00:17:40,942 --> 00:17:43,044 you're putting on them, right? 336 00:17:43,044 --> 00:17:44,279 But you're charging them. 337 00:17:44,279 --> 00:17:45,880 And if they're charged and they're in a field, 338 00:17:45,880 --> 00:17:48,416 and there are these tiny, little microscopic droplets of oil, 339 00:17:48,416 --> 00:17:50,418 they're going to maybe slow down or suspend or maybe even go 340 00:17:50,418 --> 00:17:50,952 the other way. 341 00:17:50,952 --> 00:17:53,254 And what he observed by doing this over and over again, 342 00:17:53,254 --> 00:17:57,592 that there was some multiple that you never got below, 343 00:17:57,592 --> 00:17:58,960 right? 344 00:17:58,960 --> 00:18:01,963 And you couldn't say necessarily for 100% certainty 345 00:18:01,963 --> 00:18:03,965 that it was the fundamental charge of electrons. 346 00:18:03,965 --> 00:18:05,300 It might have been a multiple of that. 347 00:18:05,300 --> 00:18:06,468 But it was pretty clear. 348 00:18:06,468 --> 00:18:07,836 You could never get below that. 349 00:18:07,836 --> 00:18:11,673 So that was the charge discovery in this detective story, 350 00:18:11,673 --> 00:18:17,212 and it allowed us to understand that the electron had a mass 351 00:18:17,212 --> 00:18:18,713 and it had a charge. 352 00:18:18,713 --> 00:18:20,448 And that we knew both of them at the time. 353 00:18:20,448 --> 00:18:25,787 So at the time then, we already had a much deeper 354 00:18:25,787 --> 00:18:27,555 understanding, if you think about it, 355 00:18:27,555 --> 00:18:31,292 just four years later than the time of Dalton because 356 00:18:31,292 --> 00:18:32,527 of these two experiments. 357 00:18:32,527 --> 00:18:33,194 Yeah? 358 00:18:33,194 --> 00:18:39,467 So we said that Thomson was 56-- he'd be [INAUDIBLE] 1940s, 359 00:18:39,467 --> 00:18:41,436 so wouldn't that be 1907? 360 00:18:41,436 --> 00:18:43,471 1856 to huh? 361 00:18:43,471 --> 00:18:45,240 Thomson was alive. 362 00:18:45,240 --> 00:18:45,974 Alive. 363 00:18:45,974 --> 00:18:48,376 [INAUDIBLE] 364 00:18:48,376 --> 00:18:50,779 I think I read in my-- 365 00:18:50,779 --> 00:18:51,846 he was ahead of his time. 366 00:18:51,846 --> 00:18:52,380 [LAUGHTER] 367 00:18:52,380 --> 00:18:54,115 Thank you very much. 368 00:18:54,115 --> 00:18:56,718 I think I read this wrong, and that's 369 00:18:56,718 --> 00:18:58,820 because I don't have my glasses on. 370 00:19:02,323 --> 00:19:04,192 So now that I look at that, it wasn't 371 00:19:04,192 --> 00:19:06,027 that small of a distance. 372 00:19:06,027 --> 00:19:09,330 [LAUGHTER] 373 00:19:09,330 --> 00:19:12,400 Thank you very much. 374 00:19:12,400 --> 00:19:13,268 You make mistakes. 375 00:19:13,268 --> 00:19:17,305 You make mistakes, and that is how you learn! 376 00:19:17,305 --> 00:19:20,608 [LAUGHTER] 377 00:19:20,608 --> 00:19:24,312 You don't make progress. 378 00:19:24,312 --> 00:19:27,115 Progress has almost nothing to do with success. 379 00:19:27,115 --> 00:19:27,816 I mean this. 380 00:19:27,816 --> 00:19:29,651 I'm using my own mistake here as an example. 381 00:19:29,651 --> 00:19:31,586 Progress has almost nothing to do with success. 382 00:19:31,586 --> 00:19:33,722 Progress has only to do with what 383 00:19:33,722 --> 00:19:34,956 you choose to do with failure. 384 00:19:37,826 --> 00:19:39,327 Did somebody just say, whoa? 385 00:19:39,327 --> 00:19:40,195 Thank you. 386 00:19:40,195 --> 00:19:40,862 Thank you. 387 00:19:40,862 --> 00:19:42,363 That hit me here. 388 00:19:42,363 --> 00:19:44,532 All right. 389 00:19:44,532 --> 00:19:46,000 OK, let's do a why this matters. 390 00:19:46,000 --> 00:19:49,170 Let's do a why this matters. 391 00:19:49,170 --> 00:19:50,038 Why does this matter? 392 00:19:50,038 --> 00:19:53,608 Well, because, see, these guys were 393 00:19:53,608 --> 00:19:56,678 trying to figure out what was inside of an atom. 394 00:19:56,678 --> 00:19:59,380 But other people, like John Baird, 395 00:19:59,380 --> 00:20:00,448 said, well, wait a second. 396 00:20:00,448 --> 00:20:02,617 You just gave me a paintbrush. 397 00:20:02,617 --> 00:20:04,185 You gave me a paintbrush. 398 00:20:04,185 --> 00:20:05,120 Look at this. 399 00:20:05,120 --> 00:20:09,190 That's a paintbrush, painting with a magnet. 400 00:20:09,190 --> 00:20:09,891 It really is. 401 00:20:09,891 --> 00:20:12,127 And the screen would light up over here. 402 00:20:12,127 --> 00:20:14,929 And so he said, well, I can paint. 403 00:20:14,929 --> 00:20:16,731 I can paint pictures. 404 00:20:16,731 --> 00:20:19,467 And this really is the first television screen. 405 00:20:19,467 --> 00:20:22,537 This was the first TV screen, and all they 406 00:20:22,537 --> 00:20:23,705 needed to do, there-- 407 00:20:23,705 --> 00:20:26,674 OK, right? 408 00:20:26,674 --> 00:20:27,442 There it is. 409 00:20:27,442 --> 00:20:27,942 Look! 410 00:20:27,942 --> 00:20:30,612 That's a cathode ray tube. 411 00:20:30,612 --> 00:20:35,817 Maybe that's one of Thomson's students who won a Nobel Prize. 412 00:20:35,817 --> 00:20:38,386 And but now, you put this down, and you put 413 00:20:38,386 --> 00:20:39,687 these sort of things around it. 414 00:20:39,687 --> 00:20:40,789 What are those things? 415 00:20:40,789 --> 00:20:43,091 Magnetic fields, that's all it is. 416 00:20:43,091 --> 00:20:44,225 It's just magnetic fields. 417 00:20:44,225 --> 00:20:46,461 It's Thomson's experiments, right? 418 00:20:46,461 --> 00:20:51,933 But now they're using it to zip the beam around 419 00:20:51,933 --> 00:20:54,636 faster than you're [INAUDIBLE] keep up with so 420 00:20:54,636 --> 00:20:55,870 that it looks like a picture. 421 00:20:55,870 --> 00:20:59,474 Now, electron painting had never been done before, 422 00:20:59,474 --> 00:21:03,411 because we didn't know that we had these electrons. 423 00:21:03,411 --> 00:21:05,680 But as soon as we knew, boy, did that 424 00:21:05,680 --> 00:21:10,585 launch a completely new era of screens. 425 00:21:10,585 --> 00:21:11,085 Right? 426 00:21:11,085 --> 00:21:12,787 The era of screens. 427 00:21:12,787 --> 00:21:14,722 We don't use cathode ray tubes. 428 00:21:14,722 --> 00:21:16,491 I was going to say, ask your grandparents. 429 00:21:16,491 --> 00:21:18,293 They'll tell you about the cathode ray tube 430 00:21:18,293 --> 00:21:22,230 TVs, which they all had. 431 00:21:22,230 --> 00:21:25,700 But you know, we don't use cathode ray tubes 432 00:21:25,700 --> 00:21:28,703 to paint with electrons today in that way, 433 00:21:28,703 --> 00:21:31,739 but we still paint with electrons today. 434 00:21:31,739 --> 00:21:32,407 Right? 435 00:21:32,407 --> 00:21:35,743 Youre OLED screen is still simply 436 00:21:35,743 --> 00:21:40,381 an electron-based painting tool, right? 437 00:21:40,381 --> 00:21:42,016 OK, we're just pumping the electrons 438 00:21:42,016 --> 00:21:45,086 into the phosphor in a different way, 439 00:21:45,086 --> 00:21:47,288 and we'll be talking about that as we 440 00:21:47,288 --> 00:21:49,357 go through the rest of this week and we understand 441 00:21:49,357 --> 00:21:52,060 how electrons interact with light 442 00:21:52,060 --> 00:21:54,762 coming in and out of an atom. 443 00:21:54,762 --> 00:21:57,532 OK, so that's my why this matters. 444 00:21:57,532 --> 00:21:59,234 And by the way, a side note here is 445 00:21:59,234 --> 00:22:05,106 that when TVs first came along, green was pretty easy. 446 00:22:05,106 --> 00:22:08,776 There were a lot of chemistries that were used for this screen. 447 00:22:08,776 --> 00:22:10,378 You put a different chemistry here, 448 00:22:10,378 --> 00:22:13,848 and it lights up differently when electrons hit it. 449 00:22:13,848 --> 00:22:14,482 Why? 450 00:22:14,482 --> 00:22:17,418 Wait until Wednesday. 451 00:22:17,418 --> 00:22:18,553 Green was easy. 452 00:22:18,553 --> 00:22:19,254 Yellow was easy. 453 00:22:19,254 --> 00:22:20,421 Red was hard. 454 00:22:20,421 --> 00:22:21,723 Red was hard. 455 00:22:21,723 --> 00:22:23,424 They couldn't get a good red. 456 00:22:23,424 --> 00:22:28,529 And of course, that's essentially the reason 457 00:22:28,529 --> 00:22:32,934 there were no color TVs until the '60s, right? 458 00:22:32,934 --> 00:22:35,303 And the answer, of course, was here. 459 00:22:35,303 --> 00:22:38,806 The answer was that there was a phosphor that worked, 460 00:22:38,806 --> 00:22:43,111 but it was yttrium orthovanadate with a little bit of europium 461 00:22:43,111 --> 00:22:44,946 added to it. 462 00:22:44,946 --> 00:22:46,180 Just a little bit of europium. 463 00:22:46,180 --> 00:22:47,048 Why did that work? 464 00:22:47,048 --> 00:22:49,884 Again, we need to understand how electrons interact with matter, 465 00:22:49,884 --> 00:22:52,120 which is where we're going. 466 00:22:52,120 --> 00:22:56,224 And speaking of yttrium, speaking of yttrium, 467 00:22:56,224 --> 00:22:59,127 this is also a side story, but it's kind of worth noting. 468 00:22:59,127 --> 00:23:01,963 You know, elements are named often-- well, 469 00:23:01,963 --> 00:23:04,565 elements can be named after many things, right? 470 00:23:04,565 --> 00:23:08,202 In this case, it was named after Ytterby, Sweden. 471 00:23:08,202 --> 00:23:10,471 Ytterby, Sweden, is a pretty cool place. 472 00:23:10,471 --> 00:23:12,473 They had this one cave. 473 00:23:12,473 --> 00:23:13,708 You got to go to it. 474 00:23:13,708 --> 00:23:17,245 It's really cool, because four elements of the periodic table 475 00:23:17,245 --> 00:23:19,714 were all discovered from Ytterby, Sweden, 476 00:23:19,714 --> 00:23:21,883 in this one cave. 477 00:23:21,883 --> 00:23:22,717 Four! 478 00:23:22,717 --> 00:23:24,919 And I keep thinking, where is that cave in Cambridge? 479 00:23:24,919 --> 00:23:26,454 [LAUGHTER] 480 00:23:26,454 --> 00:23:28,056 Hm? 481 00:23:28,056 --> 00:23:29,924 They cannot discover any more elements, 482 00:23:29,924 --> 00:23:33,261 because there's no other way to mess with the name Ytterby. 483 00:23:33,261 --> 00:23:35,196 [LAUGHTER] 484 00:23:35,196 --> 00:23:38,199 That's pretty cool. 485 00:23:38,199 --> 00:23:38,967 That's pretty cool. 486 00:23:38,967 --> 00:23:40,868 That's a cool cave. 487 00:23:40,868 --> 00:23:42,203 That's worth visiting. 488 00:23:42,203 --> 00:23:45,640 All right, so the structure of the atom in the 1900s-- 489 00:23:45,640 --> 00:23:47,075 thank you for the date correction. 490 00:23:47,075 --> 00:23:51,312 1900, the structure of the atom was as follows, all right? 491 00:23:51,312 --> 00:23:54,716 The atom is electrically neutral, 492 00:23:54,716 --> 00:23:56,784 but there are these negative charges. 493 00:23:56,784 --> 00:23:59,020 These negative charges called electrons. 494 00:23:59,020 --> 00:24:00,888 The electron has a very small mass. 495 00:24:00,888 --> 00:24:04,759 We know that from the oil drop experiments, right? 496 00:24:04,759 --> 00:24:06,527 Which means that the bulk of the atom 497 00:24:06,527 --> 00:24:10,398 is positive, because the bulk of the mass of the atom-- 498 00:24:10,398 --> 00:24:12,500 you know, the atom weighs one thing. 499 00:24:12,500 --> 00:24:14,102 The electron is just a small part 500 00:24:14,102 --> 00:24:19,374 of that in terms of the mass, so most of the mass is positive. 501 00:24:19,374 --> 00:24:21,509 And there was a question. 502 00:24:21,509 --> 00:24:22,010 OK? 503 00:24:22,010 --> 00:24:24,178 So we're getting somewhere, but how 504 00:24:24,178 --> 00:24:26,681 do these charges really look? 505 00:24:26,681 --> 00:24:29,550 How do these charges arrange inside of this atom? 506 00:24:29,550 --> 00:24:30,785 Is Thomson really right? 507 00:24:30,785 --> 00:24:33,821 And that was the next part of our detective story, 508 00:24:33,821 --> 00:24:35,823 was to really understand what these charges are 509 00:24:35,823 --> 00:24:36,824 doing inside the atom. 510 00:24:36,824 --> 00:24:38,526 I'm carrying this around like my security 511 00:24:38,526 --> 00:24:42,363 blanket, which is making me feel very secure right now. 512 00:24:42,363 --> 00:24:44,632 And you should all carry it too. 513 00:24:44,632 --> 00:24:49,771 This part of the story relies on another type of energy 514 00:24:49,771 --> 00:24:53,975 that was being discovered at the time. 515 00:24:53,975 --> 00:24:58,479 And you all may recognize this as radiation. 516 00:24:58,479 --> 00:25:05,720 Now, radioactivity, radioactivity 517 00:25:05,720 --> 00:25:10,591 is nothing more than ray activity. 518 00:25:10,591 --> 00:25:14,996 This is what they named it after, ray activity. 519 00:25:17,632 --> 00:25:19,700 That's radioactivity. 520 00:25:19,700 --> 00:25:24,038 And it was these three people who really pioneered 521 00:25:24,038 --> 00:25:26,074 the understanding-- gesundheit-- 522 00:25:26,074 --> 00:25:29,077 of radioactivity and in particular, 523 00:25:29,077 --> 00:25:32,580 the fact that some elements seemed to just be radioactive. 524 00:25:32,580 --> 00:25:33,381 What did that mean? 525 00:25:33,381 --> 00:25:37,819 Well, it means they gave off these rays of energy. 526 00:25:37,819 --> 00:25:39,921 That means they gave off these rays of energy. 527 00:25:39,921 --> 00:25:42,323 You had Henri Becquerel. 528 00:25:42,323 --> 00:25:45,960 You had Marie Curie and Pierre Curie. 529 00:25:45,960 --> 00:25:47,195 Notice I didn't even try. 530 00:25:47,195 --> 00:25:48,029 Where's Jerome? 531 00:25:48,029 --> 00:25:50,932 I didn't even try with the French. 532 00:25:50,932 --> 00:25:57,205 But anyway, they took these materials, like uranium ore, 533 00:25:57,205 --> 00:25:59,507 minerals made out of uranium, and they said, 534 00:25:59,507 --> 00:26:02,610 this stuff looks like it's got something coming off of it. 535 00:26:02,610 --> 00:26:05,947 Let's see if we can find what elements are causing that. 536 00:26:05,947 --> 00:26:07,482 All right? 537 00:26:07,482 --> 00:26:13,588 And Marie Curie, she found polonium and radium this way, 538 00:26:13,588 --> 00:26:14,388 those two elements. 539 00:26:14,388 --> 00:26:18,092 Polonium named after her home country, her native country 540 00:26:18,092 --> 00:26:20,695 of Poland. 541 00:26:20,695 --> 00:26:24,899 And this stuff gave off all this energy, 542 00:26:24,899 --> 00:26:27,735 and it was so energetic they could literally put it in a cup 543 00:26:27,735 --> 00:26:30,805 and have some writing there and put it in a super-dark room, 544 00:26:30,805 --> 00:26:33,141 and it would illuminate the picture. 545 00:26:33,141 --> 00:26:33,674 Right? 546 00:26:33,674 --> 00:26:35,009 It would illuminate the picture. 547 00:26:35,009 --> 00:26:40,281 But this is not a recommended way to light your pictures. 548 00:26:40,281 --> 00:26:48,389 So Pierre carried a vial of radium, because it was so cool 549 00:26:48,389 --> 00:26:50,458 and it glowed! 550 00:26:50,458 --> 00:26:52,693 He was like, look at this stuff! 551 00:26:52,693 --> 00:26:53,394 Right? 552 00:26:53,394 --> 00:26:57,465 And he actually did have bouts of radiation poisoning, 553 00:26:57,465 --> 00:26:59,934 although he died, tragically, when 554 00:26:59,934 --> 00:27:04,105 he was struck by a horse-drawn carriage in 1906. 555 00:27:04,105 --> 00:27:07,408 Marie lived much longer and won two-- 556 00:27:07,408 --> 00:27:09,744 but also died because of radiation poisoning. 557 00:27:09,744 --> 00:27:12,813 But she won two Nobel prizes. 558 00:27:12,813 --> 00:27:17,485 First woman to win a Nobel Prize, only woman to win two, 559 00:27:17,485 --> 00:27:20,021 and for only person ever to win one 560 00:27:20,021 --> 00:27:23,558 in two different disciplines, physics and chemistry. 561 00:27:23,558 --> 00:27:27,461 She was quite a brilliant scientist. 562 00:27:27,461 --> 00:27:28,796 And they gave us this radiation. 563 00:27:28,796 --> 00:27:32,400 Now, OK, but see, OK, they were really into radiation, 564 00:27:32,400 --> 00:27:35,770 but it was Rutherford who said, well, wait a second. 565 00:27:35,770 --> 00:27:40,541 Maybe I can use this stuff to keep going along this story. 566 00:27:40,541 --> 00:27:41,042 Right? 567 00:27:41,042 --> 00:27:42,910 Rutherford was like, well, maybe there's 568 00:27:42,910 --> 00:27:45,379 something we could do with this radiation. 569 00:27:45,379 --> 00:27:49,150 And so what happened? 570 00:27:49,150 --> 00:27:50,451 Oh, there it is! 571 00:27:50,451 --> 00:27:53,254 And so I'm going to put it right above it so you can see. 572 00:27:53,254 --> 00:27:58,426 So he said, well, OK, if I take something that glows, maybe 573 00:27:58,426 --> 00:28:01,829 some radium, and I carve out a little thing here, 574 00:28:01,829 --> 00:28:06,067 and maybe I put some plates to kind of protect scatter, 575 00:28:06,067 --> 00:28:10,404 I can make a beam of this stuff, OK? 576 00:28:10,404 --> 00:28:15,109 So this would be like maybe radium, let's say. 577 00:28:15,109 --> 00:28:17,645 And then what he did is he took this photographic plate 578 00:28:17,645 --> 00:28:20,448 and he curved it, just so he could really collect 579 00:28:20,448 --> 00:28:22,550 as much as you could, right? 580 00:28:22,550 --> 00:28:25,519 So you can really collect at really high angles. 581 00:28:25,519 --> 00:28:26,988 And he did the same thing. 582 00:28:26,988 --> 00:28:31,492 He put charged plates on either side, 583 00:28:31,492 --> 00:28:37,365 and he found that you had three different types of beams. 584 00:28:37,365 --> 00:28:40,301 He had one that went like this, one that went like this, 585 00:28:40,301 --> 00:28:41,469 and one that went like this. 586 00:28:41,469 --> 00:28:46,140 And we now know that this is beta, this is gamma, 587 00:28:46,140 --> 00:28:47,942 and that's alpha. 588 00:28:47,942 --> 00:28:49,343 Let's put this right above. 589 00:28:49,343 --> 00:28:51,045 This is so cool to watch. 590 00:28:51,045 --> 00:28:53,514 So what Rutherford did is kind of like this. 591 00:28:53,514 --> 00:28:56,284 He had stuff and he made a beam of it, 592 00:28:56,284 --> 00:29:00,221 and then he put stuff around it. 593 00:29:00,221 --> 00:29:01,622 But if you got a beam of something 594 00:29:01,622 --> 00:29:02,556 that might be charged, you're not 595 00:29:02,556 --> 00:29:04,091 going to smash it or burn it. 596 00:29:04,091 --> 00:29:05,893 You're going to put some plates around it, 597 00:29:05,893 --> 00:29:07,561 or a magnetic field or electric field. 598 00:29:07,561 --> 00:29:09,196 And that's what he did. 599 00:29:09,196 --> 00:29:14,068 And notice, alpha bends down, towards the positive charge, 600 00:29:14,068 --> 00:29:15,236 towards the negative charge. 601 00:29:15,236 --> 00:29:18,406 So alpha must be positively charged. 602 00:29:18,406 --> 00:29:20,141 I said, OK, that sounds cool. 603 00:29:20,141 --> 00:29:22,943 Maybe I can use this. 604 00:29:22,943 --> 00:29:27,682 Maybe I can use this on this, on this question 605 00:29:27,682 --> 00:29:30,951 of how these things are spatially distributed. 606 00:29:30,951 --> 00:29:34,422 So then he said, OK, let's just screen out 607 00:29:34,422 --> 00:29:35,656 those alpha particles. 608 00:29:35,656 --> 00:29:39,960 And let's just take these alpha particles and let's shoot them. 609 00:29:39,960 --> 00:29:43,097 So now comes the famous experiment, 610 00:29:43,097 --> 00:29:47,101 so you had stuff in here, radium making a beam. 611 00:29:47,101 --> 00:29:51,672 And you collect it and you make it just alpha particles. 612 00:29:51,672 --> 00:29:52,173 OK? 613 00:29:52,173 --> 00:29:57,611 So now he's just got all alpha particles, and what he did 614 00:29:57,611 --> 00:30:00,381 is he also had a photographic plate, 615 00:30:00,381 --> 00:30:02,116 or this could also be a phosphorus screen, 616 00:30:02,116 --> 00:30:03,217 and you can watch where-- 617 00:30:03,217 --> 00:30:07,488 You know, if it's a photographic plate, you record events, 618 00:30:07,488 --> 00:30:09,757 and you can see it like a picture, a film. 619 00:30:09,757 --> 00:30:13,127 And if it's a screen, then you watch it. 620 00:30:13,127 --> 00:30:14,829 Either one was used. 621 00:30:14,829 --> 00:30:20,568 But what he did is he put a very thin strip of gold in there. 622 00:30:20,568 --> 00:30:33,247 0.7 microns of gold, so that's a 0.7 thickness is 0.7 microns. 623 00:30:33,247 --> 00:30:37,351 Really, really thin, like a tissue paper of gold. 624 00:30:37,351 --> 00:30:39,019 And he shot these alpha particles at it. 625 00:30:39,019 --> 00:30:40,821 Now, why is this important? 626 00:30:40,821 --> 00:30:45,059 Because-- gesundheit. 627 00:30:45,059 --> 00:30:50,865 Because if this model of the atom was correct, 628 00:30:50,865 --> 00:30:54,168 then, you know, he'd isolated these positive particles 629 00:30:54,168 --> 00:30:56,170 that he's shooting at these atoms. 630 00:30:56,170 --> 00:30:59,106 If that model of the atom is correct, 631 00:30:59,106 --> 00:31:03,644 then this is kind of like distributed all over. 632 00:31:03,644 --> 00:31:04,178 Right? 633 00:31:04,178 --> 00:31:08,115 And so it's kind of neutral almost anywhere you look. 634 00:31:08,115 --> 00:31:10,017 It's kind of neutral. 635 00:31:10,017 --> 00:31:14,121 And so you would expect that a positive charge-- you'd maybe 636 00:31:14,121 --> 00:31:15,489 feel a little bit, but it kind of 637 00:31:15,489 --> 00:31:17,725 feels mostly neutral because it's evenly distributed. 638 00:31:17,725 --> 00:31:20,428 So it just kind of come out, but that's not what he saw. 639 00:31:20,428 --> 00:31:22,129 That's not what he saw. 640 00:31:22,129 --> 00:31:25,866 What he saw is that sometimes that would happen. 641 00:31:25,866 --> 00:31:27,635 You'd get a little bit of deflection. 642 00:31:27,635 --> 00:31:30,371 And other times, you'd get a signal 643 00:31:30,371 --> 00:31:38,546 out here, or there, or even right back at the radium. 644 00:31:38,546 --> 00:31:41,148 How is that possible? 645 00:31:41,148 --> 00:31:42,049 This did not work. 646 00:31:42,049 --> 00:31:43,551 That model of the atom did not work. 647 00:31:43,551 --> 00:31:47,688 And Rutherford himself said, it was almost as incredible 648 00:31:47,688 --> 00:31:50,491 as if you fired a 15-inch shell at a piece of tissue paper 649 00:31:50,491 --> 00:31:53,894 and it came back and hit you, because they just 650 00:31:53,894 --> 00:31:57,832 didn't expect that a positive particle could 651 00:31:57,832 --> 00:32:02,570 bounce backward off of an even distribution of charge. 652 00:32:02,570 --> 00:32:06,640 And so this is a picture from your textbook, Avril, 653 00:32:06,640 --> 00:32:09,877 that I know you are actively reading on a daily basis. 654 00:32:09,877 --> 00:32:11,312 So there's the alpha particles. 655 00:32:11,312 --> 00:32:13,347 That's what I drew, that they would expect-- 656 00:32:13,347 --> 00:32:14,381 or that what I mentioned. 657 00:32:14,381 --> 00:32:16,650 They would expect it to just kind of pass through. 658 00:32:16,650 --> 00:32:20,120 This is what was found, where they kind of come back 659 00:32:20,120 --> 00:32:21,956 at very sharp angles. 660 00:32:21,956 --> 00:32:25,326 That's what he actually observed, right? 661 00:32:25,326 --> 00:32:29,930 And it led to the Rutherford atom. 662 00:32:29,930 --> 00:32:32,533 And the Rutherford atom was that, look, 663 00:32:32,533 --> 00:32:36,270 the only way that this can happen, 664 00:32:36,270 --> 00:32:39,240 that you get these events, where a positive charge comes back 665 00:32:39,240 --> 00:32:42,877 like that, is if there's a very, very strong concentration 666 00:32:42,877 --> 00:32:46,313 of the positive charge in one small place. 667 00:32:46,313 --> 00:32:48,148 And so he said-- 668 00:32:48,148 --> 00:32:49,984 let me see if I can read my date correctly-- 669 00:32:49,984 --> 00:32:52,987 1912. 670 00:32:52,987 --> 00:33:02,496 And so Rutherford, he said that the positive charge 671 00:33:02,496 --> 00:33:06,033 has to be in the middle, and then the electrons 672 00:33:06,033 --> 00:33:09,336 must be far away. 673 00:33:09,336 --> 00:33:14,909 And that is also sometimes, it was called the planetary model. 674 00:33:14,909 --> 00:33:16,911 And in fact, there was another scientist 675 00:33:16,911 --> 00:33:22,616 from Japan, Nangaioka, who, five years earlier, had predicted-- 676 00:33:22,616 --> 00:33:26,320 although he sort of predicted this, because he loved Saturn. 677 00:33:26,320 --> 00:33:27,821 [LAUGHTER] 678 00:33:27,821 --> 00:33:28,856 He really did. 679 00:33:28,856 --> 00:33:33,260 So he said, you know, it seems that maybe we 680 00:33:33,260 --> 00:33:34,261 have a Saturn situation. 681 00:33:34,261 --> 00:33:35,829 So he called it the Saturnian model. 682 00:33:35,829 --> 00:33:40,935 But I want to mention it because it was part of the way there. 683 00:33:40,935 --> 00:33:42,036 Right? 684 00:33:42,036 --> 00:33:43,571 But in this planetary model, which 685 00:33:43,571 --> 00:33:47,107 came from Rutherford's experiments, 686 00:33:47,107 --> 00:33:49,677 you had a very different picture now of the atom. 687 00:33:49,677 --> 00:33:51,111 You had a very different picture. 688 00:33:51,111 --> 00:33:53,047 You had all the positive charge really 689 00:33:53,047 --> 00:33:55,950 concentrated in the middle. 690 00:33:55,950 --> 00:34:01,722 Now, the thing is also that the atom, 691 00:34:01,722 --> 00:34:03,257 what they were starting to understand 692 00:34:03,257 --> 00:34:05,225 is that the atom isn't just-- 693 00:34:05,225 --> 00:34:08,429 like, there isn't just all this space here. 694 00:34:08,429 --> 00:34:09,463 Right? 695 00:34:09,463 --> 00:34:11,931 They knew the electrons weren't distributed evenly in here. 696 00:34:11,931 --> 00:34:13,900 They knew they were sort of far away. 697 00:34:13,900 --> 00:34:15,703 There is a lot of space. 698 00:34:15,703 --> 00:34:19,672 There's a lot of space, that the radius of the atom 699 00:34:19,672 --> 00:34:26,112 is 100,000 times bigger than the radius of the nucleus. 700 00:34:26,112 --> 00:34:27,614 Right? 701 00:34:27,614 --> 00:34:29,416 So just to put that in perspective, 702 00:34:29,416 --> 00:34:30,650 so here's a picture. 703 00:34:30,650 --> 00:34:32,987 There would be the nucleus, the head of a pin. 704 00:34:32,987 --> 00:34:36,489 And this would be the atom, a stadium. 705 00:34:36,489 --> 00:34:37,458 OK? 706 00:34:37,458 --> 00:34:44,565 If you took all this stuff, all the stuff in a human body-- 707 00:34:44,565 --> 00:34:49,003 that means the protons, all the stuff 708 00:34:49,003 --> 00:34:53,239 in the nuclei, the neutrons-- ooh, I'm getting there! 709 00:34:53,239 --> 00:34:57,544 And the electrons, and you take 7 billion people, 710 00:34:57,544 --> 00:35:00,347 and you put that all into one volume, 711 00:35:00,347 --> 00:35:03,517 it is the volume of a sugar cube. 712 00:35:03,517 --> 00:35:07,688 So you know, so things are pretty empty out there. 713 00:35:07,688 --> 00:35:09,123 Things are pretty empty out there. 714 00:35:09,123 --> 00:35:10,557 How empty are they? 715 00:35:10,557 --> 00:35:13,293 But this is where you find true meaning. 716 00:35:13,293 --> 00:35:14,895 This is deep! 717 00:35:14,895 --> 00:35:16,030 How empty is stuff? 718 00:35:16,030 --> 00:35:23,804 Look, if I weigh the universe, you know, the universe itself, 719 00:35:23,804 --> 00:35:26,607 we're going from the atom and the electron, all the way up 720 00:35:26,607 --> 00:35:27,508 to the universe. 721 00:35:27,508 --> 00:35:30,044 The universe weighs something like, oh, 10 722 00:35:30,044 --> 00:35:33,747 to 60-ish kilograms. 723 00:35:33,747 --> 00:35:38,986 That's over roughly $30 billion lightyears. 724 00:35:42,356 --> 00:35:46,060 OK, now, if we just do the math, so you weighed the universe. 725 00:35:46,060 --> 00:35:47,828 You know how big it is, roughly. 726 00:35:47,828 --> 00:35:49,797 Plus of give or minus-- 727 00:35:49,797 --> 00:35:50,964 plus or minus. 728 00:35:50,964 --> 00:36:00,774 Then that means that 10 to the minus 20% of the whole universe 729 00:36:00,774 --> 00:36:01,275 is stuff. 730 00:36:03,911 --> 00:36:04,812 And the rest is empty. 731 00:36:09,650 --> 00:36:15,522 That's deep, because we find meaning in all of it, don't we? 732 00:36:15,522 --> 00:36:16,256 That's deep. 733 00:36:16,256 --> 00:36:18,459 Think about that. 734 00:36:18,459 --> 00:36:21,395 That's not just the universe, but the atom. 735 00:36:21,395 --> 00:36:22,096 It's the same. 736 00:36:22,096 --> 00:36:25,799 You know, so much of everything is nothing. 737 00:36:29,203 --> 00:36:31,338 OK, planetaria, but there was a problem 738 00:36:31,338 --> 00:36:33,107 with the planetary model. 739 00:36:33,107 --> 00:36:38,245 There was a big problem with the planetary model, 740 00:36:38,245 --> 00:36:44,351 because classical ENM told us that if, you know, 741 00:36:44,351 --> 00:36:48,822 if a charge is accelerating, if a charge is accelerating, which 742 00:36:48,822 --> 00:36:51,158 it is, because it's ughhh! 743 00:36:51,158 --> 00:36:53,193 We think it's going around. 744 00:36:53,193 --> 00:36:55,462 We think they're going around here. 745 00:36:55,462 --> 00:36:56,930 They are not! 746 00:36:56,930 --> 00:36:59,032 Friday, we'll know more about that. 747 00:36:59,032 --> 00:37:00,667 But we thought they were. 748 00:37:00,667 --> 00:37:04,705 But if it's accelerating, you know, to keep the circle, 749 00:37:04,705 --> 00:37:06,540 then it's got to be radiating energy. 750 00:37:06,540 --> 00:37:08,342 That's what classical ENM tells us. 751 00:37:08,342 --> 00:37:10,277 An accelerating charge loses energy. 752 00:37:10,277 --> 00:37:16,016 So if you go with that, then the electron-- 753 00:37:16,016 --> 00:37:19,620 let's see, the stability analysis that we get out 754 00:37:19,620 --> 00:37:22,122 of something like that-- 755 00:37:22,122 --> 00:37:31,431 hang on-- is that the atom would be stable, 756 00:37:31,431 --> 00:37:42,009 four-ish 10 to the minus 11 seconds, five times, roughly. 757 00:37:42,009 --> 00:37:45,245 That's not giving us a lot of time. 758 00:37:45,245 --> 00:37:45,746 All right? 759 00:37:45,746 --> 00:37:47,447 And so we were getting so far. 760 00:37:47,447 --> 00:37:52,886 We had this great model, but it didn't go with classical ENM. 761 00:37:52,886 --> 00:38:01,595 So atom's stable for that with classical ENM, oh! 762 00:38:01,595 --> 00:38:02,963 Because you know what's coming. 763 00:38:06,767 --> 00:38:08,101 We're going quantum. 764 00:38:08,101 --> 00:38:11,605 Not now, I'm just preparing you for later-- 765 00:38:11,605 --> 00:38:13,240 Did I hear an oh? 766 00:38:13,240 --> 00:38:13,941 Thank you. 767 00:38:13,941 --> 00:38:15,242 Friday. 768 00:38:15,242 --> 00:38:17,711 OK, so Bohr comes along, and on Wednesday, we're 769 00:38:17,711 --> 00:38:20,781 really going to go into Bohr. 770 00:38:20,781 --> 00:38:22,449 And we're going to talk about how Bohr's 771 00:38:22,449 --> 00:38:26,386 model, how Bohr's model of the atom 772 00:38:26,386 --> 00:38:30,958 allowed us to understand how light and matter interacts. 773 00:38:30,958 --> 00:38:34,494 That's a big deal, right? 774 00:38:34,494 --> 00:38:35,996 But we'll go into that on Wednesday. 775 00:38:35,996 --> 00:38:37,531 For now, I just want to tell you what 776 00:38:37,531 --> 00:38:41,635 he did, which is he thought about this problem a lot. 777 00:38:41,635 --> 00:38:44,371 He's like, this can't be. 778 00:38:44,371 --> 00:38:46,640 We need some way out of this. 779 00:38:46,640 --> 00:38:51,879 And he wrote this paper, where he said the following. 780 00:38:51,879 --> 00:38:53,914 He said, let's go over here. 781 00:38:53,914 --> 00:38:56,783 He said, OK, "In order to explain 782 00:38:56,783 --> 00:39:00,888 the results of experiments on scattering of alpha rays"-- 783 00:39:00,888 --> 00:39:04,157 we just did that, scattering of alpha rays-- 784 00:39:04,157 --> 00:39:06,994 "by matter, Professor Rutherford has given a theory 785 00:39:06,994 --> 00:39:08,128 of the structure of atoms." 786 00:39:08,128 --> 00:39:10,964 And then Bohr goes on. 787 00:39:10,964 --> 00:39:15,269 And he says, "Great interest is to be 788 00:39:15,269 --> 00:39:17,704 attributed to this atom model." 789 00:39:17,704 --> 00:39:21,208 Now, that is a serious diss. 790 00:39:21,208 --> 00:39:22,409 Great interest? 791 00:39:22,409 --> 00:39:26,213 If somebody calls your model of great interest, 792 00:39:26,213 --> 00:39:29,316 you know you are in trouble. 793 00:39:29,316 --> 00:39:31,618 He might as well have said back then, 794 00:39:31,618 --> 00:39:34,521 you know, this model is of great interest 795 00:39:34,521 --> 00:39:35,822 because it's totally wrong! 796 00:39:35,822 --> 00:39:37,557 [LAUGHTER] 797 00:39:37,557 --> 00:39:39,726 And if you want to know what's up, come over to me 798 00:39:39,726 --> 00:39:41,695 and I'll tell you. 799 00:39:41,695 --> 00:39:45,065 And that's what Bohr helped with in his work. 800 00:39:45,065 --> 00:39:48,869 And what he did is he wrote down some postulates. 801 00:39:48,869 --> 00:39:52,539 He said, look, the Rutherford atom is correct, 802 00:39:52,539 --> 00:39:56,610 but the problem here is classical ENM theory. 803 00:39:56,610 --> 00:39:57,344 Right? 804 00:39:57,344 --> 00:40:01,682 You cannot apply classical ENM theory to the orbiting 805 00:40:01,682 --> 00:40:02,683 electron. 806 00:40:02,683 --> 00:40:04,851 You can't, because that's what happened. 807 00:40:04,851 --> 00:40:07,454 So something's wrong with literally nothing 808 00:40:07,454 --> 00:40:10,290 less than classical physics. 809 00:40:10,290 --> 00:40:11,625 Right? 810 00:40:11,625 --> 00:40:14,394 He said, Newtonian mechanics still works, right? 811 00:40:14,394 --> 00:40:17,331 Newtonian mechanics works, but not ENM. 812 00:40:17,331 --> 00:40:19,032 So he said, we can go classical on how 813 00:40:19,032 --> 00:40:20,267 things are moving around. 814 00:40:20,267 --> 00:40:23,804 F equals MA stuff, but not ENM. 815 00:40:23,804 --> 00:40:27,908 He also postulated that the energy 816 00:40:27,908 --> 00:40:31,745 of these electrons, the energy that these electrons sit at 817 00:40:31,745 --> 00:40:33,013 or have-- 818 00:40:33,013 --> 00:40:36,149 we'll get into this a lot-- 819 00:40:36,149 --> 00:40:39,720 he hypothesized that that's quantized. 820 00:40:39,720 --> 00:40:41,421 Quantized. 821 00:40:41,421 --> 00:40:43,323 Quantized. 822 00:40:43,323 --> 00:40:44,691 We're saying that word. 823 00:40:44,691 --> 00:40:45,826 It's a beautiful moment. 824 00:40:45,826 --> 00:40:46,626 Quantized. 825 00:40:46,626 --> 00:40:49,396 Quantized means that it could only 826 00:40:49,396 --> 00:40:55,035 have certain values, certain values, which 827 00:40:55,035 --> 00:40:57,604 means that if this electron were to be in one value 828 00:40:57,604 --> 00:41:02,309 and then change into another, that the transitions are 829 00:41:02,309 --> 00:41:03,110 quantized. 830 00:41:03,110 --> 00:41:08,382 The transitions of an electron from one energy 831 00:41:08,382 --> 00:41:11,918 level of the atom to another can only have certain values. 832 00:41:11,918 --> 00:41:14,888 And that is what we get from the Bohr model. 833 00:41:14,888 --> 00:41:16,790 And boy, did it explain things! 834 00:41:16,790 --> 00:41:20,627 That's going to be the subject of Wednesday, as I mentioned. 835 00:41:20,627 --> 00:41:21,128 Right? 836 00:41:21,128 --> 00:41:28,702 But this gets us to the Bohr model. 837 00:41:28,702 --> 00:41:31,138 And here's Bohr. 838 00:41:31,138 --> 00:41:33,807 We're now-- oh, I got to draw this. 839 00:41:33,807 --> 00:41:36,643 We got all the positive charge in the middle, 840 00:41:36,643 --> 00:41:44,051 and then these electrons can only have certain radii, 841 00:41:44,051 --> 00:41:46,053 certain energies. 842 00:41:46,053 --> 00:41:49,122 They cannot continuously roam around. 843 00:41:49,122 --> 00:41:49,923 They can't. 844 00:41:49,923 --> 00:41:50,724 They're quantized. 845 00:41:50,724 --> 00:41:52,926 That's the Bohr model. 846 00:41:52,926 --> 00:42:01,802 And this is closed in 1912. 847 00:42:01,802 --> 00:42:07,808 This is closing in on the last and final model, which 848 00:42:07,808 --> 00:42:10,377 we'll get to this week, which is where quantum mechanics comes 849 00:42:10,377 --> 00:42:11,645 in. 850 00:42:11,645 --> 00:42:15,048 OK, now, we got one more thing to talk about. 851 00:42:15,048 --> 00:42:16,850 So this is where we're going. 852 00:42:16,850 --> 00:42:21,521 We're building up the full understanding of the atom. 853 00:42:21,521 --> 00:42:24,825 But there's one more thing that I've got to come back to, 854 00:42:24,825 --> 00:42:27,194 and that is that, look, we just said 855 00:42:27,194 --> 00:42:28,929 there are these positive charges in there 856 00:42:28,929 --> 00:42:30,730 and these negative charges out there. 857 00:42:30,730 --> 00:42:34,101 But it turns out that atoms can have different masses 858 00:42:34,101 --> 00:42:35,769 and stay neutral. 859 00:42:35,769 --> 00:42:36,269 All right? 860 00:42:36,269 --> 00:42:39,239 So we're now in a position to talk about this thing 861 00:42:39,239 --> 00:42:41,708 that we've been calling the nucleus and the fact 862 00:42:41,708 --> 00:42:44,111 that it's not just protons. 863 00:42:44,111 --> 00:42:45,979 We're in a position to talk about that. 864 00:42:45,979 --> 00:42:48,682 And so, you know, Rutherford gave us 865 00:42:48,682 --> 00:42:53,386 this idea of the proton, the particle, the proton. 866 00:42:53,386 --> 00:42:57,157 And it wasn't until 12 years later 867 00:42:57,157 --> 00:43:01,161 that Chadwick found the neutron. 868 00:43:01,161 --> 00:43:08,135 What the neutron does is it adds mass to the atom. 869 00:43:08,135 --> 00:43:10,604 See, the mass didn't work. 870 00:43:10,604 --> 00:43:14,441 So again, you know, if you know what the mass of the proton is, 871 00:43:14,441 --> 00:43:17,410 then you can add it up, and you get the mass of the electron, 872 00:43:17,410 --> 00:43:20,080 although it's like 1,000th as much. 873 00:43:20,080 --> 00:43:21,214 But it doesn't work. 874 00:43:21,214 --> 00:43:23,350 There's something else in there that's adding mass. 875 00:43:23,350 --> 00:43:25,852 But it's not adding charge, because these things 876 00:43:25,852 --> 00:43:28,355 are electrically neutral. 877 00:43:28,355 --> 00:43:30,824 So those other particles are called neutrons, 878 00:43:30,824 --> 00:43:37,564 and what it does is it gives us ranges of masses for the atoms. 879 00:43:37,564 --> 00:43:38,064 Right? 880 00:43:38,064 --> 00:43:39,799 It gives us ranges. 881 00:43:39,799 --> 00:43:43,303 So because, you know, hydrogen now. 882 00:43:43,303 --> 00:43:46,540 If you think about it, 99.99% of all hydrogen 883 00:43:46,540 --> 00:43:50,410 is just that thing on the left, a proton and an electron. 884 00:43:50,410 --> 00:43:52,345 But sometimes, it has a neutron. 885 00:43:52,345 --> 00:43:53,079 So it weighs more. 886 00:43:53,079 --> 00:43:54,481 It's still got neutral charge. 887 00:43:54,481 --> 00:43:56,683 It's called deuterium. 888 00:43:56,683 --> 00:43:58,652 And then you've got tritium, which is unstable. 889 00:43:58,652 --> 00:44:01,021 You can make it. 890 00:44:01,021 --> 00:44:03,490 And these are called isotopes, as many of you, I'm sure, 891 00:44:03,490 --> 00:44:04,224 already know. 892 00:44:04,224 --> 00:44:12,465 These are called isotopes, and so the isotopes of an atom, 893 00:44:12,465 --> 00:44:33,453 the isotopes of an atom with the same atomic number, 894 00:44:33,453 --> 00:44:43,863 different number of neutrons, different number of neutrons. 895 00:44:43,863 --> 00:44:46,199 So atoms have these three things in them. 896 00:44:46,199 --> 00:44:48,535 They've got the neutrons, the protons, and the electrons. 897 00:44:48,535 --> 00:44:51,304 We've been talking today about the protons and the electrons, 898 00:44:51,304 --> 00:44:53,306 but they got to have these other things in them, 899 00:44:53,306 --> 00:44:54,741 or the mass doesn't work. 900 00:44:54,741 --> 00:44:58,979 And so if you look at something like carbon, 901 00:44:58,979 --> 00:44:59,980 you see the same thing. 902 00:44:59,980 --> 00:45:01,948 And now I want to get into how these things are 903 00:45:01,948 --> 00:45:03,950 written, right? 904 00:45:03,950 --> 00:45:07,354 Because here's carbon, and there are those electrons. 905 00:45:07,354 --> 00:45:08,622 They're now put out there. 906 00:45:08,622 --> 00:45:10,657 OK, we didn't draw circles, so maybe this 907 00:45:10,657 --> 00:45:15,662 is around Rutherford's time when Avril made this picture. 908 00:45:15,662 --> 00:45:19,032 And there is the pluses, and there's other things in there. 909 00:45:19,032 --> 00:45:19,733 Right? 910 00:45:19,733 --> 00:45:20,867 And those are the neutrons. 911 00:45:20,867 --> 00:45:24,938 And so what we do is we have a nomenclature for this, 912 00:45:24,938 --> 00:45:26,373 which is the standard. 913 00:45:26,373 --> 00:45:31,111 And that is that if we write an element like this, 914 00:45:31,111 --> 00:45:32,212 this would be the element. 915 00:45:34,748 --> 00:45:39,019 And we put something here and something here, this 916 00:45:39,019 --> 00:45:43,189 is the mass number. 917 00:45:43,189 --> 00:45:44,424 OK? 918 00:45:44,424 --> 00:45:48,161 So that's how many protons plus neutrons. 919 00:45:51,264 --> 00:45:53,033 Neu-- ah! 920 00:45:53,033 --> 00:45:54,701 Neutrons. 921 00:45:54,701 --> 00:45:56,736 And this is called the atomic number. 922 00:45:59,439 --> 00:46:01,641 Now, the atomic number, I've been referencing 923 00:46:01,641 --> 00:46:03,009 this thing, this atomic number. 924 00:46:03,009 --> 00:46:05,045 I've said, Mendeleev and many others, 925 00:46:05,045 --> 00:46:08,448 they put things in order of atomic number, 926 00:46:08,448 --> 00:46:10,350 maybe mass sometimes. 927 00:46:10,350 --> 00:46:12,986 Mendeleev did the thing where he did those things 928 00:46:12,986 --> 00:46:15,689 and properties, right? 929 00:46:15,689 --> 00:46:20,593 And the atomic number was just a number, until later. 930 00:46:20,593 --> 00:46:23,530 And once we discover another type of ray, 931 00:46:23,530 --> 00:46:28,234 x-rays, then this has real physical meaning, right? 932 00:46:28,234 --> 00:46:29,903 But right now, it's still just a number. 933 00:46:29,903 --> 00:46:33,406 It's a number of the element in the periodic table. 934 00:46:33,406 --> 00:46:33,907 Right? 935 00:46:33,907 --> 00:46:38,211 But that mass number tells me how many neutrons I have 936 00:46:38,211 --> 00:46:39,546 and what the isotope is. 937 00:46:39,546 --> 00:46:41,514 So now I go back. 938 00:46:41,514 --> 00:46:45,285 I can go back, and I can look at carbon. 939 00:46:45,285 --> 00:46:47,520 Oh, by the way, carbon-- 940 00:46:47,520 --> 00:46:50,357 I wanted to write this down before I say that. 941 00:46:50,357 --> 00:46:58,631 Carbon, when you look it up in the table, you have 12.011 AMU, 942 00:46:58,631 --> 00:46:59,366 or grams per mole. 943 00:46:59,366 --> 00:47:01,101 That's what's in the table. 944 00:47:01,101 --> 00:47:04,671 But now you know why, because it's just an average. 945 00:47:04,671 --> 00:47:07,574 And in fact, the IUPAC, which is a rocking body 946 00:47:07,574 --> 00:47:11,077 of the International Union of Pure and Applied Chemistry, 947 00:47:11,077 --> 00:47:13,980 I'm sure you've seen them on the feeds. 948 00:47:13,980 --> 00:47:18,184 They hold meetings and in Vienna on a nonstop night 949 00:47:18,184 --> 00:47:21,554 of intense meeting and partying. 950 00:47:21,554 --> 00:47:25,024 And they said, hey, maybe in 2011, 951 00:47:25,024 --> 00:47:27,394 maybe we should actually do this with the periodic table, 952 00:47:27,394 --> 00:47:29,229 just to show that actually there really is 953 00:47:29,229 --> 00:47:31,398 a range of-- if I take carbon out of the ground here 954 00:47:31,398 --> 00:47:32,799 or carbon out of the ground there, 955 00:47:32,799 --> 00:47:35,101 maybe they have different AMUs, right? 956 00:47:35,101 --> 00:47:37,670 These are averages, and there are ranges. 957 00:47:37,670 --> 00:47:40,306 So they're actually talking about different ways 958 00:47:40,306 --> 00:47:45,011 of even putting this into periodic tables, right? 959 00:47:45,011 --> 00:47:45,812 But this is it. 960 00:47:45,812 --> 00:47:47,380 And this gets us back. 961 00:47:47,380 --> 00:47:51,951 Oh, there's boron and nitrogen ranges, right? 962 00:47:51,951 --> 00:47:56,956 But what we do now, at least, is we just look at the one number, 963 00:47:56,956 --> 00:47:59,192 because that's sort of averaged over, 964 00:47:59,192 --> 00:48:01,494 you know, basically anywhere you can dig this stuff up. 965 00:48:01,494 --> 00:48:03,730 And the stable isotopes. 966 00:48:03,730 --> 00:48:04,697 Stable isotopes. 967 00:48:04,697 --> 00:48:07,834 And it gets me to my last slide, which 968 00:48:07,834 --> 00:48:10,804 just comes back to the beginning, as I promised. 969 00:48:10,804 --> 00:48:11,538 Right? 970 00:48:11,538 --> 00:48:13,940 Because now I can say not only how many gallium atoms are 971 00:48:13,940 --> 00:48:15,508 in this strip, but I can tell you, 972 00:48:15,508 --> 00:48:17,110 gallium has two stable isotopes. 973 00:48:17,110 --> 00:48:19,179 By the way, it's got a bunch of unstable ones, 974 00:48:19,179 --> 00:48:23,616 but it's got two stable ones, gallium 69 and gallium 71. 975 00:48:23,616 --> 00:48:26,886 And I can now ask you, how many gallium 69 atoms 976 00:48:26,886 --> 00:48:27,987 are in this strip? 977 00:48:27,987 --> 00:48:28,788 How? 978 00:48:28,788 --> 00:48:35,462 Because I can again look up in the periodic table 979 00:48:35,462 --> 00:48:40,767 and find that in a gallium strip, 980 00:48:40,767 --> 00:48:42,669 I know how many atoms I have. 981 00:48:42,669 --> 00:48:49,309 But if I say, well, 69 times x plus 71 times 1 982 00:48:49,309 --> 00:48:57,116 minus x, that's going to equal 69.723 AMU. 983 00:48:57,116 --> 00:48:58,618 That's what's in the periodic table. 984 00:48:58,618 --> 00:49:00,220 But I now know that that's simply 985 00:49:00,220 --> 00:49:01,988 an average over all the-- 986 00:49:01,988 --> 00:49:02,489 right? 987 00:49:02,489 --> 00:49:11,898 So this would be the fraction of 69 GA, right? 988 00:49:11,898 --> 00:49:14,567 Because that's how simple these isotope problems are, right? 989 00:49:14,567 --> 00:49:17,871 It's simply averaging over the potentially stable isotopes. 990 00:49:17,871 --> 00:49:21,608 And I leave you with one more great mystery. 991 00:49:21,608 --> 00:49:28,214 As you are pondering this, think about this. 992 00:49:28,214 --> 00:49:31,551 There are two elements, ah! 993 00:49:31,551 --> 00:49:37,156 There are two elements for which there are no stable isotopes. 994 00:49:37,156 --> 00:49:39,425 Technetium and promethium. 995 00:49:39,425 --> 00:49:41,127 Nothing! 996 00:49:41,127 --> 00:49:42,729 Technetium, we use all the time. 997 00:49:42,729 --> 00:49:45,431 80% of all MRIs use technetium, but it's not stable. 998 00:49:45,431 --> 00:49:47,400 We got to make it, right? 999 00:49:47,400 --> 00:49:48,601 That's pretty deep too. 1000 00:49:48,601 --> 00:49:49,802 How did that happen? 1001 00:49:49,802 --> 00:49:52,405 Why did that happen? 1002 00:49:52,405 --> 00:49:55,074 For the answer, you'll have to stay tuned. 1003 00:49:55,074 --> 00:49:59,012 Have a good night, and see you guys on Wednesday.