1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,300 Commons license. 3 00:00:03,300 --> 00:00:05,650 Your supports will help MIT OpenCourseWare 4 00:00:05,650 --> 00:00:09,760 continue to offer high quality educational resources for free. 5 00:00:09,760 --> 00:00:12,355 To make a donation or to view addional materials 6 00:00:12,355 --> 00:00:16,125 from hundreds of MIT courses visit MIT OpenCourseWare 7 00:00:16,125 --> 00:00:16,920 at ocw.mit.edu. 8 00:00:20,040 --> 00:00:22,600 CATHERINE DRENNAN: We gotta move up to the next lowest energy 9 00:00:22,600 --> 00:00:26,958 orbital, which we'll also talk about today. 10 00:00:26,958 --> 00:00:28,530 All right. 11 00:00:28,530 --> 00:00:33,950 So that kind of completes our one electron systems, 12 00:00:33,950 --> 00:00:36,912 and now we're gonna talk about multi-electrons. 13 00:00:36,912 --> 00:00:39,900 But we're not going to move far away from the topics 14 00:00:39,900 --> 00:00:43,170 that we've discussed because with multi-electrons there 15 00:00:43,170 --> 00:00:46,592 are several things that are the same and some things that 16 00:00:46,592 --> 00:00:47,175 are different. 17 00:00:47,175 --> 00:00:48,550 But a lot of things are the same, 18 00:00:48,550 --> 00:00:51,230 so we're going to come back to radial probability 19 00:00:51,230 --> 00:00:55,260 distributions and energy levels and things like that. 20 00:00:55,260 --> 00:00:56,760 So we'll move on to today's handout. 21 00:01:00,030 --> 00:01:03,420 So similarities and differences. 22 00:01:03,420 --> 00:01:04,617 Similarities. 23 00:01:04,617 --> 00:01:07,280 We have the same shapes of orbitals, 24 00:01:07,280 --> 00:01:10,450 whether we're talking about a one electron system or more. 25 00:01:10,450 --> 00:01:14,240 So again, we're gonna have our s orbital. 26 00:01:14,240 --> 00:01:17,250 So this is my version of an s orbital 27 00:01:17,250 --> 00:01:20,800 that you can hold in your hand, where you have it spherically 28 00:01:20,800 --> 00:01:24,210 symmetric and you have probability out 29 00:01:24,210 --> 00:01:28,710 in very direction of r that you might find an electron, 30 00:01:28,710 --> 00:01:30,720 except at our radial nodes. 31 00:01:30,720 --> 00:01:33,770 And then we have our p orbitals, which 32 00:01:33,770 --> 00:01:37,100 can be in three different directions, where 33 00:01:37,100 --> 00:01:38,280 you have nodal planes. 34 00:01:38,280 --> 00:01:41,620 So it doesn't matter what you're talking about-- one electron 35 00:01:41,620 --> 00:01:43,130 system or more-- you still are going 36 00:01:43,130 --> 00:01:47,211 to have those same shapes of the wave functions. 37 00:01:47,211 --> 00:01:51,350 Other similarities again include the nodal structure, 38 00:01:51,350 --> 00:01:52,490 which I just mentioned. 39 00:01:52,490 --> 00:01:55,380 Whether s is always going to be spherically symmetrical. 40 00:01:55,380 --> 00:01:58,675 p is gonna have those nodal planes. 41 00:01:58,675 --> 00:02:00,500 The angular nodal planes. 42 00:02:00,500 --> 00:02:02,910 So that's the same. 43 00:02:02,910 --> 00:02:07,100 Differences, though, have to do with the energy levels. 44 00:02:07,100 --> 00:02:09,959 So orbitals in a multi-electron atoms 45 00:02:09,959 --> 00:02:12,400 are lower-- more negative-- in energy 46 00:02:12,400 --> 00:02:17,670 than their corresponding orbitals in the H atom. 47 00:02:17,670 --> 00:02:22,472 So let's take a look at this and think about why this is true. 48 00:02:22,472 --> 00:02:25,590 So here we have our friend the hydrogen atom. 49 00:02:25,590 --> 00:02:27,900 It has a small Z. Z equals 1. 50 00:02:27,900 --> 00:02:29,285 Doesn't get smaller than that. 51 00:02:29,285 --> 00:02:31,040 We have our 1s. 52 00:02:31,040 --> 00:02:33,350 Then we have our n equals 2. 53 00:02:33,350 --> 00:02:36,545 And we talked about the fact that in a 1 electron 54 00:02:36,545 --> 00:02:39,480 system like hydrogen-- these are all 55 00:02:39,480 --> 00:02:42,350 degenerate in energy with respect to each other, 56 00:02:42,350 --> 00:02:44,370 so they're all equal in energy. 57 00:02:44,370 --> 00:02:46,200 And then we have our n equals 3. 58 00:02:46,200 --> 00:02:50,810 Again, degenerate energy levels and n equals 4 up here. 59 00:02:50,810 --> 00:02:53,760 But when you go to a multi-electron system, 60 00:02:53,760 --> 00:02:56,395 the first thing that I'll point out 61 00:02:56,395 --> 00:02:58,820 is that the energy level is lower. 62 00:02:58,820 --> 00:03:00,570 So we have 1s over here. 63 00:03:00,570 --> 00:03:04,325 Now 1s is a more negative lower number. 64 00:03:04,325 --> 00:03:07,960 And we can think about this in multi-electron systems-- you 65 00:03:07,960 --> 00:03:11,995 have a bigger Z. So you have more positive charge 66 00:03:11,995 --> 00:03:16,820 and it's kind of hauling all of those energy levels closer. 67 00:03:16,820 --> 00:03:20,760 So it's a lower energy for all of those. 68 00:03:20,760 --> 00:03:25,310 So 1s, again, lower, and we have 2s. 69 00:03:25,310 --> 00:03:28,310 Again, that's lower in energy. 70 00:03:28,310 --> 00:03:32,620 And now importantly, 2s and 2p are no longer 71 00:03:32,620 --> 00:03:35,610 degenerate with respect to each other in energy. 72 00:03:35,610 --> 00:03:40,300 So now the 2p system is higher in energy than the 1s 73 00:03:40,300 --> 00:03:41,310 and so on. 74 00:03:41,310 --> 00:03:46,416 So we have 3s down here and then the 3p's. 75 00:03:46,416 --> 00:03:52,660 So compared to hydrogen in a multi-electron atom, 76 00:03:52,660 --> 00:03:57,690 n is not the sole determinate of what the energy levels are. 77 00:03:57,690 --> 00:04:02,090 Now, instead of just n we have n and l. 78 00:04:05,145 --> 00:04:06,565 So let's review. 79 00:04:06,565 --> 00:04:07,810 This is good. 80 00:04:07,810 --> 00:04:09,650 This is all getting ready for the test. 81 00:04:09,650 --> 00:04:12,505 Some equations that you've seen before. 82 00:04:12,505 --> 00:04:16,490 And again, equations will be on the equations sheet. 83 00:04:16,490 --> 00:04:18,379 You don't have to memorize them. 84 00:04:18,379 --> 00:04:20,649 And the equation sheet for the exam 85 00:04:20,649 --> 00:04:23,880 is handed out today, so you can take a look a it 86 00:04:23,880 --> 00:04:25,050 and see where things are. 87 00:04:25,050 --> 00:04:27,635 If I forgot anything you can ask me questions 88 00:04:27,635 --> 00:04:30,660 and maybe we'll add some more if I forgot any ones. 89 00:04:30,660 --> 00:04:34,700 So for a one electron system, binding energy 90 00:04:34,700 --> 00:04:40,250 equals minus the ionization energy equals minus Z squared 91 00:04:40,250 --> 00:04:43,912 RH, the Rydberg constant, over N squared. 92 00:04:43,912 --> 00:04:48,550 And so for hydrogens Z is 1, but there are other 1 electron 93 00:04:48,550 --> 00:04:53,950 at least ions and then you have to worry about Z. 94 00:04:53,950 --> 00:04:57,400 What about a multi-electron system? 95 00:04:57,400 --> 00:05:01,500 Looks pretty much the same, but now instead of just having n we 96 00:05:01,500 --> 00:05:06,250 have n and l because l matters with a multi-electron system. 97 00:05:06,250 --> 00:05:09,130 So the binding energy for the electron 98 00:05:09,130 --> 00:05:11,237 is minus the ionization energy-- that's always 99 00:05:11,237 --> 00:05:12,320 going to [INAUDIBLE] true. 100 00:05:12,320 --> 00:05:13,990 The binding energy is always going 101 00:05:13,990 --> 00:05:16,720 to be equal to minus the ionization energy, which 102 00:05:16,720 --> 00:05:19,790 is equal to minus Z. But now we have 103 00:05:19,790 --> 00:05:23,630 a different Z. We have Z effective, 104 00:05:23,630 --> 00:05:27,910 which is abbreviated Z eff. 105 00:05:27,910 --> 00:05:32,940 So this is the effective charge, not the actual charge. 106 00:05:32,940 --> 00:05:34,350 And that's squared. 107 00:05:34,350 --> 00:05:37,660 And then we have Rydberg constant and n again. 108 00:05:37,660 --> 00:05:43,230 So Z effective is not the same as Z, 109 00:05:43,230 --> 00:05:48,010 and it's not the same Z because of shielding. 110 00:05:48,010 --> 00:05:49,800 So we talked about shielding a little bit, 111 00:05:49,800 --> 00:05:51,684 and I made this point that you need 112 00:05:51,684 --> 00:05:53,600 to think about shielding a little differently. 113 00:05:53,600 --> 00:05:58,820 It's not just about the average size of the orbital. 114 00:05:58,820 --> 00:06:01,960 It's more about the likelihood-- the probability-- 115 00:06:01,960 --> 00:06:03,750 that they're going to be electrons close 116 00:06:03,750 --> 00:06:07,140 to the nucleus that will participate in-- that 117 00:06:07,140 --> 00:06:09,660 will be affected by shielding. 118 00:06:09,660 --> 00:06:12,180 So let's look at some extreme cases 119 00:06:12,180 --> 00:06:15,410 now and think about what's happening 120 00:06:15,410 --> 00:06:18,600 in terms of this shielding. 121 00:06:18,600 --> 00:06:20,140 So extreme case one. 122 00:06:20,140 --> 00:06:21,770 Extreme shielding. 123 00:06:21,770 --> 00:06:23,947 Maximum shielding. 124 00:06:23,947 --> 00:06:24,780 So we have the case. 125 00:06:24,780 --> 00:06:26,270 We have the helium nucleus. 126 00:06:26,270 --> 00:06:28,280 We have moved far in the periodic table. 127 00:06:28,280 --> 00:06:31,740 We've left hydrogen. And so what is the charge going 128 00:06:31,740 --> 00:06:36,370 to be on helium nucleus? 129 00:06:36,370 --> 00:06:37,910 Plus 2. 130 00:06:37,910 --> 00:06:39,820 So now we have this electron one. 131 00:06:39,820 --> 00:06:42,140 We're interested in how much this electron is 132 00:06:42,140 --> 00:06:46,830 shielded by electron two because we've got two electrons. 133 00:06:46,830 --> 00:06:50,170 Now say electron two is close to the nucleus 134 00:06:50,170 --> 00:06:53,700 and it's maximally shielding electron one 135 00:06:53,700 --> 00:06:56,620 from this positive charge down here. 136 00:06:56,620 --> 00:06:58,340 So think about this electron. 137 00:06:58,340 --> 00:07:01,450 I like to think about this electron two 138 00:07:01,450 --> 00:07:05,300 as kind of the elasti-girl of electrons. 139 00:07:05,300 --> 00:07:08,600 So elasti-girl electron is shielding everywhere, 140 00:07:08,600 --> 00:07:12,350 stretching everywhere around that nucleus. 141 00:07:12,350 --> 00:07:15,800 Just completely shielding it from electron 142 00:07:15,800 --> 00:07:21,720 one, using the full negative charge to really shield. 143 00:07:21,720 --> 00:07:29,390 So in that kind of exaggerated case, the Z effective is not 2. 144 00:07:29,390 --> 00:07:30,520 It's 1. 145 00:07:30,520 --> 00:07:33,330 So this electron basically cancels 146 00:07:33,330 --> 00:07:36,480 the whole positive charge of this nucleus. 147 00:07:36,480 --> 00:07:38,890 Really shields that away. 148 00:07:38,890 --> 00:07:42,740 And so this is the effective charge with total shielding. 149 00:07:42,740 --> 00:07:46,350 Maximal shielding that you can get. 150 00:07:46,350 --> 00:07:52,310 So now we can calculate what the binding energy-- or if we 151 00:07:52,310 --> 00:07:55,290 wanted, what the ionization energy would 152 00:07:55,290 --> 00:08:01,180 be for this case, given that particular effective charge. 153 00:08:01,180 --> 00:08:03,970 So again, the binding energy of electron one 154 00:08:03,970 --> 00:08:08,700 is equal to minus its ionization energy equal to minus the Z 155 00:08:08,700 --> 00:08:12,780 effective squared R H over n squared. 156 00:08:12,780 --> 00:08:14,670 And we can plug in those numbers. 157 00:08:14,670 --> 00:08:16,040 Don't forget the minus. 158 00:08:16,040 --> 00:08:22,540 We can have Z effective 1 over 1 and we calculate this value, 159 00:08:22,540 --> 00:08:24,990 which is, of course, just the Rydberg constant 160 00:08:24,990 --> 00:08:29,880 or minus the Rydberg constant minus 2.18 times 10 161 00:08:29,880 --> 00:08:32,020 to the minus 18th joules. 162 00:08:32,020 --> 00:08:35,980 So this is just like it was a hydrogen atom. 163 00:08:35,980 --> 00:08:37,720 A one electron system. 164 00:08:37,720 --> 00:08:41,620 So it had two electrons but one of the electron shielded 165 00:08:41,620 --> 00:08:45,660 so completely it was like a one electron system. 166 00:08:45,660 --> 00:08:50,540 Again, this is an exaggerated case of total shielding. 167 00:08:50,540 --> 00:08:53,840 So now let's go to the other extreme 168 00:08:53,840 --> 00:08:56,720 and consider zero shielding. 169 00:08:56,720 --> 00:08:59,150 No shielding at all. 170 00:08:59,150 --> 00:09:01,630 So here we have the helium nucleus. 171 00:09:01,630 --> 00:09:04,330 We have electron one is now close 172 00:09:04,330 --> 00:09:07,570 and electron two is pretty far away. 173 00:09:07,570 --> 00:09:11,800 So again, we're asking, how much is this electron one going 174 00:09:11,800 --> 00:09:14,840 to shielded by electron two? 175 00:09:14,840 --> 00:09:19,180 And in this extreme scenario, electron two 176 00:09:19,180 --> 00:09:22,150 is not participating in shielding at all. 177 00:09:22,150 --> 00:09:23,470 It's far away. 178 00:09:23,470 --> 00:09:25,310 I'm not sure what it's doing. 179 00:09:25,310 --> 00:09:27,706 Maybe it lost its super suit and can't find it. 180 00:09:27,706 --> 00:09:29,080 It might be at the dry cleaner's. 181 00:09:29,080 --> 00:09:30,610 We don't know what's going on. 182 00:09:30,610 --> 00:09:33,040 In any case, it is out of the game. 183 00:09:33,040 --> 00:09:34,410 It is not involved. 184 00:09:34,410 --> 00:09:38,730 It is not shielding at all. 185 00:09:38,730 --> 00:09:42,940 So in this extreme case, electron one 186 00:09:42,940 --> 00:09:46,810 feels that full force of the nucleus. 187 00:09:46,810 --> 00:09:52,720 So its effective charge is the full complete plus 2. 188 00:09:52,720 --> 00:09:57,040 So we can plug that in now and calculate 189 00:09:57,040 --> 00:09:59,330 what the binding energy is going to be. 190 00:09:59,330 --> 00:10:01,620 Again, binding energy for electron one 191 00:10:01,620 --> 00:10:05,290 equals minus the ionization energy for this electron. 192 00:10:05,290 --> 00:10:06,730 Same equation. 193 00:10:06,730 --> 00:10:11,130 We put this in but now the Z effective is 2 194 00:10:11,130 --> 00:10:13,690 and we can calculate that. 195 00:10:13,690 --> 00:10:17,710 And now we get a value of minus 8.72 times 10 196 00:10:17,710 --> 00:10:20,250 to the minus 18th joules. 197 00:10:20,250 --> 00:10:22,940 And this is actually the same as you 198 00:10:22,940 --> 00:10:27,840 would get for the scenario of helium plus, which 199 00:10:27,840 --> 00:10:30,590 is a one electron system. 200 00:10:30,590 --> 00:10:32,810 Then, in a one electron system you 201 00:10:32,810 --> 00:10:36,080 can use a formula of just Z where Z is 2 202 00:10:36,080 --> 00:10:37,770 and get your value. 203 00:10:37,770 --> 00:10:40,510 So here are the two extreme cases for helium. 204 00:10:40,510 --> 00:10:44,200 One it's like a hydrogen atom one electron system, and one 205 00:10:44,200 --> 00:10:45,320 it's like helium plus. 206 00:10:45,320 --> 00:10:48,060 And one case is like it just has one electron, 207 00:10:48,060 --> 00:10:52,520 and in the other case you have zero shielding. 208 00:10:52,520 --> 00:10:57,090 So extreme case one the Z effective is 1, 209 00:10:57,090 --> 00:10:59,430 and we have the binding energy that's 210 00:10:59,430 --> 00:11:04,680 very similar to hydrogen. And so this is a total shielding case. 211 00:11:04,680 --> 00:11:09,110 It's shields so much it's like a one electron case. 212 00:11:09,110 --> 00:11:12,250 Extreme case two you have the full force. 213 00:11:12,250 --> 00:11:13,540 So there's zero shielding. 214 00:11:13,540 --> 00:11:15,190 No shielding at all. 215 00:11:15,190 --> 00:11:19,090 And then, this is like helium plus case 216 00:11:19,090 --> 00:11:21,509 where you've actually lost that other electron. 217 00:11:21,509 --> 00:11:22,300 It's not shielding. 218 00:11:22,300 --> 00:11:24,060 It's not even there. 219 00:11:24,060 --> 00:11:26,150 So no shielding. 220 00:11:26,150 --> 00:11:30,900 And the reality in most cases is that you're in between, 221 00:11:30,900 --> 00:11:33,720 and you can determine this experimentally. 222 00:11:33,720 --> 00:11:37,070 You can measure ionization energies. 223 00:11:37,070 --> 00:11:40,720 So if we measured the ionization energy for helium 224 00:11:40,720 --> 00:11:46,200 we would find that it's 3.94 times 10 to the minus 18th. 225 00:11:46,200 --> 00:11:53,130 So it's greater than the 2.18 and less than the 8.72. 226 00:11:53,130 --> 00:11:57,450 So it's in between, and that's what you find most of the time. 227 00:11:57,450 --> 00:12:00,692 The Z effective is in between zero shielding 228 00:12:00,692 --> 00:12:01,525 and total shielding. 229 00:12:04,030 --> 00:12:08,920 So we can calculate what the Z effective is actually 230 00:12:08,920 --> 00:12:12,970 in this case if we know the ionization energy. 231 00:12:12,970 --> 00:12:15,800 So if you know the ionization energy 232 00:12:15,800 --> 00:12:17,770 you can calculate the Z effective, 233 00:12:17,770 --> 00:12:19,700 or if you're given a Z effective you 234 00:12:19,700 --> 00:12:21,820 can calculate what the ionization 235 00:12:21,820 --> 00:12:24,100 energy should be for that case. 236 00:12:24,100 --> 00:12:29,160 And so I told you that it was measured at 3.94 times 10 237 00:12:29,160 --> 00:12:31,030 to the minus 18th. 238 00:12:31,030 --> 00:12:33,820 And so in that case the Z effective, 239 00:12:33,820 --> 00:12:40,040 if you plug the numbers in would be 1.34. 240 00:12:40,040 --> 00:12:44,870 And so this number is in fact in between the two cases. 241 00:12:44,870 --> 00:12:49,960 With total shielding you would have a Z effective of 1 242 00:12:49,960 --> 00:12:53,280 and with no shielding you have a Z effective of 2. 243 00:12:53,280 --> 00:12:55,180 And so in reality, we're somewhere 244 00:12:55,180 --> 00:12:58,740 in between in this case. 245 00:12:58,740 --> 00:13:02,200 So thinking about this now, let's try your hand 246 00:13:02,200 --> 00:13:05,320 at a clicker question and see if you can tell me 247 00:13:05,320 --> 00:13:11,680 which of these is a possible Z effective for an element 248 00:13:11,680 --> 00:13:12,917 with a Z equals 3. 249 00:13:40,164 --> 00:13:41,655 I'm doing good time wise. 250 00:14:18,381 --> 00:14:18,880 OK. 251 00:14:18,880 --> 00:14:19,546 10 more seconds. 252 00:14:39,520 --> 00:14:44,060 So does someone want to say why the other ones are not correct? 253 00:14:44,060 --> 00:14:47,500 Why don't you run up there and you can give them this pen? 254 00:14:47,500 --> 00:14:49,400 American Chemical Society pen. 255 00:14:49,400 --> 00:14:50,230 Hand up there. 256 00:14:54,600 --> 00:14:56,442 AUDIENCE: Who had their hand up? 257 00:14:56,442 --> 00:14:57,900 CATHERINE DRENNAN: I think it's on. 258 00:15:01,100 --> 00:15:02,860 AUDIENCE: So in the case of no shielding 259 00:15:02,860 --> 00:15:06,420 the charge should be 3, and in the case of maximum shielding 260 00:15:06,420 --> 00:15:08,750 there's only two electrons in the 1s so it would be 1. 261 00:15:08,750 --> 00:15:11,430 So anything below 1 would be beyond maximum shielding, 262 00:15:11,430 --> 00:15:13,120 so it's gotta be between 1 and 3. 263 00:15:13,120 --> 00:15:14,703 CATHERINE DRENNAN: Yeah, that's right. 264 00:15:18,410 --> 00:15:21,580 So now we're going to talk more about 265 00:15:21,580 --> 00:15:24,900 why shielding is important. 266 00:15:24,900 --> 00:15:29,260 And shielding really has to do with this order 267 00:15:29,260 --> 00:15:37,250 of orbital energy that we all know and are very fond of. 268 00:15:37,250 --> 00:15:40,040 So when I show you this we want to ask the question, 269 00:15:40,040 --> 00:15:45,060 why is 2s lower than 2p or 3s lower than 3p? 270 00:15:48,130 --> 00:15:49,850 So let's take a look at this, and we're 271 00:15:49,850 --> 00:15:52,385 going to go back to our radial probability distribution. 272 00:15:52,385 --> 00:15:54,510 I told you I wasn't going to leave it for very long 273 00:15:54,510 --> 00:15:57,680 and we're back again. 274 00:15:57,680 --> 00:16:01,410 So here, again, if we're considering 2s and 2p-- 275 00:16:01,410 --> 00:16:04,020 so we'll consider this case here-- 276 00:16:04,020 --> 00:16:08,050 the maximum probable radius is longer. 277 00:16:08,050 --> 00:16:11,530 It's greater for 2s than for 2p. 278 00:16:11,530 --> 00:16:13,190 But what we really care about when 279 00:16:13,190 --> 00:16:17,262 we're talking about shielding is this part right here. 280 00:16:17,262 --> 00:16:19,470 And so there are different ways you can express this. 281 00:16:19,470 --> 00:16:22,200 You can say that the electrons in the orbital with lower 282 00:16:22,200 --> 00:16:26,780 values of l, like 2s is lower than 2p-- 283 00:16:26,780 --> 00:16:30,250 those electrons penetrate closer to the nucleus, 284 00:16:30,250 --> 00:16:35,030 even though we have this trend where the radius decreases 285 00:16:35,030 --> 00:16:36,250 with increasing l. 286 00:16:36,250 --> 00:16:39,340 So despite this size difference, when 287 00:16:39,340 --> 00:16:42,570 you compare this dotted line with this line here, 288 00:16:42,570 --> 00:16:44,150 there's more probability that they're 289 00:16:44,150 --> 00:16:47,570 going to be electrons near the nucleus with 2s. 290 00:16:47,570 --> 00:16:52,030 And that results in this lower energy. 291 00:16:52,030 --> 00:16:55,940 So there's less shielding for the s orbitals 292 00:16:55,940 --> 00:16:58,090 than for the p orbitals. 293 00:16:58,090 --> 00:17:00,880 And as a result of that, with less shielding 294 00:17:00,880 --> 00:17:04,359 because their probability is that they're closer, 295 00:17:04,359 --> 00:17:07,859 that they're bound more tightly, you have this lower energy. 296 00:17:07,859 --> 00:17:12,130 So that explains this energy difference. 297 00:17:12,130 --> 00:17:14,849 So we can look at this now for the three system 298 00:17:14,849 --> 00:17:17,319 again for n equals 3. 299 00:17:17,319 --> 00:17:21,810 And here we see that p electrons are also less shielded 300 00:17:21,810 --> 00:17:25,579 than the d electrons, despite the fact if you look 301 00:17:25,579 --> 00:17:30,890 that the most probable radius over here is longer for p 302 00:17:30,890 --> 00:17:32,120 than for d. 303 00:17:32,120 --> 00:17:36,120 But now, if we look near the nucleus at the probability 304 00:17:36,120 --> 00:17:39,022 that they're going to be electrons near the nucleus, 305 00:17:39,022 --> 00:17:40,730 there's a higher probability that they'll 306 00:17:40,730 --> 00:17:44,690 be electrons near the nucleus with p than with d. 307 00:17:44,690 --> 00:17:49,380 So these are going to be less shielded and lower in energy. 308 00:17:49,380 --> 00:17:52,940 And if I throw s on there now we see 309 00:17:52,940 --> 00:17:56,320 s has the most probability here of being closer. 310 00:17:56,320 --> 00:17:58,592 Then p then d. 311 00:17:58,592 --> 00:18:03,660 So for a multi-electron atom, the order of energy-- 312 00:18:03,660 --> 00:18:09,080 we have s lower than p, p lower than d, d lower than f due 313 00:18:09,080 --> 00:18:11,000 to this shielding. 314 00:18:11,000 --> 00:18:15,390 So again, we want to be able to draw 315 00:18:15,390 --> 00:18:19,070 some version of these diagrams with appropriate features 316 00:18:19,070 --> 00:18:22,360 to explain answers. 317 00:18:22,360 --> 00:18:24,600 And this all leads into what we're 318 00:18:24,600 --> 00:18:28,686 doing next, which is electron configurations. 319 00:18:28,686 --> 00:18:30,060 So we're going to think about how 320 00:18:30,060 --> 00:18:32,590 we're going to write the electron configurations 321 00:18:32,590 --> 00:18:38,140 and this indicates how you build up in energy. 322 00:18:38,140 --> 00:18:40,006 So electron configurations. 323 00:18:43,160 --> 00:18:50,210 So first, we're going to fill our electrons in to the energy 324 00:18:50,210 --> 00:18:51,770 states that are our lowest. 325 00:18:51,770 --> 00:18:53,440 Nature doesn't want to do a lot of work, 326 00:18:53,440 --> 00:18:56,030 so it's going to put them in the lower states. 327 00:18:56,030 --> 00:18:59,260 And again, where those energy levels are 328 00:18:59,260 --> 00:19:04,930 depends on, for multi-electron atoms, both n and l 329 00:19:04,930 --> 00:19:08,300 And we're going to put them in one electron at a time, 330 00:19:08,300 --> 00:19:10,690 starting with the lowest energy state 331 00:19:10,690 --> 00:19:12,860 and heading the following rules. 332 00:19:12,860 --> 00:19:16,380 So there are some rules in doing this, and most of you 333 00:19:16,380 --> 00:19:18,550 have probably heard some of these before. 334 00:19:18,550 --> 00:19:20,442 And if you haven't I'm sure you'll like them. 335 00:19:20,442 --> 00:19:22,650 I know you heard this because I already just told you 336 00:19:22,650 --> 00:19:24,120 about that a few minutes ago. 337 00:19:24,120 --> 00:19:26,440 We have Pauli's Exclusion Principle, 338 00:19:26,440 --> 00:19:30,050 which says that you can't have the same four quantum numbers. 339 00:19:30,050 --> 00:19:34,240 So if you put one electron in and it's spin up the next one 340 00:19:34,240 --> 00:19:37,360 has to be spin down. 341 00:19:37,360 --> 00:19:41,950 And Hund's Rule, which is one of my favorite rules in chemistry, 342 00:19:41,950 --> 00:19:46,060 and that is when you're adding electrons to the same state 343 00:19:46,060 --> 00:19:48,300 you're going to put them in singly, when there's 344 00:19:48,300 --> 00:19:52,590 degenerate energy orbitals, before a second one enters 345 00:19:52,590 --> 00:19:53,870 the same orbital. 346 00:19:53,870 --> 00:19:58,130 And the spins remain parallel as you're adding them across. 347 00:19:58,130 --> 00:20:02,370 So let's consider these and put some electrons into these. 348 00:20:02,370 --> 00:20:07,360 And we'll do this for oxygen, which is a Z equals 8 system. 349 00:20:07,360 --> 00:20:11,140 So first, I want to put them in the lowest energy state. 350 00:20:11,140 --> 00:20:12,960 So that's 1s. 351 00:20:12,960 --> 00:20:15,700 So I'll put one electron in there. 352 00:20:15,700 --> 00:20:17,990 And then I'm going to put the second electron in there 353 00:20:17,990 --> 00:20:21,070 because it's the lowest energy state, 354 00:20:21,070 --> 00:20:22,410 so I'm going to fill it up. 355 00:20:22,410 --> 00:20:25,930 But I'm going to pay attention to Pauli's Exclusion Principle 356 00:20:25,930 --> 00:20:27,790 and put it in spin down. 357 00:20:27,790 --> 00:20:28,980 One electron spin up. 358 00:20:28,980 --> 00:20:30,460 One electron spin down. 359 00:20:30,460 --> 00:20:32,670 I can't put them both spin up because they would 360 00:20:32,670 --> 00:20:34,590 have the same four quantum numbers, 361 00:20:34,590 --> 00:20:38,430 and that would violate Pauli's Exclusion Principle. 362 00:20:38,430 --> 00:20:41,510 So next I'm going to put electrons in 2s because that's 363 00:20:41,510 --> 00:20:44,150 the next lowest energy state. 364 00:20:44,150 --> 00:20:46,900 I'll put one in spin up. 365 00:20:46,900 --> 00:20:52,250 And then, because of Pauli, I'll put the other one in spin down. 366 00:20:52,250 --> 00:20:55,100 Next, we come to the 2p system. 367 00:20:55,100 --> 00:20:57,770 And I'm going to put the first electron in, 368 00:20:57,770 --> 00:21:00,240 but I'm not going to pair the second one. 369 00:21:00,240 --> 00:21:04,300 I'm going to put electron in the second orbital, both 370 00:21:04,300 --> 00:21:06,540 being spin up. 371 00:21:06,540 --> 00:21:08,370 And then I'm going to do that again. 372 00:21:11,250 --> 00:21:13,130 Most people kind of refer to the Hund's Rule 373 00:21:13,130 --> 00:21:16,740 as kind of the rule of seating on a bus, 374 00:21:16,740 --> 00:21:19,190 where it always seems that one person goes in-- 375 00:21:19,190 --> 00:21:21,830 and even though there are two seats, one person takes it. 376 00:21:21,830 --> 00:21:23,580 The next person gets on the bus. 377 00:21:23,580 --> 00:21:26,530 Doesn't sit next to the person who's already there. 378 00:21:26,530 --> 00:21:29,440 They take another empty seat and so on and so on. 379 00:21:29,440 --> 00:21:33,830 And so you fill up the bus with one person per bench 380 00:21:33,830 --> 00:21:36,240 before all those seats are taken. 381 00:21:36,240 --> 00:21:37,490 Then the next person comes on. 382 00:21:37,490 --> 00:21:40,030 Sadly has to sit with someone else. 383 00:21:40,030 --> 00:21:42,710 So if you think about that, that's the Hund's Rule. 384 00:21:42,710 --> 00:21:44,850 You'll always remember to do that. 385 00:21:44,850 --> 00:21:48,970 And once you put one in each, then the next person on the bus 386 00:21:48,970 --> 00:21:51,360 has to sit next to someone and they're 387 00:21:51,360 --> 00:21:54,360 going to do that spin down. 388 00:21:54,360 --> 00:21:56,210 Because if it was spin up again you'd 389 00:21:56,210 --> 00:21:59,990 have the same four quantum numbers. 390 00:21:59,990 --> 00:22:03,210 So these are the rules that you need to know to put these in. 391 00:22:03,210 --> 00:22:06,730 And then, after you do that you can write an electron 392 00:22:06,730 --> 00:22:10,590 configuration that says what you did. 393 00:22:10,590 --> 00:22:15,270 So here we can write it this way. 394 00:22:15,270 --> 00:22:17,540 We'll say we have 1s two. 395 00:22:17,540 --> 00:22:19,670 There are two electrons in the 1s. 396 00:22:19,670 --> 00:22:20,640 2s two. 397 00:22:20,640 --> 00:22:22,580 Two electrons in 2s. 398 00:22:22,580 --> 00:22:23,870 And 2p four. 399 00:22:23,870 --> 00:22:27,570 We have four electrons in the 2p orbitals. 400 00:22:27,570 --> 00:22:29,670 And if for some reason the question 401 00:22:29,670 --> 00:22:35,130 asks you to specify m sub l, you can do that too. 402 00:22:35,130 --> 00:22:41,230 So then instead of just saying 2p4 then you would say 2px2. 403 00:22:41,230 --> 00:22:44,490 So there were two electrons in px. 404 00:22:44,490 --> 00:22:49,900 And then to 2pz1 and 2py1. 405 00:22:49,900 --> 00:22:53,190 And if you don't write 1, 1 is implied. 406 00:22:53,190 --> 00:22:54,830 So you will see this both ways. 407 00:22:54,830 --> 00:22:57,270 You will see the 1 indicated and then 408 00:22:57,270 --> 00:23:00,050 you'll see the orbital just listed with nothing. 409 00:23:00,050 --> 00:23:03,310 If you do that you are implying there is one electron in there. 410 00:23:03,310 --> 00:23:06,090 If you don't mean to imply there is one electron in there 411 00:23:06,090 --> 00:23:07,870 don't write it that way because that's 412 00:23:07,870 --> 00:23:10,710 what is implied if that's the way it is. 413 00:23:10,710 --> 00:23:12,565 So now let's do a clicker question. 414 00:23:46,380 --> 00:23:46,880 OK. 415 00:23:46,880 --> 00:23:48,000 10 more seconds. 416 00:23:48,000 --> 00:23:50,954 I think we can get at least 93% on this one. 417 00:24:07,370 --> 00:24:08,380 That's not bad. 418 00:24:08,380 --> 00:24:09,400 90. 419 00:24:09,400 --> 00:24:10,690 That was a decent guess. 420 00:24:10,690 --> 00:24:13,430 That was a decent guess. 421 00:24:13,430 --> 00:24:17,370 So I think the trick here was just counting. 422 00:24:17,370 --> 00:24:19,810 So the bottom one that some people 423 00:24:19,810 --> 00:24:24,640 liked-- there are only six electrons and that has more. 424 00:24:24,640 --> 00:24:25,680 AUDIENCE: Question. 425 00:24:25,680 --> 00:24:26,638 CATHERINE DRENNAN: Yep. 426 00:24:26,638 --> 00:24:30,997 AUDIENCE: Are the p and sub l notations always 427 00:24:30,997 --> 00:24:33,062 filled in with order x, z, y or can it 428 00:24:33,062 --> 00:24:36,950 be x, y, z, or [INAUDIBLE]? 429 00:24:36,950 --> 00:24:39,950 CATHERINE DRENNAN: Yeah, you don't have to worry about that. 430 00:24:39,950 --> 00:24:43,260 That's kind of arbitrary what you put for that. 431 00:24:43,260 --> 00:24:46,690 So if it is indicated then that's fine, 432 00:24:46,690 --> 00:24:49,210 but we didn't give you an option that would be different. 433 00:24:52,530 --> 00:24:56,580 So now you can imagine that if you were writing these electron 434 00:24:56,580 --> 00:24:59,190 configurations and you were asked to write an electron 435 00:24:59,190 --> 00:25:02,450 configuration for something way down on the periodic table 436 00:25:02,450 --> 00:25:06,040 you would be writing for a very, very, very long time. 437 00:25:06,040 --> 00:25:11,760 So you can use information about core electrons and valence 438 00:25:11,760 --> 00:25:14,130 electrons to make your life easier. 439 00:25:14,130 --> 00:25:17,890 Now, sometimes you will be asked to write the full electron 440 00:25:17,890 --> 00:25:18,800 configuration. 441 00:25:18,800 --> 00:25:21,770 It will say don't use the noble gas 442 00:25:21,770 --> 00:25:24,136 short hand, then that's what you have to do. 443 00:25:24,136 --> 00:25:25,260 Very important on the exam. 444 00:25:25,260 --> 00:25:27,460 Read questions carefully. 445 00:25:27,460 --> 00:25:31,080 So let's introduce this concept of core electrons and valence 446 00:25:31,080 --> 00:25:32,140 electrons. 447 00:25:32,140 --> 00:25:38,440 So if we look at the periodic table we have our 1s1, 1s2, 448 00:25:38,440 --> 00:25:39,820 and then we come down. 449 00:25:39,820 --> 00:25:43,810 We have our 2s's and our 2p's, and then we have a noble gas, 450 00:25:43,810 --> 00:25:47,800 and then we come down and we have our first 3s1. 451 00:25:47,800 --> 00:25:49,940 And so if we're talking about sodium, 452 00:25:49,940 --> 00:25:51,910 we have core electrons, which are 453 00:25:51,910 --> 00:25:57,850 the electrons that make up the noble gas element that's 454 00:25:57,850 --> 00:26:00,260 on the row before. 455 00:26:00,260 --> 00:26:03,310 And then, those are usually not very reactive. 456 00:26:03,310 --> 00:26:05,260 They're held pretty tight. 457 00:26:05,260 --> 00:26:07,380 And the valence electrons-- the valence electrons 458 00:26:07,380 --> 00:26:10,725 are the electrons that do all the exciting chemistry-- lose 459 00:26:10,725 --> 00:26:12,790 and gain valence electrons. 460 00:26:12,790 --> 00:26:14,100 They're the fun ones. 461 00:26:14,100 --> 00:26:17,710 And so those are going to be beyond that noble gas 462 00:26:17,710 --> 00:26:18,620 configuration. 463 00:26:18,620 --> 00:26:20,140 Those core electrons. 464 00:26:20,140 --> 00:26:24,470 And so here sodium has one 3s1. 465 00:26:24,470 --> 00:26:29,520 So we can also write sodium then as bracket neon, 466 00:26:29,520 --> 00:26:32,220 indicating the noble gas before, and then 467 00:26:32,220 --> 00:26:34,480 just put the valence electrons. 468 00:26:34,480 --> 00:26:36,070 The 3s1. 469 00:26:36,070 --> 00:26:39,220 And unless it's clearly specified 470 00:26:39,220 --> 00:26:43,170 you're not supposed to do that this will be acceptable. 471 00:26:43,170 --> 00:26:46,480 So we can go on in that row of the periodic table 472 00:26:46,480 --> 00:26:47,920 in the third period. 473 00:26:47,920 --> 00:26:52,180 And so the next one we would have 3s2, 474 00:26:52,180 --> 00:26:56,130 and then we jump over here to the 3p's and so 475 00:26:56,130 --> 00:27:00,850 on and so on until we get to our next noble gas. 476 00:27:00,850 --> 00:27:02,590 So this is pretty straightforward. 477 00:27:02,590 --> 00:27:06,280 There are no exceptions on the third period. 478 00:27:06,280 --> 00:27:11,800 But when we get to the fourth period of the periodic table 479 00:27:11,800 --> 00:27:14,200 we start to have a couple of exceptions 480 00:27:14,200 --> 00:27:17,740 that you will be responsible for. 481 00:27:17,740 --> 00:27:20,020 So it's looking good up here in the beginning. 482 00:27:20,020 --> 00:27:24,000 We have now our noble gas, and then we 483 00:27:24,000 --> 00:27:28,420 have our valence electrons 4s1, 4s2, 484 00:27:28,420 --> 00:27:32,770 and then we start down in the 3d's over here. 485 00:27:32,770 --> 00:27:37,780 And when we get halfway through we have an exception. 486 00:27:37,780 --> 00:27:41,950 So you have exceptions halfway through and also 487 00:27:41,950 --> 00:27:44,140 when you're almost at the end. 488 00:27:44,140 --> 00:27:50,230 So instead of having 4s2 3d4, it really 489 00:27:50,230 --> 00:27:54,100 wants to have 5 electrons. 490 00:27:54,100 --> 00:27:58,530 And in this case, instead of 4s2 3d9 it wants 10. 491 00:27:58,530 --> 00:28:01,520 So it like things being half full or totally full, 492 00:28:01,520 --> 00:28:04,150 and so you have these exceptions there. 493 00:28:04,150 --> 00:28:08,070 So you don't have a d4 and you don't have d9. 494 00:28:08,070 --> 00:28:10,450 And you're responsible for these two, 495 00:28:10,450 --> 00:28:15,060 and then in the period right below same position 496 00:28:15,060 --> 00:28:16,720 same exception. 497 00:28:16,720 --> 00:28:18,590 So you can think about this-- I don't 498 00:28:18,590 --> 00:28:23,750 know if it's four exceptions or really just two in two places, 499 00:28:23,750 --> 00:28:26,410 but these are the exceptions you need to know. 500 00:28:26,410 --> 00:28:30,160 So in the fifth it's the ones right below are the same. 501 00:28:30,160 --> 00:28:33,670 You're not going to have d4 or a d9 here. 502 00:28:33,670 --> 00:28:36,690 So you bump one up because it's just happier 503 00:28:36,690 --> 00:28:41,470 when it has 5 or 10 electrons in the d orbitals. 504 00:28:41,470 --> 00:28:44,840 You're also responsible for knowing the orders. 505 00:28:44,840 --> 00:28:46,260 We have a question way up there. 506 00:28:46,260 --> 00:28:47,242 Who wants to run? 507 00:28:51,930 --> 00:28:53,180 Get some exercise. 508 00:28:53,180 --> 00:28:56,140 I don't always see questions so yell out my name 509 00:28:56,140 --> 00:28:58,230 if I don't see people. 510 00:28:58,230 --> 00:28:59,400 I am wearing my glasses. 511 00:28:59,400 --> 00:29:00,150 That's good. 512 00:29:00,150 --> 00:29:00,770 Yeah? 513 00:29:00,770 --> 00:29:03,190 AUDIENCE: So what happens when we have an ion? 514 00:29:03,190 --> 00:29:04,399 Like a titanium [INAUDIBLE]-- 515 00:29:04,399 --> 00:29:05,773 CATHERINE DRENNAN: Good question. 516 00:29:05,773 --> 00:29:06,710 I'm getting to that. 517 00:29:06,710 --> 00:29:07,220 AUDIENCE: All right. 518 00:29:07,220 --> 00:29:07,520 Cool. 519 00:29:07,520 --> 00:29:08,020 Thanks. 520 00:29:08,020 --> 00:29:09,810 CATHERINE DRENNAN: Hold on a minute. 521 00:29:09,810 --> 00:29:11,914 Yes. 522 00:29:11,914 --> 00:29:12,830 We're getting to ions. 523 00:29:12,830 --> 00:29:14,920 They're right at the end. 524 00:29:14,920 --> 00:29:17,280 So first, let's consider what happens 525 00:29:17,280 --> 00:29:19,420 when we're not doing an ion. 526 00:29:19,420 --> 00:29:23,240 And there are a couple different ways that we can look at this. 527 00:29:23,240 --> 00:29:25,860 This is one way to remember. 528 00:29:25,860 --> 00:29:29,970 So you just write out 1s, 2s, 2p, threes, your fours, 529 00:29:29,970 --> 00:29:34,070 your fives, your six, your sevens, and then draw a line. 530 00:29:34,070 --> 00:29:44,230 First you fill 1s, then 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, et 531 00:29:44,230 --> 00:29:46,740 cetera, et cetera, et cetera. 532 00:29:46,740 --> 00:29:47,770 That works. 533 00:29:47,770 --> 00:29:49,150 That's one way to do it. 534 00:29:49,150 --> 00:29:50,890 It's kind of time consuming. 535 00:29:50,890 --> 00:29:54,620 Not sure that's the easiest way. 536 00:29:54,620 --> 00:29:57,630 The other way that you can do-- this 537 00:29:57,630 --> 00:30:00,680 is the periodic table you'll be getting on the exam, 538 00:30:00,680 --> 00:30:03,550 and you can just remember what's happening 539 00:30:03,550 --> 00:30:05,530 at these different cases. 540 00:30:05,530 --> 00:30:10,180 So over here we're filling up our s's. 541 00:30:10,180 --> 00:30:15,230 Over here we're filling up our p's, except for helium. 542 00:30:15,230 --> 00:30:19,230 Over here we're doing our d's in the transition metals. 543 00:30:19,230 --> 00:30:22,430 Over here we have f. 544 00:30:22,430 --> 00:30:26,420 And then in terms of the period numbers-- so 545 00:30:26,420 --> 00:30:28,720 we have one over here. 546 00:30:28,720 --> 00:30:36,550 So we do 1s2, 2s2, or 1s1, 1s3, 2s. 547 00:30:36,550 --> 00:30:37,590 Then we come over here. 548 00:30:37,590 --> 00:30:44,320 We have the two p's, three s's, three p's, four s's. 549 00:30:44,320 --> 00:30:51,710 Then we have four d's and go to four p's, five s's, four d's, 550 00:30:51,710 --> 00:30:55,290 five p's, and then we can't forget when we 551 00:30:55,290 --> 00:30:58,140 get down here that we have f. 552 00:30:58,140 --> 00:31:00,830 What number goes here? 553 00:31:00,830 --> 00:31:02,360 Four. 554 00:31:02,360 --> 00:31:04,640 So when you get to principal quantum 555 00:31:04,640 --> 00:31:07,940 number four is when you start having f orbitals. 556 00:31:07,940 --> 00:31:09,970 So if you just remember this, it's 557 00:31:09,970 --> 00:31:11,950 going to help you think about what 558 00:31:11,950 --> 00:31:14,601 the energy levels-- how you're going to write those 559 00:31:14,601 --> 00:31:15,226 configurations. 560 00:31:19,490 --> 00:31:24,960 So maybe I'll leave this up and we'll do a clicker question now 561 00:31:24,960 --> 00:31:25,870 on this. 562 00:31:31,000 --> 00:31:34,540 And you have your periodic table available to look at. 563 00:31:34,540 --> 00:31:35,920 And then we'll get to ions. 564 00:31:44,720 --> 00:31:46,186 This should be 93%. 565 00:31:46,186 --> 00:31:46,686 Yeah. 566 00:32:02,761 --> 00:32:03,260 OK. 567 00:32:03,260 --> 00:32:03,760 10 seconds. 568 00:32:23,730 --> 00:32:25,150 Well, 85. 569 00:32:25,150 --> 00:32:27,320 That's still pretty good. 570 00:32:27,320 --> 00:32:32,140 So this one was one of our exceptions over here. 571 00:32:32,140 --> 00:32:38,340 And we wanted to have no d9. 572 00:32:38,340 --> 00:32:41,330 So it just is happier as d10 system, 573 00:32:41,330 --> 00:32:42,980 so that's our exception. 574 00:32:42,980 --> 00:32:47,430 One of the four that you need to know for this. 575 00:32:47,430 --> 00:32:52,660 So now we have our two methods of remembering this. 576 00:32:52,660 --> 00:32:55,190 And we did the clicker question, and so I'm 577 00:32:55,190 --> 00:32:59,560 going to jump to ions. 578 00:32:59,560 --> 00:33:01,690 There's one question on problem sets 579 00:33:01,690 --> 00:33:05,080 and people have been asking about ions. 580 00:33:05,080 --> 00:33:11,710 So when we're just here filling up our 4s ones. 581 00:33:11,710 --> 00:33:14,260 4s we're putting in first and then 582 00:33:14,260 --> 00:33:18,900 we're going to our 3d, which is what we just learned about. 583 00:33:18,900 --> 00:33:24,220 But what actually happens when you start putting electrons 584 00:33:24,220 --> 00:33:27,700 in your d orbitals is that the orbital energy 585 00:33:27,700 --> 00:33:31,320 of the d orbitals drops below the 4s. 586 00:33:31,320 --> 00:33:34,380 So once they start becoming filled you 587 00:33:34,380 --> 00:33:37,700 have this change that happens. 588 00:33:37,700 --> 00:33:39,630 And this is really only important 589 00:33:39,630 --> 00:33:42,925 when you're talking about ions. 590 00:33:42,925 --> 00:33:47,800 So if we were asking here about this one-- 591 00:33:47,800 --> 00:33:53,130 so we would normally say, OK, we put our two 4s in 592 00:33:53,130 --> 00:33:57,890 and then we have two 3d electrons. 593 00:33:57,890 --> 00:34:02,640 But now, because we've started to fill the 3d, 594 00:34:02,640 --> 00:34:05,610 if we're going to really do this in terms of energy level 595 00:34:05,610 --> 00:34:07,300 we could reverse the order. 596 00:34:07,300 --> 00:34:11,380 And we would accept both of these for the neutral ion. 597 00:34:11,380 --> 00:34:13,150 You could do either here. 598 00:34:13,150 --> 00:34:17,590 But importantly, if you start ionizing it and losing 599 00:34:17,590 --> 00:34:19,820 electrons, you need to think where are 600 00:34:19,820 --> 00:34:21,400 those electrons coming from? 601 00:34:21,400 --> 00:34:25,210 Which are the electrons that are easiest to remove? 602 00:34:25,210 --> 00:34:32,560 And what happens is that you end up losing the 4s2 electrons. 603 00:34:32,560 --> 00:34:40,480 And so the configuration for titanium plus 2 is just 3d2. 604 00:34:40,480 --> 00:34:42,340 And so this is different. 605 00:34:42,340 --> 00:34:45,550 Ions behave differently, and now you 606 00:34:45,550 --> 00:34:49,139 can go run and finish problem set two. 607 00:34:49,139 --> 00:34:51,040 That last question. 608 00:34:51,040 --> 00:34:57,400 And this is because now we're at the end of exam one material. 609 00:34:57,400 --> 00:35:01,500 So one thing I didn't get to-- it said in the beginning. 610 00:35:01,500 --> 00:35:04,210 Number 12 on the problem set we're not going 611 00:35:04,210 --> 00:35:06,130 to get to until next week. 612 00:35:06,130 --> 00:35:09,410 So if you've done number 12 already you can turn it in. 613 00:35:09,410 --> 00:35:12,220 It won't be graded and you can turn it in again on problem set 614 00:35:12,220 --> 00:35:14,420 3 so you won't have wasted time. 615 00:35:14,420 --> 00:35:17,550 But that won't be on exam one material. 616 00:35:17,550 --> 00:35:21,630 So this is the end of exam one material. 617 00:35:21,630 --> 00:35:26,350 And read your instructions for the exams very carefully. 618 00:35:26,350 --> 00:35:28,730 And there's a couple questions, but I think 619 00:35:28,730 --> 00:35:30,820 it's too noisy to answer them. 620 00:35:30,820 --> 00:35:35,310 Note that not everyone is taking the exam in this room, 621 00:35:35,310 --> 00:35:37,920 and you need to go to the appropriate place. 622 00:35:37,920 --> 00:35:40,300 So please look at the instructions. 623 00:35:40,300 --> 00:35:40,800 All right. 624 00:35:40,800 --> 00:35:43,171 And who won the clicker competition for today? 625 00:35:47,330 --> 00:35:49,640 Recitation 12.