1 00:00:00,000 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,850 Commons license. 3 00:00:03,850 --> 00:00:06,840 Your support will help MIT OpenCourseWare continue to 4 00:00:06,840 --> 00:00:10,170 offer high quality educational resources for free. 5 00:00:10,170 --> 00:00:13,630 To make a donation or view additional materials from 6 00:00:13,630 --> 00:00:17,440 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:17,440 --> 00:00:18,690 ocw.mit.edu. 8 00:00:21,160 --> 00:00:23,950 PROFESSOR: So now we're going to start Part (b). 9 00:00:23,950 --> 00:00:25,730 First we're going to read the question. 10 00:00:25,730 --> 00:00:29,260 To which does an atom of argon form a stronger bond: another 11 00:00:29,260 --> 00:00:31,830 argon atom or an atom of krypton. 12 00:00:31,830 --> 00:00:34,690 Justify your choice with an explanation, using narrative 13 00:00:34,690 --> 00:00:38,010 or cartoons or both that makes reference to the operative 14 00:00:38,010 --> 00:00:39,540 chemical bonding. 15 00:00:39,540 --> 00:00:43,580 So again, he's not just asking you for one or the other, but 16 00:00:43,580 --> 00:00:46,840 that you give an explanation to show that you understand 17 00:00:46,840 --> 00:00:49,840 the concepts that are being tested. 18 00:00:49,840 --> 00:01:04,290 Here we're asked: does argon bond stronger with another 19 00:01:04,290 --> 00:01:08,580 argon or krypton. 20 00:01:08,580 --> 00:01:11,090 And some students were confused by the usage of the 21 00:01:11,090 --> 00:01:12,650 term bond here. 22 00:01:12,650 --> 00:01:16,310 But remember that bond doesn't necessarily mean a covalent or 23 00:01:16,310 --> 00:01:19,330 an ionic bond, it can also mean intermolecular bonds, 24 00:01:19,330 --> 00:01:24,810 which we have been using in Part (a), so it might seem 25 00:01:24,810 --> 00:01:27,400 logical that we would think about intermolecular 26 00:01:27,400 --> 00:01:30,350 bonds in Part (b). 27 00:01:30,350 --> 00:01:35,430 Now we need to look at the specific species we are asked 28 00:01:35,430 --> 00:01:36,960 to consider here. 29 00:01:36,960 --> 00:01:47,410 Argon and krypton are both Nobel gases, and what do we 30 00:01:47,410 --> 00:01:49,130 know about Nobel gases? 31 00:01:49,130 --> 00:01:53,420 They have complete octets; they don't bond very often, 32 00:01:53,420 --> 00:01:56,440 and so, especially to each other. 33 00:01:56,440 --> 00:01:57,170 Therefore. 34 00:01:57,170 --> 00:02:03,500 we will probably be looking at intermolecular bonds, as I 35 00:02:03,500 --> 00:02:05,720 said before, but also the weakest form, 36 00:02:05,720 --> 00:02:06,970 which is Van der Waals. 37 00:02:15,430 --> 00:02:17,510 We're looking at Van der Waals, because each of these 38 00:02:17,510 --> 00:02:19,440 are a mono-atomic species. 39 00:02:19,440 --> 00:02:24,830 That is, they just are gases as one atom of argon and one 40 00:02:24,830 --> 00:02:26,110 atom of krypton. 41 00:02:26,110 --> 00:02:29,740 And so, Van der Waals is the only type of interaction they 42 00:02:29,740 --> 00:02:34,460 can have. As I said in Part (a), Van der Waals has to do 43 00:02:34,460 --> 00:02:38,010 with the movement of electrons in an induced dipole. 44 00:02:38,010 --> 00:02:38,900 And what does that mean? 45 00:02:38,900 --> 00:02:41,070 So, we're going to talk a little bit about 46 00:02:41,070 --> 00:02:42,320 polarizability. 47 00:02:51,710 --> 00:02:55,250 And polarizability is basically how easy is it to 48 00:02:55,250 --> 00:02:59,610 induce a dipole into the atom or molecule. 49 00:02:59,610 --> 00:03:05,210 And because these are non-polar atoms, obviously, we 50 00:03:05,210 --> 00:03:07,790 want to think about what can cause a difference in 51 00:03:07,790 --> 00:03:11,200 polarizability between argon and krypton. 52 00:03:11,200 --> 00:03:13,720 So if you think about it, we're going to do a very 53 00:03:13,720 --> 00:03:15,770 simplistic model of the atom. 54 00:03:15,770 --> 00:03:19,950 We have a cloud of electrons, and to get intermolecular 55 00:03:19,950 --> 00:03:24,290 interactions, we want to have a partial negative and a 56 00:03:24,290 --> 00:03:29,360 partial positive induced in the atom. 57 00:03:29,360 --> 00:03:33,730 And this can be due to just spontaneous fluctuations in 58 00:03:33,730 --> 00:03:39,810 the electron density or to an external charge or another 59 00:03:39,810 --> 00:03:41,280 molecule or something. 60 00:03:41,280 --> 00:03:46,710 But here we have two non-polar atoms, so we want to see which 61 00:03:46,710 --> 00:03:50,760 one will inherently be easier to polarize. 62 00:03:50,760 --> 00:03:55,770 And so if we go back to our two species here, we have 63 00:03:55,770 --> 00:04:06,830 argon, which has 18 electrons and krypton, 64 00:04:06,830 --> 00:04:10,920 which has 36 electrons. 65 00:04:10,920 --> 00:04:15,100 So based on our discussion of polarizability, we can see 66 00:04:15,100 --> 00:04:18,960 that krypton is going to have a greater amount of 67 00:04:18,960 --> 00:04:23,250 polarizability, because it has a larger cloud of electrons. 68 00:04:23,250 --> 00:04:26,420 Those electrons are bound less strongly. 69 00:04:26,420 --> 00:04:30,040 As we know from our periodic trends, krypton is a larger 70 00:04:30,040 --> 00:04:35,220 atom, and so we are going to say that the polarizability of 71 00:04:35,220 --> 00:04:45,080 krypton is greater than argon. 72 00:04:45,080 --> 00:04:48,000 As we talked about in Part (a), even though these had the 73 00:04:48,000 --> 00:04:53,250 same type of intermolecular bonds, if one of them exhibits 74 00:04:53,250 --> 00:04:59,920 a higher difference in partial charges as krypton will, then 75 00:04:59,920 --> 00:05:02,220 it results in a stronger bond. 76 00:05:02,220 --> 00:05:13,500 And so the answer here would be that krypton has a stronger 77 00:05:13,500 --> 00:05:38,660 interaction because it is more polarizable. 78 00:05:41,560 --> 00:05:43,910 And that because is important. 79 00:05:43,910 --> 00:05:46,370 Remember, when you're answering a question, you 80 00:05:46,370 --> 00:05:48,880 don't just want to put down the answer, you want to show 81 00:05:48,880 --> 00:05:51,770 that you know why that's the answer, and that you 82 00:05:51,770 --> 00:05:53,540 understand the concepts.