WEBVTT 00:00:00.270 --> 00:00:01.103 - [Instructor] In this video, 00:00:01.103 --> 00:00:03.270 we're gonna introduce ourselves to a new way 00:00:03.270 --> 00:00:05.100 of visualizing atoms. 00:00:05.100 --> 00:00:06.930 And as you can imagine from the title here, 00:00:06.930 --> 00:00:09.570 that's going to be Lewis diagrams. 00:00:09.570 --> 00:00:11.310 But before I even get into that, 00:00:11.310 --> 00:00:13.710 let's do a little bit of review of what we already know 00:00:13.710 --> 00:00:15.450 about Bohr models. 00:00:15.450 --> 00:00:18.570 So let's say we take an arbitrary element here. 00:00:18.570 --> 00:00:20.070 Let's say we take nitrogen. 00:00:20.070 --> 00:00:22.890 Nitrogen, by definition, has seven protons. 00:00:22.890 --> 00:00:26.040 And so if it's neutral, it's going to have seven electrons. 00:00:26.040 --> 00:00:28.740 So a Bohr model for nitrogen, 00:00:28.740 --> 00:00:31.860 in our first shell, that first shell is going to look 00:00:31.860 --> 00:00:35.700 just like helium and it's going to have two electrons. 00:00:35.700 --> 00:00:37.860 So let me draw it like that. 00:00:37.860 --> 00:00:40.680 And then in its second shell, 00:00:40.680 --> 00:00:45.680 its second shell, it is going to have the remaining five 00:00:45.840 --> 00:00:47.880 of the seven electrons. 00:00:47.880 --> 00:00:50.220 And we are going to make them unpaired at first. 00:00:50.220 --> 00:00:52.590 So one, two, 00:00:52.590 --> 00:00:56.610 three, four, and then five. 00:00:56.610 --> 00:00:58.560 The reason why I did it this way is, 00:00:58.560 --> 00:01:01.650 a full valence shell is going to have eight electrons 00:01:01.650 --> 00:01:03.330 or four pairs. 00:01:03.330 --> 00:01:05.190 But if the electrons can spread apart, 00:01:05.190 --> 00:01:06.330 they like to spread apart. 00:01:06.330 --> 00:01:08.520 So that's why I did one, two, three, four, 00:01:08.520 --> 00:01:11.190 and then I paired this last one because there's nowhere else 00:01:11.190 --> 00:01:13.920 for it to actually go. 00:01:13.920 --> 00:01:16.530 Now, I just touched on this issue of valence electrons. 00:01:16.530 --> 00:01:18.750 Those are the electrons in your outermost shell, 00:01:18.750 --> 00:01:22.680 and they tend to be the ones that are involved in reactions. 00:01:22.680 --> 00:01:24.840 So chemists said, "Hey, just for shorthand, 00:01:24.840 --> 00:01:27.420 instead of having to draw all of this every time, 00:01:27.420 --> 00:01:31.500 why don't we just visualize the valence electrons?" 00:01:31.500 --> 00:01:34.590 And so let's do that in this nitrogen example. 00:01:34.590 --> 00:01:37.590 So a Lewis diagram, 00:01:37.590 --> 00:01:39.870 which is I'm just going to draw right now, 00:01:39.870 --> 00:01:42.930 is that simplified visualization 00:01:42.930 --> 00:01:45.780 where you write the symbol for that element, 00:01:45.780 --> 00:01:48.210 and you just depict its valence electrons. 00:01:48.210 --> 00:01:50.790 We just saw that there are five valence electrons 00:01:50.790 --> 00:01:53.370 for nitrogen, seven total, but five valence, 00:01:53.370 --> 00:01:56.460 five electrons in that outermost shell. 00:01:56.460 --> 00:02:00.840 So it is going to be one, two, 00:02:00.840 --> 00:02:04.020 three, four, and then five. 00:02:04.020 --> 00:02:07.830 So that's a Lewis diagram for a neutral nitrogen atom. 00:02:07.830 --> 00:02:10.860 It turns out we can also do this for ions. 00:02:10.860 --> 00:02:15.860 So let's say that we had a nitride ion over here. 00:02:15.870 --> 00:02:19.020 Now, a nitride ion has gained three electrons. 00:02:19.020 --> 00:02:21.810 So it actually has eight valence electrons. 00:02:21.810 --> 00:02:23.850 So if you gain three from five, you're going to have eight. 00:02:23.850 --> 00:02:26.160 So I'll go one, two, 00:02:26.160 --> 00:02:29.010 three, four, five, 00:02:29.010 --> 00:02:31.950 six, seven, eight. 00:02:31.950 --> 00:02:35.070 And because it gained three electrons from being neutral, 00:02:35.070 --> 00:02:38.010 it now has a negative three charge. 00:02:38.010 --> 00:02:39.930 And so you'll often see it written like this 00:02:39.930 --> 00:02:42.870 where they put brackets around it, 00:02:42.870 --> 00:02:45.810 and you would see three minus. 00:02:45.810 --> 00:02:48.307 Now, the last thing that you might wonder about is, 00:02:48.307 --> 00:02:49.830 "Okay, I kinda understood 00:02:49.830 --> 00:02:51.570 how you got the valence electrons for nitrogen. 00:02:51.570 --> 00:02:54.750 Is there just some general pattern in the periodic table?" 00:02:54.750 --> 00:02:56.820 And the simple answer is yes. 00:02:56.820 --> 00:02:58.380 And that's one of the useful things 00:02:58.380 --> 00:02:59.310 about the periodic table. 00:02:59.310 --> 00:03:01.260 Or as we'll learn, there's many, many other 00:03:01.260 --> 00:03:03.330 really interesting things about it. 00:03:03.330 --> 00:03:05.820 If you look at the groups, in general, 00:03:05.820 --> 00:03:08.190 you're going to have one valence electron 00:03:08.190 --> 00:03:11.430 for Group One elements, for this column over here. 00:03:11.430 --> 00:03:13.680 You're going to have two valence electrons 00:03:13.680 --> 00:03:15.630 for these Group Two elements. 00:03:15.630 --> 00:03:16.627 And I know what you're thinking, 00:03:16.627 --> 00:03:19.830 "Okay, is just the group the number of valence electrons?" 00:03:19.830 --> 00:03:23.790 Well, unfortunately, it doesn't exactly work out that way. 00:03:23.790 --> 00:03:25.800 I'm going to skip the transition metals here 00:03:25.800 --> 00:03:27.600 because those get a little bit more complicated. 00:03:27.600 --> 00:03:29.100 It's a little bit more advanced. 00:03:29.100 --> 00:03:31.530 But then if we go over here to, what is this, 00:03:31.530 --> 00:03:34.260 Group One, Two, Three, Four, Five, Six, Seven, 00:03:34.260 --> 00:03:37.500 Eight, Nine, 10, 11, 12, 13, 00:03:37.500 --> 00:03:41.280 Group 13 over here is going to have three valence electrons. 00:03:41.280 --> 00:03:44.280 Group 14, four valence electrons. 00:03:44.280 --> 00:03:46.980 Five valence electrons in Group 15, 00:03:46.980 --> 00:03:49.380 and that's why we saw five valence electrons 00:03:49.380 --> 00:03:50.880 for nitrogen here. 00:03:50.880 --> 00:03:53.430 Six for Group 16. 00:03:53.430 --> 00:03:55.470 Seven for Group 17. 00:03:55.470 --> 00:03:58.020 And then 18 for, or sorry, (laughing) 00:03:58.020 --> 00:04:02.430 I should say eight valence electrons for Group 18. 00:04:02.430 --> 00:04:03.990 So one way to remember it is, 00:04:03.990 --> 00:04:06.750 for Groups 13 through 18, 00:04:06.750 --> 00:04:09.060 you take the group number and you subtract 10, 00:04:09.060 --> 00:04:11.520 and you're going to get the number of valence electrons. 00:04:11.520 --> 00:04:13.290 And hopefully that made sense based 00:04:13.290 --> 00:04:15.060 on how we were able to figure out the valence electrons 00:04:15.060 --> 00:04:17.253 for example nitrogen.