Laster

Video transcript

- [Voiceover] In this video we'll assign formal charge to nitrogen, and just to remind you of the definition for formal charge, formal charge is equal to the number of valance electrons in the free atom minus the number of valence electrons in the bonded atom. Or another way of saying that, formal charge is equal to the number of valence electrons the atom is supposed to have minus the number of valence electrons that the atom actually has in the drawing. So let's assign a formal charge to the nitrogen in this molecule. And remember that each bond represents two electrons. So I'm gonna draw in the electrons in this bond so it's easier for us to assign a formal charge to the nitrogen. So formal charge is equal to the number of valence electrons that nitrogen is supposed to have. We know that nitrogen is supposed to have five valence electrons, because of its position on the periodic table. So this is five. And from that we subtract the number of valence electrons that nitrogen has in our drawing. So let's go back over here to the dot structure and let's look at these bonds. We know that from this bond here on the left nitrogen gets one of those electrons. And from this bond on the right nitrogen gets one of those electrons and hydrogen gets the other. And same for this nitrogen-hydrogen bond. Nitrogen gets one of the electrons and hydrogen gets the other. So how many electrons do we have around nitrogen in our drawing? Let's count them up. This would be one, two, three, and then we have a lone pair of electrons on the nitrogen, so that's four, and five. So in our drawing, nitrogen is surrounded by five valance electrons. So we put five minus five which is equal to zero. So nitrogen has a formal charge of zero. Let me go ahead and redraw that. So we had our nitrogen here with our two hydrogens and a lone pair of electrons on the nitrogen. We found the nitrogen to have a formal charge of zero. So we have a pattern. Every time that you see nitrogen with three bonds, let me draw these in here, one, two, three. So three bonds and one lone pair of electrons, the formal charge is equal to zero. So when nitrogen has three bonds and one lone pair of electrons, the formal charge is equal to zero. And sometimes you don't want to draw in lone pairs of electrons, so you could just leave those off. You could just say alright, well if I just draw this and you know the formal charge of nitrogen is zero, then it's assumed you also know there's a lone pair of electrons on that nitrogen. So this is just another way of representing the same molecule, leaving off the lone pair, because you should know it's there. Let's look at other examples where nitrogen has a formal charge of zero. So we'll start with the example on the left here and if we look at this nitrogen and we know it has a formal charge of zero, let's see how many bonds it has. Let's use red here. So here's one bond, two bonds, and then three bonds. So three bonds, and with a formal charge of zero we know there should be a lone pair of electrons on that nitrogen. So you could leave it off and just know it's there, or you could draw them in. So I'll go ahead and draw in the lone pair of electrons on the nitrogen. So formal charge of zero. Let's look at the one on the right. So if we assume that nitrogen has a formal charge of zero, let's see how many bonds we have here. So here's one, two, and three. So we have three bonds, so we'd still need one lone pair of electrons. So if you wanted to show the lone pair of electrons you could put them in there like that. Notice this gives nitrogen an octet of electrons around it. So count those up, here's two, four, six, and eight. So nitrogen would have an octet. And remember, you could just leave off that lone pair of electrons and it's assumed if we know nitrogen has a formal charge of zero that there is a lone pair and we just didn't want to take the time to draw them in. Let's assign formal charge to another nitrogen, so down here. So what is the formal charge of nitrogen now? Let's draw in our electrons. So each bond is two electrons, so I draw those in there. And the formal charge on nitrogen is equal to the number of valence electrons that nitrogen is supposed to have, which we already know is five, so we put a five in here, and from that we subtract the number of valence electrons that nitrogen actually has in our drawing. So for these bonds, hydrogen gets one electron and nitrogen gets one for each of these bonds. So that allows us to see there are four electrons around nitrogen. So here's one, two, three, and four. So in our drawing, nitrogen only has four electrons around it, so this would be five minus four, which gives us a formal charge of plus one. So it's like nitrogen lost a valence electron. It's supposed to have five and here we see only four around it, so it's as if it lost a valence electron, so it's plus one for the formal charge. Alright, let me redraw that. So we have our nitrogen with four bonds to hydrogen and then nitrogen has a plus one formal charge. You should recognize this as being the ammonium ion from general chemistry. So this has a formal charge of plus one, so we have another pattern to think about here. So let's draw that in. We have one, two, three, four bonds and zero lone pairs of electrons. So when nitrogen has four bonds, four bonds and zero lone pairs, zero lone pairs of electrons, we've already seen the formal charge be equal to plus one. So let's look at some examples where nitrogen has a formal charge of plus one. So the example on the left, we can see there are four bonds and there are no lone pairs on that nitrogen, so that's a plus one formal charge. Over here on the right, same idea. Here's one bond, two bonds, three bonds, and four bonds and no lone pairs, so a plus one formal charge on the nitrogen. Alright, finally, one more nitrogen to assign a formal charge to. So let's look at this one. Let's draw in the electrons in the bond. So here's two electrons and here's two electrons. What is the formal charge on nitrogen? Formal charge is equal to number of valence electrons nitrogen is supposed to have, which we know is five, and from that we subtract the number of valance electrons nitrogen actually has in our dot structure. So again we go over to here and we look at this bond and we give one electron to nitrogen and one electron to the other atom. And over here we give one electron to nitrogen and one electron to the other atom. And now we have two lone pairs of electrons on the nitrogen. So how many is that total? this would be one, two, three, four, five, and six. So six electrons around our nitrogen. So five minus six gives us negative one. So a formal charge of negative one. Let me go ahead and redraw that. So I could draw it out here. So nitrogen with two lone pairs of electrons we just found has a formal charge of negative one. If I wanted to leave off the lone pairs of electrons I could do that, I could just write NH here and put a negative one formal charge, and because of this pattern, you should know there are two lone pairs of electrons on that nitrogen. Let me just clarify the pattern here. The pattern for a formal charge of negative one on nitrogen would be two bonds, here are the two bonds, and two lone pairs of electrons. So when nitrogen has two bonds and two lone pairs of electrons, nitrogen should have a formal charge of negative one. Let's look at some examples of that. So down here we have nitrogen. So here's nitrogen with no lone pairs of electrons drawn in, but you know this nitrogen has a negative one formal charge, because it's telling you that right here. How many bonds do we have? Well here's one bond and here's the other bond. So we have our two bonds, but we don't have our two lone pairs drawn in. So you could just know that they are there, or I'll go ahead and add them in here. So here's one lone pair of electrons and here's the other lone pair of electrons on that nitrogen. Notice that gives that nitrogen an octet of electrons. Over here on the right, let's do the same thing. You know this nitrogen has a negative one formal charge, so you know it must have two bonds and two lone pairs of electrons. So here we can see, here are the two bonds, so that takes care of the two bonds part, and if it has a negative one formal charge it must have two lone pairs of electrons. So we could draw in those lone pairs, or we could leave them off, depending on what you're trying to show when you're drawing these out.