If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:05

Video transcript

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 valence 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 I'm going to 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 5 valence electrons because of its position on the periodic table so this is 5 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 right 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 valence electrons 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 hydrogen's 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 alright 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 0 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 can just leave those off you could just say all right well if I just draw this alright and and you know the formal charge on 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 all right 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 right and just know it's there or you could draw them in right so I'll go ahead and draw on 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 and octet of electrons around it right so count those up here's two four six and eight so the 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 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 all right 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 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 right 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 1 2 3 & 4 so in our drawing nitrogen only has four electrons around it so this would be 5 minus 4 which gives us a formal charge of +1 so it's it's like nitrogen lost a valence electron right supposed to have 5 and here we see only four around it so it's as if it lost a valence electrons so it's plus 1 for the formal charge alright let me redraw that so we have our nitrogen with four bonds to hydrogen and the nitrogen has a plus 1 formal charge you should recognize this as being the ammonium ion from general chemistry so this has a formal charge of +1 so we have another pattern to think about here so let's draw that in we have 1 2 3 4 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 should be equal to +1 so let's look at some examples where nitrogen has a formal charge of +1 so the example on the left we can see there are four bonds alright and there are no lone pairs on that nitrogen so that's a plus 1 formal charge over here on the right same idea here's one bond two bond three bonds and four bonds and no lone pairs so a plus 1 formal charge on the nitrogen alright finally one more one more nitrogen to assign a formal charge to so let's look at this one let's draw in the electrons in the bonds 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 5 and from that we subtract the number of valence electrons nitrogen actually has in our dots structure so again we go over here and we look at this bond that 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 6 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 right I could do that I could just write n H 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 alright the pattern for a formal charge of negative one on nitrogen would be two bonds alright here 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 and 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 right 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 and octet of electrons over here on the right let's do the same thing you know this nitrogen has a negative 1 formal charge so you know it must have two bonds and two lone pairs of electrons so here we can see here the two two bonds right 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 right depending on depending on what you're trying to show when you're drawing these out