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Video transcript

we've already seen that formal charge is equal to the number of valence electrons in the free atom minus the number of valence electrons in the bonded atom and another way of saying that is the formal charge is equal to the number of valence electrons the atom is supposed to have minus number of valence electrons the atom actually has in the drawing so let's assign a formal charge to oxygen in this molecule remember that each bond is made of two is made up of two electrons so this bond right here is made up of two electrons this bond over here is made up of two electrons and our goal is to find the formal charge on oxygen so the formal charge on oxygen is equal to the number of valence electrons in the free atom so the number of valence electrons that oxygen is supposed to have we know that's six right oxygen is supposed to have six valence electrons minus the number of electrons that oxygen actually has in our drawing now remember when you have a bond with two electrons we give one electron to one atom and the other electron to the other atom so from these two electrons oxygen gets one of those electrons right same thing for this other bond here oxygen gets one of those electrons and the other electron goes to hydrogen so now we have now we have a total of six electrons around our oxygen so I'll highlight those we have 1 2 3 4 5 6 so there are 6 valence electrons around the oxygen in our drawing so 6 minus 6 is equal to 0 so the formal charge on oxygen is equal to 0 and let me go in write down this this pattern that we've just seen when oxygen has two bonds and two lone pairs of electrons so one oxygen has two bonds and two lone pairs of electrons the formal charge is equal to 0 and sometimes the lone pairs are just left off for convenience reasons right so you could you could draw this with your oxygen and your hydrogen like that or you could even go like this and all of those or the star just different ways of representing the same molecule right so leaving off lone pairs of electrons let's look at some other examples where the formal charge on oxygen is equal to zero and look at the one on the left first alright so the formal charge is equal to zero on this oxygen and we can see we have two bonds here two oxygen here is one of the bonds to oxygen here's the other bond to oxygen the lone pairs of electrons have been left off this dot structure but we know since the formal charge is zero we already have our two bonds here there should be two lone pairs of electrons on those oxygens on that oxygen so you can put them in there or you could leave them off I'll go ahead and put them in alright so we have a total of eight electrons around our oxygen so oxygens following the octet rule here so I'll highlight them two four six and eight on the right we have another example where oxygen has a formal charge of zero and this oxygen has two bonds to it so here's one of the bonds and here's the other bond and this oxygen would also have to have two lone pairs of electrons on it and again I didn't draw them in here sometimes you don't draw them in here for convenience but I could go ahead and add them so it's easier to see that that oxygen has a formal charge of zero and a total of an octet of electrons around it so let me highlight those two four six and eight so these patterns are important and for oxygen two bonds and two lone pairs of electrons give us a formal charge of zero alright let's move on to another formal charge situation for oxygen let's find the formal charge on oxygen here so we start by drawing in the electrons in our bonds right each bond consists of two electrons so I draw those in what is the formal charge on oxygen this time so the formal charge is equal to the number of valence electrons oxygen is supposed to have which is six minus the number of valence electrons oxygen actually has in our drawing so we divide up our electrons again so oxygen gets one from this bond and one from this bond and one from this bond so how many electrons are around oxygen now this would be one two three four and five so six minus five is equal to plus one so it's like oxygen has lost an electron here so oxygen has a formal charge of +1 I could redraw that let me go ahead and do that over here so I could redraw that over here so we have oxygen with our bond two hydrogen's and oxygen has a lone pair of electrons on it and this oxygen has a +1 formal charge and so we can come up with another pattern here here oxygen has three bonds let me highlight those bonds let me use these red this time so here's one bond two bonds and then three bonds and then one lone pair of electrons so the pattern of three bonds plus one lone pair of electrons for oxygen will give you a formal charge of +1 and again it's good to recognize these patterns you should be able to do the calculation and then after you do enough of these problems you can just look at it and figure out what the formal charge is let's look at some more examples where oxygen has a formal charge of +1 and the lone pairs were left off of this one again for convenience reasons so we'll start with this example on the left we can see that the oxygen with a plus one formal charge has three bonds to it here's one here's two and then here's three so three bonds in order for that oxygen to have a +1 formal charge it must also have one lone pair of electrons on it so you could just leave them off and know that they're there or you could go ahead and draw them in and I'll draw them in on that oxygen all right now to R now to our other example over here on the right this is the oxygen with a plus 1 formal charge so that oxygen must have three bonds and one lone pair so here the three bonds there's 1 2 & 3 and again I didn't draw in the lone pair of electrons on the oxygen but the lone pair is there so I'll go ahead and put it in like that all right so recognize this pattern 3 bonds plus one lone pair four oxygen gives us a formal charge of plus one you could have also you can also figure out how many electrons are necessary let's use this example by let me go ahead and redraw it here so how else could we figure out how many electrons are on that oxygen if that oxygen has a plus 1 formal charge well you can say that alright the calculation for formal charge would be six minus six minus X but we don't know but we do know the formal charge is plus one so let me just put this in a little box over here so six minus X is equal to plus one obviously X would have to be equal to five meaning that oxygen would have five electrons around it let's think about how many electrons will we see right now around oxygen so let me draw in let me draw on some electrons here I'll use red so this bond is two electrons and then we have a bunch of electrons in here so how many electrons around oxygen do we have so far well we would have we would have three right just these three and we need five which means we need two more which means we need a lone pair of electrons on the oxygen so that's that's a little bit complicated a little bit too complicated I think for figuring it out but you could use that method or you could just learn this pattern right and eventually you'll have to have this pattern down pretty well let's look at another example for assigning formal charge to oxygen so our goal is to find the formal charge on oxygen in this example and we put in our electrons in this bond right each bond each bond represents two electrons so the formal charge on oxygen is equal to the number of valence electrons that oxygen is supposed to have which is six minus the number of valence electrons that oxygen actually has and in this example right we would take one of these electrons from this bond and how many electrons is that total around oxygen this would be one two three four five six seven so six minus seven gives us a formal charge of negative one so I could I could redraw that over here alright I could say oxygen has three lone pairs of electrons and a negative one formal charge so our pattern this time our pattern is one bonds right here's the one bond and then three lone pairs of electrons so let me write that down so the pattern is one bond when oxygen has one bond and three lone pairs of electrons the formal charge is negative one like we saw up here with the calculation alright so we could we could leave those electrons off we wanted to save some time we could just say oh this is oxygen with a negative 1 formal charge and we should know that there must be three lone pairs of electrons on that oxygen let's look at one more example where we formal charge is negative one so right here this oxygen has a negative 1 formal charge and we can see it already has one bond to it and so the pattern of course is one bond plus three lone pairs of electrons so we already have the one bond in order for that oxygen have a negative 1 formal charge we need three lone pairs of electrons so we could we could redraw this right so that is one way to represent that ion and we could also represent it like this with putting three lone pairs of electrons on that oxygen with a negative 1 formal charge so again become familiar with these patterns