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to assign formal charge you take the number of valence electrons in the free atom or the number of valence electrons the atom is supposed to have and from that you subtract the number of valence electrons in the bonded atom or the number of valence electrons the atom actually has in the drawing so let's assign a formal charge to carbon in the methane molecule remember that each bond consists of two electrons so I'm going to draw in these electrons in these bonds because it's going to make it easier for us to assign a formal charge to carbon so to find a formal charge for carbon the formal charge is equal to the number of valence electrons in the free atom or the number of valence electrons that carbon is supposed to have we know that carbon is supposed to have four valence electrons so I could write a four here and from that we subtract the number of valence electrons that carbon actually has in the drawing remember when we drew our dot structures all right we knew that each bond here came from one valence electron from hydrogen and one valence electron from carbon so I could give that one valence electron back to hydrogen and one valence electron to carbon and so we're going to divide up all of our bonds that way right give one valence electron to hydrogen and the other valence electron to carbon because that makes it easier for us to see that carbon has four valence electrons in our drawing so let me highlight them here one two three four so we're going to subtract four from four so four minus four is equal to zero so carbon has a formal charge of zero in methane let's do another example this one down here you can see it's different because this time we have three bonds so let me draw in the electrons in those bonds and let's find the formal charge on carbon the formal charge on carbon is equal to the number of valence electrons that carbon is supposed to have which we know is four and from that we subtract the number of valence electrons that carbon actually has in our drawing we divide up the electrons in our bonds just like we did before and we can see that carbon has only three electrons around at this time so highlight those one one two and three so four minus three is equal to plus one so carbon has a formal charge of plus one so carbons supposed to have four valence electrons it has only three around it so it lost one of its electrons which gives it a formal charge of plus one let me go ahead and redraw that so over here on the right we have carbon with three bonds to hydrogen and this carbon has a plus one formal charge so we can represent that by putting a plus charge here next to the carbon notice that carbon does not have an octet of electrons around it has only six electrons around it and that's actually okay carbon can never exceed an octet but it's okay for carbon to have less than eight electrons this is a carbon with a plus one formal charge it's a positively charged carbon we call those carbo cations so let me write that down here this is a carbo cation and carbo cations come up a lot in organic chemistry mechanism so it's really important to understand them alright let's let's think about the pattern that we see here all right we have three single bonds around this carbon let me go ahead and highlight them here so here's 1 2 & 3 so we have 3 single bonds around our carbon and we have 0 lone pairs of electrons around that carbon so 3 bonds plus 0 lone pairs of electrons will give you a positively charged carbon will give you a carbo cation what is the hybridization of this positively charged carbon well there's 1 2 3 Sigma bonds and zero lone pairs of electrons and so from the videos on hybridization you should know this carbon is sp2 hybridized and therefore this carbon will have trigonal planar geometry around it and again that's important when you do your organic chemistry mechanism so carbo cations are extremely important to understand let's look at some other examples of carbo cations and analyze them a little bit too alright so let's start with the carbo cation on the far left the carbon with the plus-one formal charge is this one in the center here and what is this carbon in red bonded to well the carbon in red is bonded to a ch3 group up here which we call a methyl group in organic chemistry the carbon in red is bonded to another ch3 group here and another ch3 group here so the carbon in red already has three single bonds and zero lone pairs of electrons and so the carbon red is a plus one formal charge let's look at this carbo cation right here and let's highlight the carbon with the plus one formal charge it's this one alright so so this carbon in red is bonded to a ch3 group on the left and a ch3 group on the right so we only have two bonds here right only have two bonds at this point but we know in order for the carbon red to have a plus one formal charge we need three bonds I like the example on the left the example on the Left we have three bonds here to that carbon and so where is the last bond the last bond of course must be to a hydrogen all right so we draw on in here like that so the carbon in red is bonded to a hydrogen usually you leave off your hydrogen's when you when you make these drawings but it's important to understand what's actually there move on to the last example right this time the positive one formal charge is on this carbon in red and that carbon in red is bonded to directly bonded to one other carbon so that's one bond but we know we need a total of three bonds so the carbon in red must have two more bonds and those two other bonds must be two hydrogen so we draw on there's one bond to hydrogen and there's another bond to hydrogen so it's important to recognize these patterns let's do another formal charge right let's assign formal charge to another carbon let's put in our our electrons and our bonds all right so we put those in and our goal is to find the formal charge on carbon so the formal charge on carbon is equal to the number of valence electrons that carbon is supposed to have which we know is 4 and from that we subtract the number of valence electrons that carbon actually has in our drawing so we we divide up these electrons here in these bonds and this time carbon has a lone pair of electrons on it so how many how many electrons are around carbon in our drawing this time there's one two three and then two more from this lone pair so four and five so 4 minus 5 gives us a formal charge of negative 1 so carbon is supposed to have four valence electrons here it has 5 so it's like it's gained an extra electron which gives it a negative 1 formal charge let me go ahead and redraw that we have carbon with three bonds to hydrogen and one lone pair of electrons on this carbon a negative 1 formal charge so we can represent that here with our negative sign next to that carbon a carbon with a negative charge is called a carb anion so this is a carb anion and let's analyze the pattern that we have for our carb anion we have 1 2 3 bonds let me write that down we have three bonds this time we have one lone pair of electrons right so we have one lone pair so three bonds plus one three single bonds plus one lone pair of electrons for a carbon alright will give us a negative 1 formal charge on that carbon will have a carb anion these also come up in mechanisms in organic chemistry so let's analyze some carb anions so down here let's start with the carb anion on the left and the negative 1 formal charge is on this carbon which I just worked in red so we should have three bonds and one lone pair of electrons on that carbon well let's analyze it the carbon in red is bonded to a ch3 a ch3 and a ch3 so that takes care of our three bonds then of course here is the one lone pair of electrons let's move on to the next example so the carbon with a negative 1 formal charge is this carbon I just marked in red the carbon in red is directly bond to a carbon here and directly bonded to a carbon here so that's two bonds I need a total of 3 bonds because I already have a lone pair of electrons right here's my lone pair of electrons on that carbon so I need one more bond and that bond of course must be to a hydrogen so I can draw in a hydrogen here again that hydrogen is usually left off when you're drawing dot structures but it's important to realize that hydrogen is actually there finally one more example right the negative one formal charge is on this carbon and that carbon is directly bonded to one other carbon it already has a lone pair of electrons so so far we have one bond and one lone pair we need total of three bonds so we need two more bonds on that carbon in red and so those last two bonds of course must be to two hydrogen's so it's important to be able to assign formal charge and to do the math writes important to be able to do these calculations but eventually you won't need to do the math anymore eventually you'll be able to look at a carbon and come up with a formal charge after you've done enough problems