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Comparing formal charges to oxidation states

Video transcript

both formal charge and oxidation states are ways of counting electrons and they're both very useful concepts let's start with formal charge so one definition for formal charge is the hypothetical charge that would result if all bonding electrons are shared equally so let's go down to the dot structure on the left here which is a dot structure for methanol and let's assign a formal charge to carbon we need to think about the bonding electrons or the electrons in those bonds around carbon and we know that each bond consists of two electrons so the bond between oxygen and carbon consists of two electrons let me go and draw in those two electrons same for the bond between carbon and hydrogen each bond consists of two electrons so I can go around and put in all of my bonding electrons so if we want to assign a formal charge to carbon we need to think about the number of valence electrons in the free atom or the number of valence electrons that carbon is supposed to have we already know that carbon is supposed to have four valence electrons so I can put a four here and from that four we're going to subtract the number of valence electrons in the bonded atom or the number of valence electrons that carbon has around it in our drawing and since we're doing formal charge we need to think about all those bonding electrons being shared equally so we think about a covalent bond so if we have two electrons in one bond and those two electrons are shared equally we could split them up we could give one electron to oxygen and one electron to carbon in that bond we go over here to this carbon hydrogen bond so we could do the same thing we have two electrons we could split up those two electrons we give one to carbon and one to hydrogen we go all the way around we do the same thing over here and split up those electrons and the same thing here so how many valence electrons do we see around carbon now so let me go ahead and highlight them there's 1 2 3 & 4 so that's the number of valence electrons around carbon in our drawing so 4 minus 4 is equal to Z so zero is the formal charge of carbon so let me go ahead and highlight that here so in this molecule the formal charge for carbon is zero now let's move on to oxidation states all right so you could also call you could also call these oxidation numbers so one definition for an oxidation state is the hypothetical charge that would result if all those bonding electrons are assigned to the more electronegative atom in the bond so let's go to the dot structure on the right of methanol and let's assign an oxidation state to that carbon we need to think about our bonding electrons again so let's go ahead and put those in alright so we know that each bond consists of two electrons so I'm putting in I'm putting in the the two electrons in each bond and let's think about on the oxidation state of that carbon well first we need to know the number of valence electrons in the free atom so just like before we know that carbon is supposed to have four valence electrons so this would be a four and from that we subtract the number of valence electrons in the bonded atom or the number of valence electrons that carbon actually has in the drawing this time we need to think about an ionic bond so we're going to pretend like a covalent bond is an an ionic bond because we're going to assign all of the bonding electrons to the more electronegative atom so there's no more sharing here winner takes all the more electronegative atom is going to get all of the electrons so let's think about the electronegativities of carbon verse oxygen right we know that oxygen is more electronegative than carbon so oxygen takes both of those electrons in that bond so oxygen gets both of those electrons next let's think about the electronegativities of carbon and hydrogen we know that carbon is a little bit more electronegative than hydrogen so for these two electrons carbon is going to take both of them since carbon is more negative then hydrogen and the same thing for our other carbon hydrogen bonds carbon is more electronegative than hydrogen so carbon takes those carbon is more electronegative than hydrogen so carbon it takes those and so how many electrons do we have around carbon now let's count them up that's one two three four five and six so now we have six electrons around carbon so 4 minus 6 gives us negative 2 so here in this example carbon has an oxidation state of negative 2 so there's no more sharing when you're doing oxidation states right think about the more electronegative atom and assign both electrons to the more electronegative atom both formal charge and oxidation states are just really extreme methods of electron bookkeeping right they're not perfect they're certainly not perfect right we're assuming we're assuming we're assuming that the electrons are either shared equally perfectly or that one atom takes both electrons and neither of those concepts is perfect in the real world but it works when we're drawing our dot structures and we're thinking about chemical reactions