all right let's talk about formal charge something we're going to need if we're going to differentiate between leis structures what do I mean well let's take a look I have a deceptively I have a deceptively simple molecule here this molecule does exist so I'm not just playing something abstract with you here the question is what is the bonding like for this molecule and you're saying goly that's a good question well if we just start with our total of our veence electrons we got five from nitrogen we've got oxygen bringing six we've got chlorine bringing seven that gives us a total of 18 veence electrons the question now becomes how do we connect these guys all all right well let's say Oxygen's in the middle there so I'm going to pretend that Oxygen's my central atom there and drawing that LE structure cost me four veence electrons that leaves me with 14 then we want to make our outling atoms happy so each nitrogen and chlorine are each going to need six more veence electrons to make eight that costs me 12 so that leaves me with two I place those on the central atom and I'm out of out of electrons oo now I have a debate because I need to form a double bond to complete Oxygen's octet the question becomes who's going to get picked on nitrogen or chlorine which one of those leis structures is correct let's make this even more complicated I just chose oxygen for the central atom because it was in the middle who says oxygen is the central atom after all it's the most electronegative not the least so why isn't nitrogen the central atom couldn't we do this again with nitrogen 18 electon but nitrogen's are Central atom bonded to oxygen or chlorine that would cost us four and once again we'd have a choice to make so which one of these four different structures is the correct one and before I forget why isn't chlorine the the uh Central atom right so we could actually have six different Le structures here let me skip to a new screen here real quickly all right so for this simple molecule we started off with we've discovered that there's actually six possible Lis structures so which of these six is correct what tools do we have available to us to help sort this out well one of the big tools we have is that of formal charge formal charg is an accounting method it's a way it's a way of seeing who gains and who loses electrons when a molecule is formed formal charge also for our purposes helps us select between competing leis structures so what is formal charge we know it's an accounting method and we know it's a saying of who gains and who loses electrons now in theory when a molecule forms everybody shares their electrons and everybody benefits and it's all one big happy family they wind up holding orbitals skipping along a path and singing Kumbaya yeah in your dream everybody who's ever shared anything knows their sharing and then they're sharing right and when you share somebody always shares more and somebody always takes more cynical yes I am because after all if you've ever tried to divide a bag of M&M's with somebody unless you've actually sat there and counted out and given one person's 10 M&M's the next person 10 M&M's and gone back and forth like you're dealing cards you know that somebody always winds up eating more M&M's than somebody else matter of fact why don't you write down M&M's same way for a CO valent compound sure they may all pull their electrons but when they pull their electrons somebody winds up gaining electrons or gaining possession of the electrons more than somebody else does inal charge is a way of seeing who's gaining disproportionately compared to the other atoms in a molecule when the molecule is formed formal charge is assigned to to each atom in a molecule the formal charge for an atom is the number of valence electrons minus the number of bonds the atom participates in minus the number of Lone electrons if we were to look at a really simple molecule here and we were to look at ammonia we could calculate the formal charge for each atom if we were to look at nitrogen nitrogen is in group 5A on the Periodic Chart so nitrogen has five veence electrons in the ammonia molecule nitrogen is participating in one to three bonds so I'm going to subtract my number of bonds three three then the number of Lone electrons does not say lone electron pairs it says number of lonely electrons right lonely electrons are twice as annoying as those couples so those lonely electrons have two electrons so minus 3 - 2 so 5 - 3 - 2 is 0er so we would say that the formal charge for nitrogen is zero when it comes to ammonia if we're to look at the hydrogens well hydrogen has one veence electron because it's in group 1 a it's participating in one Bond so minus one and it's um has zero unpaired electrons so it has a formal charge of one - one -0 or zero Ah that's kind of boring everything's zero here which brings up another point for a valid Lewis structure the sum of the formal charges for the atoms in a molecule should be zero so when we look at our ammonia here we add we add 0 + 0 + 0 is 0 so we know that's the valid leis structure for ammonia because the sum of the formal charges are zero well that's a boring molecule everything's zero let's look at something where things aren't zero for just a second here let's cheat a little bit and look at sulfate I'm sulfite sorry sulfite sh don't tell anybody when I said that eight sh I only said eight you did not hear the eight sulfite okay we look at our sulfite we draw our Lewis structure six from sulfur well I'm going to give you the Lewis structure here right off the bat you know what let's cut to the chase here there's our Lewis structure for sulfate sulfite I did it again ah um all right so there's our leis structure for for sulfite let's figure out the formal charge for each of our atoms here if we look at this oxygen here this oxygen has six veence electrons because it's in group 6A so that's my number of veence electrons then that oxygen is participating in one Bond right that one Bond right there how many lone electrons does it have now it has 2 4 six alone electrons so 6 - 1 - 6 is -1 so that oxygen there has a formal charge of ne1 how about our sulfur well before we even move on to our sulfur all of our oxygens have similar bonding right they all have similar bonding and therefore they're all going to have the same formal charge of negative 1 that Oxygen's negative 1 that Oxygen's negative 1 now we look at our sulfur sulfur has six veence electrons this sulfur is participating in one two three bonds and it has two lonely electrons right there so 6 - 3 - 2 winds up being a positive one so the formal charge for my sulfur would be a positive one okay so my so positive one for my formal charge on my sulfur here so what's the total formal charge so what's the sum of the charges for my molecule well I have three oxygens at Nega 1 each and then I have one sulfur at a positive one each so the sum of my formal charges is -2 so is my leis structure valid have I made a mistake no my leis structure is still valid the sum of the formal charges of the atoms in a molecule should be zero unless you have a polyatomic ion then the sum of the formal charges should be equal to the ions on the charge so when I added up all my formal charges I wound up with a -2 and what's the charge on sulfite 2 negative so the sum of my formal charges should also be to negative so that means I have a valid Lewis structure Dr Kelly we enjoyed ammonia we appreciated sulfite but what happened to the no chloride chloride thing the no thing that you spent so much time on at the beginning of this lecture patience we're getting to this after all I'm a doctor so you know you have to have patience to deal with me bum try the salad B he'll be I'll be here all week all right so let's go back to our previous example but before we do that let's real quickly read through those rest of the rules and I'm hoping that you're going to copy these rules down if you have to pause the video to copy down those three guidelines there I would recommend you do so I also hope you're copying down that definition of a formal charge the formal charges of the atoms in a molecule should be minimized in other words we want the formal charges of the atoms in a molecule as close to zero as possible negative 1 positive 1 -2 positive2 we want them as small in terms of magnitude as possible small in terms of magnitude we want them as close to zero as possible we've already talked about number two let's take a look at number three the more negative the formal charge should be on the more electronegative atom if we went back to that sulfur that sulfite example sulfur had the positive one charges and our oxygen had a negative one charge who's more electronegative oxygen right Oxygen's more electronegative than sulfur remember electro negativity increases that direction and we said oxygen is the second most electr negative element on the entire Periodic Chart so who has the negative formal charge oxygen so that's another sign we have a valid leis structure because the more negative formal charge is on the more electronegative atom so that's another example there all right but let's get back to the problem that started this whole discussion to begin with gwiz talk about your circumlocution all right we have six different possible Lewis structures for this same compound formal charges can help us figure out which one actually exists or at least what two are involved in some degree of resonance to do that we're going to have to assign formal charges to every atom in every structure little bit of grunt work but it's all pretty easy right we can all do subtraction so let's get to it let's start with these two first I'm going to figure out the formal charge for this nitrogen here nitrogen's in group 5 a so it has five veence electrons that's my number of valence electrons minus the number of bonds nitrogen here is participating in a double bond so that's two bonds minus the number of Lone electrons two lone electrons there two electrons there so it's got four lone electrons remember a dash Can represent a pair of electrons I introduced that in the last lecture on Lew structures but I'm going to be using it heavily here but anyway 5 - 2 - 4 is -1 so the formal charge for our nitrogen there is a 1 so the formal charge of nitrogen there is negative 1 Let's do our oxygen now Oxygen's in group 6A so it has six veence electrons it's participa in 1 2 three bonds so minus three bonds then it has two lone electrons so the formal charge is a positive one for our n oxygen our chlorine has seven valence electrons because it's in group 7A number of valence electrons come straight from that periodic table it's participating in just the one Bond there and it has 2 4 six lone electrons so the total negative charge for that is so the total charge for that is zero 7 - 1 - 6 is 0o so the formal charge for oxygen is+ one and for so we have a negative one charge on our nitrogen plus one on our oxygen and a zero on our chlorine already we're running into a little bit of an issue here right because who's the most electronegative atom in this molecule Oxygen's the most Electro negative it's if you went checked your electro negativity value it's 3.5 chlorine and nitrogen both clock in at 3.0 so oxygen is the most Electro negative so if anybody's going to have the negative one charge who should it be this guy so already our Lewis structure is running into a little bit of a problem but let's keep forward and figure out the formal charges for the atoms in this structure let's go with oxygen again here oxygen has six veence electrons it's participating in one two three bonds it has two two lone electrons a dash is a pair of electrons so it's got a positive one formal charge again here our chlorine this time is going to be different it has seven veence electrons it's participating [Music] in one two bonds and it has two four has four unpaired electrons that gives it a charge of positive one don't make the mistake remember this is chlorine right so that's not a pair of electrons that's the L on chlorine FYI there um so it has four lone electrons because that's an L not a pair of electrons so that's positive one for chlorine then we come to our nitrogen here nitrogen has five veence electrons minus one Bond minus 2 4 six lonely electrons for a total charge of -2 for the formal charge there so we've got -2 pos1 and positive1 which all equal to zero but look at the magnitude all right now that we've assigned formal charges to two of our structures let's do a a celebrity death match I know I'm dating myself '90s MTV series um between these two structures which of these two structures is the best well both have the sum of formal charges equal to Zer -2 + 1 + 1 is equal to 0 - 1 + 1 + 0 is equal to 0 so the formal charge is all equals z for both compounds so they passed that test let's think about our first rule though though our first rule says that the formal charges should be as close to zero as possible well I've got a positive 1 negative 1 and zero there down here I have a positive 1 positive 1 and -2 so the magnitude of the formal charges is higher here than it is here so the magnitude of formal charge for this structure is greater than that structure and we want to minimize the formal charges since since if we added the absolute values of these formal charges up here we'd get four we added the absolute value of the formal charges up here we'd get two four is greater than two so this actually decreases the magnitude of the formal charges so this is a much better Le structure than that so we can cross this one out already we know before we even do anything do any of our other structures that this one's out this is the better better Lewis structure this is the better Lewis structure because the magnitude of the formal charges is less for this structure than for this structure so one down four more to go so let's look at our two chlorine structures next and do their formal charges we've got an oxygen here oxygen has six veence electrons because it's in group 6A it's participating in one Bond and it has two four six lone pairs I mean six lone pairs six lonely electrons six lone Rons that gives us a formal charge of1 for our oxygen our chlorine here has seven veence electrons because it's from group 7A it's participating in one 2 three bonds and has two lone electrons so that gives it a formal charge of plus two our nitrogen has five veence electrons is participating in two bonds it has two four unpaired electrons so it has a formal charge of positive um sorry formal charge of negative 1 I can't subtract today negative one all right so we had um so we had positive2 -1 -1 we look at this our neighboring structure over here and our chlorine is going to be the same again for the same reason seven veence electrons three bonds 1 2 3 two unpaired electrons so it's going to be positive2 for our chlorine again oxygen in this structure however is going to be six veence electrons because Oxygen's in group 6A it's participating in two bonds and it has two four lone electrons so it's formal charge is going to be zero then we've got nitrogen five veence electrons I forgot to draw on a pair of electrons when I drew that Louis structure there shame on me um five veence electrons minus one Bond minus 2 4 six lone electrons for a charge of -2 for the formal charge for nitrogen so for that stru Blu structure our formal charges are going to be0 for the oxygen positive2 for the chlorine and two negative for the nitrogen now for both these structures the sum of the formal charges is zero and if we did the magnitudes we'd get the sum of the magnitudes for both structures to equal four so how do we decide between these two structures which is the best between these two structures the magnitudes both sum to four and so that's not the way to decide it look at that last criteria there under formal charges the most negative formal charge will be on the most electr negative atom if we look at our structures we have one structure where nitrogen has a -2 and oxygen has a zero in this Le structure Oxygen's a zero nitrogen's a -2 who's more electronegative oxygen or nitrogen oxygen right oxygen is the second most electronegative element after Florine so this leis structure is probably a bad leis structure because the more Electro negative charge isn't on the oxygen instead it's on the nitrogen so because that negative Char formal charge isn't on the most electr negative atom we can rule out that leis structure this leis structur is a much better one why because you have the negative charge on the more electronegative atom so we've knocked out two Lewis structures already how about we do a little bit more Celebrity Death Match here and we compare this one with this one this one the magnitude of our formal charges the absolute value of the formal charges sum to two this one down here the absolute value of the formal charges sums to four remember your first rule under formal charges is your vow Le structures will minimize the sum of the formal charges will minimize the formal charges get them as small as possible in terms of magnitude this gets them to a magnitude of two this to four so this is the better leis structure because it minimizes the formal charges so we can cross this one out crisscross applesauce all right so now we're down to three contesters in contestants in our survival pool and you know one of them's going to be brave and try pick the Detroit Lions all right so we need to determine the formal charges for the atoms in this structure our oxygen here six veence electrons two bonds four lone electrons formal charge of zero this lure structure right here five nitrogen has five veence electrons it's participating in three bonds and it has two lone electrons so it has a formal charge of zero this looks good we've got chlorine with seven veence electrons it has one Bond it has six lone electrons so it has formal charge of zero wow so we've got our formal charges there to Z all sum to zero how about this structure down here nitrogen has five veence electrons it's participating in three bonds 1 2 3 two lone electrons so its formal charge is zero our oxygen has has six veence electrons it's participating in one Bond it has six lone electrons 2 4 six so its formal charge is -1 my chlorine over here has seven veence electrons because it's in group 7 a it's participating in two bonds and it has two four lone electrons don't be fooled remember that's an l l l for chlorine not an lone pair of electrons so it has so that chlorine only has four lone electrons and that gives us a positive charge of + one so you've got + one negative 1 and zero well if I'm looking at just this Le structure and just this one who's the better Le structure that first rule says we want to minimize the formal charges you can't minimize them any more than all zeros since all the formal charges in this structure are zero it is by far a better Lo structure than this one too bad you lose new soup for you all right so there are two contestants left on the island all other structures have been eliminated so I've just got to compare this one with this one hands down not a problem we got a positive one Nega we got a positive one negative 1 and Z going up against a 0 0 0 we want those formal charges as small as possible so the triple zero wins you had a nice run first structure but you met your match so that's our best structure so that's how formal charges can help us we started off with six competing lwis structures all of them found homes for our veence electrons however we were able to determine the best Lewis structure and how the molecule would actually exist by using our formal charges so the actual molecule will be most characteristic of this structure chemistry is easy life is hard yes what is also hard is life when you don't have a Snickers bar why don't you write down Snickers everybody write down Snickers s n i c k e RS Snickers like chemistry it's really satisfying I like making sure my horses are thoroughly dead dead from the good beating before I send them off to the glue factory and people are really thinking I hate horses out there how many times do I use that expression talk about beating a dead horse I think I beat a dead horse when I use the Expression beat a dead horse whoa talk about redundant all right let's take a look at another way formal charge can help us and look at another molecule all right let's draw the Lewis structure or Lewis structures for this molecule I've got four veence electrons from my carbon I have six veence electrons from my oxygen I've got two bromines bromines in group 7 a so they each have seven ve electrons and I've got two of them for 14 so I have 24 veence electrons to build my le structure with carbons present so by default I can decide it's going to be my central atom with everybody attached to it that costs me six of my central that cost me six of my electron pool that leaves me with 18 make my outlying atoms happy 2 46 2 4 6 2 46 that uses up my last 18 electrons I'm out of electrons in my building fund and who still doesn't have eight carbon doesn't have eight so we've got a dilemma because now we have two possible suspects for double bonding is our structure going to be or now a lot of you are already saying well was Dr Kelly I know it's going to be this structure because bromine's in group 7A and only likes to form one Bond and oxygen in group 6A and we know it likes to form two bonds so this has to be the right answer okay you're right you got me but play with me anyway here right don't leave the party early I promise it's going to get good all right if we use formal charges here we can differentiate between these two structures so let's assign formal charges to the atoms in both structures over here at this oxygen Oxygen's in group 6A so it has six electrons it's participating in one Bond and it has 2 4 six lone electrons so it has a formal charge of -1 my carbon has four veence electrons it's participating in four bonds it has no loone electrons so it has a formal charge of zero this bromine here has seven veence electrons it's participating in two bonds and it has four lone pairs of electrons two four so its formal charge is positive one this bromine's bonding is different so we're going to have to do its own formal charge as well seven veence electrons one Bond six lonely electrons so it has a formal charge of zero so that gives us if I can clean it up a little bit Nega 1 positive 1 0o and zero for our formal charges for that structure all right how about this structure over here well let's do the form formal charges over here we have six veence electrons in oxygen it's participating in two bonds it has four lone electrons 2 four so that means it has a formal charge of zero my bromine right here seven veence electrons because it's in group 7A one Bond 2 4 six lone electrons so it has a formal charge of zero as well this bromine same as the other bromine 7 - 1 - 6 or zero and this carbon four veence electrons four bonds no unbonded so it has a formal charge of zero as well so we've got a form so we have a situation where we have Z 0 0 for our formal charges or a situation where we can have negative 1 and positive one for our formal charges who's going to be better the formal charge are the the structure where the formal charges of the atoms in a molecule are minimized you can't get min more minimized than zer0 0 therefore the correct and only acceptable answer on a test is that one my friends chemistry is easy life is hard yes all right just in case my last example went over like a Milk Dud why don't you write write down Milk Duds write down Milk Duds m i l k du U DS Milk Duds and yes I have had way too much candy lately all right now that you've written down Milk Duds let's take a look at our last example we've got this fun-filled compound looking at us on an exam and yes I promise you it is full of fun we've got bromine contributing seven valence electrons we have carbon with four veence electrons and nitrogen with five so our total number of valence electrons is 16 drawing our skeletal structure is pretty easy we see carbon so we know it's going to be the center so that cuts down on our work and it costs us four veence electrons for 12 left nitrogen and bromine each need six more electrons to complete their octet 2 46 2 46 that costs us 12 an electrons so we have zero left over is carbon happy at the moment no very sadly carbon is not happy because carbon at the moment only has two veence electrons and how many does it want it wants four so we're going to have to share some electrons in order to do this we could have an Equitable system where bromine shares one and nitrogen shares a pair in which case our leis structure would be now the problem with that assumption is do bromine and nitrogen have equal attraction for their electrons no so in reality we've got a situation where we could also have greed play a role where we could have whoops I erased too much there where we could have bromine refusing to share any of its electrons and nitrogen sharing two pairs or for that matter we could have a situation where bromine shares two Pairs and nitrogen is the greedy selfish Pig and doesn't share any what are we to do how are we to resolve this dilemma in reality which of these leis structures will be favored by the molecule and the molecule will show signs most of which of these Le structures how can we differentiate between them that's where our formal charges come in I formally charge you to use your formal charges to to solve this dilemma all right let's do it let's start with this Le structure right here this Le structure right here we've got bromine bromine's in group 7A so it has seven veence electrons it's participating in two bonds it has four lone electrons two four so in this structure it has a formal charge of positive 1 then we've got carbon over here carbon has four veence electrons is participating in four bonds so it has a formal charge of zero then a nitrogen over here five veence electrons because it's in group 5A it's participating in two bonds it has four lone electrons so it has a charge of ne1 in this Le structure all right let's move over here to this one we've got bromine with seven veence electrons because it's in group 7A then we have one bond that bromine's participating in and then we have two four six clone electrons so that gives bromine a formal charge of zero then we've got carbon here four veence electrons is participating in four bonds no unbonded so that gives it a formal charge of zero our nitrogen over here has five veence electrons is participating in three bonds it has two lone electrons so that gives it a formal charge of zero whoa our second attempt and what did we discover we discover right off the bat we've got a situation where we have formal charges of zero Z and zero so right away because we know we want to minimize the formal charges of atoms in a molecule that this is probably our best leis structure and the molecule most probably shows characteristics of this connectivity however let's finish the exercise and make sure there's not a resonance situation so if we come down here and do our formal charges we've got bromine seven veence electrons participating in three bonds two unbonded and that gives it a formal charge of plus two all right bro means plus two so already we know that this formal charge is worse than the is worse than this formal charge so there's really no point in even continuing any farther so the answer on the test would be that one one more thing in this class one of the benefits is you're using a test Bank on the computer and therefore they do things that you find on nationally standardized exams as a hold over from the days it was near impossible to put graphics on a typewriter or to put graph in a computer a lot of times you'll see exams test leis structures by asking you the formal charge on a specific atom for example you could see a question that would read so often you'll see on standardized exams instead of them asking you to draw the Le structure of this compound what they do is they would say what is the formal charge on bromine in the correct leis structure of this so what you'd have to do is you'd have to draw all those possible structures determine which of the structures of best is best and what the formal charge is of bromine and of course we saw from the last slide that that was Zero because our best Le structure was that one where the formal charge of bromine was zero so that's just something to keep in mind and that's place you're actually quite frankly most likely to be encountering um formal charge on this level is an exercises like that in an indirect way of asking you about LS structures well I hope I've provided you with a great deal of Joy during this lecture nearly as much as an Almond Joy why don't you write down Almond Joy everybody's writing down Almond Joy a l m o n d space j y that's it I'm out of here