today we will take a look at resonance structures sometimes more than one Lou structures can be written for a molecule for example for on all three so I'll give you a couple minutes to try to write the Lou structure for it you got it okay let's see so when you first write low structure you probably have the Dilemma should I use the L pair of electrons on the left oxygen to share with the middle oxygen or should I use the electrons on the right oxygen to share with the middle one actually you can do both right so these are two possible resonance structures for onzone all right now let's calculate the formal charges first on these on oxygen atoms okay so uh the metal oxygen will have a positive charge and the single bonded oxygen will have a negative charge same for the right structure again positive charge on the mdle oxygen negative charge on the single bondet oxygen on the left so these two structures are the two possible Louis structures for the same molecule Ono both of them are correct and both of them will be contribute to the real structure why do I say that the real structure of onzone is neither of these two It's a combination of these two structure okay so both of these two contribute to the real structure of onzo molecu but neither of them is the real structure okay so we call these two Lou structures as the resonance structure and we use this type of double-headed Arrow to relate the two structures together what is the real structure it's a little complicated the real structure of ozone is a hybrid of the two structures that you just drawn we do have a double bond uh we do sorry do have a positive charge on The Middle on the middle oxygen but the oxygen oxygen Bond are neither single or double bond the both of these two bonds are in between a single and a double bond and the two terminal oxygens remember in the real in the in the Louis structures uh they have one negative charge either on the left side or on the right side in the two structures in the real structure these two terminal oxygens will share that negative charge so each will bear 1/2 of the negative charge okay so this is the real structure and it does not look like the structures that we have seen before so in most cases for molecule like this we would use one of the resonance structure to represent its structure but you need to keep in mind we have another structure or maybe more than one other structures that will contribute to the real structure of the molecule okay we will see more examples now let me first give you the key points to write uh for for the resonance structures so first resonance structures will have the same atomic positions so no at will change position in two or more resonance structures first the second octet all the resonance structures should satisfy the octet okay so all the atom should have an octet and the third they should have the same number of electrons and the same net charges for example for omom the net charge is zero so no matter how many resonance structures you have how many positive charge or negative charge in one structure the sum of those charges should equal to zero okay now there are some other attribute that will tell you which structure is more important in different resonance structures so I'm going to tell you now and use examples to illustrate it later so small charge separation means stable that means in one resonance structure if we have small charge separation that resonance structure will be more stable in the other resonance structure if charges are separated more that means more positive or more negative charges are present the resonance structure will be less stable less stable means it will contribute less to the real structure and also when the negative charge is on the most electr negative element that means the structure is more stable okay and electron delocalization stabilizes a molecule so when we are able to write different resonance structures what we are doing is to move the electrons around the molecule in most cases you will have multiple bonds present in different types of Louis structure and those multiple bonds will change their positions it's actually the electrons are changing positions okay when the electrons can move to different atoms the electrons are not localized at one position it's delocalized and the delocalization can stabilize a molecule I'll give you some examples but uh let me first use an analogy to represent this so imagine you have a few like three year or four year old kids in a room let's say in our classroom our classroom is relatively big and we have all these seats there let's say those kids are electrons if you want them to sit in a particular seat they are localized at one place and can you imagine what they will do if you ask them to sit still at one place okay they don't want to do that they want to run around they are going to scream if you you know force them to sit at one place right so if you allow them to run around the room you don't have to you know watch them for every second they probably will play with this seat play with that seat and uh play with something else in the room and you can do whatever you can do with your own stuff right so that's an example when electrons are delocalized it's more stable okay so now let's take a look at an ion NO3 minus which is the nitrate so let's do the L structure for nitrate the ve the total number of veence electrons okay five from nitrogen three oxygens six of each we have a negative charge so the last electron is from the negative charge okay the total is 24 nitrogen is the central atom three oxygens uh around the nitrogen and we use three single bonds to connect the electrons so we use the six electrons and we have 18 left to distribute to make the atom satisfied octet as I have explained earlier we are going to start with the surrounding atoms so the oxygen atoms now each has one single bond in order to satisfy the octed we need six electrons for each oxygen so after we put electrons around the three three oxygens oxygens are fine they have octed however we have used all 18 electrons we don't have any electrons left now let's look at the nitrogen only six electrons around nitrogen nitrogen does not satisfy octet nitrogen needs two more electrons what could we do multiple bonds right so let's move one pair of electrons on one oxygen to share with the nitrogen and we can move this pair from any of the three oxygens right so let's redraw the structures the first one the the second one and the third one okay so these are the three possible Louis structures for this nitrate ion if you calculate the formal charge you will see that the central nitrogen atom will have a positive charge the two single bonded oxygen will have negative charges and the double bonded oxygen will have a charge of zero okay that is true for all three structures the net charge will be1 okay that's the overall charge of this ion and again it's true for all three structures so now you can see in all the three fluid structures they have the same atomic positions okay nitrogen and oxygen are in the same place all the atoms satisfi octet same uh electrons that means they have total of 24 valence electrons and the same net charges Nega one okay and some of you may say if I turn the mod around these three structures are really identical I would say yes they are the same so these three structures will contribute the same to the real structure of nitrate ion the real structure of nitrate ion we have all this nitrogen oxygen bond in between a single and a double bond and the three oxygen atoms will share the two negative charges all right now let's take a look at another example NC minus okay so let's do the Louis structure first Total number of valence electrons five from nitrogen four from carbon six from oxygen and one from that negative charge so 16 electrons and if we use the single bonds to connect the atoms we use the four electrons there are 12 left we're going to distribute the 12 electrons around the surrounding atoms to make them satisfied octed now we have used all 12 but the central carbon atom has only four electrons around it now so carbon needs two more pair four more electrons what can we do we can move two pairs of electrons from nitrogen to share with carbon that will give give the first resonance structures we can also share two pairs of electrons from the oxygen with carbon that will give the second resonance structures we can also share one pair from nitrogen one pair from oxygen right so that will give us the Third resonance structures so now calculate the formal charges in the first structure oxygen has the negative charge in the second structure nitrogen has -2 oxygen has positive one the last structure nitrogen has1 okay same atomic position same net charge Nega one for all of them and all atoms satisfy the octed okay but as you can tell this example is not like the previous one we've talked about the previous one all the three structures are actually identical but this one all three structures are totally different okay so let's see which one is not stable so we have talked about uh charge separation when we have more positive or more negative charges that means more charge separation charge Separation Will un stabilize the structure so the structure in the middle which has the two negative on the nitrogen as the charge whenever you see two or three as the number in front of the charge that will actually raise a red flag okay positive 1ga 1 that's perfectly fine but positive2 -2 or positive 3 three that's the red flag that tells you more charges are separate like in the middle structure in the blue structure here even though the net charge is -1 the charges are separated more so the metal structure is less stable compared to the structures on the left and the structures on the right so if we lab the three structures as a b and c we would say structure B is the least stable resonance structures and it will contribute the least to the real structure of this ion all right now let's take a look at a and C A and C have only one negative charge in a the negative charge is on the oxygen in C the negative charge is on the nitrogen so we have learned that when the negative charge is on a more electr negative element the structure will be more stable so if we compare oxygen to nitrogen oxygen is more electr negative right right so structure a when the electron is on a more electr negative element oxygen will be more stable than structure C so a is the most stable structure among the three and a will contribute the most to the real structure of this ion okay so I hope now you have a basic basic understanding for resonance structures now let's practice to write resonance structures for this molecules first carbonate CO3 2 minus there are three resonance structures okay so write them out and then determine if they are identical or not if not try to find out which one is more stable which one is less stable the second molecule ch3 Co this is acetic acid let me give you the atomic position okay the first carbon has three hydrogens attached to it the two carbons are link together the second carbon has two oxygen attached to it and the last hydrogen is on one of the oxygen okay so please note the two oxygens are not linked together all right so draw the possible resonance structures for this molecule so I'm going to tell you there are at least two structures and they are not the same so you need to determine which one is more stable which one is less stable all right all right