in chemistry for the same molecule it can be represented by many lewis structure let's take a look at so42 minus ion sulfate icon it has at least six lewis structure and if we look in detail it has more than six how amazing is that as you can see in this picture it has a mixture of many types of covalent bonds however out of all of these amazing structures which one of it is the most stable structure of sulfuric ion is there any way that we could determine [Music] it hello everyone welcome back to siri bichara kimiyawa so back to my question just now is there any way that we could determine the most stable structure yes there is the method that we are going to use is to determine the most stable or in another fancy word we used to call as most plausible structure is by calculating the formal charge of the respective atom in the molecule formal charge is defined as a charge of an individual atoms would have if the bonding electrons were shed equally in the molecule and there is a very simple formula that we can use to calculate the formal charge of a letter the formula is formal charge is equal to number of valence electron of the atom minus number of electrons surrounding the atom and after we do the calculation of the formal charge on the atom of the molecule the most stable or the most plausible restructure that the molecule can have if the atom bear the smallest monocharge the negative formal charge appears on most electronegative atoms the positive formal charge appears on the least electronegative atom so in this video i will show you on how to use the formula that i mentioned just now and on top of that i will guide you on how to determine the most plausible lewis structure [Music] so just a recall back just now i also have informed to you that we have a formal we have a formula that we can use which is formal charge is equal to the number of valence electrons of the atom minus the number of electrons surrounding the atom so for the number of valence electrons of the atom you should know yourself at this level what is the valence electron for the basic elements i guess i've learned so far it is very similar like your spn before the basic one like nitrogen oxygen fluorine and so on and the number of electrons surrounding the atom this is something new for you which is this when you are encounter any single bond that is attached to the to the atom it will contribute one electron when it counts a double bond two electrons and triple bond will contribute three electrons so it looks very vague now so let's try the example [Music] the first example calculate the formal charge of both structure of co2 below next determine the most plausible lewis structure for co2 now let's focus on structure one so i enlarged it and become like this and as you can see in structure one we have carbon as the central atom and we have two oxygen one oxygen with triple bond and one oxygen with one single bond so just like i told you just now when you see single bond it will contribute one electron double bond two electrons and triple bond three electrons since this molecule have triple bond and single bond so we only look at these two single bond and also triple dot and do not forget to mention again the formula of the formal charge number of valence electron of the atom minus number of electrons surrounding the atom so let's look at oxygen here as you guys know yourself oxygen is group 16 so when it is group 16 it has six valence electron minus we have to look at the electron surrounding the oxygen as you can see this oxygen has two electrons and it has one triple bond so what we can do it has two electrons that come next to it another one comes from the triple bond so 2 plus 3 is 5. so if you minus this according to the formula 6 minus 5 it will become positive 1. a common mistake that student will do is they forgot to put positive remember again this is a formal charge when we talk about charge you must have a positive charge or negative charge zero means neutral so you don't have to put any charge in front of it all right okay now we go for carbon carbon is in group 14 so it has four valence electron and now we look at the electron surrounding the carbon minus so you can see we have one triple bond contributing three electron and it has one single bond contributing one electron so if you can see if you minus this 4 minus 4 the value is 0 since carbon is neutral so you don't have to put any charge in front next we look at this oxygen this oxygen again is come from group 16 so it has six valence electron minus the electron surrounding it as you can see it already has six electrons which is come from the three lone pairs six plus with the single bond it has here so 1 so 6 minus 7 is equal to negative 1. so there you go this is the way how we calculate the formal charge this is for structure one so now we go for structure two [Music] and see that it's a bit different where you have carbon and two oxygens however there are two oxygen with two double bonds now so let's calculate again one by one if we go to this oxygen oxygen comes from group 16 so it has six valence electron minus electrons around the oxygen it already has four electrons that comes from the lone pair and it has one double bond here so you're contributing two electrons so six minus six therefore zero next we go for carbon similarly carbon comes from group fourteen so it has four valence electron minus now you can see the carbon are surrounded with two double bonds so two electrons each from each of the double bond so four minus four this is equal to zero and for this oxygen you have to calculate again as you can see this oxygen is exactly similar like this so the six minus six the value is also zero so now we have two structures that we have calculated structure one we form a charge positive one zero negative one and structure two with all zero value for the formal charge so that's the first question has been answered second question which one of these are the most plausible structure if you guys remember before that i told you there are three ways that we can determine the most possible structure which is the first one all the atoms bears the lowest formal charge in other words it has all zero value the second one the more electronegative atom bears a negative formal charge so as you can see like this oxygen on structure one it has negative one charge and the last one the more electro positive atom will have a positive form of charge as you can see none of it have it here so we can conclude that structure two here all the three atom has a lowest formal charge which is zero zero and zero for structure one unfortunately oxygen that has a triple bond is a positive one if you recall back last time electronegativity of oxygen is very high in other words it preferred to has a negative value so that is why structure 1 is not stable thus we can conclude that structure 2 is the most plausible structure as all atoms has the lowest formal charge lowers here representing the zero value [Music] all right so i'm going to give you a second example to enhance this understanding all right so now let's go for a second example so if you look at here a very similar question right now we have pocl3 as example as you can see pocl3 also has two structures one with all single bonds and one with one double bond towards the oxygen let's calculate all the formal charge for structure one and structure two for structure one okay that's right you go for oxygen oxygen comes from group 16 so it has six valence electron minus the electrons from the oxygen one two three four five six and don't forget it has one single bond so six plus one you become seven so six and seven in the current negative one and as you can see for all these three chlorines they are all identical because it has a single bond with three lone pairs so chlorine comes from group 17 so it has seven valence electron and the electron around surrounding chlorine one two three four five six six that comes from this three on band plus one that comes from the single point so the total is seven so seven minus seven is zero so this one also seven minus seven zero zero seven minus seven zero and don't forget about the central header phosphorus phosphorus comes from group fifteen so it has five valence electron minus as you can see around this phosphorus there are no electron but it has four single bonds so one electron each from them so four five minus four is equal to positive one so these are the formal charges of all the atoms instruction line so let's move on for structure two [Music] now it's a bit different whereby the oxygen now has a double bond so let's start with it again oxygen comes from group 16 so it has six valence electron minus you have four electrons that come from two lone pairs plus with one double one double bond will contribute two electrons so one two three four five six six electron so six minus six the value is zero and now you can see for these two chlorine they are identical because it has single one and three lone pairs so chlorine comes from group seventeens it has several electron and the electrons surrounding chlorine is one two three four five six plus with the single bond so we have seven so seven minus zero seven minus seven is equal to zero similarly with this moment seven and seven is equal to zero similarly as well seven minus seven is equal to zero well for this phosphorus your gain comes from root fifteen so i have five minus next one minus now you can see that these phosphorus is one with three single bond and one double bond so one single point will contribute one electron each one two three and double bond contribute two electrons so five in total so five minus five is equal to 0. so as you can see these are all the formal charges for structure 2. if we compare the value of formal charges between structure 1 and structure 2 as you can see oxygen on structure one has a negative one for machas which is very stable however phosphorus if you recall back in your chapter three last time phosphorus is an electronegative element thus it preferred to have a negative formal charge if you look at structure one it has a positive one formal charge that is why this structure is not stable thus that is why you can see structure two all the atoms has a lower formal charge which is zero because of this reason we also can conclude that structure two is the most possible structure because it has the lowest formal charge [Music] well there you go i have shown you on how to calculate the formal charge of an atom in the molecule and also i have shown you two ways on how to determine the most plausible structure of your structure so guys after looking at this video please give it a try and do more exercise on this all right so hope you enjoyed this video don't forget to like subscribe and share and please click the bell icon below right so whenever the new videos come out you guys will get the notification alright so that's all for me today thank you have a nice day see you again [Music] you