drawing Lewis structures part one bum bum bum in this part we're going to introduce you to the basic concept of Lewis structures and tell you how to draw Lewis structures for some very simple molecules in part two we'll kick it up and deal with some formal charges and some more complicated scenarios but let's get right off to the start a quick start by talking about what we're talking about well there was a welldo statement if I ever heard one a le structure is a way of visually displaying the two-dimensional connectivity of a molecule a le structure is a in a le structure a straight line is used to represent a pair of electrons usually in the context of a covealent bond a lone pair of electrons can be represented by two dots a single dot represents a single electron you'll also see me use X's to represent single electrons the different elements and atoms themselves are represented using their elemental symbols a lot of the functionality of a molecule stems from its three-dimensional shape and its three-dimensional shape stems from its two connect two-dimensional connectivity so there's big money and big importance placed on understanding this two-dimensional connectivity and Lewis structures is one of the ways we display this information in order to understand how we're going to construct Lewis structures it helps to quickly remind ourselves of the octet rule remember the octet rule is the desire of elements is the desired bell must have a completely filled outermost SNP subshells and remember since the S holds two and the P holds six for most elements that means they want eight electrons in their veilent shell hence the octet rule how different how different elements obey this octet rule is going to differ that too bears reminding we're going to look at the different groups on the periodic chart real quickly it turns out that elements in group 4 A attempt to achieve the octet rule by forming four bonds elements in group 5 A attempt to achieve the octet rule by forming three bonds and guess how many bonds group 6A is going to form two bonds and elements in group 7 A are going to achieve the octet rule by forming one bond how about elements in group 8 A they already have eight veence electrons because there are noble gases and they're not going to do anything but that's just a quick reminder and you're saying Dr kelly what about the metals well we already talked about ionic compounds in the context of what we're doing currently with our simple Lewis structures we're not going to worry about metals we're not going to be dealing with any coordination compounds quite yet here but to keep it's very important that you keep this preferred bonding in mind how is this important well if we look at our example I drew on the other side there where I drew the Lewis structure of water i never actually acknowledged its existence on the last slide but I drew it for you if you look at this le structure of water oxygen is in group 6A right and how many covealent bonds does it have it winds up forming two if I was to look at let's say natural gas methane look how many bonds carbon has 1 2 3 4 what group is carbon in carbon's in group 4 A knowing how many bonds an element wants to participate in will help us draw our Lewis structures and save us some work later on so those are things to keep in your back of your mind as you move forward also moving forward it would be a great idea if you wrote down Jolly Ranchers why don't you write down Jolly Ranchers everybody write down Jolly Ranchers all right how can we draw Lewis structures in just four simple strikes steps or your money back guaranteed what you didn't pay anything for this lecture why not if I give you my mailing address will you write me a check oh wait i'm sorry that's oldfashioned if I put a link down below will you donate something to a PayPal account or wait no better yet maybe I should use GoFundMe tell you what how about we do L structures now and I'll figure out a way for you guys to give me money later but at any rate step one the total number of veence electrons so if we're wanting to draw the le structure for something let's take a look at a very simple molecule for starters carbon tetrachloride the first thing to do is total the number of veence electrons i have carbon and carbon's in group 4 A if you look at your periodic table and that means it has four veence electrons we have four atoms of chlorine and chlorine's in group 7A so each one of those chlorine atoms brings seven veence electrons to the table 4 * 7 is 28 that gives us a total of 32 veence electrons we have to find a home in our molecule for all 32 of those veence electrons not only that but we can't exceed 32 veence electrons in our structure i like to think of this total number of veence electrons as my building fund that's how much money I have to spend to build my Lewis structure so that's step oneished our building fund our next thing to do is to draw our skeletal structure what do I mean by our skeletal structure what we want to do now is we want to determine a who our central atoms going to be and then b how those things connect to it most of the structures we're going to look out look start looking at with le structures are all going to s have a format where you have one central atom and a bunch of other things connected to it the hard part is figuring out who that central atom is there's a couple easy ways to do this first carbon loves to be the central atom carbon actually has the elemental symbol C not because carbon begins with C but because C is for central c is for central which is good enough for me all right um so C is for central so carbon is going to be our central atom i'm then going to connect all the other atoms in my structure to that central atom so I have four atoms of chlorine that I want to connect to my carbon so I drew four lines between my I so I connected my chlorines to my carbons by drawing four lines now these lines represent a covealent bond which as you know is a shared pair of electrons and how many is a pair two so how many electrons did I use to draw my skeletal structure i used 2 4 6 8 electrons to build that skeletal structure right there so I have to come over here to my building fund and subtract eight electrons so I've used eight eight electrons to build my skeletal structure that means I have 24 electrons left to complete my le structure for step three we want to complete the octed of the non central atoms at the moment each one of our chlorine atoms has two electrons each one of our chlorine atoms is participating in one bond so if I focus just on this chlorine atom here it has two electrons from the bond to complete its octet how many more electrons does chlorine need chlorine needs six more electrons to have a total of eight since it already has two from that bond so if I'm to give this chlorine six electrons 1 2 3 4 5 6 it now has eight each one of these chlorines only has one bond so each one will need six more electrons to complete its octet now I have four chlorines and I gave each one of those chlorines six electrons that means I used a total of 24 electrons so I have to subtract those 24 electrons from my building fund oh no that exhausts my building fund so I'm out of electrons that means it's time for my last step i'm going to check and see is everybody happy all right I'm out of electrons in my building fund so I need to check and see is everybody happy and by happy I mean do they all have a complete octet so my chlorine has 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 Are you asleep yet by me saying 2 6 4 8 2 6 So all my chlorines are happy right they all have eight electrons how about carbon let's count for carbon carbon has two electrons from this bond two electrons from this bond two electrons from that bond two electrons from that bond so carbon has 2 4 6 eight electrons as well so it has a complete octet so it's happy so everybody got what they wanted by participating in this molecule that means we're done and that's our Lewis structure for our our carbon tetrachloride now some of you are saying "Wait a second Dr kelly are you trying to wrap me up in some sort of weird pyramid scheme is what what sort of racket are you chemists pulling here after all I distinctly remember you looking at this chlorine right here and counting 2 4 6 8 and saying that these electrons right here were two electrons belonging to chlorine and you're right i did say that but then you're saying "Wait wait a second Dr kelly when you were counting for the carbon you went two you went 2 4 6 8 and you counted these two electrons as belonging to carbon but but you had just counted those two electrons as belonging to chlorine you're absolutely right we double counted those two electrons and that's allowed matter of fact that's the reason these these covealent compounds form our electron the electrons in our covealent bond here count as two electrons for our carbon and two electrons for our chlorine we're at in essence double counting them because they're a shared pair of electrons it was this type of accounting that got us to mortgage back securities but at any rate yes when it comes time to count the electrons belonging to each atom we do double count those electrons in the coalent bond let's look at another le structure all right let's take a look at phosphorus triromide anybody else hungry around here this always makes me want a peanut butter and jelly sandwich um but at any rate if I ever get to discover an element I'm giving it the elemental symbol J and I don't care what IPAC says just so that I can have PB&J sandwiches on my periodic table i'm just saying and PJs how cool would that be but at any rate I digress we want to draw the Lewis structure for this compound so the first thing we're going to do is we're going to establish our building fund i have one phosphorus phosphorus is in group 5A on my periodic table so it has five veence electrons i have three broines and bromine just like chlorine is in group 7 A so it has seven veence electrons so 3 * 7 is 21 + 5 is 26 so I have 26 electrons with which to build my molecule the next thing I'm going to do is draw my skeletal structure however to draw my skeletal structure to draw my skeletal destruction um to draw my skeletal structure I have to know who the central atom is i don't have carbon so that makes my decision a little bit harder now the formal rule says that I'm going to place the least electrogative atom at the center of my molecule in this case phosphorus is my least electrogative atom remember electro negativity increases in the direction of those arrows when in doubt left and right takes precedence but that's a pinky way out of determining it a cheat way that I like to use I call the bar rule if there is one girl in a bar and three guys in the bar what's going to happen all three guys are going to crowd around and pay attention to the female in the bar correct what happens if there's one guy in a bar and three girls the three girls ignore him because they wonder why the loser is there alone but at any rate we have one girl in the bar and we have three guys so they're all going to pay crowd around and pay attention to her so we have just one phosphorus and three broines so all our broines are going to crowd around and pay attention to our phosphorus so in drawing our Lewis structure phosphorus is not only the least electronegative but it's the only girl in the bar so all my broines are going to bond with my phosphorus and before you ask it doesn't matter how you connect these um and what I mean by that is it would have been just as correct for you to have done that or for you to have done that or any other way you can turn it all that matters is that you have your three bro broines attached to your phosphorus via a coalent bond i will mention one thing I was negligent in mentioning earlier when we write Lewis structures what we like to pretend is we like to pretend that each side of our elemental symbols is an orbital we're trying to complete an octet and you have one orbital in the s and three orbitals in the p so if we were to look around our phosphorus we would say that we have 1 2 3 four orbitals in our veance shell so we like to pretend that we have four orbitals around our atom around our atoms when we draw them in case you were wondering how that convention came about whoa I just hit the wrong button there apparently because my skeletal structure just disappeared on me um but that's not a problem so we draw our skeletal structure how many electrons did it cost us to draw that skeletal structure two electrons here two electrons here and two electrons here so that's 2 4 6 so drawing our skeletal structure cost us six electrons that leaves us with 20 electrons remaining so now we're onwards to step three three complete the octet of the non- central atoms place any remaining electrons on the central atom so I'm looking at bromine and bromine has two electrons currently from it the covealent bond so to get an octet or eight it needs six more electrons so I'm going to give it six more electrons 1 2 3 4 5 6 now each one of these broines is in the same situation they each need six more electrons to complete their octet and I have three broines so six electrons to each of my bromines times three broines means that to complete my skeletal structure I just used 18 electrons 2 4 6 8 10 12 13 14 16 yay dr kelly can count by twos um so I used 18 electrons for my building fund that leaves me with two veence electrons i have to use all my electrons this isn't private industry where I'd get rewarded for saving veence electrons i have to use them all according to my rule I place any remaining electrons on the central atom so I have two veence electrons left over so I'm going to place those two electrons on my phosphorus that leaves me with zero electrons so now rule four check and see is everybody happy all right so let's count and see if everybody has an octet our bromine has 2 4 6 8 so that bromine's happy 2 4 68 so that broine's happy 2 4 6 8 so that bromine's happy and our phosphorus 2 4 6 eight electrons so our phosphorus is happy so we're done and just for kicks and giggles our clean Lewis structure would look like because I've written all over this one so our clean le structure would look like that but you know what I always say one good example deserves another let's do another example just to make sure we're all on the same page here and let's take a look at water i mean we can't really get much simpler than water matter of fact why don't you try pausing me and doing water on your own all right I hope everybody's had a nice drink and is coming back to me with a complete le structure on their paper start by counting up your total number of veence electrons we have one oxygen oxygen is in group 6A we have two hydrogens hydrogen's in group 1 A so it has one veence electrons two hydrogen's time one times one veence electron each gives me two veence electrons for my hydrogen for a total of eight veence electrons draw my skeletal structure well one girl two guys so oxygen is going to be my central atom and notice how that actually um plays into my rule there remember hydrogen can never be your central atom that's another way we could have gotten the fact that oxygen was going to be so oxygen's my central atom to draw that le structure it cost me two four electrons because I have to draw two bonds at two electrons each so I used four electrons to draw my skeletal structure now I want to complete the octet on my outlying atoms right now hydrogen has two two electrons and it turns out that makes hydrogen happy remember hydrogen is in the first period that means its veilent shell is your first energy level and the first energy level only has the one s subshell and the s subshell can only hold two electrons so hydrogen's veence shell can only hold two electrons which means a long way short hydrogen can only handle two electrons no more can't take any more than two all right so we've taken care of that now we've still not done yet because we have leftover electrons our hydrogen's are happy we've made our hydrogen's happy here so now we're going to place our remaining electrons on our oxygen we have four remaining electrons so we're going to place all four around our oxygen now we check and see is everybody happy well our hydrogen's are happy with two each and our oxygen 2 4 6 8 so our oxygen is happy and we're good chemistry is easy life is hard all right let's take a look at this compound we have carbon in group 4 A so it's four veence electrons we have two hydrogens and hydrogen's in group 1 A so each has one veence electron and we've got two of them for a total of two we have one oxygen in group 6 A so it has six veence electrons so we have a total of 12 veence electrons to build our le structure here with carbon's present so it's going to be our central atom so I'm going to connect my other three things to carbon that means I have to draw three bonds and used up six of my veence electrons now we want to make our outlying atoms happy each hydrogen's happy because they already have two electrons my oxygen however is not happy my oxygen only has two electrons it wants eight so it needs six more electrons 2 4 6 and now it has eight but that used up the rest of my building fund now I have to check and see is everybody happy well oxygen has 2 4 6 8 each hydrogen has two and my carbon has two four six uhoh what am I going to do after all I'm out of building funds i don't work for the government i can't just get more money out of thin air well actually I do work for the government but anyway I just can't get more electrons out of thin air so how in the world am I going to complete the octet for this carbon more creative accounting remember an electron pair in a bond counts as two electrons for each atom so to complete the octet for carbon I'm going to move one of the lone pairs from around the oxygen so that it's between the oxygen and the carbon when I do that how does that change my electron count well I now have 2 4 6 8 electrons around carbon how about my oxygen my oxygen still has 2 4 6 8 so everybody's happy it's a win-win scenario you know what is also a win-win scenario a Reese's Peanut Butter Cup if you want chocolate you get chocolate if you want peanut butter you get peanut butter so why don't you write down Reese's peanut butter cup in completing this structure I've also done something else very important i've introduced you to a double bond a double bond is when you have two covealent bonds between two atoms so that's one new concept I wanted to introduce let's see if we can really master this let's take a deep breath in and a deep breath out and look at some carbon dioxide carbon four veence electrons because it's in group 4 A we have two oxygen oxygen's in group 6A so each oxygen has six veence electrons for a total of 12 that gives us 16 total veence electrons to build our le structure with well carbon loves to be the central atom so I draw my le structure that cost me four veence electrons i have 12 left over i want to make my outlying atom happy 1 2 3 four five six one two three four five six Now I've completed the octet for my oxygen i've given each oxygen now a total of eight electrons 2 4 6 8 2 4 6 8 and one of my dots isn't very big i'm going to make that dot there we go i didn't want it to have electron inferiority complex so I needed to use six lone electrons to complete each of my oxygen i have two oxygens so I use 12 electrons and that cost all of my building fund i'm out of money and I'm out of my money before my carbon's happy carbon only has two four electrons so you say "Don't worry i see some lone pairs of electrons on oxygen you can form a double bond no problem." So let's form a double bond i form my double bond and let's see everybody should be happy now 2 4 6 8 oxygen's happy carbon has 2 4 6 carbon's still not happy i still don't have any electrons what can I do well if sharing one pair was good sharing another pair must be better all right I could take another pair of electrons from that oxygen but that seems awfully unfair when this guy over here hasn't been made to share anything so it turns out that this guy is going to be forced to share another pair or I mean this guy's going to be forced to share a pair we'll go into the reasons for that in part two of our Lewis structure lecture but right now take my word for it it creates a nice symmetry it also does something else important first it completes our octet rule one two three four well 2 4 68 carbon's happy 2 4 6 8 That oxygen's happy 2 4 6 8 that oxygen's happy but in doing this structure for my carbon dioxide here I'm doing something else remember we started this lecture by saying elements in group 4 A want four bonds elements in group 5 A want three bonds elements in group 6A want two bonds 7 a one bond look at our structure now by sharing our elect our by sharing our second pair of electrons with this oxygen atom instead of forcing this guy to share three how many bonds have we given this oxygen two how many does it want it wants two bonds because it's in group 6A so oxygen wants to form two bonds so that's one way we know that this is going to be a good structure we've given each oxygen's two bonds carbons in group 4 A and we managed to give it four bonds so that's another way we know that this is a very good structure let's take a look at another case scenario here let's take a look at sulfur triioxide sulfur is in group 6 A so it has six veence electrons we have three oxygens oxygen's in group 6A so each of them has six veence electrons 6 * 3 is 18 18 + 6 is 24 so that's our building fund we have one girl in the bar and three guys so there's our skeletal structure that cost us six electrons it leaves us with 18 left over each oxygen right now has two electrons and we'll need six more to complete its octet so 2 4 6 2 4 6 2 4 6 2 4 6 so that cost me 18 electrons 6 * 3 18 that cost me all my building funds and sulfur is still not happy sulfur only has 2 4 6 electrons oh no what are we going to do and you're saying not a problem we got this how do we got this we got this because we know we can form a double bond and that's exactly what we're going to do we're going to take a pair from one of those oxygens and form a double bond and when we do that And when we do that we get that structure now is everybody happy let's see 2 4 6 8 so that oxygen's happy what about my sulfur 2 4 6 8 so that sulfur is happy and this oxygen 2 4 68 it's happy kind of what do I It's happy kind of it's being treated unfairly can't you see that there are three oxygen atoms out there and what did we do we randomly picked on one and forced it to give us give up its electrons is there any justice in that i ask you is there no there's no justice in that what we did was randomly bully one of those oxygens and that's just not right and mother nature doesn't think it's right either so guess what that's not what happens instead what happens instead of one oxygen being forced to surrender that pair of electrons to form that double bond a pair of electrons is going to float between all of these oxygen atoms in reality what's going to happen is we're going to get a structure that acts as if a double bond exists between not just one oxygen and the sulfur but exists between all the oxygens and the sulfur so what we have is we have a situation where this double bond can be thought of as existing between all of these atoms you'll see textbooks attempt to draw this sometimes or attempt to illustrate this concept by drawing something that looks like Whoops by drawing something that looks like this to try to represent that you've got a double bond kind of floating between all of them the situation gets even more complex because this double bond here doesn't exist between all of them doesn't exist for a second time temporarily instead it's a synthesis this is you know what let's just cut to the chase here's the way to think about what's going on between our oxygens and our sulfur has anybody here seen a tuning fork everybody here has seen a tuning fork you hit a tuning fork on a desk and what happens it vibrates the tongs vibrate back and forth and when you look at it it looks like instead of there being just one tuning fork it looks like you have lots of tuning forks the same way we can think of what's going on with this double bond this double bond can be thought of as resonating or vibrating between each one of these oxygens and the sulfur because this is a convenient way of thinking about this we actually refer to the three structures that I've drawn here as resonance structures none of these resonance structures actually exist what actually exists is a combination of all three however we don't have a good way to draw a combination of all three so what we do as chemists is we draw all three separately with double-headed arrows between them and this is the concept of resonance you can think of it as a floating double bond you can think of it as a tuning fork but resonance structures are used to represent that floating double bond or that communal pair of electrons let's kick up the difficulty a notch and begin looking at scenarios where you have multiple central atoms let's add a degree of difficulty here and let's look at some propane and propane accessories here we've got a larger molecule than we're dealing with than we've ever dealt with before we have three carbons and eight hydrogens the steps are going to be the same but with a twist we have three carbons and carbon is in group 4 A so it has four veence electrons so 3 * 4 is 12 then we have eight hydrogens hydrogen's in group 1 A so each hydrogen has one veence electrons electron 8 x 1 is eight so we have 20 total electrons to build our structure with next we need to determine who the central atom is all right well carbon likes to be the central atom but the problem becomes for us now is how do we define it as a central atom i mean do we do something like this the answer turns out to be yes we want to have one atom viewed as the central atom in this case I'm going to write it a little bit neater so I don't run out of room here i'm going to attach my two carbons two of my carbons to one of the other carbons so I've got two carbons attached then I want to attach my hydrogens now just in theory let's say that we attached or tried attaching all eight of our hydrogens to our carbon if we do that and I'm not even sure I've got eight there and now I've got eight we have a problem because how many electrons do we have around our carbon 2 4 6 8 um uh we've got 16 we got 20 electrons around our carbon carbon's in the second energy level it can only hold eight veence electrons in its veence shell it can't hold 20 electrons so that can't be right so what can we do here whoops just a second here now things are a little bit cleaner for us so we obviously can't place all the hydrogens's around that one central carbon so what can we do and this is where what I was saying at the very beginning of the lecture comes into play we said that elements in group 4 A want four bonds elements in group 5 A want three bonds 6a two bonds and 7A want one bond and group 8A think Shan Connory was the best one you will be tested over that later um Daniel Craig and Roger Moore are also acceptable answers george Lozby just earned you bonus credit and a lot of sympathy from me but at any rate I digress carbon is in group 4 A that means it only wants to form four bonds so we're going to try to make our structure so that each carbon is only participating in those four bonds so our central carbon right now has two bonds so it can form two more so it's going to form bonds with two more hydrogens this carbon right here is only participating in one bond so it can participate in four more i mean three more so it can bond with three more hydrogens this carbon over here has only one bond so it can participate in three more so it can bond with three more hydrogens and what do you know we found a partner for all of our hydrogens because we've bonded 1 2 3 4 5 6 7 8 hydrogen so we now have found a home in our skeletal structure for all of our carbons and all of our hydrogens if we were to count up our number of electrons we would say that we've used 1 two well actually I need to count by twos don't I very good and I have the PhD here um 2 4 6 8 10 12 and I lost count 2 4 6 8 10 12 14 16 18 20 woohoo i can now pass kindergarten um so I used 20 electrons to build my le structure that exhausts my building fund so I must be done is everybody here happy well each hydrogen has two electrons so it's definitely happy because carbon because hydrogen's happy with two how about our carbons 2 4 6 8 so that carbon's happy how about this carbon 2 4 6 8 so it's happy and this carbon right here 2 4 6 8 so it's happy everybody's happy boom all right let me see if I can simplify this down for you again what we've seen here is a pattern that I call a carbon chain whenever you have a bunch of carbons those carbons are going to want to form a chain with each other or form a big molecule with each other so if you have a molecule with a lot of carbons in it like our methane here and I'm sorry I don't know why I said methane it's not methane propane and propane accessories um so when you have a compound in front of you with a lot of carbons take those carbons and connect them all together in a series that's the first thing you'd want to do then complete your molecule in such a fashion that each one of those carbons has four bonds so when drawing the le structure for a compound with more than one carbon make a carbon chain then complete the Lewis structure so that each carbon is participating in four bonds so we drew our carbon chain for our methane then we want to complete it so that each one's completing in four bonds this carbon has one bond one two three one two so that we've got one two three four one two three so 1 2 3 4 and then everything else is going to be hydrogen at the end of one of those bonds because that's all we've got left so that's what you do when you're faced with multiple hydrogens i mean multiple multiple hydrogen's faced with multiple carbons if we wanted to we could do another one like that and I don't know about you but I surely do all right we've taken a look at the barbecue let's go camping we've got that compound looking at us on an exam so you're going to say I've got a lot of carbons so the first thing I'm going to do is I'm going to connect all those carbons 1 2 3 4 and I'm going to count my veence electrons real quickly four carbons each carbon has four veence electrons because it's in group 4 A so that's a total of 16 veence electrons i have 10 hydrogens hydrogen's in group 1 A so it each hydrogen has one veence electron for a total of 11 um 11 i can't multiply today 10 * 1 is 10 i promise you guys I did go to kindergarten i even got good grades except I did have to stay in one time during recess because I was told I had slo scissor skills but that's a story for another day um so I've drawn my carbon chain connecting my four carbons now I want to make sure that each one of these carbons has the possibility of doing four bonds so I'm going to have to draw one two three more bonds for my terminal carbon here then this carbon already has two bonds so it's going to want two more same for this guy and then this guy down here only has one bond so it's going to need three more since it wants to do four bonds now can we fill all these bonds with hydrogen 1 2 3 4 5 6 7 8 9 10 tada so that's 10 hydrogen which is the number we have in our problem so now we need to count and see if our veence electron total is correct 2 4 6 8 10 12 14 16 18 20 22 24 26 and yay Dr kelly can count again and there's your Lewis structure for butane all right let's look at another scenario here let's look at the compound this is actually one of those polyatomic ions you've memorized acetate one of the reasons we write the chemical structure for acetate like we do is to help provide us with information on the connectivity but before I get ahead of myself let's total up our veence electrons we have two carbons at four veence electrons each we have two oxygens at six veence electrons each because oxygen's in group 6 A so that's 12 veence electrons then we have three hydrogens each bringing one veence electron to the table for a total of three so that gives us 15 + 8 is 23 then we have another thing we have a negative charge if a species has a negative charge does it have an extra electron or is it shortened electron if it has a negative charge it has an extra electron so because we have a negative one charge that means we have one extra electron so we actually have a total of our building fund or total number of veence electrons of 24 so we're going to draw our skeletal structure notice how our two carbons are separated sign that we have a carbonarbon chain so we're going to make two central atoms carbon then we're going to connect things to the carbon remember a carbon can only be connected to four things and I see these three hydrogens here next to that carbon so I'm going to place three hydrogens's around that carbon then I see these two oxygens here next to this carbon so I'm going to place the two oxygen next to that carbon to draw that skeletal structure cost me 2 4 6 8 10 12 electrons so I have to subtract 12 electrons from my building fund now I need to make my outlying atoms happy well hydrogen's happy with two so they're all set each of my oxygen however only has two currently they want eight so they each need six more 1 2 3 4 five 6 1 2 3 4 five 6 so 6 * 2 is 12 so I had to draw 12 dots so that cost me my last 12 veence electrons i'm out of electrons in my building fund how many electrons does everybody have each hydrogen has two so it's h all my hydrogen's are happy this carbon has 2 4 6 8 so this carbon's happy this oxygen is 2 4 68 so it's happy this oxygen is 2 4 68 so it's happy this carbon however only has 2 4 6 so it's unhappy it wants eight where am I going to get two more electrons to make it happy well the only place that I can find extra electrons is those lone pairs on oxygen so completely in a world of unjustness and randomness I'm going to take two electrons from that oxygen and form a double bond and that gives me now is there any reason all right now let's check and see is everybody happy now my carbon has 2 4 6 8 so that carbon's happy how about this oxygen 2 4 6 8 so this oxygen's happy everybody's happy so I've got the complete structure with one problem still is there any reason I picked on this is there any reason I picked on this oxygen as opposed to that oxygen no is that fair no so what must that mean that must mean that we have resonance so I have to draw both the possible structures or both of the structures that the real molecule has characteristics of connected by double-headed arrows but wait there's more that's right this is the gift that keeps on giving this is the gift that keeps on giving i have an extra electron in the structure right because I have acetate whenever you have a Lewis structure for a polyatomic ion you place the Lewis structure in staple brackets and then you put the charge out to the side out to the right hand side as a superscript so I put my entire Lewis structure in staple brackets and I put that charge as a superscript to the right hand side let me place that in law form just so that people are happy with it well first of all so there's the answer right there is the le structure for acetic for the acetate ion just to formulate a rule real quickly whenever you have a le structure of a polyatomic ion place the structure in staple brackets with the charge as a super structure to the right hand side as long as we're putting things in writing add one more thing in writing as well if you have an annion add one electron for each negative charge to your building fund for cations ions subtract one electron for each positive charge from your building fund let's look at a cation real quickly first cation that pops to my mind here the hydrronium ion if we were to tally our building fund for this we have one oxygen with six electrons we have three hydrogens each bring one veence electron to the table that gives me nine veence electrons but then I have a positive one charge over here and if I have a positive one charge that means I've lost an electron somewhere along the way so I'm going to subtract an electron from my building fund before I even begin to build now I draw my skeletal structure well only one girl in the bar here so now all my hydrogens's crowd around to draw that skeletal structure cost me 2 4 six that leaves me two veence electrons left over hydrogen's happy with two so they're all happy place all remaining electrons on the oxygen my oxygen's happy so I'm done because my oxygen has 2 4 6 8 now because I have a polyatomic ion I want to place the Lewis structure in stable brackets with the charge as a superscript chemistry is easy life is hard yes why don't you write down Hershey's write down Hershey's how can we think of chocolate without thinking of Hershey's everybody write down Hershey's i hope everybody is writing down Hershey's one last thing before wrapping up what has already been one of my longer lectures here let's take a look at the Lewis structure of sulfur hexaflloride we have sulfur with six veence electrons we have six fluorides six florines group 7 A each with seven veence electrons for a total of 42 + 6 is 48 so we have 48 total veence electrons to build our structure with sulfur is going to be our central atom here because it's the least electrogative and because it's the only girl in the bar so how many electrons does it cost us to draw this structure 2 4 6 8 10 12 veence electrons so that gives us 36 veence electrons left over each florine at the moment has two veence electrons and wants a total of eight so each one needs six more one two three four five six one two three four five six one two three four five six 1 2 3 4 5 6 6 * 6 is 36 so that uses up the rest of our building fund so we made all our florines happy how about our sulfur is our sulfur happy sure it has 2 4 6 8 10 12 how does sulfur have 12 veence electrons well first of all let me tell you this isn't a mistake this compound can can and does exist how is it possible that sulfur has 12 veence electrons well locate sulfur on the periodic table sulfur is in the third period so the sulfur veence shell has a 3s a 3p and a 3d how many electrons does sulfur have how many electrons does sulfur have in the 3d none right if you wrote the electron configuration for sulfur you'd say sulfur is neon 3s2 3 p2 right nothing in its 3d that means that the sulfur atom has a completely empty d subshell so it has room for a lot more electrons so not only does the sulfur have room for four more electrons in its 3p but it has room for 10 more electrons in its 3d so it's entirely possible that the sulfur could have 12 veence electrons now let me ask you another question as long as we're on this subject how about oxygen could oxygen have 12 veence electrons no it couldn't right no why not oxygen is in the second period so its veilent shell is the second energy level its veilent shell only has the 2s and the 2p so it can only hold eight electrons maximum it doesn't have that emptyd to summarize elements in the third period or higher and by higher I mean [Music] numerically may may exceed the octet rule because they all have an empty D now this is an exception this is a rare exception it usually occurs when the element is bonding to a very small atom that is highly electrogative in other words florine or chlorine just because this can occur doesn't mean it will occur don't try to force this into happening only exceed the octet rule when you're dealing with something where when you follow those rules you wind up with more around that central atom elements in the first and second period cannot exceed the octet rule elements in the second period cannot exceed the octet rule only elements in the third period are higher and even then that's an exception that wraps it up good luck on the homework