welcome to section 13.12. all right gentle people in this lecture what we're going to go ahead and discuss are exceptions to the octet rule and the first one we want to go ahead and discuss are compounds that are going to go electron deficient and that means that they are going to have less than eight electrons around one single atom and so they are not going to complete the octet so let's go ahead and take a look at this real-life compound bf3 it has 24 valence electrons that you have to distribute among these four atoms now there are two ways that we can go ahead and try to envision a lewis dot structure one way is that we're going to obey the octet rule and we're going to have a double bond between the boron and the fluorine the other way is we are going to go ahead and break the octet rule and you'll notice that boron here only has 6 electrons around it 2 4 and 6 so it does not complete its octet so why don't you guys go ahead and answer this quiz question for me what's the formal charge of this fluorine all right gentle people let's go ahead and write our formula down the formal charge equals the valence minus the lone pair electrons minus one half the bonding electrons or the number of bonds so fluorine has a valence of seven if i look at the number of lone pair electrons i have four and one half the bonding electrons or four electrons that are involved in that bond if i go ahead and do this calculation out i get plus one now this is a little bit disconcerting what i'm saying here is that i have a plus one formal charge on the most electronegative atom on the periodic table and this doesn't really make a whole lot of sense if i were to go ahead and complete the formal charges on here boron would be minus one each one of these fluorines would be zero now if i go ahead and look at the structure on the left hand side what you'll notice is everything has a formal charge of zero so here's what we're going to say there are going to be certain atoms on the periodic table that are allowed to go electron deficient those are going to be beryllium boron and sometimes aluminum now you guys already know that hydrogen is considered electron deficient it only wants two electrons around it but now what we're going to say is that these elements right here they don't necessarily need to complete its octet these elements sometimes will have six or four electrons around it and it will be completely satisfied now what i want you guys to understand about going electron deficient with these three atoms here this isn't the default choice you're only going to invoke this deficiency or this breaking of the octet if it minimizes formal charge so in this particular example you can see that i'm closer to zero with my formal charges here so a better lewis dot structure is one where i go electron deficient this is what we see experimentally what we see is the boron fluorine bond is more similar to a single bond than any of the resonance structures this structure would provide now if we go electron deficient one of the things that you might ask yourself is can we go the other way can we get more than eight electrons around a single atom now this is going to be the case for elements that are past period 2. so if we look at row 3 row 4 and so forth elements in these rows can go hypervalent and what i mean by that is that they can have more than eight electrons around the central atom for example pcl5 what you'll notice is that phosphorus has 10 electrons around it sf6 is going to have 12 electrons around it these are real compounds that we observe experimentally and these lewis structures match what we see again i want to reiterate this rule just because something can expand its octet doesn't mean it will the only reason that you're going to go hypervalent again is if you have to and it minimizes formal charge you can look at the formal charge of each one of these compounds and what you guys will find is they have a zero formal charge across the board so always remember to obey the octet rule unless you have to break it due to the number of electrons and atoms you have and that will provide a structure that minimizes formal charge all right chem 1a i hope that made sense and remember to stay safe