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Video transcript

here's some of the guidelines for drawing dot structures so let's say we want to draw the dot structure for this molecule so silicon tetrafluoride the first thing we need to do is to find the total number of valence electrons that we would account for these valence electrons in our dot structure so to find the valence electrons we need to look at a periodic table and so here I have a modified version of the periodic table and you can see what I've done is I've kind of cut out the d-block here so we can focus in on the elements that we're going to be drawing in our dot structures it also makes it easier to see how the group numbers correspond to the number of valence electrons for example if we look at elements in the first group like hydrogen lithium or sodium right the first group all have one valence electron so the group number corresponds to how many valence electrons something has so hydrogen has one valence electron if we think about the periods right so hydrogen is in the first period or the first energy level so periods are going horizontally across your periodic table hydrogen is in Group one has one valence electron and if I go over here to helium right this would be two valence electrons for helium and so in the first energy level you can fit a maximum of two electrons and so this is going to be Portmore drawing our dot structures because when we've drawing hydrogen we're always going to throughout it by two electrons or a single covalent bond when you get to the second period on the periodic table so here we have the second second period lithium has one valence electron beryllium has two boron has three carbon has four nitrogen has five oxygen has six fluorine seven and neon eight and so you have more orbitals in the second energy level and so because of that you can fit more electrons so maximum of eight electrons in the second energy level and this is where the idea of the octet rule comes in so four elements like carbon nitrogen oxygen and fluorine understanding the octet rule is going to help you when you're drawing dot structures now it is possible for some of the elements the second period to not have eight electrons so they can it's possible for them to have less than eight electrons so things like things like boron will sometimes do that but is not possible for elements to have more than eight electrons always check your dot structures and make sure that if you have an element in the second period you do not exceed eight electrons once you get to the third period you have even more orbitals available to you now so in the first energy level you have only 1s orbital in the second energy level you have s and P orbitals and a third energy level you have s P and D orbital so you can fit more than eight electrons and so therefore it's possible to exceed the octet rule for elements in the third period and beyond and we will see a few examples of that in this video and some of the ones to come here so getting back to getting back to our molecules silicon tetrafluoride if I want to find out how many total valence electrons are in this molecule I need to find these elements on my periodic table so I go over here and I find silicon and I see it's in Group four so therefore one atom of silicon has four valence electrons fluorine is over here in group seven and so therefore each atom of fluorine will have seven valence electrons and I have four of them so seven times four all right gives me 28 valence electrons for my fluorine the total number of valence electrons for our molecule will be 28 plus four so I have to account for 32 valence electrons when I draw this dot structure so let's go ahead and move on to the next step let's go back up here and look at our guidelines so we've figured out how many valence electrons we need to account for for our dot structure we don't have any kind of charges right so we don't need to worry about the rest of step one here move on to step two where we decide the the central atom of our dot structure and the way to do this is to pick the least electronegative element that we have here and then draw the bonds and so for our example right we're working with silicon and fluorine and so we can go ahead and find those again on our periodic table here's fluorine alright fluorine is the most electronegative element and so therefore for silicon tetrafluoride we're going to put the silicon atom at the center of our dot structure since it is the least electronegative of those two so I'm going to start with silicon here and I know that silicon has four bonds to fluorine atoms so go ahead and put in some fluorines right here so here's some fluorines like that so I just drew four covalent bonds and we know that each covalent bond represents two valence electrons right so here's two valence electrons here's two so that's total four six and eight so we've represented eight valence electrons so far in our dot structure so we originally had to represent thirty-two so I'm gonna go ahead and subtract eight from 32 so 32 minus eight gives me 24 so now I only have to account for 24 valence electrons let's go back up and look at our steps again so let's find out where we are so we've decided the central atom and we've drawn the bonds and we just subtracted the electrons that we use to draw those bonds from the total that we got in step one so we're on to step three where we assign the leftover electrons to the terminal atoms so in this case the terminal atoms would be the fluorines let's go back down here and look at our dot structure so fluorine will be the terminal atoms we're going to assign electrons to those fluorines but how many do we need to assign well going back to our going back to our periodic table over here so fluorine is in the second period so pretty good bet it's going to follow the octet rule here so we need to surround each fluorine atom with eight electrons each fluorine already has two electrons around it so I'm gonna go ahead and put six more around each fluorine like that so each fluorine gets six more valence electrons and since I'm assigning six valence electrons to four fluorines alright six times four gives me 24 and so therefore we've now accounted for all of the valence electrons and so this should be this should be the final structure for this should be the final dot structure here and so we don't even need to go on to step four for this molecule this is a very simple molecule to draw let's go ahead and look at another example all right so now we have a CH 2 O which is the dot structure for which is the molecular formula I should say for formaldehyde and so for following our guidelines here the first thing we need to do is find out the valence electrons so we need to look and find carbon hydrogen and oxygen on our periodic table so let's go back up here and let's find those elements here so here's carbon carbon is in Group four so therefore carbon will have four valence electrons here's hydrogen in Group one so one valence electron and oxygen is in group six so six valence electrons for oxygen so let's go back down and let's let's calculate the total number of valence electrons that we need to represent in our dot structure we have one atom of carbon so that's four valence electrons each hydrogen is one valence electron but we have two of them so one times two and each oxygen is six and we have one of them so six so six plus four plus two is twelve valence electrons so we need to represent 12 valence electrons in our dot structure all right let's go back up to our guidelines and see where we are now so we've we've already figured out the total number of valence electrons once again we don't have any charges so we don't need to worry about the rest of step one step two is decide the central atom which is the least electronegative and remember you you are going to ignore hydrogen for this example so if we ignore hydrogen the central atom is either going to be carbon or oxygen and let's look at our periodic table to figure out the relative relative values for those oxygen is more electronegative than carbon here we go here's oxygen right next to fluorine and here's carbon over here so remember your trends your trends for for electronegativity oxygen is more electronegative therefore carbon is going to be at the center of our dot structure so go ahead and put carbon in the center all right so go ahead and draw carbon and we know that the carbon is going to be bonded to two hydrogen's and we know that the carbon is going to be bonded to an oxygen here so let's see how many valence electrons we've accounted for so far two four and six so we needed 12 right we just used up six so now right now we have six valence electrons left over okay so let's let's go back up and and look at the next step here so we are two step three assign the leftover electrons to the terminal atoms here all right so the terminal atoms well in this case right our terminal atoms would be hydrogen and oxygen but we're not going to assign any electrons to hydrogen because each hydrogen is now surrounded by two electrons and so therefore we're going to assign those leftover electrons to the oxygen here and oxygen is going to follow the octet rule so it's already has two around it so it needs six more so two four and then six and so that actually takes care of all of our valence electrons right so now we have accounted for all 12 of them however we're not we're not done with our dot structure here so carbon actually follows the octet rule almost all of the time and let's go back up here and let's let's let's look at step four here so so so for step four if the central atom doesn't have an octet and it usually does have an octet you can give it an octet by creating multiple bonds so let's look at what we have so far for our dot structure and let's see if we can if we can create a multiple bond somehow so if I took if I took one of these electron pairs here right and I moved them into here let's see what that would give us for a dot structure now I have carbon alright with a double bond to oxygen and this oxygen would have two lone pairs of electrons and then these hydrogen's right down here and so this would be the correct dot structure for formaldehyde you have an octet of electrons around carbon you have an octet of electrons around oxygen and hydrogen has two electrons around it which it is happy with so this is the correct dot structure and let me just talk about some terminology really fast here so so these electrons in here alright so this would be a double bond between the carbon the oxygen these electrons you'd call bonding electrons because obviously they're involved in bonding and then these electrons out here would be nonbonding electrons or lone pairs of electrons so that's just terminology that you will hear people use when you're talking about dot structures alright so and then let me just go ahead and highlight the fact that carbon has an octet of electrons here so this would be two four six and eight electrons surrounding our carbon all right let's do one more example here let's look at this time an ion so this would be the xenon pentafluoride cation here so we need to look and find xenon and fluorine on our periodic table so we can figure out how many valence electrons we're dealing with here so let's find zina on first okay so here is xenon it is in group eight so eight valence electrons for xenon and then we've seen fluorine of course in group seven so let's go back down and figure out how many valence electrons we need to count 4 here so we have one xenon and that's 8 valence electrons all right each fluorine is 7 and we have 5 of them so 7 times 5 is 35 so we have 35 plus 8 gives us 43 now this is actually not the total number of electrons that we're going to account for because this is an ion this is a plus 1 a positive charge which means that we lost an electron remember an electron is negatively charged so if you lose a negative charge you have a positive charge leftover and so we're going to take away an electron so instead of 43 where we are now going to account for 42 in our dot structure let's go back up to our guidelines and I'll show you where I have that written down right up here so find the total number of valence electrons all right and then in this case we had a positive charge in our ion so we subtracted an electron and then again step to find the central atom least electronegative okay so we know that fluorine is the most electronegative so that means we're going to put xenon in the center so we'll go ahead and let's get some room right down here and we'll go ahead and put xenon in the center like that bond it to five fluorines so we can go ahead and put in these bonds around here like that and how many valence electrons have we accounted for so far well see that would be 2 4 6 8 and 10 so we had to represent 42 we just represented 10 so now we have 32 left over that we need to represent and notice xenon is already violating the octet rule exceeding the octet rule and it's okay it can expand its its outer shell because xenon is past the third period on the periodic table obviously like what we talked about earlier so we can go ahead and and go to the next step now we're going to assign the leftover electrons to the terminal atoms and our terminal atoms are of course the fluorines which we know are going to follow the octet rule and so once again each fluorine has two electrons around it so that means i'm going to give each of the other fluorines and each fluorine six more all right so i'm going to go ahead and put six more valence electrons on each of my fluorines and so when i'm thinking about how many how many valence electrons i've represented now so I have I have six more on five fluorines so six times five is thirty all right so I've represented thirty more electrons and so that means I have two left over so two valence electrons left over we haven't had this situation before let's go back up to the guidelines to refresh your memory what we do we have leftover electrons after step three so we get two we get to step four here so if necessary in this case it is we're going to assign any leftover electrons back to the central atom this time and if the central atom has an octet or exceeds an octet which is what we have in this example you are usually done and we're not going to worry about formal charges in this video I'll talk about them in the next video so we're going to take those two electrons and assign them to our xenon so let's go back down here and assign those two extra electrons to the xenon and so we would draw it like that and so now we are done we've represented all 42 of the valence electrons that we were supposed to so you can count all those up if you want to now most people will represent this dot structure by putting brackets here and putting a positive charge outside of it so there's your xenon pentafluoride cation so we're going to we're going to do a lot more examples for drawing dot structures in the next several videos and see how drawing dot structures allows you to predict the shapes of different molecules
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