Transcript for:
Chemical Bonding and Lewis Diagrams

introduction to bonding and Lewis dot diagrams alright so in this series of videos we're going to talk about how atoms join together to make a new compound basically a species with a new identity and this is going to happen by these atoms joining together with something called chemical bonds and in particular we're going to talk about covalent bonds which is sharing electrons so the first thing we need to do is understand electrons in atoms so that we can understand how these various atoms will join together form bonds share electrons and form compounds so chemical bonds are almost always formed by valence electrons interacting with each other so basically separate atoms are going to have their valence electrons are going to interact and the goal is to get eight electrons around each atom in the valence shell and that's called the octet rule so basically atoms like to have eight electrons in their valence shell all except for hydrogen which is satisfied with two we'll talk about that a little bit later when we talk about exceptions so the bottom line is that atoms will gain or lose electrons or they'll share electrons in order to have an octet which is eight electrons in their valence shell now we've talked about ionic bonding and so that's we're gaining electrons and losing electrons come in in this case now we're gonna start talking about sharing electrons when atoms joined together with covalent bonds now as I mentioned with hydrogen the octet rule is not always satisfied for all elements at all times but it is a really good rule of thumb and it applies in the vast majority of cases but we will learn a few specific exceptions to the octave a few specific exceptions to the octet rule that you should keep in mind okay so let's just look at the main group elements and so I've labeled them in on this periodic table and I've labeled the number of valence electrons on each of those okay now notice I've neglected transition metals and you can too all right so every element in the first column has one valence electron and so when we talked about electron configurations then that electron would be in 1s or 2's or 3s or 4s the next column over two valence electrons so beryllium magnesium calcium strontium barium all have two valence electrons then we skip over to the other side of the transition elements and we talked about boron aluminum gallium indium and all of these have three valence electrons four valence electrons four carbon silicon germanium five valence electrons four nitrogen phosphorus six valence electrons for oxygen and sulfur seven four four other halogens and then finally the noble gases here have eight valence electrons with the exception of helium which has two valence electrons and it is satisfied with that so so basically what the goal is is that each one of these elements wants to find a way to get to the same electron configuration as the nearest noble gas in order to get an octet so for instance hydrogen gains one electron through sharing in this case and will have an octet or actually it will have a full shell configuration just like helium which would be 1s2 fluorine if it gains one electron it has the same electron configuration as neon and it has an octet oxygen if it gains two electrons then it's gonna have a full configuration octet just like neon so Dini two electrons for nitrogen it's gonna be three carbon normally shares so we're gonna talk about carbon a bit and then we're going to also talk about some of these elements that actually would just prefer to lose an electron and go back to the next noble gas so for instance lithium will lose an electron and have the same configuration as helium sodium will lose an electron have the same configuration as neon museum will lose two electrons and have the same configuration as neon so you can see that you can use the periodic table to predict how many electrons would be either lost or gained by various elements on the periodic table ok so I do I talked about about electron configurations in previous videos and basically when we want to identify the valence electrons what we're looking for are those found in the outermost shell and that's going to be the highest end so looking at fluorine we have 1s2 2s2 2p5 that's the electron configuration 1s 2 those are core electrons ie naught valence electrons and equals 2 that's gonna be our our valence shell so fluorine has seven valence electrons just like we saw in that periodic table we were just looking at carbon so the core electrons are in the 1s and the valence electrons are in the 2s and the 2p and carbon has four valence electrons looking at aluminum aluminum has core electrons in 1s2 2s2 2p6 and then the valence electrons are in the N equals 3 shell and it has 3 valence electrons so you can see how you can identify how many valence electrons a specific element has if you write out its electron configuration so what about drawing Lewis dot diagrams basically what these are is a representation of the valence electron of an atom and it uses dots around the symbol of the element so for instance hydrogen has one electron and it exists in the one s subshell and so basically we're just going to put one dot by hydrogen now it doesn't matter where you put the dot you can put it on that side you can put on this side you can put it here or here that doesn't matter but the big thing is that it has one dot for every valence electron now you can't arrange any more than two valence electrons on a side so we'll see that with a few other elements we're going to look at next okay so I just mentioned hydrogen and here's the electron configuration for hydrogen so you can see there's the valence electron there's the one dot on the Lewis dot diagram representing that one electron that one valence electron helium the electron configuration one s2 has two dots arranged around helium and they're paired up together lithium so we're going over to the other side of the periodic table now where we have one valence electron so it's in the two s subshell so if we look at the electron configuration 1s 2 there's the core electrons we don't represent those around the lewis dot diagram we only put the valence electron there there's one of those so it has one electron dot shown with the lithium elemental symbol alright and finally beryllium has two valence electrons in the 2's sub shell so it has these core electrons they are not shown on the Lewis dot diagram but the valence electrons are so beryllium is shown with two dots so we've already done beryllium let's look at boron so boron has three valence electrons and so we can see the two s 2 and the 2 P 1 so we put two paired on one side those are the 2's and one unpaired and again we could put it over here we could put it here it doesn't really matter we just need three valence electrons around boron carbon has four valence electrons now there's 2 in the 2s and there's two in the two-piece so you'll see you know two paired for the 2s and then there are three 2p orbitals so you so and they'll be unpaired until they're forced to be paired so we might write it where we have one electron separated from another electron so but notice we still have four valence electrons total now carbon likes to form four bonds of one form or another whether they're double bonds for single bonds so often you will see the Lewis dot diagrams showing the four dots evenly distributed carbon so either way is completely fine and finally nitrogen has five valence electrons here they're in the 2s and 2p so again paired for the 2s and then there's the 2p unpaired until they're forced to be paired ok and that shows five dots total let's move on to oxygen now we have six valence electrons two of them paired two of them paired in a p-orbital and then two unpaired electrons fluorine similar situation paired paired paired and one unpaired in one of the two peas and then finally neon showing the eight valence electrons all the way around and neon has a full octet octet of valence electrons now just to remind you so basically look at fluorine here missing one valence electron if it can find a way to share an electron with another compound then or share a pair of electrons then it can have a noble gas configuration like neon so basically that's the goal alright so let's think about Lewis dot diagrams for particulars so finally minim on the on the periodic table and find selenium on the periodic table pause the presentation and then try it and then we'll go over the answers all right let's think about aluminum now aluminum has three s 2 & 3 P 1 as the valence configuration so remember those core electrons are still there but they're not represented on our lewis dot diagram so so we have the 3 s paired here and then we have that p electron right here and remember it could be here or here and that would be totally fine you could put the pair over here and the unpaired one over here that's totally fine what about selenium so selenium has six valence electrons okay so they're gonna be paired for the 4s and then we're gonna put one one one for each of the P orbitals but then we still have one more to place so it's going to be paired in a p orbital and the electron can figure for selenium actually looks a lot like that for oxygen okay so now what about atoms with partially filled D or F sub shells now these electrons are usually omitted from a Lewis electron dot diagram so for iron instead of putting in the four s 2 and the 3 D 6 then what we're going to do is just put in the 4 s 2 okay and so iron would just simply have these two electrons representing for s 2 now I just want to point out that introductory chemistry is not responsible for electron diagrams for transition metals so you're only responsible for those metals that are in the main main group elements that I showed at the beginning of this presentation all right now another thing that's kind of interesting about this is that elements in the same column of the periodic table have similar Lewis electron dot diagrams and basically that's because they have the same valence shell electron configuration so look at that first column of elements get out your periodic table and look at hydrogen lithium sodium potassium rubidium and cesium and you're gonna see that the that they each have one valence electron in their valence shell so this will be N equals one N equals two three four five and six but it's still just one electron in that highest and orbital now remember that monatomic ions are atoms that have either lost electrons and that's cations or gained electrons for anions and so we can draw electron dot diagrams for ions just as we can for atoms now we do have to remove those electrons for cations and then we have to add some for anions so for instance let's look at sodium we're going to take sodium metal and then we're gonna take sodium cation so the lewis dot diagram for sodium metal is just simply one unpaired electron and sodium likes to form a plus one cation and so the lewis dot diagram is just simply going to be the elemental symbol for sodium and a positive charge representing that lost electron if we compare the electron configuration here's our three s one that's where this lone electron is residing and here's the core electrons represented with the nearest noble gas configuration which is perfectly fine and then we notice for the sodium cation we just have that noble gas configuration so basically only the original valence shell is shown so it's it's occupied for this for sodium metal and unoccupied for the sodium cation now when you make cations electrons are going to be lost from the highest numbered shell but not necessarily the last sub shell filled so let's look at a neutral iron atom compared to the iron two-plus cation so let's look right here so here is iron metal and here's iron two-plus cation here's the electron configuration so if we're filling and we're following the periodic table we're gonna fill the 4s to first and then we're gonna start filling the three DS as we move along the Curie otic table now when iron forms a cation it's gonna lose the electrons in the 4s so that makes it kind of easy to remember so even though we filled these last we still take those electrons from the highest end so now for the iron two-plus cation the electron configuration is just going to be argon which is the nearest noble gas and 3d6 and we're going to indicate that this for s2 is empty by just simply not writing it now again I also want to emphasize that intro chem is not responsible for electron diagrams for transition metals now when compared to the original atom anions have an extra electron so let's look at a chlorine a chloride sorry let's look at a chlorine atom and a chloride anion so a chlorine has seven valence electrons and so we can see them shown in the electron configuration so here's neon the nearest noble gas plus the valence electrons now if we add one more electron to our chlorine atom we're going to get a chloride anion here's our electron configuration for the core electrons which is neon and then now we have eight electrons in the in the valence shell so 3s2 3p6 notice that we have one more than we had for the chlorine atom and now this electron configuration is actually like argon as opposed to neon okay so just a few comparisons everything that we've looked at so this is just a summary so you can think about what we've just done so sodium iron and chlorine and just to summarize the concepts we use electron dot diagrams to represent valence electrons around an atomic symbol and so each electron is going to be represented with a dot now when we make an electron dot diagram for cations we're gonna have fewer dots than the corresponding atom so remember we're gonna remove electron so we're going to be left with a positively charged cation if we're drawing an electron dot diagram for anions then we're gonna have more dots than the corresponding atom basically just representing those electrons that we have added