Covalent Bonding Part 2

Added: 2026-05-10

[00:00:02]all right guys so we're gonna pick up where we left off um looking at drawing dot structures molecular compounds um there are some kind of mental steps that I'll take you through for drawing these um the only way to really know how to draw these is to practice them overall so here are some of the kind of steps or things to Think Through when drawing dot structures and then we're going to do several examples together of some common ones so for molecular compounds you first want to add up the total number of valence electrons in that compound because that's what's going to actually be bonding and sharing electrons one thing to note is there are molecules that have a charge these are referred to as polyatomic ions so it will be made of only non-metals and then carry a charge remember with ions if a substance has a charge that changes the total number of valence electrons so we do have to account for that when we're finding the number of valence electrons when we start to draw the Dot Structure we want to draw the skeletal structure first to figure out what substance should go in the middle and then just have everything else around it on the outside generally whatever element is listed first is the element that goes in the middle unless it's hydrogen hydrogen will never go in the middle so if you have hydrogen first in a compound whatever the second element listed is goes in the center whenever we're drawing dot structures we want to connect all of the outer atoms to the center atom using bonds and we're just going to draw single bonds to start out with and remember that Each Bond accounts for two of your valence electrons once everything is connected to the center we're going to fill up the octet for all of our outer atoms so go around to all each outer atom and make sure that it has eight total electrons so add in those unshared electrons or those dots to fill its octet any leftover electrons you have after filling the octet of all the outer atoms any excuse me I'm skipping a step um that's fine any leftover electrons are always going to go on the center atom so any extra electrons once those outer atoms are full we'll dump onto the center and then you want to make sure that everything has a full locked head so if you have atoms that do not have a full octet once you've used up all of your electrons that is when you can add a double or a triple bond so double and triple bonds are kind of as a last resort to fill octets for atoms so that will always be kind of your last step that you do um and one thing to note is if the molecule has a charge so those polyatomic ions I mentioned you're going to note in your Dot Structure that this has a charge by putting it in Brackets with the charge on the outside that way if someone is looking at a Dot Structure they know that it has a charge and that the overall number of electrons has changed so we're going to go through several examples together so you can see what these look like we already Drew oxygen a moment ago so I'm gonna skip that one here let's look at finding the loose structure Dot Structure for Carbon dioxide which is CO2 so we said first thing you want to do is to add up the number of valence electrons so carbon is in group 4A so carbon has four valence electrons oxygen is in group 6A so oxygen has six valence electrons but note that there are two oxygens each oxygen has six so you do have to account for any of those subscripts so 2 times 6 is 12 plus 4 is 16.

[00:04:31]valence electrons that need a place to go in this Dot Structure we said you want to draw the skeletal structure first so whatever is listed first goes in the middle unless it is hydrogen so carbon will go in the middle with our oxygens floating around on the outside we said you connect everything to the center atom so both oxygens are going to connect onto the carbon that is four of our 16 electrons because of the two bonds we then said you look at the outer atoms and fill their octet so the oxygen on the left has a bond which is two electrons so in order to have eight total it needs six more electrons and we draw these in pairs just like we did with our DOT structures yesterday so since we added six dots to our two bonds that is a total of 10 electrons that are drawn so now we go to the other outer atom the oxygen on the right it has a bond which is two electrons so it needs six more to have a full octet so we can add our six dots on there that is 16 electrons that we have drawn now if you look at Carbon carbon right now only has two bonds attached to it only has four electrons so here's where we have something that is not full it does not have a flocked head so we need to start adding bonds in order to full fill the octet of carbon now we've already drawn 16 electrons we can't add any more so what we do is we simply relocate them so instead of having a lone pair I'm on the oxygen on the left I will relocate that lone pair and make it a bonded pair that way it is now being shared between the oxygen and the carbon so oxygen still has eight I didn't change how many electrons oxygen has but now carbon since it's attached to three bonds has six electrons however carbon is still not full so what it'll do is it will borrow another pair from the other oxygen to make that a bonded pair that way oxygen still has eight but now carbon also has access to eight so to clean this up a little bit carbon's Dot Structure will look like this but getting in those double bonds we simply moved around the electrons that were already there and those double Bonds were added as a last resort all right let's look at this example um here it wants us to draw the Dot Structure for the carbonate ion which has a formula of CO3 negative 2.

[00:07:46]so here carbon and oxygen are non-metals this is still made of non-metals but it does have a charge so when we're finding the number of valence electrons we said carbon is in group 4A so it has four valence electrons add to that oxygen is in group 6A so it has six valence electrons however there are three oxygens so remember each oxygen has six so 3 times 6 is 18 plus 4 is 22 but remember we said this has a charge so what that does is it changes our total number of valence electrons so just like we talked about yesterday with ions that negative 2 charge means that it gained two electrons because there are two additional negative charges so we have to add two so we actually have 24 valence electrons that we are including so you always want to do the opposite of the sign so if it's a negative you want to add that many electrons if it is a positive charge you want to subtract that many electrons so now that we know our total number of valence electrons we need to draw our skeletal structure so since carbon is listed first carbon is going to be in the middle with our oxygens kind of floating around on the outside we said we connect everything to the center atom so that's three bonds that's six electrons we have 18 left that we need to put somewhere so we said you then look at each outer atom and fill its octet so the oxygen to the left already has a bond which is two electrons so to have a full octet to have eight it needs six more looking at the oxygen on the bottom it already has a bond which is two electrons so it needs six more to have a full octet and that is 18 electrons that we have drawn so far if you add up our bonds and our dots and then looking at the oxygen on the right it already has the bond which is two electrons so it needs six more have a full octet so that is 24 electrons that have been drawn however look back at Carbon carbon has three bonds attached to it which is six electrons it means eight so you're going to have to take from one of the oxygens and it doesn't matter which one you're gonna have to take a pair of electrons and relocate it into a double bond so here we are going to have carbon with two single bonds and a double bond so I'll clean that up a little bit and like I said it didn't matter which oxygen you took from as long as you end up with two that are single Bond and one that is double bonded the last thing that we said is if the substance has a charge you need to put it in Brackets with the charge on the outside so that's an example of what's slightly different wherever you add that polyatomic ion in there all right so let's continue doing a couple other examples and I just picked examples that give you a variety of different things that you're going to see when drawing dot structures um so our next one we want to draw is water um we want to find the number of valence electrons for water hydrogen is in group 1A so it has one valence electron but note that there are two hydrogens so we have to multiply that by two add to that options in group 6A and has six valence electrons so that makes a total of eight valence electrons in this Dot Structure we said then you draw your skeletal structure so whatever is listed first goes in the middle unless it is hydrogen so here since hydrogen is listed first the next element is our Center atom so oxygen is going to be in the middle and I'll show you why hydrogen cannot be in the middle in a moment um so we said then you connect everything to the center atom so each hydrogen connects onto the oxygen that is four of our eight electrons we then said that you fill the octet of the outer atoms so if you look at the hydrogen on the left it already has a bond attached which is two electrons remember hydrogen is our exception that can only have up to two electrons you are never going to have dots on hydrogen because once it has a bond attached hydrogen is full that's also why hydrogen can never go in the middle because it cannot bond to more than one atom so the hydrogen on the left is full with two electrons the hydrogen on the right is also full with two electrons but we have only used up four of our eight electrons so we said anytime you have leftovers they go on the center atom and yet again just make sure to draw them in pairs and it doesn't matter whether you have two pairs side by side or a pair on top and a pair on bottom as long as it's distinct that you have two pairs of electrons drawn so that is our Dot Structure for water showing all eight valence electrons let's look at the next one SO2 um f is sulfur that is in group 6A so it has six valence electrons add to that excuse me that look like a multiplication sign but add to that our oxygen which is in group 6A as well so it has six valence electrons but there are two of them so this structure is going to have 18 total valence electrons that need to be drawn in so whatever comes first tends to go in the middle so sulfur is going to be in the middle here with our oxygens kind of hanging out on the outside we said we connect everything to the center atom so that's four of our 18 electrons since we drew two bonds so we have 14 left then you want to fill the octet of your outer atoms so the oxygen on the left already has a bond which is two electrons so it needs six more to have a full octet the oxygen on the right already has a bond which is two electrons so it needs six more times the octet that is 16 of our 18 valence electrons so anytime you have leftovers they always dump on the middle so since we have two left those two are going to go in the middle yet again make sure they are drawn in pairs so we have used up all 18 valence electrons however if you look at sulfur sulfur has two bonds and one pair of dots for a total of six valence electrons sulfur is not full so that's where we need to look at one of our outer oxygens borrow a pair of electrons to move as a bonded pair and it doesn't matter which oxygen you pulled from to make that bonded pair so to clean it up a little bit you're going to have sulfur with a double bond attached to an oxygen with four lone electrons and then a single Bond attached to an oxygen with six valence electrons with two lone electrons on that sulfur and one thing is whenever we have those dot structures that have a double bond in it a lot of times it will have what we refer to as a resonance structure this is a structure that occurs when it's possible to write two or more electron dot formulas for the same compound so if you look back at the carbonate ion we did a couple slides ago it had two single bonds and a double bond and I told you it didn't matter which oxygen you made that double bond on because all three structures are considered resonant structures whether you chose the oxygen on the left for the double bond the one on the bottom for the double bond or the one on the right for the double bond these three structures are equivalent to one another whenever you can have that double bond in different locations the same thing is true if we go back to our SO2 that we just had and I'll redraw it oops excuse me so we drew it with that double bond that is in resonance with the same structure with the double bond on the right oxygen instead so these structures are equivalent to one another um and resonance will go in deeper level if you decide to take AP that will kind of go more in depth to what these actually look like but know that anytime you have that double bond that can be in different locations you are going to have at least two or more resonance structures all right so last thing that I wanted to touch on today is there are molecules that are exceptions to the octet rule we've already went over hydrogen where like H2 hygiene is full with only two valence electrons that is an exception to the octet rule um there are other times where the octet rule cannot apply um so the octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number we tend to not see these too many because these tend to be the ones that form ions and either lose or gain electrons um but if there's an odd number of electrons you cannot make it to where every single atom has eight valence electrons there are also molecules in which an atom will have less than or more than a complete octet Advanced electrons the less than is primarily hydrogen there are a couple other exceptions but you guys do not need to know those at this point in time um but there are some that are going to have more than eight valence electrons especially if it is a larger atom in that Central atom so to show you what these look like the first example is the no nitrogen monoxide so if we add up our number of valence electrons nitrogen has five valence electrons oxygen has six so that is a total of 11 valence electrons so we cannot make even pairs with 11 valence electrons but we want to fill the octet of as many substances as possible so here there's not really a central atom we just have two elements next to one another they connect together and then we start filling the octet of everything um so here's two four six so that's 10 that are drawn including our bond I'm gonna put that lone extra one on nitrogen so that is 11 valence electrons that are shown oxygen has a full octet but nitrogen only has five valence electrons right now so that's where they will take a pair of electrons and change it into a bonded pair so the oxygen still has eight but now nitrogen has access to seven however that's all that we can do we do not have an extra electron to give to nitrogen to give it eight so the Dot Structure here looks like this like I said you're not going to come across these often but it is possible where you have that odd number of valence electrons what's more common that you're going to come across are some of our larger elements um that third row of the periodic table and down sometimes the central atom can have more than eight valence electrons so here a look at pcl5 um phosphorus is in group 5A so it has five valence electrons add to that chlorine is in group 7A with seven valence electrons and there are five of them so 5 times 7 is 35 Plus the 5 means here we have 40 valence electrons we'll put phosphorus in the middle and then the five chlorines are kind of going to be hanging around on the outside we connect everything to the center so there are five bonds that attach to the center meaning that that Center phosphorus is attached to 10 valence electrons it looks odd but it's possible because that third row and down also has that empty d block that they can put electrons in um so that's in our Dot Structure the five bonds is 10 of our 40 electrons each chlorine since it has a bond we'll need six more electrons so if we fill the six electrons in everything that will end up being 40 total electrons so like I said before those larger elements so the third row and down the it is possible for the center atom to have more than eight it is not required for it to have more than eight though I want to show you one more example of this expanded octet um let's do icl2 and I believe it should have a negative charge I'm just pulling one off the top of my head um here iodine has seven valence electrons add to that chlorine also has seven valence electrons but there are two of them so that's a total of 21 valence electrons but since there's a negative charge we have to add one so we're actually dealing with a total of 22 valence electrons when we draw this iodine is going to go in the Middle with the chlorines on the outside we connect everything to the center so that's four of our 22 we fill all of our outer octets so chlorine has a bond which is two electrons so it needs six more the other chlorine same deal needs six more that is 16 electrons that are drawn remember any time we have leftovers they dump on the center even if it's going to give the center more than eight so we have 16 drawn so we'll draw 17 and 18 19 and 20. 21 and 22.

[00:24:32]and last thing this does have a charge so it needs to be in Brackets with the charge on the outside so notice that this iodine has 10 valence electrons around it but it is one of those larger atoms so this is possible um the general chemistry is not going to see expanded octets very often and they're going to be more common for honors chemistry and then they will definitely show up if you move out to take AP Chemistry go ahead and you guys are going to be working on your molecular model lab um I want you to focus on finding the number of valence electrons for each one of them and drawing The Dot Structure the last column where it asks you for the shape and geometry we are going to deal with that in our next set of lessons we'll talk about going over theirs but I want you to get the hang of drawing these dot structures and like I said practice these for sure your short answer for your quiz and your tests are going to be drawing dot structures so I want to make sure that you're comfortable with this