Representing Chemical Bonds - Lesson three

okay on with part three from chapter one and i'm not going through every slide i'm just getting some highlights here when it comes to covalent bonds you want to be able to put together molecules showing the presence or absence of double bonds or triple bonds whenever those bonds show up they're important and so we want to know when they're called for if only got two or three atoms like in the previous examples you can drop them individually and just kinda connect the dots literally but when you have several atoms and when there is the possibility of these extra bonds, the double and triple, it's not always obvious where they go so these rules will kind of step you through any problem that we would encounter when it comes to building molecules as it says in rule three the connectivity has to be given apart from that we can use these rules of a valence electrons to come up with correct structures The octet rule is our constant guide but there's always going to be a set number of electrons we can use to to get us there examples like this one for formaldehyde off to the right is the calculation of how many valence electrons the whole molecule brings to the table i really don't care in my final answer which electrons came from carbon or oxygen. I just need a grand total here that total was twelve, six of which are already drawn so if I build right on top of the structure as you see here we can't get carbon and oxygen satisfied just by putting in pairs of dots we end up having to make a double bond between the carbon in the oxygen and like i say we want to know when that's called for so by knowing we can't use more than twelve electrons for this about molecule and by knowing that i have to get carbon and oxygen to satisfy the octet double bond's the only thing that works hydrogens are easy because they never make double bonds they never have a dots around them, they just make one single bond to get their two electrons this is another example where you don't have a whole lot of electrons to work with, but both that carbon and nitrogen need to satisfy the octet so this molecule only has ten total and if I considered the two bonds already shown, those have to be there plus putting those six dot surround the nitrogen well almost done but this carbon in the middle still isn't happy until we move those electrons off of nitrogen we're gonna have to make a triple bond notice that gives eighty electrons around both carbon and nitrogen and it does that using a grand total of ten so we're constantly counting electrons and then distributing them polyatomic ions like nitrate work the same way with the exception that that negative charge as you see is an extra electron we have to put in our total so whereas i would normally count twenty three electrons for nitrate that negative charge means there's twenty four if they weren't positive charge up top here we would have taken away an electron and so atoms will make those adjustments when they can do something that's more stable and as i put in those electrons i find that with twenty four grand total i'm gonna have to put a double bond between nitrogen and one of those oxygens in order to get the octet rule to be satisfied for all atoms and that's what i'm trying to figure out if i need a double bond where can it go and in this case is it says at the bottom of that a choice of where we make the double bond so there is some really three different answers and that leads into this next topic of resonance because when we have more than one way to drop the octet rule structure we generally draw all the possibilities because nitrate he really has three different answers that would satisfy our requirements resonance is this phenomenon where we can draw more than one structure that obeys the octet rule and it just changes the location of things like double bonds or triple bonds ozone's a simple example of this because you can see those two structures are very similar but the location of the double bond is different and as it says we really want to draw both of these because the real answer to what ozone looks like it's a hybrid of these two this next slide explains that as well uh... so being able to recognize the existence of resonance and when it occurs and how many structures we can draw makes for good practice but it also represents the reality of how these molecules are really made in real life ozone doesn't have a short double bond in a long single bond the real answer somewhere in the middle so we want to draw structure to reflect that