How to write Lewis structures for resonance.

TRANSCRIPT:
Here we’re asked to write the Lewis structure for NO3- and this is going to involve resonance structures. So let’s write out one example of the Lewis structure for nitrate. If you’ve seen my previous video, what I like to do is give everything an octet first, and I also like to connect the elements together. So, let’s see. This oxygen has an octet. This oxygen does, this oxygen does, and this nitrogen does. Let’s clean this up a bit. I don’t really like seeing this hanging electron pair. Also, we should calculate how many valence electrons there should be. So nitrogen has 5 valence electrons, and oxygen has 6, and there’s 3 oxygens so that’s 18 plus 5, which is 23. And don’t forget about this, because that means you’re going to add 1 more electron. So we have 24 valence electrons total. Let’s count how many we have now. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, and 26. So let’s move this over and create a double bond here, take away that other electron pair, and we now have 24 valence electrons. We also need to keep in mind that we should write this in brackets and give it an extra negative, because this is an ion. This is the ion nitrate. So basically, when you put a Lewis structure in brackets with this, it just shows it’s the NO3- ion. And for resonance, what we do is draw an arrow in between two different structures – or there could be 3 structures, depending on the compound, which shows that there’s resonance in between them.
So the next Lewis structure that you can make for this actually involves a double bond on this other oxygen. So, it looks kind of like this, and if you count, you still have 24 valence electrons. But there’s 1 more resonance structure for NO3, which is why I’m going to draw another arrow and let’s put this down here. And of course, this oxygen is going to get the double bond now, so it looks kind of like this, I’m drawing it as fast as I can. And that’s the last resonance structure. So you can see that nitrate really resonates between these 3 Lewis structures, and that really means that instead of having a real double bond or a single bond here, it’s going to have something in between a double bond and a single bond. It’s just that the bond length is going to be a little shorter than a single bond, but a little longer than a double bond because it’s somewhere in between those two. So that’s really what resonance means. It describes a compound where the bonds between the elements kind of shift back and forth, except they’re not really shifting back and forth. It’s just an intermediate between either a single and a double bond, or a double and a triple bond, something like that. So it’s just a really useful way for chemists to show the bond lengths and the forces in between elements.

свернуть