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  • - [Instructor] In this video, we're gonna start talking

  • about exceptions to the octet rule,

  • which we've talked about in many other videos.

  • The octet rule is this notion

  • that atoms tend to react in ways

  • that they're able to have a full outer shell.

  • They're able to have eight valence electrons.

  • Now, we've already talked about some exceptions,

  • things like hydrogen.

  • Its outer shell is that first shell

  • which gets full with two electrons.

  • So it's trying to get to that duet rule.

  • But as we'll see, there are other exceptions.

  • Boron and aluminum, for example,

  • they can form stable molecules where the boron

  • or the aluminum only have six valence electrons, not eight.

  • And there are exceptions in the other direction.

  • As you get to the third period and beyond,

  • we'll actually see atoms that can maintain more

  • than eight valence electrons.

  • And we're actually going to see an example

  • of that with xenon.

  • So let's just go into a few examples.

  • Given what I've told you,

  • see if you can come up with the Lewis diagram

  • for aluminum hydride.

  • So aluminum hydride has one aluminum

  • and three hydrogens.

  • See if you can draw the Lewis diagram for that.

  • All right, now let's do this together.

  • So the first thing you wanna do is account

  • for all of the valence electrons.

  • Aluminum's outer shell is the third shell,

  • so the third period here,

  • and it has one, two, three valence electrons.

  • And then we have three hydrogens,

  • and each hydrogen has one valence electron.

  • And so you add all of this up together,

  • three plus three is equal to six valence electrons

  • in aluminum hydride.

  • Now, the next step after that is

  • to try to draw the structure with some covalent bonds.

  • We don't wanna make hydrogen our central atom.

  • That would be very atypical.

  • And so let's put aluminum in the center.

  • And then we're gonna have three hydrogens.

  • So one, two, and three.

  • And then let's put some covalent bonds in here.

  • And so let's see, how many valence electrons

  • have we now accounted for?

  • This is two in this covalent bond.

  • Another two gets us to four.

  • Another two gets us to six.

  • So we have just accounted for all six valence electrons.

  • So we have no more valence electrons to play with.

  • Let's think about how the various atoms are doing.

  • So the hydrogens are all meeting their duet rule.

  • These two electrons in this bond are hanging around hydrogen

  • and around the aluminum.

  • But from hydrogen's point of view, it has a full duet,

  • that hydrogen as well and that hydrogen as well.

  • But notice the aluminum over here,

  • it has two, four, six electrons,

  • valence electrons around it, and so it's not a full octet.

  • But aluminum hydride is actually something

  • that has been observed.

  • Let's think about another example.

  • Let's think about xenon pentafluoride.

  • Xenon pentafluoride cations,

  • a positively charged ion here.

  • Pause this video and see

  • if you can draw the Lewis diagram for this.

  • All right, now let's do this together.

  • If any of this seems unfamiliar,

  • I encourage you to watch the video

  • on introduction to drawing Lewis diagrams.

  • But what we'd wanna do is first think

  • about our valence electrons.

  • So xenon right over here, it's actually a noble gas.

  • It already has a full octet in its outer shell,

  • so it has eight valence electrons.

  • So xenon has eight valence electrons.

  • And then fluorine, we've seen this multiple times,

  • has one, two, three, four, five, six,

  • seven valence electrons, but there's five of them.

  • So five times seven.

  • I'm gonna be drawing a lot of electrons in this.

  • So this gives us a total of eight plus 35,

  • which is 43 valence electrons.

  • But we have to be careful.

  • This is a cation.

  • It is a positively charged molecule.

  • It has a positive one charge.

  • So we have to take one electron away because of that.

  • So let's take away one valence electron to get that cation.

  • And so we are left with 42.

  • 42 valence electrons.

  • So the next step is to try to draw its structure

  • with some basic single covalent bonds.

  • And xenon would be our preferred central atom

  • because fluorine is more electronegative.

  • It's actually the most electronegative element.

  • So let's put xenon in the middle,

  • and then let's put some fluorines around it,

  • five of them to be specific.

  • So one, two,

  • three, four.

  • I'm having trouble writing an F.

  • Four and then five fluorines.

  • And now let me make five covalent bonds.

  • One, two, three, four, five.

  • So just like that, I have accounted for 10 valence electrons

  • because you have two valence electrons

  • in each of these covalent bonds,

  • two, four, six, eight, 10.

  • So let me subtract 10 valence electrons.

  • And then we are left with 32 valence electrons.

  • Now, the next step is to try to allocate some more

  • of these valence electrons to the terminal atom

  • so that they get to a full octet.

  • So let me do that to the fluorines.

  • Each of these fluorines already are participating

  • in a covalent bond, so they already have

  • two valence electrons hanging out with them,

  • so let's give 'em each six more.

  • So let's give that fluorine six,

  • and that fluorine gets six,

  • and that fluorine gets six valence electrons,

  • and that fluorine gets six valence electrons,

  • and then last but not least,

  • this fluorine gets six valence electrons.

  • So I have just given away six valence electrons

  • to each of five fluorine atoms.

  • So that is 30 valence electrons that I have just allocated.

  • And then what does that leave me with?

  • That leaves me with two valence electrons

  • that have gone unallocated.

  • And the only place to now put them is on the xenon.

  • And as I said, things that are lower down

  • in that periodic table of elements,

  • especially as we get below the third period,

  • these can defy the octet rule.

  • Xenon already has 10 valence electrons,

  • and I'm about to allocate it two more to it just like that.

  • So you allocate those two more.

  • And then we have allocated all of our valence electrons.

  • And I wanna make sure I remind myself and everyone

  • that this is a cation.

  • So I have to put that plus charge just like this,

  • but this is something that has been observed

  • where you can actually have a central atom like this

  • that goes beyond an octet number of valence electrons.

  • In this case, it has two, four, six,

  • eight, 10, 12 valence electrons.

  • Now, an interesting question is how do these atoms

  • that are in the third period or beyond handle more

  • than eight valence electrons?

  • And it is a matter of debate,

  • but some chemists believe that it's possible

  • because they're able to place their electrons

  • in their empty valence d-orbitals.

  • But once again, this is controversial

  • in the chemistry community.

- [Instructor] In this video, we're gonna start talking

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オクテットルールの例外|AP化学|カーンアカデミー (Exceptions to the octet rule | AP Chemistry | Khan Academy)

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    林宜悉 に公開 2021 年 01 月 14 日
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