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  • - [Instructor] In this video,

  • we're going to introduce ourselves

  • to the idea of photoelectron spectroscopy.

  • It's a way of analyzing the electron configuration

  • of a sample of a certain type of atom.

  • And so what you'll often see

  • and you might see something like this on an exam,

  • is a photoelectron spectrum

  • that looks something like this.

  • And so the first question is,

  • well, what's even going on?

  • How is this generated?

  • Well, I'm not gonna go into the details,

  • but the big picture is

  • the analysis will be done by taking a stream of that atom,

  • and so that atom, there's an atom stream

  • going in one direction,

  • and then the other direction, let me label this,

  • so that's the atoms

  • that we're trying to analyze,

  • and then the other direction,

  • you send high-energy photons

  • that are going to bombard those atoms, photons.

  • Now these photons are high enough energy,

  • in fact, they're typically x-ray photons

  • so that when they collide,

  • the photons are high enough energy

  • to overcome the binding energy

  • of even the core electrons

  • and as those electrons get knocked out,

  • they move away

  • and they enter into a magnetic field

  • that will deflect those electrons

  • and then make them hit a detector.

  • And so you can imagine

  • the electrons that are closer to the nucleus,

  • those have the highest binding energy,

  • and so more of that energy from the photon

  • is going to be used to knock it off

  • so less of it is going to be there

  • for the kinetic energy,

  • so those closer electrons aren't going to get as far

  • and the outer electrons,

  • those have the lowest electron binding energy.

  • They're the easiest to knock off

  • and so you have more of the photon's energy

  • is going to be transferred into kinetic energy.

  • And so they're going to get further away

  • and they're going to hit the detector

  • at a further point.

  • And so one way to view the photoelectron spectrum

  • is it gives you a sense of roughly how many electrons

  • have various binding energies.

  • And you can see that the binding energy increases

  • as we go to the left.

  • Now the reason why this makes sense,

  • the binding energy is inversely proportional

  • to how much kinetic energy these electrons have

  • as they actually get knocked off.

  • And so this spike on our spectrum at the extreme left,

  • these are the innermost electrons,

  • and then these would be electrons further out

  • with the next lower binding energy,

  • and then lower binding energy after that.

  • And so we can analyze this

  • to actually come up with the electron configuration

  • of this mystery element right over here.

  • What do you think that would be?

  • Pause this video and try to think about that.

  • Well as I mentioned,

  • this spike right over here would correspond

  • to detecting the innermost electrons,

  • and so the innermost electrons

  • are the one S electrons,

  • and we know that those aren't the only electrons

  • 'cause there's electrons that have lower binding energies,

  • and so we know that would have filled up

  • that innermost shell

  • and so we know that they have two one S electrons

  • and then we can then think that this next spike,

  • that's going to be the two S electrons

  • and we have more electrons than that

  • so we must have filled up the two S sub shell

  • and then this next spike,

  • this looks like two P.

  • And the reason why this really makes a lot of sense is

  • notice the detector is detecting more electrons there,

  • and we also have more electrons,

  • and so that must have been filled

  • and that makes sense,

  • and actually the way this was constructed,

  • it's not always going to be this perfect,

  • but you can see you have roughly three times as many two P

  • electrons as two S electrons, which makes sense.

  • The two P sub shell can fit six electrons.

  • Two S sub shell fits two.

  • So this next spike is going

  • to be the next highest energy shell,

  • which is going to have a lower binding energy.

  • It's easy to knock the,

  • it's easier to knock those electrons off.

  • And so this looks like it's going to be the three S two

  • and then this next spike,

  • this looks like three P six

  • and then that one gets completely filled

  • and we have one more spike after that

  • and that spike seems to get roughly the same number

  • of electrons as all of the other S sub shells

  • and we know from the Aufbau principle

  • that the next we fill is four S

  • and it looks like there's two electrons there

  • because this spike is about the same

  • as the other filled S sub shells.

  • And so just like that,

  • we're able to use a photoelectron spectrum

  • to come up with the electron configuration

  • of this mystery element.

  • Its electron configuration is one S two,

  • two S two,

  • two P six,

  • three S two,

  • three P six,

  • four S two.

  • And what element has this electron configuration?

  • Well, we've worked on it in other videos,

  • but I can get my periodic table of elements out,

  • and we can see, let's see.

  • One S two gets us to helium,

  • then you have two S two, two P six gets us to neon.

  • Three S two, three P six gets us to argon,

  • and then four S two gets us to calcium.

  • So our mystery element is

  • calcium,

  • and if someone were to ask about valence electrons,

  • that would be this outermost spike right over here.

  • The spike of electrons with the lowest binding energy.

  • They have the lowest binding energy

  • because they're the furthest out there.

  • They are the easiest to knock off,

  • and because they're the easiest to knock off,

  • most of that photon energy is leftover

  • after overcoming the binding energy

  • that gets converted into kinetic energy.

  • So those electrons get deflected further.

  • And the base of what we see here

  • are the photoelectron spectrum of calcium.

  • What would we expect the photoelectron spectrum

  • of potassium be?

  • And just as a reminder,

  • potassium has an atomic number of 19,

  • so it has 19 protons in the nucleus,

  • while calcium has 20 protons in the nucleus,

  • and we're going to assume

  • that we're talking a neutral potassium atom,

  • so it's going to have 19 electrons, as well.

  • Pause this video and think about

  • how it might be different.

  • When we think about potassium,

  • it's going to have a very similar photoelectron spectrum

  • as calcium,

  • but because it only has 19 versus 20 protons,

  • it has less positive charge in the nucleus,

  • so it pulls a little bit less hard

  • on our various shells.

  • So in potassium,

  • you're still going to have one S two,

  • but it's going to have a slightly lower binding energy

  • because it's not pulled into the nucleus as much.

  • And I'm not drawing it perfectly.

  • It might not be this much.

  • Actually, you know what?

  • It's probably more slight, probably.

  • Something like this,

  • but it's going to be a little bit to the right.

  • Similarly, two S two is going to be a little bit

  • to the right,

  • and then two P six is going to be a little bit

  • to the right,

  • and once again, I'm not drawing it completely perfectly

  • 'cause I don't have the exact data here.

  • Three S two

  • would be a little bit to the right.

  • Once again,

  • only 19 protons versus 20 for calcium,

  • so we're pulling a little bit less inwards,

  • so we have a lower binding energy

  • for any given shell or sub shell,

  • and three P six is going to be a little bit to the right,

  • like this,

  • and then what is the four S sub shell going to look like?

  • Well, it doesn't have two electrons in the four S sub shell.

  • It only has one,

  • 'cause it only has 19 electrons and not 20.

  • And so it's going to be a little bit to the right.

  • It has a lower binding energy

  • and it's only going to be half as high

  • because you only have one electron, not two.

  • So it's going to look something like that.

  • That would be the photoelectron spectrum

  • of potassium, roughly speaking.

  • Now we've already talked about that your outermost shell

  • shows where your valence electrons are.

  • So if we're thinking about potassium,

  • it would be right over there.

  • Now that also tells us,

  • when we're thinking about the binding energy over here,

  • so this binding energy,

  • that tells us how much energy do we need

  • to remove an electron?

  • And so when you're removing that first electron,

  • that's your first ionization energy.

  • Once you remove that first electron,

  • because of all of the interactions between the electrons,

  • your photoelectron spectrum would change

  • so you can't think about your second

  • or third ionization energies,

  • but your first ionization energy,

  • you just have to think about it's the binding energy

  • of your outermost electrons.

- [Instructor] In this video,

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光電子分光学入門|AP化学|カーンアカデミー (Introduction to photoelectron spectroscopy | AP Chemistry | Khan Academy)

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    林宜悉 に公開 2021 年 01 月 14 日
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