Introduction to photoelectron spectroscopy | AP Chemistry | Khan Academy

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. 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, what's even going on? How is this generated? Well, I'm not going to 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 in 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.

As those electrons get knocked out, they move away and enter into a magnetic field that will deflect those electrons and then make them hit a detector. You can imagine that the electrons that are closer to the nucleus have the highest binding energy. 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 have the lowest electron binding energy. They're the easiest to knock off, and so you have more of the photons' energy that's going to be transferred into kinetic energy. They're going to get further away and hit the detector at a further point.

One way to view the photoelectron spectrum is that it gives you a sense of roughly how many electrons have various binding energies. You can see that the binding energy increases as we go to the left. The reason why this makes sense is that the binding energy is inversely proportional to how much kinetic energy these electrons have as they actually get knocked off.

This spike on our spectrum at the extreme left corresponds to detecting the innermost electrons, which are the 1s electrons. We know that those aren't the only electrons because there are electrons that have lower binding energies. So we know that we would have filled up that innermost shell.

Thus, we know that there are two 1s electrons. Then we can think that this next spike is going to be the 2s electrons, and we have more electrons than that. So we must have filled up the 2s subshell. Then this next spike looks like 2p.

The reason why this really makes a lot of sense is that notice the detector is detecting more electrons there, and we also have more electrons, so that must have been filled. That makes sense. The way this was constructed is not always going to be this perfect, but you can see you have roughly three times as many 2p electrons as two or less 2s electrons, which makes sense.

The 2p subshell can fit 6 electrons, while the 2s subshell fits 2. This next spike is going to be the next highest energy shell, which has a lower binding energy—it's easier to knock those electrons off. So this looks like it's going to be the 3s².

Then this next spike—this looks like 3p⁶, and that one gets completely filled. We have one more spike after that, and that spike seems to get roughly the same number of electrons as all the other s subshells. We know from the Aufbau principle that the next we would fill is 4s, and it looks like there are two electrons there because this spike is about the same as the other filled s subshells.

Just like that, we're able to use the photoelectron spectrum to come up with the electron configuration of this mystery element. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². What element has this electron configuration? We've worked on it in other videos.

I can get my periodic table of elements out, and we can see—let's see—1s² gets us to helium. Then you have 2s² 2p⁶, which gets us to neon. 3s² 3p⁶ gets us to argon, and then 4s² gets us to calcium. So our mystery element is calcium.

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; they are the easiest to knock off. Because they're the easiest to knock off, more of that photon energy is left over.

After overcoming the binding energy, that gets converted into kinetic energy, so those electrons get deflected further. Now, based on what we see here, the photoelectron spectrum of calcium, what would we expect the photoelectron spectrum of potassium to be?

Just as a reminder, potassium has an atomic number of 19. It has 19 protons in the nucleus, while calcium has 20 protons in the nucleus. We're going to assume we're talking about 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.

In potassium, you're still going to have 1s², but it's going to have a slightly lower binding energy because it's not pulled into the nucleus as much. I'm not drawing it perfectly; it might not be this much. It probably looks something like this, but it's going to be a little bit to the right. Similarly, 2s² is going to be a little bit to the right, and then 2p⁶ is going to be a little bit to the right.

Once again, I'm not drawing it completely perfectly because I don't have the exact data here; 3s² 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. Thus, we have a lower binding energy for any given shell or subshell.

3p⁶ is going to be a little bit to the right like this. What is the 4s subshell going to look like? Well, it doesn't have 2 electrons in the 4s subshell; it only has one because it only has 19 electrons, not 20.

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. 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. That also tells us when we're thinking about the binding energy over here—the binding energy tells us how much energy we need to remove an electron.

When you're removing that first electron, that's your first ionization energy. Once you remove that first electron, because of all the interactions between the electrons, your photoelectron spectrum would change. 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.