In Unit 1, Section 6, we're looking at evidence for sublevels in atoms, and we're going to use something called photoelectron spectroscopy in order to demonstrate this. Now, we're going to start by talking about the fact that there are some flaws in Niels Bohr's model of the atom. So you might remember seeing pictures like this.
This is, of course, not the scale, but we see an idea of what a neon atom might look like according to the Bohr model. We basically have 10 protons here in the middle. We have 10 electrons scattered in these two energy levels.
I've, of course, left out the neutrons. Well, Niels Bohr thought that, just for example, all eight of these electrons in the second energy level were identical. As it turns out, today we have evidence that he was incorrect. Now, if we take this atom and we run it through an object called a photoelectron spectroscope, we actually can strip these electrons one by one away from the atom to determine the successive ionization energies, or how much energy is required to strip each electron away from that atom.
And if we do that, we get a spectrum that looks kind of like this. And we don't get two peaks, but we get three peaks. And This tells us that this atom has three sublevels.
So it shows us that the Bohr model is flawed. There are sublevels, not just energy levels. And if we label these from left to right, the first one on the left is 1s, and then 2s, and then 2p. And those are the sublevels that each peak will be representing.
Now if you can write an electron configuration like we did in the last video, then you can interpret a photoelectron spectroscopy graph. It's basically the same thing. You just go from left to right, 1s, 2s, 2p, and as it turns out, the last peak, the very last one here, the one that's labeled 2.08, that will be telling us about the the first or the lowest ionization energy.
Those are the easiest to remove electrons. It only takes 2.08 megajoules per mole to take those electrons away, whereas in the 1s level or sublevel, that would take 84.0 megajoules per mole, a whole lot more. Now, the height of each peak corresponds to the number of electrons in the sublevel.
So you know, that 1s has a maximum of two electrons. So I'm going to go ahead and label 1s2, 2s2. In fact, I'll just label them all so you can see this.
1s2, 2s2. But this last peak, I want you to notice that this last peak is three times taller than the other peaks. That means that it has three times as many electrons as the other peaks. So that's why we can safely say this is 2p6.
And... If you remember back from the electron configurations, you can match this up to a periodic table there and see that this is neon. That's the atom that has this electron configuration. Let's try another one. Let's try this photoelectron spectroscopy graph, and let's label each peak with its corresponding sublevel.
So this works just like an electron configuration. It goes 1s. then 2s and then 2p so that's all you have to do just label from left to right just like it were an electron configuration if you can write an electron configuration you can do these how many electrons are in each sub level well we know it goes 2 and 2 you know 1s and 2s are always maxed out at 2 but what about this peak this last peak it's half as tall as the others so guess what you it has half as many electrons, which means it's only got one in there. So it's 1s2, 2s2, 2p1.
Now, which element is this? Well, you can match it up on your periodic table, and that is boron. So if you can write an electron configuration, you can interpret a PES diagram. Now, last part here. Let's calculate the first ionization energy.
per electron for this element in joules per electron. Now, once again, like I said earlier, the first ion... Organization energy, that's the last peak.
So we're going to take the 0.80 MegaJoules per mole and we're going to convert that to Joules per electron down here at the end. So let's convert the mega joules to joules first So one mega joule goes on the bottom and joules goes on the top Hopefully you remember that there are 1 million joules in a mega joule or a million of anything in a mega of anything, right? So mega joules are out. And now let's convert from moles to electrons. So in our next conversion factor, moles have to go on top to cancel.
And there are 6.02 times 10 to the 23rd electrons in a mole. So now we can cancel moles. On our calculator, we take 0.80 times a million divided by 6.02 times 10 to the 23rd.
And we get an answer of about 1.3 times 10 to the negative 18th joules per electron. So that's all you have to do on this type of calculation. Let's try one more example here. Let's take a look at this PES diagram. And let's label it first.
And we'll identify the element. So once again, if you can do an electron configuration. You can do a PES diagram.
Just label it with the sublevels. It goes 1s, 2s, 2p, and then 3s, 3p, and then 4s, as we progress our way across the periodic table there. And now we can label the heights.
All of these s's are going to be 2, right, because they're all the same height. And we know that s maxes out at 2. And then the p's are all, these are all three times higher than the others. So that means they have three times as many electrons. So that would be 2p6 and 3p6.
Now, which element is this? Well, once again, just match it up on your periodic table and see which one ends with 4s2. And that is calcium.
So you can do photoelectron spectroscopy diagrams. Hope you enjoyed this video. If you learned something from it, please... Smash that like button. I'm Jeremy Krug, and I hope to see you on my next video, which will be Unit 1, Section 7.