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Electron configuration for d block element

Introduces rules for elements in d-block. Goes through example of Fe, and discusses relative reactivity and energy of 4s and 3d electrons.

Created by Jay.
Video transcript
Voiceover: What I want to do in this video is think about the electron configuration for an atom whose highest energy electron is not in the S block or the P block but the D block. And to help us think about that with the periodic table, I'm going to rearrange the periodic table a little bit. I'm gonna take Helium, which it makes sense that's it's right over here in the top right because it's a noble gas, it's inert like the other noble gases, has very similar properties. But for the purpose of electron configurations, because Helium's highest energy electron is in the S sub-shell it's electron configuration is essentially 1S2. I'm gonna put it over here and then that allows us to construct, essentially, an S block. So let me cut and paste that here. So what do I mean by an S block? Well that means that any of these, there are some exceptions, but the general rule is that if an element is sitting in this block it's highest energy electron is going to be in an S sub-shell. So this is the S block right over there. Now, for example, Potassium's highest energy electron is going to be in the 4S sub-shell. While Hydrogen's highest energy electron, of course, is in the 1S sub-shell. But it's going to be in an S sub-shell. Now that allows us to divide the rest of the periodic table to other blocks. This right over here is the P block, same idea. The general rule is that the highest energy electron in the electron configuration of these elements is going to be in a P sub-shell. So this is the P block right over there. And, as you might have guessed, this in the middle is the D block. This is the D block and we'll talk about the F block in a future video. This is the F block. And you can actually place it here and kind of push over the D and the P blocks to make space for this. But we'll do that in a future video. So this right over here is the D block. Now why is this helpful? Well, if something is in the D block we can say that it's highest energy electron and there's going to be some special cases, but we can say, in general, the highest energy electron is going to be sitting in a D sub-shell. So, for example, if we were to focus on Iron, it's highest energy electron is going to be in a D sub-shell. Now the D sub-shell starts to become interesting. For something like the S and P sub-shells, you can say look at Oxygen, and you can say "OK, Oxygen's in the P block it's highest "energy electron is going to be in a P sub-shell." And you can even look at its period and you can say "OK, it's going to be in the 2P sub-shell." You can actually figure out its configuration. It's going to be 1S2, 2S2, and then 1, 2, 3, 4, so 2P4. Actually let me just write that down just for fun. Just for a review so we can compare them. So Oxygen we have 1S2, you fill that sub-shell. Then you go to 2S2, you fill that up, you're in the second period. And now you're gonna have 1, 2, 3, 4 electrons in the 2P sub-shell. And how did I know it was 2P? I'm in the second period, 2P. So, 2 or the second row I guess you could say of the Periodic Table, so 2P4. So 1S2, 2S2, 1, 2, 3, 4. Now you might be tempted when you're doing the electron configuration for, say, something like Iron, you fill all these up, you got the 4S2, although you might tempted to say 4D1, 4D2, 4D3, so on and so forth, but the D sub-shell you actually back fill into the third shell. So the thing to remember is when you're thinking about which sub-shell you're filling, say, for Iron, and you might say OK, Iron is in the fourth period, but when you're thinking about the D sub-shell you're gonna take that period minus one. So its highest energy electron isn't going to be in the 4, it's not going to be in the 4D sub-shell, but it's going to be in the 3D sub-shell. But let's make that a little more concrete by figuring out its entire electron configuration. So this right over here is Iron. So let's see, 1S2, 2S2, then we fill out the P block to get up to electron configuration of Neon. So 1, 2, 3, 4, 5, 6 in the P block. So 2P6 and then we come to the third shell. We're gonna have 3S2 again. And then we're gonna fill up the P sub-shell of the third shell so this is going to be 3P6 to get to the electron configuration of Argon, so 3P6. And then we go to 4S2. And now we're going to fill, not the 4D sub-shell, but the 3D sub-shell for the D block. You take the period minus one, where in the fourth period we're gonna take one less of that. So the general rule you could think of it as 3D1 3D2, 3D3, 3D4, 3D5, 3D6, so 3D6. And so that's its electron configuration. We've gotten to the D sub-shell. Now this is an interesting thing because when you think about its outer most electrons, the outermost electrons are the ones that are in the fourth shell. They're those right over there. But if you were to think about its highest energy electrons those are the 3D6, but they go back fill. One other way to think about it is we've seen these kind of, 3D visualizations of how orbitals look. They're these probability distributions that these electrons fall into depending on their energy state. And one way to think about it is it starts to... These higher energy electrons are filling in kind of a three dimensional probability distribution gap in the third shell. So the outermost electrons are in this 4S sub-shell but the highest energy ones, one way to think about it, are in the 3D sub-shell.