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2015 AP Chemistry free response 1e

Writing the electron configuration of zinc, and figuring out which electron is removed during oxidation. From 2015 AP Chemistry free response 1e. 

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  • piceratops ultimate style avatar for user Matthew Chen
    Which electron would be oxidized with copper? The single 4s electron or one of the 10 4d electrons? And is it the same with chromium?
    (1 vote)
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  • blobby green style avatar for user Brian Hoy
    At he says that the 4s subshell is at a higher energy level than the 3d subshell. I can't find any source to back this up looking at Google Images. Can anyone point me to a source explaining why in this case the 4s subshell is at a higher energy level, or explain it themselves?
    (1 vote)
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  • duskpin seed style avatar for user Daria Rudykh
    Is the exception shown about 4s/3d orbitals at the only orbital exception we need to know for the AP exam? Or are there different exceptions - such as the highest energy electrons in an atom with orbitals 3d and 4p would be 3d? (That's probably not correct of course, but I'm just wondering if this applies to other orbitals)
    Or does this only apply to the s and d orbitals in general and not the 4s/3d orbitals specifically? (By that I mean, any atom with 5s and 4d will have higher energy electrons at 5s, etc.)
    (1 vote)
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    • mr pants purple style avatar for user Ryan W
      They’d no doubt expect you to know the electron configuration exceptions of Cr and Cu too.

      The 4s electrons are always the ones ionised first in transition metals rather than 3d. Same with 5s and 4d and 6s and 5d.

      4p is higher than 3d too. When you get to zinc the 3d electrons behave as core electrons rather than valence electrons.
      (1 vote)

Video transcript

- [Voiceover] The only common oxide of zinc has a formula ZnO, zinc and then you have your oxygen. Write the electron configuration for a zinc atom in the ground state. So there's a couple of ways that you could do this for the electron configuration. Let's first identify zinc on our periodic table of elements, and this is the actual one that they will give you when you take the AP test. And so we have zinc right over here, zinc right over there, and there's a couple of ways you could do it. The simplest way is starting with the noble gas that is before zinc, so that would be argon right over here. You could say it's going to have the same electron configuration as argon, argon. Let me... So you're gonna have argon but then you're going to go and you're gonna add the four, you're gonna add electrons to the 4s subshell. So you'll go 4s1, 4s2. So argon and then you have 4s2, and then you go and-- it's an oversimplification what I'm about to say, but when you're figuring out electron configurations, it's helpful to think of well, after you figure out fill in the s subshell, then you go and backfill the 3d subshell. So 3d1, 3d2, 3d3, 3d4, 5, 6, 7, 8, 9, 10. So 4s2 3d10. And there's two exceptions in this fourth row here. You have the exceptions of chromium and copper that don't exactly fill just like that. You can look those up on your own, but that right over here is the electron configuration for a zinc atom in the ground state. Now another way you could write this is actually writing out the electron configuration for argon and then writing 4s2 3d10, and we'll just for the sake of it, let's do that. So you fill up your 1s2. So we go 1s2, and then we're in the second row, so you're gonna have 2s2 and then 2 one, two, three, four, five, six, 2p6. 2s2 2p6. So let me write that down. 2s2 2p6. And then you're going to go to the third row, so you're gonna have-- and if this looks completely foreign to you, I encourage you to watch the Khan Academy videos on electron configuration. And then you're gonna have 3s2 3p6. You're gonna fill those two subshells next. 3s2 3p6, and then what we've just described is essentially the electron configuration of argon, and so then you get to 4s2 and 3d10. 4s2 and 3d10. So either of those would do, and then the second part. From which sublevel are electrons removed when a zinc atom in the ground state is oxidized? And this is what's interesting about d block elements, and that's why I said it's a little more nuanced than saying you're kind of constructing this electron by electron and you fill these in, the 4s2 and then you go and backfill the 3d because it actually turns out when you have a neutral zinc atom, the highest energy electrons, the ones that are the most likely to be pulled away from the zinc atom are not the d electrons. It's actually going to be the 4s2 electrons. So from which sublevel are electrons removed? Well, the most likely to be removed not the first to is going to be the 4s electrons. So the 4s sublevel. And once again, I encourage you to kinda read more about that if you're curious about why that is. You can kind of imagine in your head. Even when you look at energy levels, the 3d sublevel and 4s are very, very, very close, but it does turn out that most of the time what's going to react are the ones that are furthest out, the 4s1. So you could argue that those are at the highest energy level, and so when you oxidize, you start pulling electrons off of zinc, you can pull off the 4s. You're more likely to pull off the 4s electrons or you're going to pull off the 4s electrons.