2015 AP Chemistry free response 1e | Chemistry | Khan Academy

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.

We have zinc right over here, zinc right over there. And there are 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.

So you could say it's going to have the same electron configuration as argon. Argon! Let me... so you're going to have argon, but then you're going to go and you're going to add the four, you're going to add electrons to the 4s subshell.

So you have 4s^1, 4s^2. So argon, and then you have 4s^2. 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 filling the s shell subshell, then you go and backfill the 3d subshell.

So 3d^1, 3d^2, 3d^3, 3d^4, 3d^5, 3d^6, 3d^7, 3d^8, 3d^9, 3d^10. So 4s^2, 3d^10.

And there are 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 4s^2 3d^10. And, well, just for the sake of it, let's do that.

So you fill up your 1s^2. So we go 1s^2, and then we're in the second row, so you're going to have 2s^2 and 2p^6.

Let me write that down: 2s^2, 2p^6. And then you're going to go to the third row, so you're going to have... and if this looks completely foreign to you, I encourage you to watch the Khan Academy videos on electron configuration.

Then you're going to have 3s^2, 3p^6. You're going to fill those two subshells next: 3s^2, 3p^6. And then what we've just described is essentially the electron configuration of argon.

And so then you get to 4s^2 and 3d^10. 4s^2 and 3d^10. So either of those would do.

Then the second part: from which sub-level 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 bit more nuanced than saying you're kind of constructing this electron by electron, and you fill these in the 4s^2, 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 that zinc atom, are not the d electrons; it's actually going to be the 4s^2 electrons.

So, from which sublevel are electrons removed? Well, it's going to be the most likely to move, not from the d; it could be from the 4s electrons.

So, the 4s sublevel. And once again, I encourage you to kind of 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 4s ones. 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 can pull off—you're more likely to pull off the 4s electrons. You're going to pull off the 4s electrons.