Electron configurations of ions | Atomic structure and properties | AP Chemistry | Khan Academy

In many videos, we have already talked about electron configuration, and now in this video, we're going to extend that understanding by thinking about the electron configuration of ions. So these are going to be charged atoms.

So let's just start looking at some examples. Let's say we are dealing with fluorine. Now, we know what a neutral fluorine atom's electron configuration would be. In fact, if you want a little bit of practice, try to pause this video and think about what is the electron configuration of a neutral fluorine atom.

All right, now let's work through this together. A neutral fluorine atom has nine electrons, and we could just use our periodic table of elements. So first, we're going to have two electrons in 1s, so we'll have 1s². Then we're going to go to the second shell, so then we go to 2s². So so far, we have filled in four electrons.

Next, we go to the 2p subshell, and we are going to have, if we're talking about a neutral fluorine, we are going to have 1, 2, 3, 4, 5 electrons in that 2p subshell, so it's 2p⁵. So if that's the electron configuration for fluorine, what do you think the electron configuration for fluoride would be?

So this is just the anion that has one extra electron. It is a negatively charged ion. Pause this video and try to figure it out. Well, here, you're now going to have one extra electron. The fluorine has nabbed an electron from some place.

So where will that extra electron go? Well, our 2p subshell has space for one more electron, so that's where it will go. So the fluoride anion is going to have an electron configuration of 1s² 2s² 2p, and that's going to have an extra electron here: 2p⁶.

Now let's do another example. Let's say we wanted to figure out the electron configuration of a positively charged calcium ion. So, calcium, let's make it two plus, so it has a positive charge of two. This is, you could view this as a neutral calcium that has lost two electrons. What would be its electron configuration? Pause this video and try to figure that out.

All right, well, one way to figure this out is first we could figure out the electron configuration of a neutral calcium atom. From that, we can take two of the highest energy electrons away. So neutral calcium, you could view it actually—let's do it in noble gas configuration.

The noble gas that comes right before calcium is argon, so it's going to have the electron configuration of argon, and then we are going to have two electrons for that fourth shell. It's going to fill in the 4s subshell. So we're going to have argon, and then we're going to have—let me do this in a new color—let's call this 4s².

Now, what do you think is going to happen if we were to lose two electrons? Well, those two electrons in that 4s subshell and that fourth shell are going to go away. So the electron configuration here for calcium with a positive 2 charge, this calcium cation, is going to be the electron configuration of argon and no 4s².

So it's actually going to have the exact same electron configuration as argon. So I will leave you there with just a couple of examples, and I encourage you, if you're feeling in the mood, just pick any of these atoms, any of these elements, and think about what would happen if they gained or lost an electron and what their electron configurations might be.