So remember, exceptions start with chromium. So let's look at chromium. If we're to determine its electron configuration. Initially we would see that it would look like argon 4S²3D⁴. Now here what do we have? Here we have 3D with four electrons within it. But remember earlier we said that S&D subshells or sub levels have this urge to try to be half filled or totally filled.

Now we're going to say an S orbital electron can be promoted to create half filled orbitals with D 4 electrons. So what we're saying here is if you're doing the electron configuration of chromium, you're going to end with AD⁴. That's a key to tell you that, oh D⁴, we have only four electrons within these D orbitals. But if I could somehow get one more electron in there, those D orbital will be half filled.

So what's going to happen here is we're going to take one electron from the 4S and donated over here. So our 3D⁴ is going to become AD⁵ and our 4S² just gave up an electron so it becomes 4S¹. So it now looks like this. This would be the correct electron configuration of Chromium. So again, remember chromium has this type of exception and the driving force is trying to get a half filled set up D orbitals.

So here we're not going to land, we're not going to stay as D⁴ when it's neutral, it's going to become a D⁵. Now that we've seen this with the first column, let's see what happens with the second column. So click on the next video and let's see what happens with them.