Electron Configuration for Cr^2+

[Erin 2I]

The correct electron configuration for Cr²⁺ is said to be [Ar]3d⁴.

I thought that in general, we move the s electrons to the d when we want to make the d shell half or completely filled. But in this case, moving the 4s electrons will not even make the d shell half filled. So, how do we know when to move the s orbital electrons to the d?

[Annie Qing 2F]

I think the difference with Cr^2+ is that it's an ion so we're technically removing electrons. When removing electrons, we remove from the valence shell first, in this case from n=4. Thus, we remove the first electron from 4s and the second from 3d to create the ion.

Let me know if this makes sense?

[Omar Habib 3L]

When dealing with anions, I believe you first remove the highest energy electrons first, which are the 2 electrons from the 4s orbital, opposed to taking one from the 4s orbital and one from the 3d orbital. Therefore, you wouldn't have any left from the 4s to fill the 3d orbital, leaving it as 3d^4.