## Electron configuration when 4s and 3d

yast_27
Posts: 29
Joined: Fri Sep 28, 2018 12:19 am

### Electron configuration when 4s and 3d

Hi,

I'm still really confused about how we should represent atoms that have either 3d or 4s shell.
Should we fill the 4s first and then then 3d? (4s^2, 3d^10) Or should we fill 3d before 4s? (3d^10,4s^2).

Thank you very much!

Maria Solis Disc 1G
Posts: 63
Joined: Fri Sep 28, 2018 12:27 am
Been upvoted: 2 times

### Re: Electron configuration when 4s and 3d

From my understanding you fill the 4s before the 3d usually. Only when the 3d is an exception, such as when you are 1 e from completing the 3 d orbital, do you complete the 3d first, since youre ultimate goal is to get a more stable atom. But when you write down the electron configurations first make sure to write it as 3d4s even if the 3d is incomplete.

armintaheri
Posts: 68
Joined: Fri Sep 28, 2018 12:26 am

### Re: Electron configuration when 4s and 3d

You fill the 4s first, just like the Aufbau principle tells you to. The only exceptions are Chromium and Copper. The reason those two are different is that orbitals want to be either full or half-full whenever possible. Aufbau tells you chromium should be [Ar]3d44s2, but the actual configuration is [Ar]3d54s1, because it gives you a half-full 4s and a half-full 3d, so both of them are happy. Same thing for copper. You would expect the configuration to be [Ar]3d94s2, but the 3d orbital is really close to being full, so it's going to take an electron from 4s, giving you [Ar]3d104s1, so that both orbitals are either full or half-full. So the short answer is, fill 4s first, but remember the two expections.