Electron Configurations

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Ashley Lopez 3J
Posts: 90
Joined: Wed Sep 30, 2020 10:06 pm

Electron Configurations

Postby Ashley Lopez 3J » Sun Nov 01, 2020 11:38 pm

Can someone please explain why is it that 4s comes before 3d?

Connie Liang 3L
Posts: 59
Joined: Wed Sep 30, 2020 10:00 pm
Been upvoted: 1 time

Re: Electron Configurations

Postby Connie Liang 3L » Sun Nov 01, 2020 11:51 pm

It's because of the Aufbau Principle which states that electrons will always fill up oribtals with lower energy first. The 4s orbital is slightly closer to the nucleus than the 3d orbital and therefore is considered a lower energy orbital.That's why electrons fill up the 4s orbital before the 3d one.

David Chibukhchian 2G
Posts: 158
Joined: Wed Sep 30, 2020 9:50 pm

Re: Electron Configurations

Postby David Chibukhchian 2G » Mon Nov 02, 2020 9:58 am

I agree with the previous answer and would just like to add that once the 3d subshell is actually filled, it then becomes lower in energy than the 4s one. This is the reason why we usually write 3d before 4s for electron configurations even though on the periodic table itself, 4s comes before 3d. It's just because 3d stops being higher in energy than 4s once it's filled up. Until that, though, 4s is lower. I hope that helps!


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