## 3d fills before or after 4s?

Angel More
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### 3d fills before or after 4s?

For textbook problem 2A.5 it asks to give the ground-state electron configuration expected for Cu+

I thought the answer would be [Ar]3d^8 4s^2 since 4s has a lower energy than 3d and would fill up first. However the answer is [Ar]3d^10. Why doesn't the 4s fill up first?

What is the order of the electron configuration? I thought it is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f. Is that correct?

Melody Haratian 1B
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### Re: 3d fills before or after 4s?

Hello!
Cu is part of the exceptions in electron configuration. So its configuration for a neutral atom of Cu is [Ar]3d104s1, because it prefers a full d shell. When Cu loses an electron, it will lose its outermost electron. In this case, its outer most electron is in the 4s shell ( since 4s1 is the last electron shown on its electron configuration). When you remove that electron, you’ll get an electron configuration of [Ar]3d10.

Gabby Magat 3F
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### Re: 3d fills before or after 4s?

The order you listed is the correct order electrons fill subshells! This makes sense when you refer to the periodic table to write electron configurations. But when you change the configuration to remove electrons (like when writing the configurations for ions), you have to group the s, p, and d orbitals with the same energy level together (i.e. ...3s^2 3p^6 3d^10 4s^2), which is different from the order the electrons actually fill the subshells. That's why when Cu loses the electron, you remove one electron from the 4s shell (even though the 4s shell fills first).

SavannahScriven_1F
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### Re: 3d fills before or after 4s?

Adding onto the previous responses, 4s fills up before 3d, but once 3d is occupied, it has a lower energy than 4s. So when electrons enter the 3d state, the electron configuration is written with 3d before 4s.

Angel More
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### Re: 3d fills before or after 4s?

Oh ahhhh that makes more sense, thank you guys!

YUNALEE
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### Re: 3d fills before or after 4s?

after, and you can also think about it in the order of the n number! 3 before 4 so 3d, 4s

Kayla Booker 1F
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### Re: 3d fills before or after 4s?

My TA said that you could write it either way but it flows better if you put 3d before 4s.

VincentLe_3A
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### Re: 3d fills before or after 4s?

Since the energy of 3d orbital becomes lower than that of the 4s orbital once it starts getting filled with electrons, it makes sense to write 3d before 4s in terms of energy order of the orbital. It is important to note that the 4s orbital is lower energy than the 3d orbital until the 3d orbital starts getting filled with electrons.