3d before 4s for Z > 10

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Christine Chen 1H
Posts: 64
Joined: Fri Sep 28, 2018 12:18 am

3d before 4s for Z > 10

Postby Christine Chen 1H » Sun Oct 28, 2018 9:05 pm

For clarification, when Z (atomic number) > 10, 3d comes before 4s, 4d comes before 5s, 5d comes before 6s, etc... ?

Netpheel Wang 4L
Posts: 60
Joined: Fri Sep 28, 2018 12:15 am

Re: 3d before 4s for Z > 10

Postby Netpheel Wang 4L » Sun Oct 28, 2018 9:11 pm

I belive so as long as there is an electron occupying an s-orbital

Jennifer Lathrop 1F
Posts: 71
Joined: Sat Dec 02, 2017 3:00 am

Re: 3d before 4s for Z > 10

Postby Jennifer Lathrop 1F » Sun Oct 28, 2018 9:12 pm

I was confused on this as well, in high school I was always taught to write 4s3d but in the book and in class it has 3d before 4s. Is there a difference between the two?

Jonathan Omens 1K
Posts: 60
Joined: Fri Sep 28, 2018 12:15 am

Re: 3d before 4s for Z > 10

Postby Jonathan Omens 1K » Sun Oct 28, 2018 9:19 pm

This rule only comes into effect for elements above argon (Z=18).

You are correct that 3d is written before 4s, etc. when writing electron configurations

However the 4s shell will fill before the 3d shell. This can be seen for the elements Potassium and Calcium (19 & 20). K: [Ar] 4s1, Ca: [Ar] 4s2. Notice how these elements have electrons in the 4s orbital but none in any 3d orbitals.

After this, the 3d subshell is filled but it is written before the 4s subshell because it has less energy. For example: Scandium: [Ar] 3d14s2.


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