## Electronic configuration after Z=20

ckim_1I
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

### Electronic configuration after Z=20

I have trouble understanding this topic.
Whenever we write the electron configuration, the 3d is written before the 4s, but the 4s orbital is filled before the 3d orbital?
Can someone please tell me why this is so?

Vivian Tan 3F
Posts: 34
Joined: Fri Sep 25, 2015 3:00 am

### Re: Electronic configuration after Z=20

When we write the electron configuration for elements Z=20, the d-subshells are in front of the s-subshells because they are lower in energy.

marina_salama1A
Posts: 8
Joined: Thu Dec 11, 2014 3:00 am

### Re: Electronic configuration after Z=20

Yes. When looking at it, the way my ta does it actually helps me put everything in perspective. He first writes everything as you see it in order on the periodic table starting from left to right ( so the s orbital the then the d if applicable then the f orbital again if applicable then finally the p orbital all the way to the right.) Then you come back and arrange them all from lowest to highest energy. So basically I try look at the number before the s,p, etc orbital and put them in order based on that in increasing order. For example, 3d comes before 4s because the 3 is smaller than the 4 ! Hope this helps :)

Chem_Mod
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### Re: Electronic configuration after Z=20

The reasons for this ordering is a result of very complicated solution to quantum mechanics (Schrodinger equation), but to keep it short, the energy levels of orbitals also depend on many factors. Some of the factors include nuclear charge (protons) and electron shielding. But the result is that as you go from Ca to Sc, the 3d orbitals become lower in energy than the 4s orbitals.

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