## 4s 3d or 3d 4s?

StephanieDoan3B
Posts: 21
Joined: Fri Sep 29, 2017 7:07 am

### 4s 3d or 3d 4s?

When filling up orbitals with electrons, we fill 4s before 3d. However, we write the electron configuration of V, for example, as [Ar] 3d3 4s2 because here, the 4s is now higher energy than the 3d. Does this mean that the energy levels of orbitals change relative to each other when they become filled? If so, is the Aufbau diagram (which charts orbital levels relative to energy levels n) only used to chart the filling of electrons but not the energy levels of filled orbitals themselves?
Last edited by StephanieDoan3B on Sun Oct 22, 2017 5:34 pm, edited 1 time in total.

Vincent Grospe 3C
Posts: 20
Joined: Fri Sep 29, 2017 7:07 am

### Re: 4s 3d or 3d 4s?

It is relative to the number of electrons that occupy each orbital. Usually before the 3d orbital is filled, electrons enter 4s orbital first. But, as you write configurations for transition elements (after the 4s orbital is occupied and configuration starts on the 3d orbital), the 3d goes before 4s (or has lower energy).

The energies of the two orbitals fluctuate as the 3d orbital is filled and during ionization, the 4s orbital is removed first, which means that the 4s orbital has higher energy.

JonathanLam1G
Posts: 25
Joined: Fri Sep 29, 2017 7:04 am

### Re: 4s 3d or 3d 4s?

StephanieDoan3B wrote:When filling up orbitals with electrons, we fill 4s before 3d. However, we write the electron configuration of Scandium, for example, as [Ar] 3d3 4s2 because here, the 4s is now higher energy than the 3d. Does this mean that the energy levels of orbitals change relative to each other when they become filled? If so, is the Aufbau diagram (which charts orbital levels relative to energy levels n) only used to chart the filling of electrons but not the energy levels of filled orbitals themselves?

I believe if you are writing for an element in the first two groups (s orbital) , you wouldn't need to write 3d^0 because the elements are occupying the 4s orbital. Although, the 3d is a higher orbital level than 4s, if no electrons are occupying the 3d orbital, then you don't have to write it.

Chem_Mod
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### Re: 4s 3d or 3d 4s?

Always write it as 3d4s. After the 3d orbitals are filled, the 4s energy level drops so during ionization, 4s electrons go first.