THE ORDER OF FILLING 3d AND 4s ORBITALS

This page looks at some of the problems with the usual way of explaining the electronic structures of the d-block elements based on the order of filling of the d and s orbitals. I am grateful to Dr Eric Scerri from UCLA, who pointed these problems out to me and provided me with some useful academic papers I wouldn't otherwise have been able to get hold of.

The way that the order of filling of orbitals is normally taught gives you an easy way of working out the electronic structures of elements. However, it does throw up problems when you come to explain various properties of the transition elements.

This page takes a closer look at this, and offers a more accurate explanation which avoids the problems.

  
The commonly taught version
This section is just a summary of the way this is currently taught. It is taken from things you have probably already read elsewhere on Chemguide. You shouldn't find anything unfamiliar in it.

The order of filling orbitals
Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible.
The diagram (not to scale) summarises the energies of the orbitals up to the 4p level.

The oddity is the position of the 3d orbitals.

They are shown at a slightly higher level than the 4s - and so it is the 4s orbital which will fill first, followed by all the 3d orbitals and then the 4p orbitals. Similar confusion occurs at higher levels, with so much overlap between the energy levels that the 4f orbitals don't fill until after the 6s, for example.

To continue reading click on the link below:

http://www.chemguide.co.uk/atoms/properties/3d4sproblem.html