[zilalti]

I recently have been reading about the Afbau principle but im a little confused about how it works for subshells. I.e. if all sub shells within a principle quantum shell are degenerate, why does an electron enter a S shell before a P. Can anyone help me out here?

[Yggdrasil]

The subshells are degenerate only in the hydrogen atom where there are no electron-electron repulsions.  When you add in electron-electron repulsions, then the s subshell has a lower energy than the p subshell.

[zilalti]

Thanks for your reply. Much appreciated.

[zilalti]

Just one more question. Are the different type of subshells all degenerate i.e are Px Py and Pz all at the same energy level once the 1s2 orbtals and 2S orbitals are filled? If so which p orbital will be filled first?

[Yggdrasil]

The different types of subshells are all degenerate (because of symmetry).  However, if you break the symmetry, for example by applying a magnetic field, you can make them non-degenerate.

[zilalti]

So in the absence of a magnetic field, will the order in which the P orbitals are filled be random?

[Yggdrasil]

Basically.  You still have to remember the rule of maximum multiplicity.  In other words, every orbital in a given subshell must have at least one electron before any orbital in that subshell will have two electrons.

http://en.wikipedia.org/wiki/Maximum_Multiplicity

[deepground]

Just one more question. Are the different type of subshells all degenerate i.e are Px Py and Pz all at the same energy level once the 1s2 orbtals and 2S orbitals are filled? If so which p orbital will be filled first?

Px will be filled first with 1 electron, followed by Py with 1 electron, last Pz with one electron. As the electrons start to group together it goes in the same order, Px, Py, Pz. Correct me if I am wrong.

[agrobert]

Px Py and Pz are all the same energy.  The x y and z just denote what direction of the plane they are in.  Conventionally you would fill Px first, but it really doesn't matter.