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Topic: How does the Afbau principle apply to subshells.  (Read 8508 times)

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Offline zilalti

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How does the Afbau principle apply to subshells.
« on: October 27, 2007, 02:22:12 PM »
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?

Offline Yggdrasil

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Re: How does the Afbau principle apply to subshells.
« Reply #1 on: October 27, 2007, 06:27:04 PM »
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.

Offline zilalti

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Re: How does the Afbau principle apply to subshells.
« Reply #2 on: October 28, 2007, 12:16:09 PM »
Thanks for your reply. Much appreciated.

Offline zilalti

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Re: How does the Afbau principle apply to subshells.
« Reply #3 on: October 28, 2007, 12:34:02 PM »
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?

Offline Yggdrasil

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Re: How does the Afbau principle apply to subshells.
« Reply #4 on: October 28, 2007, 12:44:20 PM »
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.

Offline zilalti

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Re: How does the Afbau principle apply to subshells.
« Reply #5 on: October 28, 2007, 01:15:47 PM »
So in the absence of a magnetic field, will the order in which the P orbitals are filled be random?

Offline Yggdrasil

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Re: How does the Afbau principle apply to subshells.
« Reply #6 on: October 28, 2007, 01:29:46 PM »
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

Offline deepground

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Re: How does the Afbau principle apply to subshells.
« Reply #7 on: October 30, 2007, 07:21:20 PM »
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.

Offline agrobert

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Re: How does the Afbau principle apply to subshells.
« Reply #8 on: October 30, 2007, 07:34:20 PM »
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.
In the realm of scientific observation, luck is only granted to those who are prepared. -Louis Pasteur

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