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Atomic Structure Links

Pauli Exclusion Principle and Hund's Rule

Pauli Exclusion Principle

An orbital can hold 0, 1, or 2 electrons only, and if there are two electrons in the orbital, they must have opposite (paired) spins. Therefore, no two electrons in the same atom can have the same set of four Quantum Numbers. 

incorrect; electrons must spin in opposite directions

correct; the electrons have opposite spins 

Hund’s Rule

When filling sublevels other than s, electrons are placed in individual orbitals before they are paired up.

Electrons fill like people do on a bus. You would never sit right next to someone you did not know if there are free seats available, unless of course all the seats are taken then you must pair up.

So when working with the p sublevel, electrons fill like this....up, up, up...down, down, down...take a look

atom	orbital box diagram
B	1s	2s	2p
C	1s	2s	2p
N	1s	2s	2p
O	1s	2s	2p
F	1s	2s	2p
Cl	1s	2s	2p	3s	3p
Mn	1s	2s	2p	3s	3p	4s	3d

Here is an animation of how the orbitals would look as you build out from the 1s.

So why is this such a big deal?  Recall that electrons have a “spin” and therefore are like little magnets.  If an element has many unpaired electrons, a sample of it can become magnetic if all of the atoms in the sample are oriented properly.  We see this most commonly with the elements iron, cobalt and nickel.  All three have unpaired electrons in their “d” orbitals and if oriented properly, large samples of these elements can become a magnet.  Most magnets you are familiar with are made of iron. 

Quantum Numbers

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