Pauli Exclusion Principle, Aufbau Principle and Hund's Rule
These rules are designed to help you remember what you can and can't do with electrons and quantum numbers. Check out the sample questions at the bottom of the page to see what kinds of questions could be asked of you.
Pauli Exclusion Principle
No two electrons in an atom can have the same four quantum numbers.
Aufbau Principle
Fill from the bottom up
Hund's Rule
If multiple orbitals have the same energy, one electron goes into each of them before they start to double up.
Sample Question
Which of the rules do each of the following electron energy diagrams violate?
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a)
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b)
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c)
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ANSWERS ARE BELOW
Orbitals of the lowest energy are always filled first.
a) violates Hund's Rule, since electrons in 2p are doubled up before each of the orbitals at that energy have one in them
b) violates the Aufbau principle, because the 1s orbital is missing an electron. There should not be electrons in 2s until 1s is filled!
c) violates the Pauli Exclusion Principle since both electrons in the 1s orbital are "spin up". This means that both have the same ms and so they have the same four quantum numbers.
b) violates the Aufbau principle, because the 1s orbital is missing an electron. There should not be electrons in 2s until 1s is filled!
c) violates the Pauli Exclusion Principle since both electrons in the 1s orbital are "spin up". This means that both have the same ms and so they have the same four quantum numbers.
