Rules Regarding Electron Configurations

I don’t know if it was just me, but when I was learning about the quantum orbitals and electron configurations, I never could find a complete list of the different rules to keep in mind. Additionally, they were worded in the most indirect language imaginable. Were they trying to confuse people? Here, I’ll try to clear up some of that confusion.

Aufbau Principle

When filling electrons in orbitals, always fill from the lowest energy level to the higher energy levels. This means to fill 1s then 2s then 2p then 3s and so on. Because of the Aufbau Principle, Chlorine’s electron configuration is 1s^22s^22p^63s^23p5 and not something like 1s^23s^24p^65s^23p5.

If you ask why this is true? Well it seems that all things in nature like to be in its least energy state, so why hold an electron way out where it’s highly energetic when you have space in the “ground level” so to speak?

A notable exception is when you have excited electrons. These electrons will seemingly “jump” into a higher energy level and violate the Aufbau Principle, but all you have to keep in mind is that the atom is not in its ground state and you’ll be okay.

Hund’s Rule

Electrons want to be as spread out as possible to minimize electron-election repulsion. Thus, fill one electron in each orbital first and then come back to fill the second orbital. Again, this should be rather intuitive after introduced. This of electrons as living, breathing creatures! I don’t know anybody who would rather be cramped in a small space when there are clearly other open spaces available!

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Pauli Exclusion Principle

No two electrons can have the exact same four quantum numbers and live in the same “address” or place in the atom. The closest you can get is having the first 3 quantum numbers the same, but then the last number, the spin, must be different.


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