Rules for filling orbitals... summarized:
Aufbau Principle: electrons fill from the lowest energy levels first.
*when filling in electrons, start at the bottom and work up.
Pauli Exclusion Principle: no two fermions can have the same quantum number.
*you can't have 2 electrons in the same place doing the same thing. Electrons can have spin up or spin down (this is 2 different quantum numbers). Therefore, if you have 2 electrons in 1 orbital, you must always have 1 up and 1 down. You may not have 2 ups or 2 downs. This is why we are limited to two electrons in each orbital.
Hunds rule: every orbital in a sublevel occupied by a single electron (going in the same spin direction) first before anything is doubly occupied.
*when given the choice, electrons choose to have their own orbital. Therefore, if there are 3 electrons in 3p, 1 will be in 3px, 1 in 3py, and 1 in 3pz.
Fill up each level with an up arrow on every line first before adding the second (down) arrow.
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