Aufbau / Hund / Pauli
MCAT glossary · MCAT
Definition
Aufbau: fill lowest energy first; Hund: parallel spins in degenerate orbitals; Pauli: no two electrons share all 4 quantum numbers.
Aufbau / Hund / Pauli in plain English
These three rules tell you where electrons live around an atom, like a seating chart for a theater. Aufbau says fill the cheapest seats first — electrons take the lowest-energy spots before higher ones. Pauli says every electron needs a unique "ticket": no two can have the exact same set of four quantum numbers, so a single seat (orbital) holds at most two electrons and only if they spin opposite ways. Hund says when there's a row of equal-priced seats (orbitals of the same energy), electrons spread out one-per-seat with matching spins before doubling up — people take their own empty row before squeezing next to a stranger.
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