Chemistry / Unit 5 Chem 0847 short lesson, about 20 seconds
The Aufbau Principle
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The idea
The Aufbau principle states electrons fill an atom's orbitals from lowest available energy upward as more electrons are added.
In practice
Building carbon's six electrons fills 1s first, then 2s, then two of the three 2p orbitals, in increasing energy order.
Where people slip
Filling order tracks energy, not shell number — the 4s orbital fills before 3d despite its lower principal quantum number.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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