The Aufbau Principle
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State
The Aufbau principle states electrons fill an atom's orbitals from lowest available energy upward as more electrons are added.
Show
Building carbon's six electrons fills 1s first, then 2s, then two of the three 2p orbitals, in increasing energy order.
Watch for
Filling order tracks energy, not shell number — the 4s orbital fills before 3d despite its lower principal quantum number.