Aufbau Principle
The rule that electrons occupy the lowest-energy orbital available before filling higher-energy ones.
Formula / equation
fill in order of increasing (n + l)
Unit
Dimensionless
In depth
Orbital energies follow the (n + l) rule: lower n + l fills first, and where two orbitals tie, the one with lower n wins. This gives the sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and explains why the 4s orbital fills before 3d yet empties first on ionisation.
Examples in the real world
Potassium's nineteenth electron enters 4s rather than 3d, giving [Ar] 4s¹ and placing it in group 1.