Filling of Orbitals in Atom

The filling of electrons into the orbitals of different atoms takes place according to the aufbau principle — but the aufbau principle itself is not a standalone rule. It rests on three ideas working together: the relative energies of the orbitals (the order they fill in), the Pauli exclusion principle (how many electrons an orbital can hold), and Hund's rule of maximum multiplicity (how electrons spread across orbitals of equal ene …