Filling of orbitals

Once the shapes and relative energies of orbitals are known, the next question is a practical one: in the ground state of a real atom, exactly which orbitals get filled with electrons, and in what order? Three separate rules, used together, answer this completely: the Aufbau principle (which decides which orbital to fill first), Pauli's exclusion principle (which limits how many electrons any one orbital can hold), and Hund's rule of maximum multiplicity (which decides how electrons distribute the …