Electrons fill orbitals from lowest energy to highest energy {Aufbau principle}. Before using f orbitals, orbital hybridization causes one electron to go into a d orbital.
Electrons tend to enter all shell orbitals before they fill any orbital with two opposite-spin electrons {Hund's rule} {Hund rule}. Hund's rule is true for small atoms, because it takes more energy to put two electrons into one orbital than into two different orbitals.
Fermions are electrons, neutrons, protons, and the like. Because fermions have half-unit spins, when identical fermions interchange, their wavefunctions become the negative of the other. Therefore, no two fermions can have same energy quanta {Pauli exclusion principle, fermion}|.
Hund's rule is true for small atoms. Other rules {Slater's rules} {Slater rules} apply for large atoms.
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Date Modified: 2022.0225