What rule/principle states that electrons in the same sublevel (p, d, or f) are placed in individual orbitals, before they are paired up, to increase atomic stability?

Hund’s rule says electrons fill orbitals of the same energy one at a time before any orbital receives a second electron. This arrangement reduces electron repulsion and increases stability, so the correct choice is Hund’s rule. The Aufbau principle describes filling lower-energy orbitals first, while the Pauli exclusion principle limits the electrons that can occupy one orbital.

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