Electrons occupy orbitals singly before pairing

• A. Pauli Exclusion Principle
• B. Hund's Rule
• C. Aufbau Principle
• D. Electron configuration

Answer: (B)

The key idea is that electrons prefer to keep their spins parallel when occupying orbitals of the same energy. Hund’s rule says that in a given subshell with multiple degenerate orbitals, each orbital gets one electron with the same spin before any orbital gets a second electron. This arrangement maximizes the number of unpaired electrons and lowers the atom’s energy due to exchange interactions, making it more stable overall. For example, in a p subshell with three orbitals, three electrons will occupy each orbital singly with parallel spins before any pairing occurs. Only after all three orbitals are singly occupied do electrons begin to pair up in the same orbitals. The Pauli Exclusion Principle governs that no two electrons in an atom can have the same set of quantum numbers and thus restricts how electrons can share an orbital (two electrons per orbital with opposite spins), but it doesn’t prescribe filling all degenerate orbitals singly first. The Aufbau Principle describes the order in which orbitals are filled by increasing energy, not the specific distribution among degenerate orbitals. Electron configuration is the overall arrangement of all electrons, not a rule by itself.