Orbitals of equal energy are each singly occupied before any orbital is occupied by a second electron, and all electrons in singly occupied orbitals must have the same spin is described by

• A. Hund's rule
• B. Aufbau principle
• C. Pauli exclusion principle
• D. Noble gases

Answer: (A)

Hund’s rule is the guideline here: when filling orbitals of equal energy, electrons first occupy each degenerate orbital singly with parallel spins before any orbital gets a second electron. This arrangement maximizes the total spin and lowers the system’s energy through exchange interactions, making it the most stable configuration for those electrons. The statement directly describes this setup where all singly occupied orbitals share the same spin. The Aufbau principle deals with the order in which orbitals are filled by increasing energy, not the preference for single occupancy in degenerate orbitals. The Pauli exclusion principle says no two electrons in the same orbital can have the same set of quantum numbers, which governs pairing in an orbital but not the parallel-spin filling of multiple orbitals. Noble gases are just a group of elements, not a rule governing electron occupancy.