According to Hund's Rule, how do electrons fill degenerate orbitals?

• A. In pairs
• B. Singly before pairing
• C. All electrons must be paired
• D. They fill lowest energy first without considering spin

Answer: (B)

Hund’s Rule states that when electrons occupy orbitals of the same energy (degenerate orbitals), they first fill those orbitals singly with spins aligned before any pairing occurs. This arrangement maximizes the total spin and lowers energy through exchange interactions, making it more stable. So in a p subshell with three degenerate orbitals, the first three electrons will occupy each orbital once with the same spin, and only after that do electrons begin to pair in orbitals. The description that electrons fill degenerate orbitals singly before pairing best captures this behavior. The other ideas—pairing in the same orbital right away, everyone ever being paired, or filling by energy ignoring spin—do not reflect how degeneracy and spin interactions guide electron placement.