Hund's rule governs electron placement in degenerate orbitals. Which statement correctly describes it?

• A. When filling degenerate orbitals, electrons occupy empty orbitals singly before pairing to maximize unpaired spins.
• B. When filling, electrons always pair in the first available orbital.
• C. Two electrons in the same orbital must have same spin.
• D. Subshell energy is determined solely by n.

Answer: (A)

Hund's rule says that within a subshell with degenerate orbitals, electrons will first occupy empty orbitals one each with parallel spins before any pairing occurs. This arrangement maximizes the number of unpaired electrons and the total spin, which lowers the energy through exchange interactions and leads to a more stable configuration. That’s why the statement describing filling degenerate orbitals singly before pairing to maximize unpaired spins is the correct description.

Think of why the other ideas don’t fit: electrons don’t always pair in the first available orbital—they delay pairing so that each degenerate orbital has one electron before any pairing happens. Two electrons in the same orbital must have opposite spins due to the Pauli exclusion principle. And subshell energy isn’t determined solely by the principal quantum number; both n and the orbital angular momentum l (and electron-electron interactions) influence the energy ordering.