Which rule states that electrons fill all empty orbitals in a subshell with parallel spins before pairing?

• A. Hund's Rule
• B. Aufbau Principle
• C. Pauli Exclusion Principle
• D. Heisenberg Uncertainty Principle

Answer: (A)

Hund's Rule states that within a subshell with degenerate orbitals, electrons occupy empty orbitals singly with parallel spins before any pairing occurs. This maximizes the total spin and lowers the energy through exchange interactions, making the arrangement more stable. For example, in a p subshell with three orbitals, three electrons will first fill each orbital singly with the same spin before any pairing happens, so you’d have one electron in each of the three p orbitals. Only after those are occupied would electrons start to pair in the orbitals. This is exactly why configurations like 2p^3 place one electron in each of the three 2p orbitals with the same spin.

The other rules describe different ideas: Aufbau explains the order of filling by energy, Pauli Exclusion says no two electrons can have the same set of quantum numbers, and the Heisenberg Uncertainty Principle concerns the limits of simultaneously knowing position and momentum. None of these specifically describe filling empty degenerate orbitals first with parallel spins—that distinction belongs to Hund's Rule.