Which principle states that electrons fill the lowest-energy orbitals first before occupying higher-energy orbitals?

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

Answer: (C)

The main idea being tested is that electrons fill the lowest-energy orbitals first, which is described by the Aufbau principle. This rule explains why electron configurations are built from the bottom up, starting with the 1s orbital and moving to higher-energy subshells in that characteristic order, because occupying the lowest available energy states minimizes the atom’s overall energy. Two electrons can share an orbital only if their spins are opposite (Pauli exclusion), and when filling orbitals of the same energy level, electrons first occupy separate degenerate orbitals with parallel spins before pairing (Hund’s rule). It’s helpful to keep in mind that the Heisenberg principle deals with limits on knowing position and momentum, not how electrons fill orbitals, so it doesn’t describe this pattern. For example, carbon ends up as 1s^2 2s^2 2p^2, and neon as 1s^2 2s^2 2p^6, reflecting the stepwise, energy-minimizing filling dictated by the Aufbau principle.