Which principle guides the order in which orbitals are filled to achieve minimum energy?

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

Answer: (A)

The order in which orbitals are filled to achieve minimum energy is guided by the Aufbau principle. This rule says electrons occupy the lowest-energy orbitals available before moving to higher-energy ones. The typical filling sequence follows increasing orbital energy (roughly 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on), reflecting how energy rises as you add electrons into more distant or more complex subshells. Because orbitals differ in energy due to nuclear attraction, shielding, and electron–electron repulsion, placing electrons in the lowest-energy states first lowers the atom’s total energy. Within a given subshell, the Pauli exclusion principle prevents more than two electrons from sharing the same orbital with the same spin, and Hund’s rule promotes keeping electrons unpaired in separate degenerate orbitals before pairing, which increases stability. Noble gases aren’t a rule for filling order; they’re the result of having completely filled electron shells.