Which principle states that electrons fill the lowest energy level first?

• A. Aufbau Principle
• B. Pauli Exclusion Principle
• C. Hund's Rule
• D. Moseley's Law

Answer: (A)

Electrons arrange themselves to minimize the atom’s energy, so they fill the lowest-energy orbitals first. This behavior is described by the Aufbau principle, the rule used to build electron configurations step by step. In practice, you predict which orbitals get electrons by ordering their energies and placing electrons into the lowest available state while obeying the Pauli exclusion principle (no two electrons share the same set of quantum numbers) and Hund’s rule (unpaired electrons occupy separate degenerate orbitals before pairing). Moseley’s Law, on the other hand, deals with X-ray frequencies and atomic number, not how electrons fill orbitals. So the statement about filling the lowest energy level first is explained by the Aufbau principle.