Which principle describes that electrons fill lower energy orbitals first?

• A. Aufbau Principle
• B. Heisenberg Uncertainty Principle
• C. Atomic Radius
• D. Ionization Energy

Answer: (A)

Electrons fill the lowest available energy orbitals first. This behavior is described by the Aufbau principle, which guides building up electron configurations by placing electrons in orbitals in order of increasing energy (roughly following the n+l rule). As a result, electrons occupy 1s before 2s, then 2p, then 3s, and so on, because doing so minimizes the atom’s total energy and yields the most stable arrangement for a given number of electrons. The other concepts describe different ideas: the Heisenberg principle concerns limits on knowing position and momentum simultaneously, atomic radius relates to the size of the electron cloud, and ionization energy is the energy needed to remove an electron, not how electrons fill orbitals.