Which principle describes the order in which electrons fill atomic orbitals to achieve the lowest energy configuration?

• A. Valence electrons
• B. Ionization energy
• C. Aufbau principle
• D. Electronegativity

Answer: (C)

The Aufbau principle describes how electrons fill atomic orbitals to reach the lowest energy configuration. Electrons occupy orbitals in order of increasing energy, following the n + l (Madelung) rule, so the typical filling sequence is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. This ordering explains why electrons occupy the lowest available energy levels first and how the ground-state electron configuration is built, even though small exceptions exist. The other terms refer to properties of atoms—valence electrons are about bonding, ionization energy is the energy to remove an electron, and electronegativity is how strongly an atom attracts electrons in a bond—rather than the rule that governs how orbitals are filled.