An electron occupies the lowest-energy orbital that can receive it is described by the

• A. Aufbau principle
• B. Pauli exclusion principle
• C. Hund's rule
• D. Noble-gas configuration

Answer: (A)

Electrons fill the lowest-energy available orbital first. This is the Aufbaub principle. When adding electrons, you place each new one into the first orbital that can receive it—the orbital with the smallest energy that isn’t already full. Since an orbital holds at most two electrons with opposite spins, the next electron goes into the next available slot in the lowest-energy orbitals (1s, then 2s, then 2p, then 3s, and so on). In practice, the energy order can be nuanced due to electron–electron interactions, which is why sometimes 4s fills before 3d, but the core idea is building the configuration from the bottom up in energy. Pauli exclusion sets the two-electron limit per orbital, Hund’s rule describes how electrons are arranged among degenerate orbitals, and noble-gas configurations are just shorthand for filled shells.