Aufbau principle
Summary
The Aufbau principle says that in an atom's ground state, electrons generally fill orbitals from lowest to highest energy (1s, 2s, 2p, 3s, 3p, 4s, 3d...). The most testable point is that 4s fills before 3d. A few elements, such as Cr and Cu, are exceptions.
Detail
Meaning: The Aufbau principle (German for "building up") says that in the ground state of an atom, electrons generally fill the lowest-energy available orbitals first. The usual order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d... It follows increasing n + l, and when two orbitals tie, the one with lower n fills first. It works together with the Pauli exclusion principle (two electrons per orbital, opposite spins) and Hund's rule (degenerate orbitals fill singly with parallel spins before pairing).
On the MCAT: You may be asked to write a ground-state configuration, count unpaired electrons, or decide whether an atom is paramagnetic or diamagnetic. Example: Fe is [Ar] 4s² 3d⁶, with four unpaired electrons, so it is paramagnetic. You may also be shown a configuration and asked whether it is a ground state or an excited state. Cr ([Ar] 4s¹ 3d⁵) and Cu ([Ar] 4s¹ 3d¹⁰) are the standard exceptions, where a half-filled or completely filled d subshell is favored.
Common confusion: Students assume that transition metal cations form by undoing the fill order, so 3d electrons would leave first. Electrons are removed from the highest-n shell first, so 4s electrons go before 3d. Fe²⁺ is [Ar] 3d⁶, not [Ar] 4s² 3d⁴.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.