Orbital Filling
Definition and meaning of Orbital Filling in chemistry.
Orbital filling is the process by which electrons occupy an atom's atomic orbitals in order of increasing energy, following the Aufbau principle, while obeying the Pauli exclusion principle and Hund's rule.
In more detail
Orbitals are filled in the order predicted by the Madelung (n+l) rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on, with lower-energy orbitals filled first. The Pauli exclusion principle limits each orbital to two electrons of opposite spin, while Hund's rule requires that degenerate orbitals (like the three 2p orbitals) each receive one electron with parallel spin before any pairing occurs, since this minimizes electron-electron repulsion. A few elements, such as chromium and copper, deviate from the simple n+l order because a half-filled or fully filled d subshell provides extra stability.
Key facts
| Field | General Chemistry |
|---|---|
| Governing rule | Aufbau principle (Madelung n+l order) |
| Companion rules | Pauli exclusion principle; Hund's rule |
| Common exceptions | Cr: [Ar]4s¹3d⁵; Cu: [Ar]4s¹3d¹⁰ |
Nitrogen (Z = 7) has the ground-state configuration 1s² 2s² 2p³. The three 2p electrons each occupy a separate 2p orbital with parallel spins rather than pairing up, in accordance with Hund's rule.
Frequently asked questions
Why doesn't chromium follow the expected 4s²3d⁴ filling order?
Chromium adopts [Ar]4s¹3d⁵ instead of [Ar]4s²3d⁴ because a half-filled 3d subshell, with one electron in each of the five d orbitals, gains extra stabilization from exchange energy that outweighs the usual 4s-before-3d energy preference.
What is the difference between the Aufbau principle and Hund's rule?
The Aufbau principle determines the sequence in which subshells of increasing energy are filled across the whole atom, while Hund's rule governs how electrons are distributed among the degenerate orbitals within a single subshell, favoring unpaired, parallel-spin electrons.