Aufbau Principle
The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy, beginning with the lowest-energy orbital available, before occupying higher-energy orbitals. The filling order follows the (n + l) rule: orbitals with lower (n + l) values are filled first; when two orbitals have the same (n + l) value, the one with the lower n is filled first. This principle, together with the Pauli exclusion principle and Hund's rule, allows chemists to predict the ground-state electron configuration of any element.
Worked Example
Problem
Write the ground-state electron configuration of iron (Fe, Z = 26) using the Aufbau principle.
Solution
Step 1 — Filling order by increasing energy: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p … Step 2 — Distribute 26 electrons: 1s: 2 electrons → 1s² 2s: 2 electrons → 2s² 2p: 6 electrons → 2p⁶ 3s: 2 electrons → 3s² 3p: 6 electrons → 3p⁶ 4s: 2 electrons → 4s² 3d: remaining 6 electrons → 3d⁶ Step 3 — Verify: 2+2+6+2+6+2+6 = 26 ✓
Answer
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ (or [Ar] 4s² 3d⁶)
Orbital Filling Order by Aufbau Principle
| Fill Order | Orbital | n + l Value | Capacity | Cumulative Electrons |
|---|---|---|---|---|
| 1 | 1s | 1 | 2 | 2 |
| 2 | 2s | 2 | 2 | 4 |
| 3 | 2p | 3 | 6 | 10 |
| 4 | 3s | 3 | 2 | 12 |
| 5 | 3p | 4 | 6 | 18 |
| 6 | 4s | 4 | 2 | 20 |
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Related Terms
Atomic Orbital
An atomic orbital is a mathematical function describing the wave-like behaviour and probable location of an electron in an atom, representing a region of space where there is a high probability (typically 90–95%) of finding the electron. Orbitals are characterised by three quantum numbers (n, l, mₗ) and have distinct shapes: s-orbitals are spherical, p-orbitals are dumbbell-shaped, and d- and f-orbitals have more complex geometries. Atomic orbitals form the basis for understanding electron configurations, chemical bonding, and molecular orbital theory.
Hund's Rule
Hund's rule of maximum multiplicity states that when electrons occupy degenerate (equal-energy) orbitals, one electron fills each orbital with parallel spins before any orbital receives a second electron. This arrangement minimises electron–electron repulsion because electrons in separate orbitals are farther apart, resulting in the lowest-energy (most stable) ground state. The rule also means that atoms with partially filled degenerate orbitals possess unpaired electrons, making them paramagnetic.
Valence Electron
Valence electrons are the electrons located in the outermost energy shell (valence shell) of an atom, and they are the primary participants in chemical bonding. The number of valence electrons determines an element's chemical reactivity and the types of bonds it can form. In main-group elements, the number of valence electrons equals the group number (using the 1–18 IUPAC numbering), making them a key predictor of periodic trends.
From German "Aufbau" meaning "building up" or "construction." The principle was articulated by Niels Bohr and Wolfgang Pauli in the early 1920s as part of the quantum mechanical description of electron configurations.