Definition · Plain-language
Electron configuration
Electron configuration is the way an atom’s electrons are arranged across its energy levels, subshells and orbitals.
The step most authors miss
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Shells, subshells and orbitals
Electrons occupy regions around the nucleus organised into energy levels, or shells, numbered from the innermost outward. Each shell is divided into subshells labelled s, p, d and f, and each subshell contains a set number of orbitals, each holding up to two electrons. The first shell has only an s subshell (2 electrons); the second adds a p subshell (up to 6 more), and so on. Configuration notation records how many electrons sit in each subshell — for example, 1s² 2s² 2p⁶ describes a neon atom with its first two shells full.
The rules that fill the orbitals
Three principles govern how electrons fill. The Aufbau principle says electrons occupy the lowest-energy orbitals available first. The Pauli exclusion principle says no orbital holds more than two electrons, and those two must have opposite spins. Hund’s rule says that within a subshell, electrons spread out singly across orbitals before pairing up. Following these rules in order reproduces the ground-state configuration of any atom and explains the structure of the periodic table, whose blocks correspond to which subshell is being filled.
Why configuration drives chemistry
An atom’s chemical character is set mainly by its valence electrons — those in the outermost shell. Atoms with full outer shells, like the noble gases, are stable and unreactive, which is why their configurations are used as shorthand (for example, writing [Ne] for neon’s core). Atoms with nearly full or nearly empty outer shells tend to react to reach a stable arrangement, by gaining, losing or sharing electrons. This is why elements in the same group of the periodic table, sharing similar outer configurations, behave alike.
Key facts
At a glance
- Definition: the arrangement of electrons across shells, subshells and orbitals
- Notation: e.g. 1s² 2s² 2p⁶ for neon
- Filling order: Aufbau — lowest energy orbitals fill first
- Per orbital: maximum 2 electrons, with opposite spins (Pauli)
- Within a subshell: electrons spread out singly first (Hund’s rule)
- Why it matters: valence electrons determine bonding and periodic-table position
Common misconceptions
What people often get wrong
Often heard: Electrons orbit the nucleus in fixed circular paths like planets.
Actually: Electrons occupy orbitals — regions of probability where an electron is likely to be found — not fixed planetary orbits. The configuration describes which orbitals are filled, not specific trajectories.
Often heard: Inner shells fill completely before any outer shell gets an electron in every case.
Actually: Mostly true for light atoms, but energy levels overlap higher up: the 4s subshell fills before 3d, for example, so the simple "inner first" picture has well-known exceptions.
Often heard: All the electrons in an atom decide how it reacts.
Actually: Chemical behaviour is driven mainly by the valence (outer-shell) electrons. The inner, core electrons are held tightly and rarely take part in bonding.
Going deeper
