Electron Configuration
Definition and meaning of Electron Configuration in chemistry.
Electron configuration is the arrangement of electrons in an atom's or ion's orbitals, written in notation such as 1s² 2s² 2p⁶ that specifies the principal energy level, subshell type, and number of electrons occupying each subshell.
In more detail
Electrons fill orbitals according to three rules: the Aufbau principle (lowest-energy orbitals fill first), the Pauli exclusion principle (each orbital holds at most two electrons, with opposite spins), and Hund's rule (electrons occupy degenerate orbitals singly before pairing up). The resulting configuration determines an element's valence electron count, which governs its chemical reactivity, bonding behavior, and placement in the periodic table. A few transition metals, such as chromium and copper, deviate from the predicted order because a half-filled or completely filled d subshell provides extra stability.
Key facts
| Field | General Chemistry |
|---|---|
| Governing rules | Aufbau principle, Pauli exclusion principle, Hund's rule |
| Common exception | Chromium: [Ar] 3d⁵ 4s¹ (not [Ar] 3d⁴ 4s²) |
| Shorthand form | Noble-gas core notation, e.g. [Ne] 3s¹ |
Sodium (Z = 11) has the electron configuration 1s² 2s² 2p⁶ 3s¹, commonly abbreviated as [Ne] 3s¹, showing its single valence electron in the 3s orbital.
Frequently asked questions
Why do chromium and copper not follow the expected filling order?
Chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹) gain extra stability from a half-filled or fully filled 3d subshell, so one electron shifts from the 4s orbital into 3d compared to the configuration predicted by strict Aufbau order.
How does electron configuration relate to the periodic table?
An element's position in the periodic table corresponds directly to its highest-energy subshell being filled; elements in the same group share the same valence-shell configuration, which explains their similar chemical properties.