Electron configuration

An electron configuration describes how electrons are distributed among an atom’s or molecule’s energy levels, subshells, or orbitals. It records occupancy and helps explain chemical behavior and spectroscopic properties.

Atomic orbital: A quantum state describing an electron’s spatial distribution and energy in an atom. Orbitals are the specific states whose electron occupancies configurations record.
Hund's rule: A rule that electrons occupy equal-energy orbitals singly with parallel spins before pairing. It predicts occupancy within a degenerate subshell such as p or d.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Its blocks and periods reflect recurring patterns in electron configurations.
Electron configuration notation: A symbolic shorthand listing occupied atomic subshells and their electron counts. Notation compresses configurations but can hide orbital-level details.
Electron shell: A group of atomic orbitals sharing the same principal quantum number. Shells organize the broad energy levels in an atomic configuration.
Madelung rule: An empirical ordering rule that fills orbitals by increasing n plus l, with lower n first in a tie. It predicts the standard sequence of subshell filling across many elements.
Valence electron: An electron in an atom’s outer shell that can participate in chemical bonding. Outer-shell occupancy is the configuration feature most directly tied to bonding.
Orbital diagram: A drawing that represents orbitals as boxes and electrons as arrows. It displays spin pairing and degeneracy more explicitly than subshell notation.
Subshell: A set of orbitals sharing principal and angular-momentum quantum numbers. Subshell notation such as 2p specifies configuration at finer resolution than shells.
Exchange interaction: A quantum effect arising from the indistinguishability and antisymmetry of identical particles. It helps explain why parallel spins can stabilize half-filled subshells.

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