An electron configuration is a compact map of exactly which orbitals an atom's electrons occupy, built one electron at a time by the Aufbau principle. This calculator fills orbitals in the standard energy order, applies the well-known transition-metal exceptions, and shows the result as a full configuration, a noble-gas shorthand, and a box-and-arrow orbital diagram.
How the Aufbau order works
Subshells fill in order of increasing energy: 1s, then 2s, then 2p, and so on through 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. Because the 4s subshell happens to sit at slightly lower energy than 3d for most atoms, it fills first even though its principal quantum number is higher — this is the single most common point of confusion when learning the pattern.
Ten transition metals break the naive pattern: Cr, Cu, Nb, Mo, Ru, Rh, Ag, Pt, and Au each shift one electron from the outer ns subshell into the adjacent (n-1)d subshell to reach a half-filled or fully-filled d subshell, which is more stable than the naive prediction. Palladium is a double exception — both would-be 5s electrons move into 4d, leaving 5s completely empty.
Reading noble-gas shorthand
Because every noble gas has a complete, stable configuration, chemists abbreviate any element's configuration by writing the bracketed symbol of the largest noble gas smaller than that element, followed only by the subshells filled after that point. Iron's full configuration 1s²2s²2p⁶3s²3p⁶4s²3d⁶ becomes [Ar]3d⁶4s² — note that the shorthand reorders the tail by increasing shell number, so 3d appears before 4s even though 4s filled first.
Ions: which electrons leave or arrive first
Cations remove electrons from the highest principal quantum number (n) first — for a transition metal, that means the outer ns electrons leave before any (n-1)d electrons, which is the reverse of the fill order. Fe²⁺ is [Ar]3d⁶ (both 4s electrons gone, 3d untouched), and Fe³⁺ is [Ar]3d⁵ (both 4s electrons plus one 3d electron gone). Anions simply continue the normal Aufbau fill from wherever the neutral atom left off, so Cl⁻ completes 3p to reach the argon configuration [Ar].
This calculator deliberately does not apply the Cr/Cu-style exceptions to ions — once an atom loses or gains electrons, its configuration is derived directly from the neutral ground state using the highest-n-first rule, which already reproduces textbook ion configurations (e.g. Cu²⁺ = [Ar]3d⁹) without any extra special-casing.
Limits and edge cases
Beyond the ten classic d-block exceptions, a handful of lanthanide and actinide elements (such as lanthanum, cerium, and several actinides) have ground-state configurations that are genuinely disputed across different chemistry references. This calculator applies plain Aufbau order to those elements so its answers stay consistent and reproducible against a standard general-chemistry textbook, rather than picking one contested convention. Valence-electron counting also becomes ambiguous for a few chemically unusual configurations, most notably palladium's empty 5s and full 4d — this calculator applies the same simplified rule (outer ns + np, plus an incomplete inner (n-1)d) uniformly, which may not match every source's treatment of these outliers.