Electron Configuration Calculator

What Is the Electron Configuration Calculator?

The Electron Configuration Calculator generates the complete and noble gas shorthand electron configuration for any element from hydrogen through lead, including configurations for common ions. Select an element or enter an atomic number, optionally set an ionic charge, and the calculator applies the aufbau principle to determine how electrons fill into subshells, accounting for known exceptions like chromium and copper. According to the Chemistry LibreTexts overview of noble gas configuration, this shorthand notation exists specifically because full configurations become unwieldy for elements with many electrons.

The calculator also produces a visual orbital box diagram following Hund's rule, and reports the valence electron count, the electrons in the outermost shell responsible for chemical bonding behavior and periodic trends.

The Aufbau Principle and Filling Order

Electrons fill atomic subshells in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Notably, 4s fills before 3d despite 3 being a lower principal quantum number, because 4s is lower in energy for the neutral, ground-state atom at that point in the filling sequence. Each subshell type has a fixed maximum capacity determined by its number of orbitals: s holds 2 electrons, p holds 6, d holds 10, and f holds 14, since the Pauli exclusion principle limits each individual orbital to 2 electrons with opposite spin.

Known Exceptions: Chromium, Copper, and Beyond

A small number of elements deviate from the straightforward aufbau prediction due to the extra stability of half-filled and completely filled d-subshells. Chromium is observed experimentally as [Ar] 3d⁵ 4s¹ rather than the predicted [Ar] 3d⁴ 4s², and copper is [Ar] 3d¹⁰ 4s¹ rather than [Ar] 3d⁹ 4s². This occurs because promoting one electron from 4s to 3d, achieving a half-filled (5 electrons) or fully filled (10 electrons) d-subshell, releases more stabilization energy from favorable electron exchange interactions than it costs to make the promotion. Similar exceptions occur for niobium, molybdenum, silver, and gold among heavier transition metals, all following the same half-filled or filled subshell stability principle.

Electron Configuration for Ions

Forming an ion changes the electron count without changing the proton count. For anions, add electrons continuing the normal aufbau order. For cations, electrons are removed from the highest-energy occupied subshell first, which for transition metals means removing ns electrons before (n-1)d electrons, even though d subshells fill after s subshells in the neutral atom. As a result, work out cation configurations by first writing the neutral atom's configuration, then stripping ns electrons before touching the d subshell. Iron's neutral configuration is [Ar] 3d⁶ 4s², but Fe²⁺ is [Ar] 3d⁶ (both 4s electrons removed) and Fe³⁺ is [Ar] 3d⁵ (both 4s electrons plus one 3d electron removed), the latter benefiting from the same half-filled d-subshell stability seen in chromium.

Accuracy and Limitations

Configurations generated by this calculator follow the aufbau principle and known experimentally observed exceptions for the most commonly taught elements (chromium, copper, and related heavier transition metals), matching standard textbook references. For ions, the calculator applies the established ns-before-(n-1)d removal rule consistent with experimentally observed transition metal ionization behavior. That said, configurations for the heaviest synthetic elements (beyond roughly atomic number 103) involve relativistic effects not captured by the standard aufbau approach and require specialized quantum chemical calculation; for authoritative reference data on all 118 elements, consult the NIST Atomic Spectra Database, which publishes experimentally measured ground-state configurations.

The Most Common Electron Configuration Mistake

The single most frequent error students make is removing 3d electrons before 4s electrons when forming a transition metal cation, simply reversing the filling order. This is backwards: even though 4s fills before 3d in the neutral atom, the 3d subshell becomes lower in energy once partially filled, so 4s electrons are always removed first during ionization. Given that this distinction trips up a large share of students encountering transition metal chemistry for the first time, I would recommend memorizing the rule explicitly as "ns before (n-1)d for cation formation" rather than trying to infer it from the filling order, since the two processes follow genuinely different energy orderings.