Electron Configuration Calculator

About the Electron Configuration Calculator

Type any element — by symbol, name or atomic number — or an ion such as Fe³⁺, Cu⁺ or O²⁻, and get its full and noble-gas (shorthand) electron configuration, written in filling order (4s² 3d⁶) or shell order (3d⁶ 4s²). Neutral atoms use the measured ground states from the NIST Atomic Spectra Database, so the exceptions to the aufbau rule — chromium, copper, palladium, gadolinium and the others — come out right, with the configuration the simple rule would have predicted shown alongside.

Each result includes an orbital box diagram filled by Hund’s rule, the number of unpaired electrons (paramagnetic or diamagnetic), valence and core electrons, a shell (Bohr) diagram, the block, period and group, and the four quantum numbers of the last electron. You can also go the other way: type a configuration such as 1s² 2s² 2p⁶ 3s¹ or [Kr] 4d⁵ and find out which atom or ion it belongs to — or whether it is an excited state.

Common uses

• Chemistry homework: configurations, orbital diagrams and quantum numbers for any element or ion.
• Checking which elements break the aufbau rule, and what they would be without the exception.
• Working out whether an ion is paramagnetic (has unpaired electrons) — useful for transition-metal chemistry.
• Recognising an element or ion from its configuration, including isoelectronic series such as N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺.

Filling order, Pauli and Hund

Electrons occupy the lowest-energy subshells first (the aufbau principle). The order follows the n + l (Madelung) rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Each orbital holds two electrons of opposite spin (the Pauli exclusion principle), so s, p, d and f subshells hold 2, 6, 10 and 14 electrons. Within a subshell, electrons spread out one per orbital with parallel spins before any pair up (Hund’s rule), which is how the box diagram is filled (OpenStax Chemistry 2e §6.4).

The exceptions

Twenty elements have measured ground states that differ from the simple filling order: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. In Cr and Cu (and Mo, Ag and Au below them) a half-filled or completely filled d subshell is preferred: Cr is [Ar] 3d⁵ 4s¹, not 3d⁴ 4s². In the others the (n−1)d and ns, or the 4f/5f and 5d/6d, energies are so close that small effects decide, and there is no simple rule to predict them (OpenStax §6.4). The calculator uses the NIST Atomic Spectra Database ground states for elements 1–108 and the predicted ones for 109–118.

Ions

For a cation, remove electrons from the outermost shell first: for a d- or f-block metal the ns electrons go before the (n−1)d or (n−2)f ones, even though ns filled first — Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴ 4s². In the p block the np electrons go before ns (Pb²⁺ is [Xe] 4f¹⁴ 5d¹⁰ 6s²). Anions add electrons in the normal filling order (O²⁻ is [Ne]). This is the rule chemists use for ions in compounds. For a few free gas-phase ions spectroscopy finds a different ground state — V⁺ is [Ar] 3d⁴, Co⁺ [Ar] 3d⁸, Ni⁺ [Ar] 3d⁹, Lu⁺ [Xe] 4f¹⁴ 6s² — and the calculator says so for the singly charged d-block ions and Lu²⁺.

Valence electrons and quantum numbers

• Valence electrons (OpenStax §6.4): for main-group elements, the electrons in the outermost shell (gallium has 3; its filled 3d¹⁰ counts as core); for transition metals, the ns and (n−1)d electrons; for inner transition metals, ns, (n−1)d and (n−2)f.
• Quantum numbers: n is the shell, l the subshell (0, 1, 2, 3 for s, p, d, f), m_l the orbital (−l … +l) and m_s the spin (+½ or −½). For the last electron the calculator fills the boxes from m_l = −l upward with spin up first; some textbooks start at +l, which you can choose. Within a degenerate subshell, any m_l is physically equivalent.

Sources

• A. Kramida, Yu. Ralchenko, J. Reader and the NIST ASD Team, NIST Atomic Spectra Database: ground-state configurations and levels of the neutral atoms (Z = 1–108) and of the ions Sc II–Zn II, Y II–Cd II, La II, Lu II, Lu III and Hf II–Hg II. Configurations for Z = 109–118 are predictions, as listed by PubChem.
• OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms: filling order, Hund’s rule, exceptions, ions, valence electrons and Examples 6.10–6.11.

Limitations

• Configurations of elements 109–118 are theoretical predictions; their chemistry is barely known.
• Ion configurations follow the textbook removal rule; ground states of free gas-phase ions, and of lanthanide and actinide ions other than 3+, can differ.
• A configuration describes which subshells are occupied, not the full quantum state: the atom’s term symbol (shown from NIST for neutral atoms) carries the rest.
• Identifying a configuration lists only common, plausible ions (anions of non-metals reaching a noble-gas configuration, metal cations up to +4).

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