# Electron Configuration & Orbital Diagram

> Electron configuration of any element or ion: orbital boxes, unpaired electrons, exceptions. Type an element or an ion — Fe, Fe3+, Cu+, O2-, 26 or “iron” — and get its ground-state electron configuration in full and in noble-gas shorthand, the orbital box diagram drawn with Hund’s rule, the number of unpaired electrons (paramagnetic or diamagnetic), the electrons per shell and the quantum numbers of the last electron. The 20 elements whose measured ground state breaks the Aufbau rule, such as chromium, copper, palladium and gold, use the NIST configuration and say what the rule would have predicted. Part of the Proofline math notebook.

Part of usefulweb.app (https://usefulweb.app/). Part of [Proofline](https://proofline.usefulweb.app/) (ownerApp: proofline). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://prooflineelectronconfig.usefulweb.app/ (interactive tool at https://prooflineelectronconfig.usefulweb.app/tool)
- JSON API: POST https://prooflineelectronconfig.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://prooflineelectronconfig.usefulweb.app/api/v1/tool)
- OpenAPI: https://prooflineelectronconfig.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://prooflineelectronconfig.usefulweb.app/mcp - tool `proofline_electron_config`

## Inputs
- species (text, required): Element or ion - A symbol, name or atomic number, optionally with a charge: Fe3+, Fe^3+, Fe+3, O2-, Cl-.
- diagram (select): Orbital boxes

## Method
- Subshells fill in Madelung order (increasing n + l, then n): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. For Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr the measured ground state of the NIST Atomic Spectra Database replaces the prediction.
- Cations lose electrons from the subshell with the highest n first (for equal n the highest l), so transition metals lose their s electrons before d electrons (Fe²⁺ = [Ar] 3d⁶). Anions add electrons in Madelung order.
- Orbital boxes follow Hund’s rule: within a subshell each orbital (mℓ = −l … +l, left to right) receives one spin-up electron before any is paired. Unpaired electrons = Σ min(e, 2(2l + 1) − e) over the subshells.
- Verified against the PubChem/NIST configurations of all 118 elements (the only difference is lawrencium, where NIST gives 7p¹ and PubChem still 6d¹).

## Limits
- Configurations are for free atoms and ions in the gas phase; in compounds ligands can pair electrons (low-spin complexes). Configurations beyond meitnerium are theoretical predictions.