# What is the electron configuration?

Gives the ground-state electron configuration of any element from hydrogen to oganesson, in full and noble-gas form, with electrons per shell, outer-shell electrons and unpaired electrons, using NIST’s ground states.

- Page: https://www.acalculator.org/chemistry/electron-configuration-calculator
- JSON spec: https://www.acalculator.org/chemistry/electron-configuration-calculator.json
- Version: 972e5eef7cd9

## Default answer

Example with the default inputs (Element 26 Fe (Iron)): The electron configuration of Fe (Iron), Z = 26 is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s².

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| z | Element | The element, by atomic number, symbol and name. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| full | Electron configuration | Every occupied subshell with its electrons, in shell order. |
| noble | Noble gas notation | The configuration after the previous noble gas’s core, written in brackets. |
| element | Element | The element’s symbol and name. |
| shells | Electrons per shell | The electrons in shells n = 1, 2, 3, and so on. |
| outer | Outer-shell electrons | The electrons in the highest occupied shell (largest n). |
| unpaired | Unpaired electrons | Electrons alone in an orbital, by Hund’s rule, in this ground configuration. |
| basis | Source of this configuration | Whether the configuration is the plain filling order, a NIST exception, or a prediction beyond NIST’s table. |

## Method

Fill subshells in order of n + l (lower n first on a tie): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p, with s 2, p 6, d 10 and f 14 electrons; use NIST’s ground state where it differs.

## Assumptions

- Neutral atoms in their ground state; ions are not covered.
- Subshells are written in shell order (3d before 4s), as NIST writes them.
- Elements 109 to 118 are not in NIST’s table; their configurations follow the filling order and are predictions.

## Worked examples

1. z = 8 gives full = 1s² 2s² 2p⁴, noble = [He] 2s² 2p⁴, shells = 2, 6, outer = 6, unpaired = 2. Source: OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations) (aufbau order 1s 2s 2p 3s 3p 4s 3d 4p …, Hund’s rule; Cr is [Ar]3d⁵4s¹ and Cu [Ar]3d¹⁰4s¹). https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations, retrieved 2026-10-02: oxygen is 1s²2s²2p⁴ with two unpaired electrons.
2. z = 26 gives noble = [Ar] 3d⁶ 4s², shells = 2, 8, 14, 2, unpaired = 4. Source: NIST Standard Reference Database 111, Ground Levels and Ionization Energies for the Neutral Atoms (ground configurations, Z = 1 to 108). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms, retrieved 2026-10-02: Fe [Ar]3d⁶4s².
3. z = 24 gives noble = [Ar] 3d⁵ 4s¹, unpaired = 6, basis = NIST ground state, an exception to the filling order.. Source: NIST Standard Reference Database 111, Ground Levels and Ionization Energies for the Neutral Atoms (ground configurations, Z = 1 to 108). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms, retrieved 2026-10-02: Cr [Ar]3d⁵4s¹; OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations) (aufbau order 1s 2s 2p 3s 3p 4s 3d 4p …, Hund’s rule; Cr is [Ar]3d⁵4s¹ and Cu [Ar]3d¹⁰4s¹). https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations, retrieved 2026-10-02.
4. z = 29 gives full = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹, noble = [Ar] 3d¹⁰ 4s¹, unpaired = 1. Source: NIST Standard Reference Database 111, Ground Levels and Ionization Energies for the Neutral Atoms (ground configurations, Z = 1 to 108). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms, retrieved 2026-10-02: Cu [Ar]3d¹⁰4s¹; OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations) (aufbau order 1s 2s 2p 3s 3p 4s 3d 4p …, Hund’s rule; Cr is [Ar]3d⁵4s¹ and Cu [Ar]3d¹⁰4s¹). https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations, retrieved 2026-10-02.
5. z = 103 gives noble = [Rn] 5f¹⁴ 7s² 7p¹, outer = 3. Source: NIST Standard Reference Database 111, Ground Levels and Ionization Energies for the Neutral Atoms (ground configurations, Z = 1 to 108). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms, retrieved 2026-10-02: Lr [Rn]5f¹⁴7s²7p¹.
6. z = 118 gives noble = [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶, shells = 2, 8, 18, 32, 32, 18, 8, unpaired = 0. Source: OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations) (aufbau order 1s 2s 2p 3s 3p 4s 3d 4p …, Hund’s rule; Cr is [Ar]3d⁵4s¹ and Cu [Ar]3d¹⁰4s¹). https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations, retrieved 2026-10-02.

## FAQ

### How do I write an electron configuration?

Fill subshells in the aufbau order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p until all the electrons are placed. An s subshell holds 2 electrons, p 6, d 10 and f 14. Oxygen’s 8 electrons give 1s² 2s² 2p⁴.

### What is noble gas notation?

A short form that writes the electrons of the previous noble gas as its symbol in brackets. Iron, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², is [Ar] 3d⁶ 4s².

### Why are chromium and copper exceptions?

Their measured ground states put one 4s electron into 3d: chromium is [Ar] 3d⁵ 4s¹, not 3d⁴ 4s², and copper is [Ar] 3d¹⁰ 4s¹, not 3d⁹ 4s². Half-filled and filled d subshells are especially stable. NIST lists 20 such exceptions up to element 108, and this calculator uses them all.

### Why is 3d written before 4s?

The page writes subshells by shell, as NIST does, so 3d comes before 4s even though 4s fills first. Many textbooks write them in filling order instead ([Ar] 4s² 3d⁶); both describe the same configuration.

### How many unpaired electrons does an atom have?

By Hund’s rule, electrons spread out one per orbital before pairing. A p subshell has 3 orbitals, so oxygen’s 2p⁴ has 2 unpaired electrons; iron’s 3d⁶ (5 orbitals) has 4.

### What about elements beyond 108?

NIST’s table of measured and calculated ground states ends at hassium (108). For meitnerium (109) to oganesson (118) the page shows the configuration from the filling order and marks it as a prediction.

## Sources

- OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations): the aufbau order, s 2, p 6, d 10 and f 14 electrons, Hund’s rule, oxygen 1s²2s²2p⁴ with two unpaired electrons, and the chromium ([Ar]3d⁵4s¹) and copper ([Ar]3d¹⁰4s¹) exceptions. https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations (retrieved 2026-10-02)
- NIST Standard Reference Database 111, Ground Levels and Ionization Energies for the Neutral Atoms (ground-state configurations for Z = 1 to 108, in its HTML table in the NIST Atomic Spectra Database). https://www.nist.gov/pml/ground-levels-and-ionization-energies-neutral-atoms (retrieved 2026-10-02)
- CIAAW (IUPAC Commission on Isotopic Abundances and Atomic Weights), Standard atomic weights: element names and symbols for Z = 1 to 118. https://www.ciaaw.org/atomic-weights.htm (retrieved 2026-10-02)