What is the electron configuration?
Pick an element. The calculator writes its ground-state electron configuration, in full and with a noble gas core, and counts its electrons per shell, outer-shell electrons and unpaired electrons.
- Electron configuration
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
The electron configuration of Fe (Iron), Z = 26 is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s².
- Noble gas notation
- [Ar] 3d⁶ 4s²
- Element
- Fe (Iron), Z = 26
- Electrons per shell
- 2, 8, 14, 2
- Outer-shell electrons
- 2
- Unpaired electrons
- 4
- Source of this configuration
- NIST ground state, the same as the filling order.
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². The electron configuration of Fe (Iron), Z = 26 is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s².
How to calculate
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.
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.
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.
- 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
Each example is checked against the calculator on every build.
- Element 8 O (Oxygen) gives Electron configuration 1s² 2s² 2p⁴, Noble gas notation [He] 2s² 2p⁴, Electrons per shell 2, 6, Outer-shell electrons 6, Unpaired electrons 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
- Element 26 Fe (Iron) gives Noble gas notation [Ar] 3d⁶ 4s², Electrons per shell 2, 8, 14, 2, Unpaired electrons 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
- Element 24 Cr (Chromium) gives Noble gas notation [Ar] 3d⁵ 4s¹, Unpaired electrons 6, Source of this configuration 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
- Element 29 Cu (Copper) gives Electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹, Noble gas notation [Ar] 3d¹⁰ 4s¹, Unpaired electrons 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
- Element 103 Lr (Lawrencium) gives Noble gas notation [Rn] 5f¹⁴ 7s² 7p¹, Outer-shell electrons 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
- Element 118 Og (Oganesson) gives Noble gas notation [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶, Electrons per shell 2, 8, 18, 32, 32, 18, 8, Unpaired electrons 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
How it works
Filling order (aufbau, Madelung rule). Subshells fill in order of n + l, and of two with the same n + l, the one with lower n first: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. Here l = 0, 1, 2, 3 for s, p, d, f, and the subshells hold 2, 6, 10 and 14 electrons. A neutral atom of atomic number Z has Z electrons.
NIST exceptions. For these 20 elements NIST’s ground state differs from the filling order, and the page uses NIST’s:
| Z | Element | Ground state |
|---|---|---|
| 24 | Cr | [Ar] 3d⁵ 4s¹ |
| 29 | Cu | [Ar] 3d¹⁰ 4s¹ |
| 41 | Nb | [Kr] 4d⁴ 5s¹ |
| 42 | Mo | [Kr] 4d⁵ 5s¹ |
| 44 | Ru | [Kr] 4d⁷ 5s¹ |
| 45 | Rh | [Kr] 4d⁸ 5s¹ |
| 46 | Pd | [Kr] 4d¹⁰ |
| 47 | Ag | [Kr] 4d¹⁰ 5s¹ |
| 57 | La | [Xe] 5d¹ 6s² |
| 58 | Ce | [Xe] 4f¹ 5d¹ 6s² |
| 64 | Gd | [Xe] 4f⁷ 5d¹ 6s² |
| 78 | Pt | [Xe] 4f¹⁴ 5d⁹ 6s¹ |
| 79 | Au | [Xe] 4f¹⁴ 5d¹⁰ 6s¹ |
| 89 | Ac | [Rn] 6d¹ 7s² |
| 90 | Th | [Rn] 6d² 7s² |
| 91 | Pa | [Rn] 5f² 6d¹ 7s² |
| 92 | U | [Rn] 5f³ 6d¹ 7s² |
| 93 | Np | [Rn] 5f⁴ 6d¹ 7s² |
| 96 | Cm | [Rn] 5f⁷ 6d¹ 7s² |
| 103 | Lr | [Rn] 5f¹⁴ 7s² 7p¹ |
Every other element up to Z = 108 has the filling-order configuration in NIST’s table. Z = 109 to 118 are not in it; the page gives the filling order and says it is a prediction.
How the result is written.
- Electron configuration: every occupied subshell with its electrons as a superscript, in shell order (by n, then s, p, d, f), separated by spaces: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s².
- Noble gas notation: the largest noble gas with fewer electrons (He 2, Ne 10, Ar 18, Kr 36, Xe 54, Rn 86) in brackets, then the subshells whose electrons differ from that core, in shell order. Hydrogen and helium have no core.
- Electrons per shell: the electrons with n = 1, 2, 3, … in order, comma separated.
- Outer-shell electrons: the electrons in the highest occupied shell n (for iron, the two 4s electrons).
- Unpaired electrons (Hund’s rule): a subshell with k orbitals (1, 3, 5, 7 for s, p, d, f) and e electrons has e unpaired when e ≤ k, and 2k − e otherwise; the page adds them up.
Neutral atoms only; ions are not covered.
Worked examples by hand
Oxygen (Z = 8). 1s² 2s² 2p⁴ = 8 electrons: 1s² 2s² 2p⁴, or [He] 2s² 2p⁴. Shells 2, 6; outer shell 6. 2p⁴ over 3 orbitals: 2 × 3 − 4 = 2 unpaired.
Iron (Z = 26). After [Ar] (18): 4s² then 3d⁶. Shell order: [Ar] 3d⁶ 4s². Shells 2, 8, 14, 2. 3d⁶: 2 × 5 − 6 = 4 unpaired.
Chromium (Z = 24). NIST: [Ar] 3d⁵ 4s¹. 5 + 1 = 6 unpaired.
Copper (Z = 29). NIST: [Ar] 3d¹⁰ 4s¹, in full 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹. 1 unpaired.
Lawrencium (Z = 103). NIST: [Rn] 5f¹⁴ 7s² 7p¹; the outer shell (n = 7) holds 3 electrons.
Oganesson (Z = 118, predicted). After [Rn] (86): 7s² 5f¹⁴ 6d¹⁰ 7p⁶ = 32 electrons: [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶. Shells 2, 8, 18, 32, 32, 18, 8; every subshell is full, so 0 unpaired.
Other questions people ask
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.