Electron Configuration Calculator
This electron configuration calculator writes the configuration of any element from hydrogen to hassium (Z = 1–108) and of its ions. Enter the atomic number, for iron 26, and a charge if needed; it shows the Aufbau order, the shell order, the noble-gas short form and the exceptions listed by NIST.
Example: Atomic number Z 26 · Charge 0 → Electron configuration: Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. Source: OpenStax Chemistry 2e, 6.4 Electronic Structure of Atoms (Electron Configurations): Aufbau principle, filling order 1s 2s 2p 3s 3p 4s 3d …, Hund’s rule, exceptions Cr/Cu/Nb, noble-gas core notation, ions (Example 6.11: Na⁺, P³⁻, Al²⁺, Fe²⁺, Sm³⁺) (read 10.10.2026). Updated: .
How it is calculated
The electron configuration lists how many electrons sit in each subshell: s holds at most 2, p 6, d 10 and f 14 electrons, filled in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d … This is the Aufbau principle as described in OpenStax Chemistry: each added electron occupies the subshell of lowest energy available. The calculator applies it and compares the result with the ground states published by NIST.
Electron configurations of common elements
| Element | Z | Configuration |
|---|---|---|
| Hydrogen (H) | 1 | 1s¹ |
| Carbon (C) | 6 | 1s² 2s² 2p² |
| Oxygen (O) | 8 | 1s² 2s² 2p⁴ |
| Sodium (Na) | 11 | 1s² 2s² 2p⁶ 3s¹ |
| Chlorine (Cl) | 17 | 1s² 2s² 2p⁶ 3s² 3p⁵ |
| Calcium (Ca) | 20 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² |
| Iron (Fe) | 26 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ |
| Bromine (Br) | 35 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵ |
The short form replaces the inner electrons with the preceding noble gas: sodium is [Ne] 3s¹, iron [Ar] 3d⁶ 4s². The calculator also shows the shell order (all n = 3 subshells before 4s), which is how NIST writes it.
Exceptions: chromium, copper and 18 more
For 20 of the elements 1–108 the ground state listed by NIST differs from the Aufbau prediction. Chromium is [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s², copper [Ar] 3d¹⁰ 4s¹ instead of [Ar] 3d⁹ 4s². OpenStax explains these two with the stability of half-filled and filled subshells and notes that there is no simple method to predict all exceptions. The calculator therefore uses the NIST values for Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr and shows the Aufbau prediction next to them.
Ions
| Ion | Z | Electrons | Configuration |
|---|---|---|---|
| Na⁺ | 11 | 10 | 1s² 2s² 2p⁶ |
| P³⁻ | 15 | 18 | 1s² 2s² 2p⁶ 3s² 3p⁶ |
| Al²⁺ | 13 | 11 | 1s² 2s² 2p⁶ 3s¹ |
| Fe²⁺ | 26 | 24 | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ |
| Sm³⁺ | 62 | 59 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 4f⁵ |
- Main-group cations lose the electrons that were added last; transition metals lose the highest s electrons first (Fe²⁺ loses 4s², not 3d).
- Anions add electrons in Aufbau order (P³⁻ fills 3p to 3p⁶).
- The table shows the five ions worked out in OpenStax Example 6.11.
The element data come from the periodic table; the mass of a compound follows from the molar mass calculator.
Frequently asked questions
How do you write an electron configuration?
Fill the subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p … with at most 2 (s), 6 (p), 10 (d) or 14 (f) electrons until the number of electrons is reached. Iron (26 electrons) ends with 4s² 3d⁶.
What is the electron configuration of iron?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, short [Ar] 3d⁶ 4s². Fe²⁺ loses the two 4s electrons: [Ar] 3d⁶.
Why are chromium and copper exceptions?
NIST lists the ground states [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. OpenStax explains them with the extra stability of a half-filled or filled 3d subshell; there is no simple rule that predicts every exception.
How do you find the configuration of an ion?
Start from the neutral atom. For a negative ion add electrons in Aufbau order, for a positive ion remove them – for transition metals from the highest s subshell first. Enter the charge in the calculator, e.g. −3 for P³⁻.
Which elements does the calculator cover?
Z = 1 to 108 (hydrogen to hassium), the range of the NIST table. For heavier elements PubChem lists only calculated or predicted configurations.
What does the noble-gas notation mean?
The symbol in brackets stands for the electrons of that noble gas: [Ne] means 1s² 2s² 2p⁶. Sodium is then written [Ne] 3s¹.
Sources and legal basis
- OpenStax Chemistry 2e, 6.4 Electronic Structure of Atoms (Electron Configurations): Aufbau principle, filling order 1s 2s 2p 3s 3p 4s 3d …, Hund’s rule, exceptions Cr/Cu/Nb, noble-gas core notation, ions (Example 6.11: Na⁺, P³⁻, Al²⁺, Fe²⁺, Sm³⁺) (read 10.10.2026)
- NIST SP 966, Periodic Table: Atomic Properties of the Elements (2019) – ground-state configurations Z = 1–108 (read 10.10.2026)
- PubChem Periodic Table (NCBI/NLM), ElectronConfiguration – cross-check; identical to NIST except Lr (read 10.10.2026)
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