🔟 Electron Configuration Calculator
Enter an element and get its electron configuration — the observed notation, the idealised Aufbau order, and the number of electrons in each shell.
Iron (Z = 26) — full configuration
[Ar]3d⁶4s²
Aufbau (idealised) order
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
Fills subshells in increasing energy (Madelung rule).
Electrons per shell (K, L, M…)
2, 8, 14, 2
Sum = 26 electrons
The full configuration is the experimentally observed one; the Aufbau order is the idealised filling. Some d- and f-block elements deviate from the simple prediction (e.g. Cr, Cu). 🔒 Computed in your browser.
How the electron configuration calculator works
Electrons fill orbitals from lowest energy upward (the Aufbau principle / Madelung rule): 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. The tool fills that order for the element’s atomic number to build the configuration and the electrons-per-shell breakdown, and shows the experimentally observed configuration alongside it.
The observed configuration is the real one; the Aufbau order is the textbook prediction. A handful of d- and f-block elements (chromium, copper and others) deviate because half-filled and filled subshells are extra stable.
Frequently asked questions
How do I write an electron configuration?
Fill subshells in order of increasing energy (1s, 2s, 2p, 3s, 3p, 4s, 3d…) until you have placed all the element’s electrons. Iron (26) is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², written [Ar]3d⁶4s².
What is the Aufbau principle?
The rule that electrons occupy the lowest-energy orbitals first. The Madelung (n+l) rule gives the filling order, which is why 4s fills before 3d.
What are the electrons per shell?
The count in each principal shell (K, L, M…). For iron it is 2, 8, 14, 2. They sum to the atomic number.
Why do chromium and copper break the pattern?
A half-filled (d⁵) or fully filled (d¹⁰) d-subshell is unusually stable, so chromium is [Ar]3d⁵4s¹ and copper [Ar]3d¹⁰4s¹ rather than the naïve prediction. The tool shows the real configuration.
What is noble-gas shorthand?
Replacing the inner electrons with the previous noble gas in brackets — e.g. [Ar] for argon’s 18 electrons — so iron is [Ar]3d⁶4s² instead of the full string.