Electron Configuration Calculator
Aufbau order, with the elements that break it built in.
Work out Electron Configuration. Aufbau order, with the elements that break it built in. Says where the model stops being reliable.
Iron is 26, chromium 24, copper 29.
0 for a neutral atom, 2 for Fe²⁺, −1 for Cl⁻.
Iron (Fe)
[Ar] 3d6 4s2
26 electrons, 2 in the outer shell
Configurations are written in shell order rather than filling order, which is why iron reads [Ar]3d⁶4s² despite the 4s filling first. Both conventions appear in textbooks; this is the more common one and the one that makes ion behaviour obvious.
How the Electron Configuration Calculator works
Enter an atomic number and an optional charge to get the ground-state configuration in both full and noble-gas notation. The elements that break the Aufbau order — chromium, copper and eleven others — return their measured configurations rather than the naive prediction.
Also known as: noble gas configuration calculator · aufbau principle calculator · electron configuration of iron · orbital diagram calculator
Frequently asked questions
What is the electron configuration of iron?
[Ar] 3d⁶ 4s², or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² in full. It is written with 3d before 4s even though 4s fills first, because that is the conventional shell order.
Why is chromium 3d⁵ 4s¹ and not 3d⁴ 4s²?
Because a half-filled d subshell is stable enough to pull an electron across from the 4s. Copper does the same to reach a full 3d¹⁰. These are measured configurations, not predictions from the rule.
Why does Fe²⁺ lose its 4s electrons rather than 3d?
Because once 3d is occupied it drops below 4s in energy. The atom fills 4s first, but the ion empties it first, so Fe²⁺ is [Ar]3d⁶ rather than [Ar]3d⁴4s². Filling order and emptying order are genuinely different.
What is noble gas notation?
A shorthand replacing the inner electrons with the preceding noble gas in brackets. Iron's [Ar] stands for the eighteen electrons of argon, leaving only the chemically interesting outer ones written out.
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