Electron Configuration Calculator

Work out electron configuration instantly with clear inputs, formula shown and shareable results.

About Electron Configuration Calculator

Electrons fill subshells in order of increasing n + l, the Madelung rule, which is why 4s fills before 3d. Each subshell holds 2(2l+1) electrons, so s holds 2, p holds 6, d holds 10 and f holds 14. Iron (Z = 26) therefore reads 1s2 2s2 2p6 3s2 3p6 4s2 3d6, with two electrons in the fourth shell. Ionic charge is applied by removing that many electrons before filling.

Formulas

Filling order

Madelung rule: increasing n + l, then increasing n

Subshell capacity

capacity = 2(2l + 1)

Worked example

With the default values below, Electron Configuration Calculator returns Outer-Shell Electrons of 2. Change any input above and every figure here is recalculated in your browser.

Inputs used

Atomic number Zaccepts 1–118: 26
Ionic chargeone of Neutral atom, 1+ cation, 2+ cation, 3+ cation, 1- anion, 2- anion: Neutral atom

Results

Outer-Shell Electrons: 2
Electron Configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Total Electrons: 26
Highest Occupied Shell: 4

Step by step

1Values used
2Filling order
3Subshell capacity
4Outer-Shell Electrons= 2
5Electron Configuration= 1s2 2s2 2p6 3s2 3p6 4s2 3d6
6Total Electrons= 26
7Highest Occupied Shell= 4

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does 4s fill before 3d?

Because 4s has the lower n + l sum (4 versus 5), so it lies lower in energy in the neutral atom despite the larger principal number.

Does this handle the chromium and copper exceptions?

No. A handful of elements promote an s electron to half-fill or fill the d subshell; those need to be corrected by hand.