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Electron Configuration Calculator

Enter the atomic number (1 to 118).

Iron is 26, copper is 29, gold is 79.

This result is an estimate only. Double-check important figures before relying on them.

How it is calculated

  • Fill subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
  • Capacities: s 2, p 6, d 10, f 14
  • Known exceptions are applied, such as Cr and Cu
  • Shorthand replaces the inner electrons with the nearest noble gas in brackets

For atomic mass and moles instead, use the mole calculator.

Worked example

Iron (Fe, 26):

  • Full: 1s2 2s2 2p6 3s2 3p6 3d6 4s2
  • Shorthand: [Ar] 3d6 4s2

Why your number may differ

Textbooks sometimes write the 4s before the 3d. The meaning is the same. Exceptions beyond the common ones can differ between sources.

Frequently asked questions

How is the order of filling decided?

By the Madelung rule: subshells fill in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on, each holding up to 2, 6, 10 or 14 electrons for s, p, d and f.

Why are chromium and copper different?

A half-filled or completely filled d subshell is slightly more stable, so one electron moves from 4s to 3d. Several other elements have such exceptions.

Are the heaviest elements reliable?

Beyond about element 103, configurations are predicted rather than measured, so treat them as the expected result.

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