Electron Configuration Calculator (all elements 1–108, NIST ground states)

Understanding the electron configuration calculator — concept and principle

This calculator shows the ground-state electron configuration of any element from hydrogen to hassium (1–108), in noble-gas notation and shell by shell, with the number of unpaired electrons and a check against the aufbau filling rule. Students use it for chemistry homework, to predict magnetism and to understand the layout of the periodic table.

Iron fills 4s before finishing 3d, leaving four unpaired 3d electrons.

How it is calculated

Each subshell nℓ holds at most 2(2ℓ + 1) electrons: s 2, p 6, d 10, f 14.
The configuration shown is the measured ground state of the neutral atom (NIST data), not just a rule.
Noble-gas notation replaces the inner shells with the previous noble gas in brackets.
Unpaired electrons (Hund's rule): in a subshell of capacity c holding k electrons, k are unpaired if k ≤ c/2, otherwise c − k.
The aufbau line compares with the Madelung (n + ℓ) order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p.

The principle

Electrons fill the lowest available energy levels first, two per orbital with opposite spins (Pauli principle). Within a subshell they spread out into separate orbitals before pairing (Hund's rule), which lowers their repulsion. The Madelung rule, fill by increasing n + ℓ and then by n, predicts most configurations, but about 20 elements, such as Cr, Cu, Mo, Ag, Pt and Au, prefer a different arrangement because half-filled or filled d subshells are especially stable.

Worked example

Default input: iron (Fe, Z = 26).

Argon accounts for 18 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶.
The remaining 8 go into 4s² and 3d⁶, giving [Ar] 3d⁶ 4s².
Electrons per shell: K 2, L 8, M 3s² 3p⁶ 3d⁶ = 14, N 4s² = 2, so 2, 8, 14, 2.
3d holds 6 of 10: 6 > 5, so unpaired = 10 − 6 = 4.
The Madelung rule predicts the same configuration as the measured ground state.

How to read the result

Valence electrons in the outermost shell, plus d electrons in transition metals, decide chemical behaviour. Unpaired electrons make atoms paramagnetic; iron's four help explain its magnetism. If the aufbau line differs, the element is one of the known exceptions.

Common mistakes

Writing 4s after 3d when filling but forgetting that 4s electrons are removed first when ions form (Fe²⁺ is [Ar] 3d⁶).
Assuming Cr is [Ar] 3d⁴ 4s²; it is [Ar] 3d⁵ 4s¹.
Counting shells by subshell order instead of by n.

Keep in mind: The configurations are for isolated neutral atoms in the gas phase. In compounds and ions the electron arrangement can differ.

Frequently asked questions

Why is copper [Ar] 3d¹⁰ 4s¹?

A completely filled 3d subshell is lower in energy than the 3d⁹ 4s² arrangement the simple rule predicts.

How many electrons fit in the third shell?

Up to 18: 3s 2, 3p 6 and 3d 10.

Why stop at element 108?

For heavier elements the ground states are theoretical predictions, not measurements.

Sources: Particle Data Group, Review of Particle Physics 2024, Table 5.1 "Electronic structure of the elements" · NIST Atomic Spectra Database (A. Kramida, Yu. Ralchenko, J. Reader and NIST ASD Team)

Checked 9 Nov 2026. Particle Data Group, Review of Particle Physics 2024 (S. Navas et al., Phys. Rev. D 110, 030001), Table 5.1 “Electronic structure of the elements” — ground-state configurations from the NIST Atomic Spectra Database (A. Kramida, Yu. Ralchenko, J. Reader and NIST ASD Team).

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