Electron configuration calculator

Type an element to get its full and noble-gas electron configuration. Exceptions like chromium and copper use the measured ground state from NIST.

By the Calcist team · Checked against known answers · Updated October 27, 2026

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What it does

An electron configuration lists which orbitals (subshells) an atom's electrons occupy in its lowest-energy state. This calculator gives the full configuration, the noble-gas shorthand, the electrons in each shell and the period for every element from hydrogen to oganesson. Where nature doesn't follow the textbook filling order, it gives the measured ground state from the NIST Atomic Spectra Database and shows what the simple rule would have predicted.

How it's worked out

Electrons fill subshells in order of increasing n + l (the Aufbau or Madelung rule), taking the lower n first when there's a tie. An s subshell holds 2 electrons, p holds 6, d holds 10 and f holds 14.

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

The noble-gas shorthand replaces the inner electrons with the symbol of the previous noble gas in brackets, so iron's 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ becomes [Ar] 4s² 3d⁶. The calculator lists subshells in filling order; it also shows them ordered by shell (3d before 4s), which is how NIST writes them.

Worked example: chromium

Chromium has 24 electrons. Following the filling order gives [Ar] 4s² 3d⁴. But spectroscopy shows chromium's ground state is actually [Ar] 3d⁵ 4s¹: one electron moves from 4s to 3d, giving a half-filled d subshell. Copper does the same thing with a full d subshell: [Ar] 3d¹⁰ 4s¹, not 4s² 3d⁹.

The exceptions

Twenty elements up to hassium (108) have a measured ground state that differs from the filling order:

Element	Filling order predicts	Measured (NIST)
Cr (24)	[Ar] 4s² 3d⁴	[Ar] 3d⁵ 4s¹
Cu (29)	[Ar] 4s² 3d⁹	[Ar] 3d¹⁰ 4s¹
Nb (41)	[Kr] 5s² 4d³	[Kr] 4d⁴ 5s¹
Mo (42)	[Kr] 5s² 4d⁴	[Kr] 4d⁵ 5s¹
Ru (44)	[Kr] 5s² 4d⁶	[Kr] 4d⁷ 5s¹
Rh (45)	[Kr] 5s² 4d⁷	[Kr] 4d⁸ 5s¹
Pd (46)	[Kr] 5s² 4d⁸	[Kr] 4d¹⁰
Ag (47)	[Kr] 5s² 4d⁹	[Kr] 4d¹⁰ 5s¹
La (57)	[Xe] 6s² 4f¹	[Xe] 5d¹ 6s²
Ce (58)	[Xe] 6s² 4f²	[Xe] 4f¹ 5d¹ 6s²
Gd (64)	[Xe] 6s² 4f⁸	[Xe] 4f⁷ 5d¹ 6s²
Pt (78)	[Xe] 6s² 4f¹⁴ 5d⁸	[Xe] 4f¹⁴ 5d⁹ 6s¹
Au (79)	[Xe] 6s² 4f¹⁴ 5d⁹	[Xe] 4f¹⁴ 5d¹⁰ 6s¹
Ac (89)	[Rn] 7s² 5f¹	[Rn] 6d¹ 7s²
Th (90)	[Rn] 7s² 5f²	[Rn] 6d² 7s²
Pa (91)	[Rn] 7s² 5f³	[Rn] 5f² 6d¹ 7s²
U (92)	[Rn] 7s² 5f⁴	[Rn] 5f³ 6d¹ 7s²
Np (93)	[Rn] 7s² 5f⁵	[Rn] 5f⁴ 6d¹ 7s²
Cm (96)	[Rn] 7s² 5f⁸	[Rn] 5f⁷ 6d¹ 7s²
Lr (103)	[Rn] 7s² 5f¹⁴ 6d¹	[Rn] 5f¹⁴ 7s² 7p¹

Limitations

Elements 109 to 118 (meitnerium to oganesson) have never had their ground states measured; only a few atoms have ever been made. For these the calculator shows the filling-order configuration and labels it as predicted.
These are configurations of neutral atoms in the gas phase. Ions are different: transition metals lose their outer s electrons first, so Fe²⁺ is [Ar] 3d⁶, not [Ar] 4s² 3d⁴.
Some school courses only expect the filling-order answer. If your teacher hasn't covered exceptions, check which one they want.

Sources

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

Iron: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ ✓
Chromium is an exception: [Ar] 3d⁵ 4s¹ (NIST) ✓
Atomic number 29 (copper): [Ar] 3d¹⁰ 4s¹ ✓
Gold by name: [Xe] 4f¹⁴ 5d¹⁰ 6s¹ ✓
Palladium has no 5s electrons: [Kr] 4d¹⁰ ✓
Lawrencium: [Rn] 5f¹⁴ 7s² 7p¹ ✓
British spelling aluminium: [Ne] 3s² 3p¹ ✓
Hydrogen, the smallest: 1s¹ ✓
Oganesson is labeled as predicted ✓
Not an element is refused ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

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Questions people ask

How do you write an electron configuration?

Fill subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on, putting at most 2 electrons in s, 6 in p, 10 in d and 14 in f, until you've placed as many electrons as the atomic number.

What is the electron configuration of iron?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or [Ar] 3d⁶ 4s² in shorthand. Iron has 26 electrons.

Why is chromium's configuration an exception?

Its measured ground state is [Ar] 3d⁵ 4s¹ rather than [Ar] 3d⁴ 4s². The 3d and 4s levels are very close in energy, and the arrangement with one electron in each of six orbitals ends up lower in energy.

What is noble gas notation?

A shortcut that writes the previous noble gas's configuration as its symbol in brackets. Sodium's 1s² 2s² 2p⁶ 3s¹ becomes [Ne] 3s¹.

Which elements are exceptions to the Aufbau principle?

Up to element 108, NIST's measured ground states differ from the filling order for Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr.

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