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Electron Configuration Calculator

Get the full and noble-gas electron configuration of any element or ion, with an orbital diagram, valence count and the last electron's quantum numbers.

26
Atomic number Z
26
Electrons
8
Valence electrons
4
Unpaired electrons

Iron (Fe)

Full notation
1s2 2s2 2p6 3s2 3p6 3d6 4s2
Noble-gas shorthand (core: Ar)
[Ar] 3d6 4s2
Classification
Transition metal · block d · period 4 · group 8

Orbital diagram

1s
↑↓
2 of 2 e⁻
2s
↑↓
2 of 2 e⁻
2p
↑↓↑↓↑↓
6 of 6 e⁻
3s
↑↓
2 of 2 e⁻
3p
↑↓↑↓↑↓
6 of 6 e⁻
3d
↑↓↑↑↑↑
6 of 10 e⁻
4s
↑↓
2 of 2 e⁻

Hund's rule: every box in a subshell takes one spin-up electron before any box takes a second, spin-down one.

Quantum numbers of the last electron

  • Principal n = 3
  • Azimuthal ℓ = 2 (d subshell)
  • Magnetic mℓ = -2
  • Spin ms = −1/2

mℓ counts from −ℓ upwards as the boxes fill left to right, so this is one valid assignment of the last electron rather than the only one.

Electrons per shell

  • Shell n = 1: 2 electrons
  • Shell n = 2: 8 electrons
  • Shell n = 3: 14 electrons
  • Shell n = 4: 2 electrons
  • Outermost shell n = 4 — 3d6 4s2

Subshells are filled in Madelung order (increasing n + ℓ, then increasing n): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. The twenty ground-state exceptions from chromium to lawrencium come from measured NIST spectroscopic data. Ions are built by taking electrons from the highest n first, which is why iron loses its 4s electrons before its 3d ones.

What is the Electron Configuration Calculator?

An electron configuration lists which orbitals an atom's electrons occupy. Fill them in Madelung order — 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on — until every electron is placed. Iron is 1s2 2s2 2p6 3s2 3p6 3d6 4s2, or [Ar] 3d6 4s2 in shorthand.

  • All 118 elements plus cations and anions
  • Full notation and noble-gas shorthand side by side
  • Orbital box diagram with spin arrows and per-subshell counts
  • Valence and unpaired electron counts, and electrons per shell
  • Flags the measured exceptions and shows the Madelung prediction they break
  • Quantum numbers n, l, ml and ms for the last electron placed

How to use the Electron Configuration Calculator

  1. 1

    Type an element symbol, full name or atomic number in the element box.

  2. 2

    Set a charge for an ion — 2 for Fe2+, or -1 for the chloride ion — or leave it at zero.

  3. 3

    Read the full notation and noble-gas shorthand in the results panel.

  4. 4

    Scroll to the orbital diagram to see how the boxes fill under Hund's rule.

  5. 5

    Check the quantum numbers panel for n, l, ml and ms of the last electron.

About the Electron Configuration Calculator

The ByteTools Electron Configuration Calculator writes out the orbital arrangement for any of the 118 elements, or for an ion once you set a charge. Type Fe, iron or 26 and you get the full notation, the noble-gas shorthand, an orbital box diagram with Hund's rule arrows, the valence electron count and the four quantum numbers of the last electron placed.

Orbitals are filled in Madelung order, increasing n plus l and then increasing n. The twenty measured exceptions — chromium, copper, palladium, gold and the rest — are held in a lookup list taken from spectroscopic ground-state data, and the page tells you when the element you asked about is one of them and what the naive prediction would have been.

Ions are built by removing electrons from the highest principal shell first, which is why iron loses its 4s electrons before its 3d ones. Every calculation happens in your browser with no network request, so the tool is fast, private and works offline.

Frequently asked questions

What order do electrons fill orbitals in?

In Madelung order: lowest n plus l first, and where two subshells tie, the lower n wins. That gives 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p — which is why 4s fills before 3d.

Why is chromium 3d5 4s1 instead of 3d4 4s2?

A half-filled d subshell is unusually stable, so chromium's measured ground state moves one electron from 4s into 3d. Copper does the same thing to complete its d subshell. There are about twenty such exceptions and this calculator uses the measured configuration, not the prediction.

Which electrons does an atom lose first when it forms a cation?

The ones in the highest principal quantum number, not the last ones added. Iron fills 4s before 3d but loses 4s first, so Fe2+ is [Ar] 3d6 rather than [Ar] 3d4 4s2. This tool applies that rule automatically.

How do you write a noble-gas shorthand configuration?

Replace the innermost complete shells with the symbol of the previous noble gas in square brackets, then write out whatever comes after it. Iron becomes [Ar] 3d6 4s2 because argon accounts for its first eighteen electrons.

What are valence electrons and how are they counted here?

Valence electrons are the ones available for bonding. This tool counts everything in the outermost shell plus any partly filled d subshell, which is the convention that makes transition-metal chemistry work. The result is shown alongside the number of unpaired electrons.

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