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Electron Configuration Calculator

Write out where the electrons sit in any element or ion (its ground-state configuration), with noble-gas shorthand, outer-shell and unpaired electrons.

Level: Grade 10

Printable worksheet

Electron Configuration Calculator

Inputs

0 for a neutral atom, a positive number for a cation (Fe³⁺ is 3) or a negative one for an anion (Cl⁻ is −1).

Result

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Fe: [Ar] 4s² 3d⁶

Noble-gas shorthand
[Ar] 4s² 3d⁶
Written in shell order
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
Electrons
26
Electrons in the outermost shell (n = 4)
2
Unpaired electrons (Hund's rule)
4

See the step-by-step solution ↓

Step-by-step solution

  1. A neutral atom has as many electrons as protons: the atomic number of iron is 26

    electrons=26

  2. Fill the subshells in the standard (Aufbau) order, lowest energy first. Each s holds up to 2 electrons, p 6, d 10 and f 14

    1s → 2s → 2p → 3s → 3p → 4s → 3d

  3. Write out the full lowest-energy (ground-state) configuration of Fe

    1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

  4. For the short version, swap the inner electrons that match Ar (18 electrons) for [Ar]

    [Ar] 4s² 3d⁶

  5. Count the electrons in the outermost shell (shell 4), since these do most of the chemistry

    4s² → 2  electrons

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

See exactly how the electrons are arranged in any element or ion. An electron configuration lists the electrons in each subshell, the small groups of spaces electrons fill around an atom. For oxygen it's 1s² 2s² 2p⁴. That means two electrons in the 1s subshell, two in 2s and four in 2p. This arrangement explains a lot of chemistry. It shows why sodium gives up one electron, and why iron can form both Fe²⁺ and Fe³⁺.

This calculator writes the ground-state configuration, the lowest-energy arrangement, for every element from hydrogen to oganesson. It works for a neutral atom or an ion (an atom with extra or missing electrons). It fills subshells in the Aufbau order, the standard filling order. It also applies the 20 measured exceptions, such as chromium and copper. You'll get the noble-gas shorthand and the same configuration in shell order. You'll also see the electrons in the outermost shell and the number of unpaired electrons.

Where it's used

Why this math matters outside the classroom:

  • Chemistry class. Students write oxygen as 1s² 2s² 2p⁴ and see its 6 outer electrons explain why it gains 2 to form O²⁻.
  • Ion charges. A student finds Na⁺ has the same configuration as neon, which is why sodium loses exactly one electron.
  • Magnetism. Chemists predict Fe³⁺, with 5 unpaired 3d electrons, is strongly attracted to a magnet.
  • Periodic trends. Students see that elements in the same column share an outer configuration, such as ns¹ for the alkali metals.
  • Exam review. A student checks the copper exception, [Ar] 4s¹ 3d¹⁰, before a test on transition metals.

How to use it

  1. Choose the element.
  2. Enter the charge. Use 0 for a neutral atom, a positive number for a cation (an ion that lost electrons) or a negative one for an anion (an ion that gained electrons).
  3. Read the full configuration and the noble-gas shorthand. Then check the outer-shell and unpaired electron counts.

What the inputs mean

Ion charge
Cations lose their outermost electrons first. Fe²⁺ loses its two 4s electrons, not 3d ones. Charges are limited to ones that remove only electrons outside the noble-gas core (the inner electrons that match a noble gas).

Formula and method

electrons = atomic number − charge
filling order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
capacity: s 2, p 6, d 10, f 14
unpaired electrons in a subshell = e if e ≤ orbitals, otherwise 2 × orbitals − e

The atomic number is the element's number of protons. Each subshell is named by a shell number (1, 2, 3 and so on) and a letter: s, p, d or f. An s subshell holds up to 2 electrons, p holds 6, d holds 10 and f holds 14. In the last line, e is the number of electrons in a subshell. Orbitals are the slots inside it, and each holds up to two electrons.

The Aufbau (or Madelung) rule fills subshells in order of n + l. Here n is the shell number and l is the subshell number: 0 for s, 1 for p, 2 for d and 3 for f. When two subshells tie, the lower shell fills first. That's why 4s fills before 3d. Hund's rule says electrons spread out one per orbital before they pair up. That's what gives the unpaired count.

Twenty elements up to lawrencium have measured ground states that break the simple order. Chromium (4s¹ 3d⁵) and copper (4s¹ 3d¹⁰) are examples, where a half-filled or filled d subshell is lower in energy. The calculator applies these from a table checked against the NIST atomic spectra data. Beyond lawrencium, the configurations are predictions.

Worked example

What are the electron configurations of iron and of the Fe³⁺ ion?

  1. Iron has atomic number 26, so the neutral atom has 26 electrons.
  2. Fill in order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, which is [Ar] 4s² 3d⁶; 3d⁶ has 4 unpaired electrons.
  3. Fe³⁺ has 26 − 3 = 23 electrons. Remove 2 from 4s and 1 from 3d.
  4. Fe³⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵, or [Ar] 3d⁵, with 5 unpaired electrons.

Result: Iron is [Ar] 4s² 3d⁶, and Fe³⁺ is [Ar] 3d⁵, a half-filled d subshell.

Understanding the result

For main-group elements, the electrons in the outermost shell are the valence electrons. They set how the element's group behaves in chemistry: 1 for sodium, 6 for oxygen, 7 for chlorine.

Unpaired electrons make a substance attracted to magnets, which chemists call paramagnetic. Fe³⁺, with five unpaired electrons, is strongly paramagnetic. Ions with none, such as Zn²⁺, are not.

Assumptions and limitations

  • Atoms and ions are on their own and in their ground states, as in gas-phase spectroscopy (measuring light from single atoms in a gas).
  • Anions add electrons to the next open subshells in filling order.
  • In compounds, d and f electrons can rearrange. So a metal's configuration inside a complex (a metal bonded to surrounding molecules) may differ.
  • Configurations for elements 104 to 118 are predictions. Some calculations suggest slightly different ground states.
  • Some textbooks write subshells in shell order ([Ar] 3d⁶ 4s²). Both orders describe the same configuration, and both are shown.

Frequently asked questions

Why does 4s fill before 3d?

While a neutral atom is being built up, 4s has the lower n + l value (4 + 0 = 4, against 3 + 2 = 5), so it fills first. Once 3d has electrons in it, the order of energies shifts. That's why transition metals lose 4s electrons first when they form ions.

Why are chromium and copper exceptions?

For chromium, moving one 4s electron into 3d gives a half-filled 3d⁵ subshell. For copper, it gives a completely filled 3d¹⁰. Measurements show both arrangements are lower in energy than the simple filling order.

What is noble-gas shorthand?

It swaps the inner electrons that match a noble gas for that gas's symbol in brackets. Sodium's 1s² 2s² 2p⁶ 3s¹ becomes [Ne] 3s¹.

How many valence electrons does an element have?

For main-group elements, count the electrons in the outermost shell, which the calculator shows. For transition metals, the d electrons also take part in bonding, so definitions vary.

How this calculator is checked

Method: Aufbau (Madelung n + l) filling, Hund's rule for unpaired electrons; ground-state exceptions checked against the NIST Atomic Spectra Database ground states for Z = 1–103. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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