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Electron Configuration Calculator

Get the electron configuration of any element or simple ion, in full and in noble-gas shorthand, with an orbital diagram and the number of unpaired electrons. Configurations are the measured ground states, and exceptions such as chromium and copper are explained.

Chemistry · Atomic structureUpdated Sources cited

Symbol, name or atomic number: Fe, iron or 26
e.g. 2 for Fe²⁺, −1 for Cl⁻
Examples:

Result

Iron (Z = 26, 26 electrons)

Noble-gas notation

[Ar] 3d6 4s2[\text{Ar}]\,3d^{6}\,4s^{2}
Full configuration
1s2 2s2 2p6 3s2 3p6 3d6 4s21s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\,3d^{6}\,4s^{2}
In filling order
1s2 2s2 2p6 3s2 3p6 4s2 3d61s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\,4s^{2}\,3d^{6}
Electrons per shell
K 2 · L 8 · M 14 · N 2
Unpaired electrons
4 (paramagnetic)
Orbital diagram (Hund's rule)
Assumptions behind this result
  • Ground-state configurations of neutral atoms are from the NIST Atomic Spectra Database where measured.

Step-by-step solution

  1. Find the number of electrons

    Iron has atomic number 26, so a neutral atom has 26 electrons.

  2. Fill subshells in order of increasing energy (Aufbau principle)

    Electrons occupy the lowest-energy subshells first. The order follows the n + l rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. An s subshell holds 2 electrons, p holds 6, d holds 10 and f holds 14.

    1s2 2s2 2p6 3s2 3p6 4s2 3d61s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\,4s^{2}\,3d^{6}

  3. Check against the measured ground state

    The prediction matches the ground state measured spectroscopically (NIST Atomic Spectra Database).

  4. Write the noble-gas shorthand

    Replace the inner electrons that match the previous noble gas with its symbol in square brackets.

    [Ar] 3d6 4s2[\text{Ar}]\,3d^{6}\,4s^{2}

  5. Apply Hund's rule to count unpaired electrons

    Within a subshell, electrons occupy separate orbitals with parallel spins before pairing up. This gives 4 unpaired electrons, so the atom is paramagnetic (attracted by a magnetic field).

What the calculator does

For any element from hydrogen to oganesson, or a simple ion of it, the calculator gives:

  • the full electron configuration and the noble-gas shorthand;
  • the configuration written in filling order;
  • an orbital box diagram following Hund’s rule;
  • the number of electrons in each shell and the number of unpaired electrons;
  • a note when the element is an exception to the Aufbau principle, with the reason.

How to use it

Enter an element by symbol (Fe), name (iron) or atomic number (26). For an ion, add its charge: 2 for Fe²⁺, −1 for Cl⁻.

The rules

The Aufbau principle

Electrons fill subshells in order of increasing energy, which follows the n + l (Madelung) rule: lower n + l first, and for equal n + l, lower n first.

1s→2s→2p→3s→3p→4s→3d→4p→5s→4d→5p→6s→4f→5d→6p→7s→5f→6d→7p1s \to 2s \to 2p \to 3s \to 3p \to 4s \to 3d \to 4p \to 5s \to 4d \to 5p \to 6s \to 4f \to 5d \to 6p \to 7s \to 5f \to 6d \to 7p

Each subshell holds 2(2l + 1) electrons: 2 in s, 6 in p, 10 in d and 14 in f.

The Pauli principle and Hund’s rule

Each orbital holds at most two electrons, with opposite spins. Within a subshell, electrons occupy separate orbitals with parallel spins before any pair up. That is why nitrogen’s three 2p electrons are all unpaired.

Exceptions

About twenty elements have ground states that differ from the simple prediction. Chromium and copper are the best known: 3d⁵ 4s¹ and 3d¹⁰ 4s¹ instead of 3d⁴ 4s² and 3d⁹ 4s². The calculator uses the measured ground states from the NIST Atomic Spectra Database, so these come out right.

Worked example: iron, Z = 26

  1. Fill in order. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² uses 20 electrons; the remaining 6 go into 3d.
  2. Shorthand. The first 18 electrons match argon, so Fe is [Ar] 3d⁶ 4s².
  3. Hund’s rule. Six d electrons in five orbitals: one pair and four single electrons, so 4 unpaired electrons.
  4. The Fe³⁺ ion. Remove the two 4s electrons first, then one 3d electron: [Ar] 3d⁵, with 5 unpaired electrons.

Common mistakes

  • Removing 3d electrons before 4s when forming transition-metal cations.
  • Pairing electrons too early in an orbital diagram, against Hund’s rule.
  • Forgetting the exceptions. Cr, Cu, Mo, Ag and Au are the ones most often tested.
  • Miscounting for ions. Subtract electrons for positive ions and add them for negative ions.

Practical applications

  • Chemical bonding. The outer electrons determine how an element bonds.
  • Magnetism. Unpaired electrons make substances paramagnetic, which is the basis of MRI contrast agents based on gadolinium.
  • Colour. Partially filled d subshells give transition-metal compounds their colours.
  • Spectroscopy. Each configuration has characteristic energy levels that identify elements in stars and samples.

Assumptions and limitations

Neutral-atom configurations are the measured ground states (NIST) for elements up to Z = 108, and theoretical predictions beyond that. Ion configurations use the standard rule of removing electrons from the outermost shell first. This is right for the main-group and first-row transition-metal ions met in most courses, but the true ground state of some lanthanide and actinide ions differs.

Frequently asked questions

Why is chromium [Ar] 3d⁵ 4s¹ and not [Ar] 3d⁴ 4s²?

The 3d and 4s subshells are very close in energy. Moving one electron from 4s into 3d gives a half-filled 3d⁵ subshell, and the lower electron repulsion and favourable exchange energy make that arrangement the ground state. Spectroscopy confirms it.

Why does iron lose its 4s electrons first when it forms Fe²⁺?

Once electrons are removed, the 3d subshell drops below 4s in energy. The 4s electrons are then the outermost and are lost first, giving Fe²⁺ as [Ar] 3d⁶.

Should I write 4s before or after 3d?

Both orders are used. Writing subshells by shell number ([Ar] 3d⁶ 4s²) is the convention used by NIST and IUPAC; writing them in filling order ([Ar] 4s² 3d⁶) follows the Aufbau sequence. The calculator shows both.

What does it mean for an atom to be paramagnetic?

It has one or more unpaired electrons, so it is attracted into a magnetic field. Atoms with every electron paired are diamagnetic and are weakly repelled.

Are the configurations of superheavy elements known?

Not by measurement. For elements beyond about rutherfordium (Z = 104), only a few atoms have ever been made, so their configurations come from relativistic quantum calculations. The calculator marks these as predictions.

References

  1. Kramida, A., Ralchenko, Yu., Reader, J. & NIST ASD Team. NIST Atomic Spectra Database. National Institute of Standards and Technology.
  2. Housecroft, C. E. & Sharpe, A. G. (2018). Inorganic Chemistry (5th ed.), chapter 1. Pearson.
  3. Scerri, E. R. (2019). The Periodic Table: Its Story and Its Significance (2nd ed.). Oxford University Press.

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