Electron configuration calculator

An electron configuration calculator shows how an atom's electrons are arranged in subshells, such as 1s² 2s² 2p⁴ for oxygen. Pick any element from hydrogen to oganesson in our calculator, and any charge to make an ion, and it gives the full configuration, the noble gas shorthand, an orbital box diagram and the number of unpaired electrons, and explains exceptions such as chromium and copper.

Each subshell is written as its shell number, its letter (s, p, d or f) and the number of electrons in it as a superscript. The calculator also gives the electrons per shell, such as 2, 6 for oxygen, the block and the period.

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Electron Configuration Tool

Fill the shells and orbitals of any element

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How to use the electron configuration calculator

1. Pick the element from the periodic table.
2. For an ion, set Charge on the ion: positive for electrons lost, negative for electrons gained.
3. Read the full configuration and the noble gas shorthand.
4. Check the readouts: the number of electrons, electrons per shell, unpaired electrons, and the block and period.
5. Look at the orbital box diagram, which shows each electron as an arrow following Hund's rule and the Pauli principle.

If the element is one of the exceptions, the calculator says so and explains why. For elements beyond lawrencium, element 103, it notes that the configurations are predicted rather than measured.

How electrons fill subshells

Electrons fill the lowest-energy subshells first (the Aufbau principle). The filling order is:

Within a subshell, electrons spread out one per orbital before pairing up (Hund's rule), and two electrons in the same orbital have opposite spins (the Pauli principle). 4s fills before 3d, but the calculator lists subshells in order of shell number, so iron is written 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Many textbooks write 4s² 3d⁶ instead, in filling order; both describe the same configuration.

Worked examples you can check in the calculator

Example 1: oxygen

Oxygen has 8 electrons. Fill 1s with 2, 2s with 2, and put the last 4 in 2p:

The shorthand replaces the helium core: [He] 2s² 2p⁴. In the 2p boxes, Hund's rule puts one arrow in each of the three, then pairs one, leaving 2 unpaired electrons. The calculator shows the same.

Example 2: iron

Iron has 26 electrons. After [Ar], which accounts for 18, 4s takes 2 and 3d takes 6:

The calculator gives 2, 8, 14, 2 electrons per shell and 4 unpaired electrons, all in 3d.

Example 3: transition metal ions

When a transition metal forms a positive ion, it loses its 4s electrons before its 3d ones. So Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵, with 5 unpaired electrons. Set the charge to +2 or +3 in the calculator to see this.

Example 4: the chromium and copper exceptions

The calculator flags both and gives these reasons. These two are the exceptions most courses expect you to know.

Example 5: negative ions

Negative ions gain electrons into the next free subshell. S²⁻ has 18 electrons, 1s² 2s² 2p⁶ 3s² 3p⁶, the same as argon. Cl⁻ has the same configuration. Set the charge to −2 for sulfur or −1 for chlorine.

Try filling the shells in the course, then press Mark:

Blocks of the periodic table

The block an element is in tells you which subshell its last electron goes into. The calculator gives the block and period for every element.

For the simple shell picture (2, 8, 8), use the Atom Builder; our build an atom guide explains it.

Common mistakes

• Filling 3d before 4s. 4s fills first, even though it is written after 3d in shell order.
• Removing 3d electrons first from a transition metal. Positive ions lose 4s electrons first.
• Ignoring the exceptions. Chromium and copper have only one 4s electron.
• Putting too many electrons in a subshell. s holds 2, p holds 6, d holds 10 and f holds 14.
• Pairing electrons too early. In an orbital diagram, fill each box with one arrow before adding a second.

Practise atomic structure in a short timed paper

Electron configurations come up in Honors Chemistry and High School Chemistry practice, and in A-level Chemistry: Year 1 in the UK. Each daily paper is a short timed paper of 8 questions worth 25 to 35 marks, and the papers mix topics. The Atomic Structure course builds up to s, p and d subshells.

Read next

Chemistry · Atoms and the periodic tableWhat is an ion? Positive and negative ions explainedWhat an ion is, how atoms become positive or negative ions, how to work out an ion's charge and particles, ion names and formulae, with worked examples.