Electron Configuration Calculator

Last updated: June 2026

Use this electron configuration calculator to find the full electron configuration, condensed noble-gas configuration and electrons per shell for neutral atoms and common ions. It is designed for O-Level, IGCSE, A-Level and Singapore H2 Chemistry.

Key takeaway: Electrons fill subshells in an energy order, but the final configuration is normally written with 3d before 4s. When transition-metal ions form, electrons are removed from 4s before 3d.
Scope: The calculator supports elements from hydrogen to radon and common school-level ions. It includes important observed exceptions such as Cr, Cu, Mo, Pd, Ag, Pt and Au. Very highly charged ions and advanced f-block cases may require a specialist data source.

What is electron configuration?

Electron configuration shows how electrons are distributed among shells and subshells. Oxygen has atomic number 8, so a neutral oxygen atom contains eight electrons:

O: 1s2 2s2 2p4

Each superscript gives the number of electrons in that subshell. The maximum capacities are 2 electrons for an s subshell, 6 for p, 10 for d and 14 for f.

Electron-filling order

For the usual aufbau model, electrons are added in the order:

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

This is a filling sequence, not always the order used when the final configuration is written. For transition elements, the conventional condensed form is normally written with the d subshell before the outer s subshell, such as [Ar] 3d6 4s2 for iron.

Condensed noble-gas configuration

Condensed notation replaces the inner-shell configuration with the symbol of the largest noble-gas core contained in the species.

Fe: [Ar] 3d6 4s2

This notation is shorter and makes valence electrons easier to identify.

Electron configuration of ions

A positive charge means electrons have been removed, while a negative charge means electrons have been added.

O2−: 1s2 2s2 2p6 = [Ne]

For transition-metal cations, remove electrons from the subshell with the highest principal quantum number first. Therefore, 4s electrons are removed before 3d electrons.

Fe: [Ar] 3d6 4s2

Fe2+: [Ar] 3d6

Fe3+: [Ar] 3d5

Exam trap: The 4s subshell fills before 3d in the simple filling sequence, but 4s electrons are removed before 3d electrons when first-row transition-metal cations form.

Electrons per shell

Shell notation adds together all electrons with the same principal quantum number. It is especially useful at Lower Secondary and O-Level.

Na: 2,8,1

Na+: 2,8

At A-Level, subshell notation is still needed because shell notation does not distinguish between s, p and d electrons.

Important exceptions: Cr and Cu

A simple aufbau count predicts [Ar] 3d4 4s2 for chromium and [Ar] 3d9 4s2 for copper. Their observed ground-state configurations are instead:

Cr: [Ar] 3d5 4s1

Cu: [Ar] 3d10 4s1

These arrangements reflect the very small energy differences between the 3d and 4s subshells. It is safer to learn the observed configurations than to describe them only as a simple “half-filled and fully filled stability” rule.

Worked examples

Oxide ion, O2−

Oxygen has atomic number 8. Gaining two electrons gives 10 electrons, so O2− has the same electron configuration as neon:

O2−: 1s2 2s2 2p6 = [Ne]

Iron(II) ion, Fe2+

Neutral iron is [Ar] 3d6 4s2. Remove the two 4s electrons first:

Fe2+: [Ar] 3d6

Copper(I) ion, Cu+

Neutral copper is [Ar] 3d10 4s1. Removing the 4s electron gives:

Cu+: [Ar] 3d10

Common exam traps

Trap 1: Do not remove a 3d electron before a 4s electron when forming Fe2+, Fe3+, Cu+ or similar transition-metal ions.
Trap 2: An anion has more electrons than its neutral atom; a cation has fewer.
Trap 3: Do not write Cr as [Ar] 3d4 4s2 or Cu as [Ar] 3d9 4s2.
Trap 4: The noble-gas core must actually be contained in the final configuration. It is not simply the noble gas closest to the species by electron count.

Frequently asked questions

Why does 4s fill before 3d but get removed first?

The relative energies of 4s and 3d change as electrons are added. Once the 3d subshell is occupied, 4s is the outer, higher-energy subshell and is removed first during cation formation.

What is the electron configuration of Fe2+?

Fe2+ is [Ar] 3d6. The two 4s electrons are removed from neutral iron.

What is the electron configuration of Cu?

The observed ground-state configuration is [Ar] 3d10 4s1.

Does the calculator support every possible ion?

It is intended for common school-level atoms and ions. Extremely highly charged species and advanced lanthanide chemistry may require more specialised treatment.

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