Electron Configuration Calculator

The Electron Configuration Calculator shows how the electrons of any element or ion are arranged among its atomic subshells. It generates both the full and the noble-gas (condensed) electron configuration, draws an orbital box diagram with spin arrows, traces the Aufbau diagonal-filling order, and reports the number of unpaired electrons, whether the species is paramagnetic or diamagnetic, its valence electrons, and the quantum numbers of its last electron. It also handles the tricky d- and f-block exceptions such as chromium, copper, and silver that many calculators get wrong.

What Is an Electron Configuration?

An electron configuration is a shorthand notation that tells you exactly which subshells the electrons of an atom or ion occupy, and how many electrons are in each. It is written as a sequence of subshells — 1s, 2s, 2p, and so on — with the electron count of each shown as a superscript. For example, the configuration of oxygen is 1s² 2s² 2p⁴, meaning two electrons in the 1s subshell, two in the 2s, and four in the 2p.

The Three Rules That Govern Filling

Electron configurations follow three fundamental principles:

• Aufbau principle: electrons occupy the lowest-energy subshell available before filling higher ones.
• Pauli exclusion principle: each orbital holds at most two electrons, and they must have opposite spins.
• Hund's rule: within a set of equal-energy orbitals, electrons spread out singly with parallel spins before any orbital is doubly occupied.

The Aufbau (Madelung) Order

The order in which subshells fill is set by the Madelung rule: subshells fill by increasing value of n + ℓ, and when two subshells have the same n + ℓ, the one with the lower n fills first. This is why the 4s subshell fills before the 3d subshell.

Full vs Noble-Gas Configuration

The full configuration lists every subshell from 1s upward. The noble-gas configuration is a condensed form that replaces the inner core electrons with the symbol of the previous noble gas in square brackets, leaving only the valence-region subshells. For iron:

• Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
• Noble-gas: [Ar] 3d⁶ 4s²

Electron Configurations of Ions

To write the configuration of an ion, start from the neutral atom and adjust the electron count:

• Anions (negative ions) gain electrons, which are added following the normal Aufbau order. For example, O²⁻ is 1s² 2s² 2p⁶, the same as neon.
• Cations (positive ions) lose electrons from the outermost shell first — the highest principal quantum number n, before the inner d electrons. This is why iron loses its 4s electrons before its 3d electrons: Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵.

The d- and f-Block Exceptions

A few elements do not follow the simple Aufbau prediction because a completely filled or exactly half-filled d or f subshell is unusually stable. The most commonly tested exceptions are:

This calculator stores all of the established ground-state exceptions, so the configurations it returns match the experimentally observed values.

Paramagnetic vs Diamagnetic

The orbital box diagram reveals whether a species has unpaired electrons. If there is at least one unpaired electron, the species is paramagnetic and is weakly attracted to a magnetic field. If every electron is paired, it is diamagnetic and is weakly repelled. Iron, with four unpaired 3d electrons, is strongly paramagnetic; zinc, with all electrons paired, is diamagnetic.

How to Use This Calculator

1. Enter the element: type a symbol (Fe), a name (Iron), or an atomic number (26).
2. Choose the charge: leave it neutral, or pick a charge to get an ion such as Fe³⁺ or O²⁻.
3. Click Calculate: the configuration is generated instantly.
4. Review the results: read the full and noble-gas configurations, study the orbital box diagram and Aufbau chart, and check the unpaired-electron count, magnetism, and quantum numbers.

Frequently Asked Questions

What is an electron configuration?

An electron configuration describes how the electrons of an atom or ion are distributed among its atomic subshells, such as 1s, 2s, and 2p. It is written as a list of subshells with the number of electrons in each shown as a superscript, for example carbon is 1s² 2s² 2p².

What is the noble-gas (condensed) configuration?

The noble-gas or condensed configuration replaces the inner core electrons with the symbol of the previous noble gas in square brackets. For iron, instead of writing 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² you write [Ar] 3d⁶ 4s², which is shorter and highlights the valence electrons.

What is the Aufbau principle?

The Aufbau principle states that electrons fill atomic orbitals from the lowest available energy level upward. The order of filling follows the Madelung rule, which orders subshells by increasing n + ℓ, and is summarized by the diagonal-arrow chart used in this calculator.

How do you find the electron configuration of an ion?

For a negative ion, add the extra electrons following the normal Aufbau order. For a positive ion, remove electrons from the outermost shell first, taking the highest principal quantum number before the inner d electrons. For example iron loses its 4s electrons before its 3d electrons, so Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵.

Why are chromium and copper exceptions?

A completely filled or exactly half-filled d subshell is especially stable. Chromium and copper each move one electron from the 4s subshell into the 3d subshell to reach a half-filled (3d⁵) or fully filled (3d¹⁰) d subshell, giving [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ instead of the expected 4s² configurations.

What does paramagnetic and diamagnetic mean?

A species with one or more unpaired electrons is paramagnetic and is weakly attracted to a magnetic field. A species in which all electrons are paired is diamagnetic and is weakly repelled. The calculator counts unpaired electrons from the orbital box diagram to classify each atom or ion.

Additional Resources

• Electron Configuration - Wikipedia
• Aufbau Principle - Wikipedia