Electron Configuration Calculator

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How to Use the Electron Configuration Calculator

1. 1. Choose the element. Type a symbol (Fe), a name (iron) or an atomic number (26). Case does not matter.
2. 2. Add a charge for an ion. Write it after the symbol, as in Fe3+, Cl- or O2-, or use the separate charge box.
3. 3. Read the configurations. The full and noble-gas forms are shown with real superscripts, along with the electron count, unpaired electrons, magnetism and outer shell. Tick the options to write subshells in filling order or show only the outer part of the diagram.
4. 4. Read the orbital diagram. Each box is one orbital, with arrows for electrons, filled by Hund's rule. Atoms with unpaired electrons are paramagnetic; those with none are diamagnetic.

The Filling Rules

Three principles describe how electrons occupy orbitals. The Aufbau principle says electrons fill the lowest-energy subshell available. The Pauli exclusion principle says an orbital holds at most two electrons, with opposite spins. Hund's rule says electrons occupy the orbitals of a subshell one at a time, with parallel spins, before any orbital gets a second electron.

The Madelung rule gives the usual order: subshells fill by increasing n + l, and by lower n when n + l ties. That produces the familiar sequence below. A subshell holds 2 (s), 6 (p), 10 (d) or 14 (f) electrons.

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

s 2, p 6, d 10, f 14

Worked Example: Iron and Its Ions

Iron has 26 electrons. Filling in order gives 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, which is written by shell as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Replacing the argon core 1s² 2s² 2p⁶ 3s² 3p⁶ with [Ar] gives the noble-gas form [Ar] 3d⁶ 4s². In the 3d subshell, five orbitals take one electron each and one is then paired, leaving 4 unpaired electrons, so iron is paramagnetic. Its outer shell is n = 4, with 2 electrons.

Cations lose electrons from the outermost shell first, so Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵, a half-filled d subshell with 5 unpaired electrons. That is the reverse of the filling order, where 4s fills before 3d, because once the 3d electrons are present, the 4s level is the higher one.

Exceptions to the Filling Order

For these twenty elements the measured ground state differs from what the Madelung rule predicts. The calculator takes every neutral configuration from NIST's table of ground levels, so it gets them right; the table lists the prediction beside the measurement.

In chromium and copper the 3d and 4s levels are so close that a half-filled or filled 3d subshell wins. Palladium has no 5s electron at all. The lanthanide and actinide cases come from 4f and 5d (or 5f and 6d) being almost equal in energy. None of this is a rule you can predict from the table of orbitals alone, which is why the data come from measurement.

Frequently Asked Questions

How do you write the electron configuration of an element?

Find the number of electrons (the atomic number for a neutral atom) and fill subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on, up to each subshell's capacity. For example, sulfur with 16 electrons is 1s² 2s² 2p⁶ 3s² 3p⁴. Check the exceptions table before relying on the pattern for transition metals.

What is the noble-gas shorthand for an electron configuration?

It replaces the inner electrons by the symbol of the previous noble gas in brackets. Sulfur becomes [Ne] 3s² 3p⁴ and iron [Ar] 3d⁶ 4s². The bracketed part must be a complete noble-gas configuration, so the calculator picks the largest one that fits.

Why do transition metals lose 4s electrons before 3d?

Once the 3d subshell has electrons, it is lower in energy than 4s, so the 4s electrons are the outermost and easiest to remove. That is why Fe²⁺ is [Ar] 3d⁶ rather than [Ar] 3d⁴ 4s².

Why are chromium and copper exceptions?

Their 3d and 4s subshells are so close in energy that a half-filled (Cr, 3d⁵ 4s¹) or completely filled (Cu, 3d¹⁰ 4s¹) d subshell is slightly more stable than the predicted arrangement. The same effect produces the exceptions in the 4d, 5d, lanthanide and actinide series.

What is Hund's rule?

Electrons fill the orbitals of a subshell singly, with parallel spins, before any orbital is doubly occupied. This is why nitrogen's three 2p electrons occupy three different orbitals, giving 3 unpaired electrons, and why the orbital diagram shows single arrows before pairs.

How do unpaired electrons relate to magnetism?

An atom or ion with one or more unpaired electrons is paramagnetic, meaning it is attracted into a magnetic field. One with all electrons paired is diamagnetic and is weakly repelled. Fe³⁺, with 5 unpaired electrons, is strongly paramagnetic; Zn²⁺, [Ar] 3d¹⁰, is diamagnetic.

What is the electron configuration of an anion?

Add the extra electrons to the next subshell in filling order. Chloride, Cl⁻, has 18 electrons, 1s² 2s² 2p⁶ 3s² 3p⁶, the same as argon, so it is written [Ar]. Oxide, O²⁻, is [Ne].