# Electron Configuration Calculator - Orbital Diagram, Ions, Unpaired Electrons

> Free electron configuration calculator for any element or ion (Fe3+, O2-): full and noble-gas notation, an orbital diagram, unpaired electrons and shells.

- URL: https://qilingtools.com/en/math/electron-config
- Category: Chemistry Tools (https://qilingtools.com/en/chemistry)
- Tags: [Chemistry](https://qilingtools.com/en/tags/chemistry), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-26
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

An electron configuration lists how an atom's electrons are distributed among subshells. Following the Aufbau principle (Madelung n + l rule), subshells fill in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, …; each orbital holds at most two electrons of opposite spin (Pauli exclusion principle), and electrons spread out over a subshell with parallel spins before pairing (Hund's rule). Iron (Z = 26), for example, is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s² in noble-gas notation; Fe³⁺ loses the two 4s electrons first and then one 3d electron, giving [Ar] 3d⁵ with five unpaired electrons (paramagnetic). Enter a symbol, an English or Chinese name, or an atomic number, add a charge for an ion (Fe3+, Cu+, Cl-, O2-), or pick from the mini periodic table. Neutral atoms use the spectroscopic ground states, including the exceptions Cr (3d⁵ 4s¹), Cu (3d¹⁰ 4s¹), Mo, Ag, Pd, Pt, Au, La, Ce, Gd and the actinides Th, U and others, with the Madelung prediction shown for comparison. You get the full configuration, noble-gas shorthand, valence configuration and count, an orbital box diagram with up/down arrows, unpaired electrons and magnetism, electrons per shell (K L M N…), an Aufbau diagonal diagram, a Bohr model, and the four quantum numbers of the last electron, with the working shown.

## How to use

1. Type an element or ion such as Fe, Fe3+, O2- or copper, or open the periodic table and click an element
2. Use the charge buttons to switch between common ions and read the full configuration, shorthand, valence electrons and unpaired electrons
3. Scroll down for the orbital box diagram, Aufbau filling order, Bohr model, quantum numbers and step-by-step working

## Features

- All 118 elements plus their cations and anions
- Real ground states including the Cr, Cu, Pd, Ag, Pt, Au, lanthanide and actinide exceptions, compared with the Madelung prediction
- Transition-metal ions lose ns electrons before (n−1)d
- Orbital box diagram with Hund's rule and Pauli arrows and m_l labels
- Unpaired electron count with paramagnetic / diamagnetic label
- Valence configuration, valence electron count and electrons per shell
- Aufbau diagonal-rule chart and Bohr model drawn as SVG
- Quantum numbers (n, l, m_l, m_s) of the last electron; one-click copy

## FAQ

### Why are chromium and copper exceptions?

The Aufbau rule predicts [Ar] 3d⁴ 4s² for Cr and [Ar] 3d⁹ 4s² for Cu, but spectroscopy shows [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. Because 3d and 4s are so close in energy, the extra exchange stabilisation of a half-filled (d⁵) or filled (d¹⁰) d subshell makes it favourable to move one 4s electron into 3d. Mo, Ag and Au behave the same way, and Nb, Ru, Rh, Pd, Pt and several lanthanides and actinides have their own exceptions; the tool flags every one.

### What is the electron configuration of Fe3+, and why are 4s electrons lost first?

Fe is [Ar] 3d⁶ 4s². To make Fe³⁺, remove three electrons: both 4s electrons first, then one 3d electron, giving [Ar] 3d⁵ (1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵). Although 4s fills before 3d, once 3d is occupied the 4s electrons are higher in energy and farther from the nucleus, so cations always lose the highest-n electrons first. Likewise Cu²⁺ is [Ar] 3d⁹ and Zn²⁺ is [Ar] 3d¹⁰.

### How do I count unpaired electrons?

By Hund's rule, electrons occupy separate orbitals of a subshell with parallel spins before any pairing. The number of unpaired electrons in a subshell is therefore min(electrons, capacity − electrons): 5 for 3d⁵, 4 for 3d⁶, 2 for 2p⁴. Add them over all subshells: Cr has 6, Fe has 4, O has 2. Species with unpaired electrons are paramagnetic; if all are paired they are diamagnetic.

### How are the four quantum numbers of the last electron found?

Take the subshell filled last to get n and l (s = 0, p = 1, d = 2, f = 3). Then place the electrons in the orbital boxes by Hund's rule; the box holding the last electron gives m_l, and its spin gives m_s = +½ (up) or −½ (down). Oxygen, 2p⁴, gets three up-arrows in m_l = −1, 0, +1 and then a down-arrow in −1, so its last electron is n = 2, l = 1, m_l = −1, m_s = −½. Some textbooks number m_l from +l instead, which changes m_l.

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