See how electrons fill orbitals for every element, one electron at a time.
Full and noble-gas notation, orbital box diagrams, the Aufbau order, Hund's rule, the Pauli principle and the exceptions (Cr, Cu and 18 more), all in your browser.
Features · Screenshots · How it works · For teachers · Quick links · FAQ
Iron (Fe) filling one electron at a time. Each step names the rule it follows: Aufbau, Hund or Pauli.
Type an element, a symbol or an atomic number and you get:
Iron (Fe), Z = 26
Full configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
Noble gas notation [Ar] 3d⁶ 4s²
Unpaired electrons 4 → paramagnetic
Shells (K L M N) 2 · 8 · 14 · 2
- Search by name, symbol or atomic number (
oxygen,Oor8), with suggestions as you type. - Browse with the atomic number box, a list of all 118 elements, Prev / Next, Random and quick picks (H, C, N, O, Na, Cl, Cr, Fe, Cu, Au, U).
- Mini periodic table coloured by s, p, d and f block. Click a tile, or move with the arrow keys and press Enter.
- Full configuration and noble-gas notation, each with superscripts and colour-coded by subshell.
- Show both or just one, and copy either with one click.
- Every orbital drawn as a box with ↑ and ↓ spins, so Hund's rule (single electrons with parallel spins first) and the Pauli principle (two opposite spins per box) are visible at a glance.
- Two layouts: filling order or grouped by shell, with a
filled / capacitycount under each subshell.
- Play, Pause, Replay and Step controls, speeds from 0.5× to 4×, and a slider to jump to any electron.
- Every step says where the electron went and why, for example "Electron 12 of 26 goes into 3s … pairs with opposite spin (Pauli exclusion principle)."
- The Aufbau track (1s → 2s → 2p → 3s → …) lights up as subshells fill.
- Respects the reduce motion setting of your device.
- The 20 elements whose real ground state breaks the simple filling order are flagged, and the tool shows the Aufbau prediction next to the accepted configuration:
Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. - The configurations of elements 104–118 are marked as predicted, because they have not been measured.
- Atom summary: protons, electrons, charge, unpaired electrons, paramagnetic or diamagnetic, and the outer shell.
- Shell distribution: a Bohr-style diagram with electrons per shell (K, L, M, N …) and each shell's capacity.
- Orbital filling table: each subshell's orbitals, capacity, electrons and unpaired electrons.
- Learning mode: short explanations of the Aufbau principle, Hund's rule, the Pauli principle and shells vs subshells.
- Shareable links: the address updates as you browse, so
?element=Feor?element=26opens straight on iron. - Light and dark themes, a layout that works on phones, and keyboard support (← → switch elements).
Orbital diagram 
Shell distribution 
Orbital filling table |
![Copper's card explaining that Aufbau predicts [Ar] 3d9 4s2 but the accepted configuration is [Ar] 3d10 4s1](https://github.laxmiang.work.gd/tooladda/electron-configuration-visualizer/blob/main/assets/exception-copper.png)
Exceptions explained |
The same diagram grouped by shell
Mini periodic table, coloured by s, p, d and f block
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Electrons go into the lowest-energy orbital that is still free. The tool uses the standard classroom model, and every visual follows from these rules:
| Rule | What it says | Where you see it |
|---|---|---|
| Aufbau principle | Subshells fill in order of increasing energy (the n + l, or Madelung, rule). | The filling-order track and the animation |
| Hund's rule | In a subshell with several orbitals, electrons take separate boxes with parallel spins before they pair. | Single ↑ arrows in p, d and f boxes |
| Pauli exclusion principle | An orbital holds at most two electrons, with opposite spins. | ↑↓ pairs |
Filling order
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
Subshell capacities
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Noble-gas notation replaces the inner electrons with the previous noble gas in brackets: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² becomes [Ar] 3d⁶ 4s².
Why some elements break the pattern. For chromium and copper, a half-filled or completely filled d subshell is more stable, so one 4s electron moves into 3d:
| Element | Aufbau predicts | Accepted ground state |
|---|---|---|
| Chromium (24) | [Ar] 3d⁴ 4s² | [Ar] 3d⁵ 4s¹ |
| Copper (29) | [Ar] 3d⁹ 4s² | [Ar] 3d¹⁰ 4s¹ |
Heavier elements such as Pd, Ag, Pt, Au, La and U have their own exceptions, and the tool flags each one.
About the data. All configurations are for neutral atoms in their ground state. Values for elements 104–118 are theoretical predictions and are labelled that way. The shell diagram counts electrons per shell; it is not a picture of real orbital shapes.
- On a projector: use Step and a slow speed so the class can predict where the next electron goes before you click.
- Learning mode keeps the three rules on screen while you explain them.
- Homework and worksheets: paste a link with
?element=and students land on the right element. - Revision: press Random, write the configuration yourself, then compare.
- Exceptions: Cr, Cu, Ag and Au make a good "why is this different?" discussion. The tool shows the predicted and the accepted configuration side by side.
| Element | Configuration | Open |
|---|---|---|
| Hydrogen (1) | 1s¹ | H |
| Carbon (6) | [He] 2s² 2p² | C |
| Oxygen (8) | [He] 2s² 2p⁴ | O |
| Sodium (11) | [Ne] 3s¹ | Na |
| Chlorine (17) | [Ne] 3s² 3p⁵ | Cl |
| Chromium (24) | [Ar] 3d⁵ 4s¹ | Cr |
| Iron (26) | [Ar] 3d⁶ 4s² | Fe |
| Copper (29) | [Ar] 3d¹⁰ 4s¹ | Cu |
| Silver (47) | [Kr] 4d¹⁰ 5s¹ | Ag |
| Gold (79) | [Xe] 4f¹⁴ 5d¹⁰ 6s¹ | Au |
| Uranium (92) | [Rn] 5f³ 6d¹ 7s² | U |
What is the electron configuration of iron?
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s² in noble-gas notation. Four of the six 3d electrons are unpaired, so iron is paramagnetic.
Why is copper 3d¹⁰ 4s¹ and not 3d⁹ 4s²?
A completely filled 3d subshell is lower in energy, so one electron moves from 4s to 3d. Chromium does the same to get a half-filled 3d⁵. The tool shows the predicted configuration next to the accepted one for all 20 exceptions.
What does noble-gas notation mean?
It writes the inner electrons as the previous noble gas in square brackets, e.g. [Ne] 3s¹ for sodium, so only the outer (valence) part is written out.
Can I link straight to an element?
Yes. Add ?element= with a symbol or an atomic number, for example ?element=Cu or ?element=29. The Copy link button does this for you.
Does it handle ions?
Not yet. The tool shows neutral atoms (charge 0).
Do I need to install anything or sign up?
No. It is a free web page that works in current versions of Chrome, Edge, Firefox and Safari, on computers, tablets and phones. The page is supported by ads.
| Tool | What it does |
|---|---|
| Interactive Periodic Table | Properties, trends and details for all 118 elements |
| Chemical Equation Balancer | Balances chemical equations |
| Molecular Structure Viewer | Rotatable 3D models of molecules |
| Half-Life Calculator | Radioactive decay and half-life calculations |
Found a configuration that looks wrong, or have an idea for a feature? Open an issue and include the element and what you expected to see.
This repository showcases the Electron Configuration Builder at tooladda.online. It does not contain the tool's source code, and no license is granted. © ToolAdda. All rights reserved.