Electron Configuration Builder
Pick any of the 118 elements, or type an ion such as Fe3+, to see its electron configuration, orbital diagram and quantum numbers — then switch to Practice and fill the orbitals yourself.
At a glance
- Gives the ground-state electron configuration of all 118 elements, in full (1s² 2s² 2p⁶ …) and in noble-gas shorthand ([Ar] 3d⁶ 4s²).
- Handles ions from 4− to 8+: electrons leave the outermost shell first, so Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴ 4s².
- Includes the 20 elements that break the filling order, such as chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹), and shows what the rule would have predicted.
- Checked against the NIST Atomic Spectra Database for every atom from hydrogen to hassium and every positive ion of those elements up to 8+.
- Draws the orbital diagram with Hund’s rule, and can fill it one electron at a time.
- Lists electrons per shell, valence electrons, unpaired electrons, whether the atom is paramagnetic or diamagnetic, its block, period and group, and the quantum numbers of the last electron.
- Practice mode: fill the boxes yourself and get told which rule — Aufbau, Pauli or Hund — the first mistake breaks.
- Free, with nothing to install; works on a phone.
Step by step
How to find an electron configuration
- 1
Choose the element
Type a symbol, name or atomic number, or click the element in the periodic table.
- 2
Set the charge
For an ion, type it (Fe3+, Cl-) or use the − and + buttons beside the symbol.
- 3
Read the configuration
The noble-gas form and the full form appear with copy buttons. Switch the order between “by shell” and “as they fill” to match your course.
- 4
Study the diagram
The orbital diagram shows paired and unpaired electrons. Drag the slider to watch the orbitals fill one electron at a time.
- 5
Practise
Choose Practice, fill the boxes for the element shown and press Check.
Features
Everything you need, nothing you don’t
Atoms and ions
Type Fe, iron, 26 or Fe3+, or click the periodic table and step the charge up and down.
Both ways of writing it
By shell ([Ar] 3d⁶ 4s²), as most textbooks and data tables print it, or in filling order ([Ar] 4s² 3d⁶), as many classes teach it.
Exceptions explained
For chromium, copper, silver, gold and the rest, the measured configuration is shown next to the predicted one, so you see exactly what differs.
A diagram you can build
Click orbitals to add and pair electrons. The check points to the first subshell that is wrong and explains the rule behind it.
Answers the follow-up questions
How many valence electrons? How many unpaired? Paramagnetic? What are n, l, mℓ and ms of the last electron? They are all on the page.
Backed by measured data
Configurations follow NIST’s published ground states, not just a rule — including the awkward 1+ ions of vanadium, cobalt and nickel.
The three rules
- Aufbau principle — electrons fill the lowest-energy subshell that has room, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
- Pauli exclusion principle — an orbital holds at most two electrons, with opposite spins. That makes the capacities s = 2, p = 6, d = 10, f = 14.
- Hund’s rule — within a subshell, every orbital gets one electron before any orbital gets a second.
Examples
| Atom or ion | Configuration | Unpaired electrons |
|---|---|---|
| Carbon, C | [He] 2s² 2p² | 2 |
| Sodium, Na | [Ne] 3s¹ | 1 |
| Chlorine, Cl | [Ne] 3s² 3p⁵ | 1 |
| Iron, Fe | [Ar] 3d⁶ 4s² | 4 |
| Chromium, Cr | [Ar] 3d⁵ 4s¹ (exception) | 6 |
| Copper, Cu | [Ar] 3d¹⁰ 4s¹ (exception) | 1 |
| Fe²⁺ | [Ar] 3d⁶ | 4 |
| Fe³⁺ | [Ar] 3d⁵ | 5 |
| Cu²⁺ | [Ar] 3d⁹ | 1 |
| Cl⁻ | [Ar] | 0 |
Elements that break the filling order
Twenty elements have a ground state the filling order does not predict: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. In most of them an electron moves from the s subshell into d, because a half-filled or filled d subshell is slightly more stable — chromium is 3d⁵ 4s¹ rather than 3d⁴ 4s², and copper 3d¹⁰ 4s¹ rather than 3d⁹ 4s².
These cannot be worked out from the rule; they are measured. The tool shows the measured configuration and, beside it, what the rule alone would give.
Why ions lose 4s before 3d
In a neutral atom 4s fills before 3d. Once 3d holds electrons, though, it lies lower in energy than 4s, so when a transition metal becomes a positive ion the 4s electrons leave first. Iron, [Ar] 3d⁶ 4s², becomes Fe²⁺ = [Ar] 3d⁶ and Fe³⁺ = [Ar] 3d⁵. Writing Fe²⁺ as [Ar] 3d⁴ 4s² — the configuration of a neutral atom with 24 electrons — is the commonest mistake, and Practice mode points it out.
FAQ
Frequently asked questions
What is the electron configuration of iron?
Should 4s or 3d be written first?
How many valence electrons does an element have?
What are the quantum numbers of the last electron?
Why is my answer for Cr or Cu marked wrong in Practice?
How reliable are the configurations of the heaviest elements?
Which ions can it show?
Keep going
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