Electron Configuration Calculator
Full and noble-gas shorthand electron configurations for any element, 1 to 118.
What it does
An electron configuration is the ledger of where an atom's electrons live: which shells, which subshells, and how many in each. Written in increasing principal quantum number order, it says which orbitals are occupied, and the outermost entries drive the element's chemistry.
Enter any atomic number from hydrogen to oganesson. The tool fills subshells in the experimentally observed order, prints the full configuration, compresses the core into the standard noble-gas bracket, and flags the twenty elements where the simple aufbau rule needs an override.
The defaults show iron: atomic number 26 comes out as 1s2 2s2 2p6 3s2 3p6 3d6 4s2, or [Ar] 3d6 4s2 in shorthand. Try chromium (24) or copper (29) to see the famous half-filled and filled d-subshell exceptions, or lawrencium (103) for one of the most recently confirmed overrides.
The fill order and exception list follow standard chemistry references: the configuration chapters in OpenStax Chemistry, the NIST Atomic Spectra Database ground-state configurations used to settle disputes, and the Royal Society of Chemistry's element pages for spot checks.
How to use
- Type an atomic number from 1 to 118.
- Read the element name and symbol back for confirmation.
- Take the full configuration for written work and exams.
- Use the shorthand when the core is understood.
- Check the note: it explains any aufbau override for that element.
Formula
- n
- Principal quantum number, the shell number.
- s, p, d, f
- Subshell types, holding 2, 6, 10, and 14 electrons.
- [X]
- Noble gas shorthand: the bracketed gas's configuration stands in for the core.
- supers
- The count written after each subshell, like the 6 in 3d6.
The aufbau principle fills the lowest available energy first, and the order above is the empirical version of that principle: 4s fills before 3d even though it carries a higher n, because penetration and shielding put it lower in energy. Display here follows increasing n order, so 3d6 4s2 for iron. NIST and many textbooks instead write the d block before the next s, as [Ar] 3d6 4s2, following the order electrons are added. Both describe the same atom; choose one convention and stay consistent.
Worked example
Example: iron, atomic number 26
Defaults: atomic number 26.
- Fill 1s2 2s2 2p6 3s2 3p6, which accounts for 18 electrons (the argon core).
- The next two go to 4s: 4s2, then six to 3d: 3d6.
- Full configuration: 1s2 2s2 2p6 3s2 3p6 3d6 4s2.
- Shorthand: [Ar] 3d6 4s2.
- No exception applies: iron fills exactly as aufbau predicts.
Reference table
| Z | Element | Aufbau predicts | Actual |
|---|---|---|---|
| 24 | Chromium | 3d4 4s2 | 3d5 4s1 |
| 29 | Copper | 3d9 4s2 | 3d10 4s1 |
| 41 | Niobium | 4d3 5s2 | 4d4 5s1 |
| 42 | Molybdenum | 4d4 5s2 | 4d5 5s1 |
| 44 | Ruthenium | 4d6 5s2 | 4d7 5s1 |
| 45 | Rhodium | 4d7 5s2 | 4d8 5s1 |
| 46 | Palladium | 4d8 5s2 | 4d10 |
| 47 | Silver | 4d9 5s2 | 4d10 5s1 |
| 57 | Lanthanum | 4f1 6s2 | 5d1 6s2 |
| 58 | Cerium | 4f2 6s2 | 4f1 5d1 6s2 |
| 64 | Gadolinium | 4f8 6s2 | 4f7 5d1 6s2 |
| 78 | Platinum | 5d8 6s2 | 5d9 6s1 |
| 79 | Gold | 5d9 6s2 | 5d10 6s1 |
| 89 | Actinium | 5f1 7s2 | 6d1 7s2 |
| 90 | Thorium | 5f2 7s2 | 6d2 7s2 |
| 91 | Protactinium | 5f3 7s2 | 5f2 6d1 7s2 |
| 92 | Uranium | 5f4 7s2 | 5f3 6d1 7s2 |
| 93 | Neptunium | 5f5 7s2 | 5f4 6d1 7s2 |
| 96 | Curium | 5f8 7s2 | 5f7 6d1 7s2 |
| 103 | Lawrencium | 5f14 6d1 7s2 | 5f14 7s2 7p1 |
Limits
- Covers neutral atoms only; ion configurations are not computed.
- Configurations for elements 104 and heavier are theoretical.
- Gas-phase and solid-state configurations can differ for some elements.
- Notation conventions vary; this tool uses increasing n order.
Common mistakes
- Expecting aufbau to be exact for every element.
- Writing 3d before 4s and then losing track of the filling order.
- Forgetting the half-filled stability behind chromium and copper.
- Mixing up valence electrons with the full configuration.
- Treating the shorthand bracket as always ending at the previous period.
Try it yourself
Quick checks. Answers sit tucked under each question.
Copper, Z = 29?
[Ar] 3d10 4s1: a full d subshell beats the predicted 3d9 4s2.
How many valence electrons does iron have?
Eight: six in 3d and two in 4s, the electrons added after the argon core.
Palladium, Z = 46?
[Kr] 4d10: uniquely among the elements so far, the 5s subshell stays empty.
Questions
What is an electron configuration?
A listing of which subshells an atom's electrons occupy, like 1s2 2s2 2p6 for neon, written as shell number, subshell letter, and electron count.
What does the bracketed symbol mean?
Noble gas shorthand: [Ar] stands for the 18 electrons of argon's configuration, and only the subshells filled after it are written out.
Why do the exceptions exist?
Half-filled and completely filled d and f subshells sit slightly lower in energy than simple aufbau predicts, so an electron shifts to reach that arrangement.
Why is potassium [Ar] 4s1 and not [Ar] 3d1?
For potassium the 4s orbital is genuinely lower in energy than 3d because of penetration and shielding; the periodic table's shape follows that fact.
Does the tool handle ions?
No. It reports neutral atoms; for ions you remove or add electrons following the parent configuration rules.
Why does NIST write 3d before 4s?
NIST orders by the sequence electrons are added and by spectroscopic convention, while this tool sorts by increasing n. Both orders describe the same occupied subshells.
Sources
- OpenStax Chemistry 2e, 6.4 Electron Configurations
- NIST Atomic Spectra Database, energy levels form
- Royal Society of Chemistry periodic table: iron