What this calculator does
Electron configuration describes how the electrons of an atom are arranged across its subshells, written as a string such as 1s² 2s² 2p⁴ for oxygen. Each part gives the shell number, the type of subshell, and how many electrons sit in it, and reading through an electron configuration chart by hand for a heavier element means counting through every preceding subshell in order.
This calculator works out the full configuration for any atomic number from 1 to 56, following the Aufbau principle of filling subshells in a fixed order, from 1s through to 6s. A small number of elements, including chromium and copper, are well-known exceptions to that plain filling order, and this calculator applies the correct published ground-state configuration for those specific cases rather than the naive Aufbau result.
The formula
Electrons fill subshells in a fixed sequence, 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, each up to its own capacity (2 electrons for an s subshell, 6 for p, 10 for d) before the next subshell begins. For most elements this Aufbau order alone gives the correct ground-state configuration; for a handful of elements, an electron shifts from an s subshell into a d subshell because the resulting arrangement is more stable, and this calculator accounts for those specific known cases.
| Term | Meaning |
|---|---|
| Subshell | A group of orbitals of the same type (s, p or d) within a shell, each holding a fixed maximum number of electrons. |
| Aufbau principle | The rule that electrons fill the lowest-energy available subshell first, giving the standard fill order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s. |
| Noble-gas shorthand | Writing a configuration starting from the nearest preceding noble gas in brackets, such as [Ar], instead of spelling out every earlier subshell. |
| Ground-state exception | An element whose lowest-energy arrangement shifts an electron out of the expected subshell, such as chromium and copper, because a half-filled or fully filled d subshell is unusually stable. |
The inputs explained
| Field | What to enter |
|---|---|
| Atomic number (Z) | The atomic number (number of protons, and of electrons in a neutral atom), from 1 to 56. |
When to use it
Working out a configuration without counting by hand
Rather than working through a printed electron configuration chart and counting off each subshell in turn, entering the atomic number directly returns the full configuration.
Checking a known exception
Chromium and copper are commonly asked about because they break the plain Aufbau pattern; this calculator applies their actual published ground-state configuration rather than the naive prediction.
Finding the outermost shell for a quick chemistry check
The outermost occupied shell is often what is actually needed for a reactivity or bonding question, and is shown separately alongside the full configuration.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Electron configuration for a spread of elements
A spread of atomic numbers, including two of the well-known ground-state exceptions.
| Atomic number (Z) | Electron configuration | Noble-gas shorthand |
|---|---|---|
| 8 | 1s² 2s² 2p⁴ | [He] 2s² 2p⁴ |
| 11 | 1s² 2s² 2p⁶ 3s¹ | [Ne] 3s¹ |
| 17 | 1s² 2s² 2p⁶ 3s² 3p⁵ | [Ne] 3s² 3p⁵ |
| 20 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² | [Ar] 4s² |
| 24 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ | [Ar] 4s¹ 3d⁵ |
| 26 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ | [Ar] 4s² 3d⁶ |
| 29 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰ | [Ar] 4s¹ 3d¹⁰ |
| 36 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ | [Ar] 4s² 3d¹⁰ 4p⁶ |
| 47 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d¹⁰ | [Kr] 5s¹ 4d¹⁰ |
Questions
How do I find electron configuration for an element by hand?
Write down the Aufbau fill order (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s) and fill each subshell to its capacity, in order, until the running total reaches the element's atomic number. This calculator does the same count, plus applies the known exceptions.
Why does the order fill 4s before 3d?
Despite 3d having a lower shell number, the 4s subshell sits at a lower energy for the elements where this matters, so it fills first under the Aufbau principle. This is a well-established feature of the standard fill order, not an error in the sequence.
Why is this calculator limited to atomic number 56?
From element 57 onward, the f subshells begin filling, and a larger number of additional ground-state exceptions appear across the lanthanide and actinide elements than can be verified as consistently and simply as the ones handled here, so the range is kept to where every result can be checked against a standard reference.
What is the point of the noble-gas shorthand?
It shortens a long configuration by replacing the electrons that exactly match the nearest smaller noble gas with that gas's symbol in brackets, leaving only the electrons added beyond that point, which is the form most textbooks use once configurations get longer than a couple of subshells.
For a related fixed-value chemistry lookup, see the electronegativity calculator.