Electron Configuration Calculator
Get the electron configuration of any element. Enter an atomic number from 1 to 118 to see the full Aufbau (fill-order) configuration, the noble-gas shorthand, and the number of valence electrons in the outer shell.
Example: with Atomic number (Z, 1–118) 26 → Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
- Noble-gas shorthand[Ar] 4s² 3d⁶
- Valence electronsFe — 2 valence electrons (outer shell n=4)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Electrons fill orbitals from lowest energy up (the Aufbau principle), following the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… with s holding 2, p holding 6, d holding 10 and f holding 14.
Filling orbitals from the bottom up
Electrons occupy the lowest-energy orbitals first, a rule called the Aufbau principle. The order is not simply by shell number: the 4s subshell fills before 3d, and 6s before 4f, because energy levels overlap. Each subshell has a fixed capacity — s holds 2 electrons, p holds 6, d holds 10, and f holds 14 — and you keep filling until you have placed all Z electrons.
Iron (Z = 26) works out to 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. The noble-gas shorthand replaces the argon core (the first 18 electrons) with [Ar], leaving [Ar] 4s² 3d⁶. The two electrons in the outermost 4s shell are its valence electrons, which is what governs most of iron's everyday chemistry.
How it’s calculated
Electrons are assigned in Aufbau fill order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, with capacities 2/6/10/14 for s/p/d/f. Shorthand replaces the largest fully filled noble-gas core (He, Ne, Ar, Kr, Xe, Rn). Valence electrons are counted as those in the highest principal quantum number n present.
Configurations follow the idealized Aufbau ordering. A handful of elements (such as chromium and copper) have known exceptions where a single electron shifts to half-fill or fill a d subshell; those anomalies are not applied here.
Electron configurations of sample elements
| Element (Z) | Configuration | Shorthand |
|---|---|---|
| Hydrogen (1) | 1s¹ | 1s¹ |
| Carbon (6) | 1s² 2s² 2p² | [He] 2s² 2p² |
| Neon (10) | 1s² 2s² 2p⁶ | [He] 2s² 2p⁶ |
| Iron (26) | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ | [Ar] 4s² 3d⁶ |
| Bromine (35) | …4s² 3d¹⁰ 4p⁵ | [Ar] 4s² 3d¹⁰ 4p⁵ |
Built from the Aufbau fill order with s/p/d/f capacities 2/6/10/14.
Common mistakes
- Filling 3d before 4s — the 4s subshell is lower in energy and fills first.
- Giving a d subshell more than 10 electrons or a p more than 6.
- Forgetting the noble-gas core must be completely filled before the shorthand starts.
- Assuming every element follows Aufbau exactly; chromium and copper are common exceptions.
Frequently asked questions
How do you write an electron configuration?
Fill orbitals in Aufbau order (1s, 2s, 2p, 3s, 3p, 4s, 3d…) until you have placed all Z electrons, respecting the s/p/d/f capacities of 2, 6, 10 and 14.
What is noble-gas shorthand?
It replaces the inner core of electrons with the symbol of the previous noble gas in brackets. Iron's [Ar] 4s² 3d⁶ stands for argon's 18 electrons plus the rest.
What are valence electrons?
They are the electrons in the outermost shell — the highest principal quantum number n. For main-group elements this count matches the group and drives bonding.
Why does 4s fill before 3d?
Energy levels overlap beyond the third shell, and the 4s orbital sits slightly lower than 3d, so it fills first even though its shell number is higher.