About the Electron Configuration Calculator
This electron configuration calculator writes the ground-state electron configuration of any element from hydrogen (1) to oganesson (118), in both full notation (1s² 2s² 2p⁶…) and the shorter noble-gas notation ([Ar] 4s² 3d⁶). It also draws a box-style orbital diagram, counts unpaired electrons using Hund’s rule, and tells you the element’s block and the number of electrons in its outermost shell.
It is meant for high-school and university chemistry students, and anyone checking homework on the Aufbau principle. Enter a charge to get the configuration of an ion: cations lose electrons from the outermost shell first — so iron(II) is [Ar] 3d⁶, not [Ar] 4s² 3d⁴ — and anions gain electrons in filling order.
The calculator follows the Madelung (n + l) filling order and applies the well-known exceptions such as chromium (3d⁵ 4s¹), copper (3d¹⁰ 4s¹), palladium (4d¹⁰), gold and several lanthanides and actinides, matching NIST ground states. Configurations for elements beyond lawrencium (104–118) are theoretical predictions, because those atoms have never been measured spectroscopically.
How to use the electron configuration calculator
- 1Choose the element from the list (sorted by atomic number).
- 2Enter a charge if you need an ion, such as +2 for Fe²⁺ or −2 for O²⁻.
- 3Read the full configuration and the noble-gas shorthand.
- 4Use the orbital diagram to see how electrons pair within each subshell.
Formula and method
Electrons occupy the lowest-energy subshells first (the Aufbau principle). Subshell energy is approximated by the Madelung rule: lower n + l fills first, and for equal n + l the lower n fills first. Each subshell holds 2(2l + 1) electrons — 2 for s, 6 for p, 10 for d and 14 for f — because each orbital takes two electrons of opposite spin (Pauli exclusion).
Within a subshell, electrons spread across separate orbitals with parallel spins before pairing (Hund’s rule), which is how unpaired electrons are counted. Twenty elements up to lawrencium have measured ground states that differ from the simple order (for example Cr, Cu, Pd, Au, La, Ce, Gd, Th, U); these are applied as corrections. For cations, electrons are removed from the highest principal shell first, then (n−1)d, then (n−2)f.
- n
- Principal quantum number (shell)
- l
- Angular momentum quantum number: s = 0, p = 1, d = 2, f = 3
- 2(2l + 1)
- Maximum electrons in a subshell
Worked examples
Iron (Fe, Z = 26)
Iron’s 26 electrons fill 1s through 4s (20 electrons) and leave 6 in 3d. Five 3d orbitals take one electron each and one pairs up, leaving 4 unpaired electrons.
Iron(III) ion, Fe³⁺
Fe³⁺ loses both 4s electrons first and then one 3d electron, leaving [Ar] 3d⁵ — a half-filled d subshell with 5 unpaired electrons.
Copper, an Aufbau exception
Simple filling predicts 4s² 3d⁹, but copper’s measured ground state is 4s¹ 3d¹⁰ because a filled d subshell is more stable. It has one unpaired electron.
Chloride ion, Cl⁻
Chlorine (3s² 3p⁵) gains one electron to fill its 3p subshell, giving 18 electrons — the same configuration as argon.
Frequently asked questions
How do you write an electron configuration?+
Fill subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d… placing up to 2 electrons in s, 6 in p, 10 in d and 14 in f, until you have placed as many electrons as the atomic number.
What is noble gas notation?+
It abbreviates the inner electrons with the symbol of the previous noble gas in brackets. Sodium’s 1s² 2s² 2p⁶ 3s¹ becomes [Ne] 3s¹, so only the valence electrons are written out.
Why are chromium and copper exceptions?+
The 4s and 3d energies are very close, and half-filled (d⁵) or completely filled (d¹⁰) subshells gain extra stability from exchange energy. Chromium is therefore [Ar] 3d⁵ 4s¹ and copper [Ar] 3d¹⁰ 4s¹.
Why does 4s lose electrons before 3d in transition-metal ions?+
Once 3d electrons are present, the 3d orbitals drop below 4s in energy, and the 4s electrons are farther from the nucleus. So Fe²⁺ is [Ar] 3d⁶ — both 4s electrons are removed first.
How do I find the number of unpaired electrons?+
Apply Hund’s rule: in each subshell put one electron in every orbital before pairing any. Oxygen’s 2p⁴ has one pair and two singles, so it has 2 unpaired electrons and is paramagnetic.
What are valence electrons in an electron configuration?+
For main-group elements they are the electrons in the highest-numbered shell, s plus p — chlorine has 7. For transition metals the (n−1)d electrons also take part in bonding, so “valence” is less clear-cut.