Electron configuration calculator
Type an element symbol, name or atomic number, such as Fe, iron or 26. The calculator writes the ground-state electron configuration in full and in noble-gas shorthand, draws the orbital diagram and counts the electrons per shell and the unpaired electrons. Exceptions such as chromium and copper use the measured configurations.
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How to use it
- Type an element symbol, its name or its atomic number.
- Choose the order of subshells: filling order or by shell.
- Choose whether to draw the orbital diagram for the outer subshells, all of them, or none.
- Copy the full configuration or the shorthand with the copy buttons.
How it works
Electrons fill subshells in the Madelung (Aufbau) order of increasing n + l, then n:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
An s subshell holds 2 electrons, p 6, d 10 and f 14. Twenty elements have measured ground states that break this order: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. They are listed in the code with their measured configurations and checked by tests. In the orbital diagram each box is one orbital: electrons enter singly with parallel spins before pairing (Hund’s rule), and the unpaired count follows from that.
Worked example
Iron, Fe, Z = 26: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or [Ar] 4s² 3d⁶. The shells hold 2, 8, 14 and 2 electrons. The 3d subshell has five orbitals for six electrons: one pair and four single electrons, so iron has 4 unpaired electrons.
Copper, Z = 29, is an exception: [Ar] 4s¹ 3d¹⁰ (by shell, [Ar] 3d¹⁰ 4s¹), not the predicted [Ar] 4s² 3d⁹.
Common mistakes
- Removing electrons for ions. Transition metals lose their 4s electrons first: Fe²⁺ is [Ar] 3d⁶, not [Ar] 4s² 3d⁴.
- Counting the core. The shorthand still means all the electrons; [Ar] stands for 18 of them.
- Superheavy elements. From rutherfordium (Z = 104) on, configurations are predictions, not measurements.
Frequently asked questions
How is an electron configuration written?
Each subshell is written as its shell number, its letter (s, p, d or f) and the number of electrons as a superscript. Iron is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, with 26 electrons in total.
What is the noble-gas shorthand?
The inner electrons that match the previous noble gas are replaced by its symbol in brackets. Iron becomes [Ar] 4s² 3d⁶, because argon accounts for the first 18 electrons.
Why are chromium and copper exceptions?
A half-filled or filled d subshell is especially stable, and the 4s and 3d energies are very close. Chromium is [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², and copper [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s².
Which elements are treated as exceptions?
Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr, using their measured ground-state configurations. A note under the result shows what the simple filling rule would predict.
Should 4s be written before or after 3d?
Both are used. The filling order (4s before 3d) follows the Aufbau rule taught in many schools; ordering by shell (3d before 4s) is used in reference tables. Switch with the Order of subshells setting.