An electron configuration lists how an atom’s electrons are distributed over its subshells. Iron, for example, is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶: the number gives the shell, the letter the subshell and the superscript how many electrons it holds. Three principles build it up – the Aufbau principle (fill the lowest-energy subshell first, in the order given by the n + l rule), the Pauli exclusion principle (at most two electrons, with opposite spins, per orbital) and Hund’s rule (spread electrons over equal-energy orbitals before pairing them).
Because the inner shells repeat the previous noble gas, chemists usually write the shorthand form: iron is [Ar] 3d⁶ 4s². About twenty elements break the simple filling order because nearly equal subshell energies trade places – chromium is [Ar] 3d⁵ 4s¹ and copper [Ar] 3d¹⁰ 4s¹ rather than 3d⁴ 4s² and 3d⁹ 4s². This calculator uses the measured ground states for all of them, including Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr.
Enter a charge to get the configuration of an ion: electrons are removed from the highest shell first, so Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴ 4s².
How to use the Electron Configuration Calculator
- Element (symbol, name or Z) — Type a symbol (Fe), a name (iron) or an atomic number (26).
- Charge (0 for a neutral atom) — E.g. 2 for Fe²⁺, −1 for Cl⁻.
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
The calculator fills the subshells in Madelung order (or uses the measured configuration for exceptions), then removes or adds electrons for an ion. It finds the largest noble-gas core for the shorthand, draws each subshell’s orbitals with Hund’s rule, counts unpaired electrons, and derives the block, group and period from the atomic number.
Worked example
Iron has Z = 26. Filling in order: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20) and the last six electrons in 3d: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or [Ar] 3d⁶ 4s². The five 3d orbitals hold six electrons as ↑↓ ↑ ↑ ↑ ↑ – four unpaired electrons, so iron atoms are paramagnetic. Shells: K 2, L 8, M 14, N 2. Iron is in period 4, group 8 of the d-block, with 2 + 6 = 8 valence electrons (4s + 3d).
For Fe³⁺ remove three electrons, the two 4s electrons first and then one 3d: [Ar] 3d⁵ with five unpaired electrons.
Exceptions to the Aufbau order
| Element | Predicted | Actual ground state |
|---|---|---|
| Cr (24) | [Ar] 3d⁴ 4s² | [Ar] 3d⁵ 4s¹ |
| Cu (29) | [Ar] 3d⁹ 4s² | [Ar] 3d¹⁰ 4s¹ |
| Nb (41) | [Kr] 4d³ 5s² | [Kr] 4d⁴ 5s¹ |
| Mo (42) | [Kr] 4d⁴ 5s² | [Kr] 4d⁵ 5s¹ |
| Ru (44) | [Kr] 4d⁶ 5s² | [Kr] 4d⁷ 5s¹ |
| Rh (45) | [Kr] 4d⁷ 5s² | [Kr] 4d⁸ 5s¹ |
| Pd (46) | [Kr] 4d⁸ 5s² | [Kr] 4d¹⁰ |
| Ag (47) | [Kr] 4d⁹ 5s² | [Kr] 4d¹⁰ 5s¹ |
| La (57) | [Xe] 4f¹ 6s² | [Xe] 5d¹ 6s² |
| Ce (58) | [Xe] 4f² 6s² | [Xe] 4f¹ 5d¹ 6s² |
| Gd (64) | [Xe] 4f⁸ 6s² | [Xe] 4f⁷ 5d¹ 6s² |
| Pt (78) | [Xe] 4f¹⁴ 5d⁸ 6s² | [Xe] 4f¹⁴ 5d⁹ 6s¹ |
| Au (79) | [Xe] 4f¹⁴ 5d⁹ 6s² | [Xe] 4f¹⁴ 5d¹⁰ 6s¹ |
| Ac (89) | [Rn] 5f¹ 7s² | [Rn] 6d¹ 7s² |
| Th (90) | [Rn] 5f² 7s² | [Rn] 6d² 7s² |
| Pa (91) | [Rn] 5f³ 7s² | [Rn] 5f² 6d¹ 7s² |
| U (92) | [Rn] 5f⁴ 7s² | [Rn] 5f³ 6d¹ 7s² |
| Np (93) | [Rn] 5f⁵ 7s² | [Rn] 5f⁴ 6d¹ 7s² |
| Cm (96) | [Rn] 5f⁸ 7s² | [Rn] 5f⁷ 6d¹ 7s² |
| Lr (103) | [Rn] 5f¹⁴ 6d¹ 7s² | [Rn] 5f¹⁴ 7s² 7p¹ |
Tips
- Type formulas with normal capitals and numbers, e.g. Ca(OH)2 or CuSO4·5H2O – brackets and hydrates work.
- Atomic masses are IUPAC standard atomic weights; radioactive elements use the mass number of their longest-lived isotope.
- Electronegativity values are on the Pauling scale.
- Everything is calculated in your browser.
Common mistakes to avoid
- Writing CO (carbon monoxide) when you mean Co (cobalt) – capitals matter.
- Forgetting to multiply the bracket contents by the subscript after the bracket.
- Mixing up atomic number (protons) and mass number (protons + neutrons).
Formula
Filling order (n + l rule): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7pCapacity: s 2, p 6, d 10, f 14 electrons · shell n holds 2n²Worked example
Inputs — Element (symbol, name or Z): Fe; Charge (0 for a neutral atom): 0.
Results — Electron configuration = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶; Noble-gas shorthand = [Ar] 3d⁶ 4s²; Element = Iron (Fe); Ordered by shell = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²; Atomic number = 26; Number of electrons = 26.
Key terms
- Aufbau principle
- Electrons occupy the lowest-energy available subshell first.
- Pauli exclusion principle
- No two electrons in an atom share all four quantum numbers, so one orbital holds at most two electrons.
- Hund’s rule
- Electrons occupy degenerate orbitals singly, with parallel spins, before any pairing.
- Valence electrons
- The outer electrons that take part in bonding.
- Noble-gas core
- The inner electrons that match the configuration of the preceding noble gas.
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, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p until all electrons are placed (s holds 2, p 6, d 10, f 14). Sulfur (16 e⁻) is 1s² 2s² 2p⁶ 3s² 3p⁴.
What is the noble-gas shorthand?
Replace the inner electrons by the symbol of the previous noble gas in square brackets: sulfur is [Ne] 3s² 3p⁴, iron [Ar] 3d⁶ 4s².
Why is copper 3d¹⁰ 4s¹ and not 3d⁹ 4s²?
The 3d and 4s energies are very close, and a completely filled 3d subshell lowers the total energy. The same happens for chromium (3d⁵ 4s¹, a half-filled d subshell), silver and gold.
Which elements are exceptions?
Among measured ground states: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. Palladium is the most unusual – [Kr] 4d¹⁰ with no 5s electron at all.
How do I write the configuration of an ion?
For cations remove electrons from the highest shell first (4s before 3d): Fe is [Ar] 3d⁶ 4s², so Fe³⁺ is [Ar] 3d⁵. For anions add electrons: O²⁻ is 1s² 2s² 2p⁶, like neon.
How many valence electrons does an element have?
For main-group elements, the s and p electrons of the outermost shell – the group number for groups 1–2 and the group number minus 10 for groups 13–18. Oxygen (group 16) has 6.
How do I count unpaired electrons?
Draw the orbital boxes with Hund’s rule. Iron’s 3d⁶ has one pair and four single electrons, so iron has 4 unpaired electrons and is paramagnetic.
What formula does the Electron Configuration Calculator use?
Filling order (n + l rule): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p.