This electron configuration calculator writes the ground-state electron configuration of any element by atomic number — in full form and noble-gas shorthand — following the Aufbau order with the well-known transition-metal exceptions built in.
How the Filling Works
Orbitals fill by rising energy — the Aufbau order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p (the n+ℓ rule draws the famous diagonal diagram) — with capacities s=2, p=6, d=10, f=14. Written order groups by shell for readability (3d after 4s in print, though 4s fills first — and empties first in ions). Hund’s rule spreads electrons across equal orbitals before pairing; the Pauli principle caps each orbital at two. Chemistry itself happens in the outermost shell: sodium’s lone 3s electron and chlorine’s 3p⁵ hunger are the entire plot of table salt. The masses behind the symbols sit in our Atomic Mass Calculator. Part of our Chemistry Calculators collection.
Worked Examples (Verified)
- Iron, Fe (Z = 26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ = [Ar] 4s² 3d⁶ — written [Ar] 3d⁶ 4s² in shell order; its magnetism and its ions (Fe²⁺ loses the 4s pair first: [Ar] 3d⁶) all follow from this line
- Copper, Cu (29) — the famous exception: predicted [Ar] 4s² 3d⁹, actual [Ar] 4s¹ 3d¹⁰ — a filled d subshell wins over the Aufbau script, as with chromium ([Ar] 4s¹ 3d⁵, half-filled)
- Bromine, Br (35): [Ar] 4s² 3d¹⁰ 4p⁵ — one electron short of krypton’s shell, hence its eagerness (the halogens all sit one short, which is a periodic-table story told entirely in configurations)
Common Pitfalls
- Ion configurations are not atom configurations minus the last-written electrons: transition metals lose 4s before 3d (Fe²⁺ is [Ar] 3d⁶, not 3d⁴ 4s²).
- The exceptions beyond Cr/Cu (Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au and several f-block seats) are empirical ground states — this calculator encodes the standard textbook set; a few heavy f-block assignments remain specialty debates.
- Configurations describe isolated gas-phase atoms; bonding rearranges the story (orbitals hybridize, electrons delocalize) without breaking the counting.
Related Tools
- Atomic Mass Calculator — isotopes into average mass
- Percent Composition Calculator — elemental makeup by mass
- Chemistry Calculators — the full collection
Frequently Asked Questions
How do you write the electron configuration of an element?
Fill subshells in Aufbau order (1s 2s 2p 3s 3p 4s 3d…) with capacities 2/6/10/14 until you reach the atomic number, then write each subshell with a superscript count. Iron: 1s²2s²2p⁶3s²3p⁶4s²3d⁶.
What is noble-gas shorthand?
The core replaced by the previous noble gas in brackets: iron’s first 18 electrons are argon’s configuration, so Fe = [Ar] 4s² 3d⁶. It spotlights the valence electrons that actually do chemistry.
Why are chromium and copper exceptions?
Half-filled (3d⁵) and fully filled (3d¹⁰) subshells carry extra stability, enough to pull one 4s electron down into 3d: Cr is [Ar] 4s¹3d⁵ and Cu is [Ar] 4s¹3d¹⁰ rather than the naive predictions. Several heavier transition metals pull the same trick, and this calculator knows the standard list.
How many electrons can each subshell hold?
s: 2, p: 6, d: 10, f: 14 — twice the number of orbitals (1, 3, 5, 7), two electrons (opposite spins) per orbital by the Pauli exclusion principle.
What are valence electrons?
The electrons in the outermost principal shell (plus, for transition metals, the near-energy (n−1)d set) — the participants in bonding. Sodium’s 1 and chlorine’s 7 in shell 3 predict NaCl’s 1:1 marriage without any further data.
Is this electron configuration calculator free?
Yes — free, instant, no sign-up, all 118 elements.