⚛️ Electron Configuration Calculator

How to Use This Calculator

Type an element name, symbol, or atomic number. You get the full configuration, noble-gas shorthand, valence count, and block.

Rules

The calculator encodes the Madelung order and the known exceptions (Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au and f-block irregularities). Configurations are for neutral ground-state atoms.

Worked Example

Iron (26): fill 1s2 2s2 2p6 3s2 3p6 4s2 3d6. Shorthand: [Ar] 4s2 3d6. Valence electrons: 8 (transition metal, counting 4s + 3d).

Exceptions to the Rules

Chromium and copper famously break the expected pattern: Cr is [Ar] 4s1 3d5 instead of 4s2 3d4, and Cu is [Ar] 4s1 3d10 instead of 4s2 3d9. Half-filled and fully-filled d subshells are extra stable, so an electron shifts from 4s to 3d. About 20 elements have such exceptions, mostly in the transition metals. This calculator includes the experimentally observed configurations, not the naive predictions.

Why Configurations Matter

An atom's chemistry lives in its outermost electrons. Elements in the same column share outer configurations, which is why the periodic table groups them: alkali metals all have one s electron to lose, halogens all need one electron to complete a p subshell. Ionization energy, atomic size, and reactivity all trace back to these arrangements.

Valence Electrons and Ions

Atoms gain or lose electrons to achieve noble gas configurations. Sodium loses its single 3s electron to become Na+ with a neon core; chlorine gains one to become Cl- with an argon core. Transition metals lose s electrons first, then d electrons, which is why iron forms both Fe2+ and Fe3+.

Writing Shorthand

Noble gas shorthand compresses configurations: iron becomes [Ar] 4s2 3d6 instead of the full 26-electron string. This works because inner electrons rarely participate in chemistry. When writing ions, remove electrons from the highest n subshell first, which for transition metals means the s electrons go before d.