PeriodicTable.world · Chemistry reference

Electron configuration reference

A printable electron-configuration guide with subshell capacities, filling order, noble-gas cores, orbital rules, and common exceptions.

Useful noble-gas cores
Core	Electrons	Core	Electrons
[He]	2	[Ne]	10
[Ar]	18	[Kr]	36
[Xe]	54	[Rn]	86

How to use it

Common construction order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p.
Pauli exclusion: an orbital holds at most two electrons, with opposite spins.
Hund’s rule: for orbitals of equal energy in a subshell, place one electron in each with parallel spins before pairing them.
Check the total: Z is the atomic number. A neutral atom has Z electrons; Na⁺ has Z − 1, while Cl⁻ has Z + 1. Remember to count the electrons in the bracketed noble-gas core.

Fe: [Ar] 3d⁶ 4s². Remove the two 4s electrons for Fe²⁺ → [Ar] 3d⁶. Remove one more 3d electron for Fe³⁺ → [Ar] 3d⁵ (23 electrons).

Neutral ground-state exceptions: Cr is [Ar] 3d⁵ 4s¹; Cu is [Ar] 3d¹⁰ 4s¹.
Use the filling order as a memory aid, then check known exceptions. Subshell energies can change when electrons are added or removed.
A p subshell has three orbitals, not one. Orbital boxes describe quantum states; arrows represent spin states, not electron paths.