Orbital / Electron Configuration Calculator

Electron configuration shows how an atom’s electrons are distributed across its subshells. The ground-state configuration follows three rules: the Aufbau principle (fill lowest energy subshells first), Pauli exclusion (max two electrons per orbital, opposite spins), and Hund’s rule (fill degenerate orbitals singly before pairing).

Subshell capacities: s holds 2, p holds 6, d holds 10, f holds 14. Each subshell is labeled by principal quantum number n and letter (1s, 2s, 2p, 3s, 3p, 3d, etc.).

The Madelung filling order — by the (n+l) rule, with smaller n breaking ties — runs: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p

Notice 4s fills before 3d, even though n=3 < n=4. The reason: 4s sits slightly lower in energy for neutral atoms despite the higher principal quantum number.

Eight known exceptions exist within elements 1-54, all involving anomalously stable half-filled or fully-filled d subshells:

Cr (24): [Ar] 3d^5 4s^1: half-full d is more stable than 3d^4 4s^2 Cu (29): [Ar] 3d^10 4s^1: full d beats 3d^9 4s^2 Nb (41), Mo (42), Ru (44), Rh (45), Ag (47): similar 4d preferences Pd (46) is unique: [Kr] 4d^10 with no 5s electron at all

The noble gas shorthand replaces the inner core with the symbol of the previous noble gas in brackets. Carbon’s full configuration 1s^2 2s^2 2p^2 becomes [He] 2s^2 2p^2 — the [He] stands for the same 1s^2 found in helium.

Valence electrons are the outermost-shell electrons, which determine chemical bonding. For main-group elements they are the s and p electrons of the highest n. For transition metals it is conventionally the (n)s plus (n-1)d electrons.