Electron Configuration
Pick any element to see its ground-state electron configuration and noble-gas shorthand.
Noble-gas shorthand: [Ar] 4s² 3d⁶
An element’s ground-state electron configuration lists how its electrons fill the orbitals from lowest energy up. You place all Z electrons in Aufbau order — 1s, 2s, 2p, 3s, 3p, 4s, 3d, … — respecting each subshell’s capacity. Iron (Z = 26) gives 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or [Ar] 4s² 3d⁶.
What an electron configuration is
An electron configuration says exactly which orbitals an atom’s electrons occupy in the ground state. The superscript on each subshell — like the 6 in 3d⁶ — is the number of electrons in that subshell. Add the superscripts and you get the atomic number (the number of electrons in a neutral atom).
The orbitals fill from lowest energy to highest. Because orbital energies overlap between shells, the order is not simply 1, 2, 3, …: the 4s subshell sits below 3d, so 4s fills before 3d. That single quirk explains most of the surprises in the transition metals.
Aufbau filling order. Capacities: s = 2, p = 6, d = 10, f = 14.
Worked example — iron (Z = 26)
Fill 26 electrons into the orbitals in order until they run out.
- 1 Get the electron count. A neutral atom has Z electrons. Iron’s atomic number is 26, so 26 electrons to place.
- 2 Fill orbitals in Aufbau order. Lowest energy first: 1s, 2s, 2p, 3s, 3p, 4s, 3d, … Note 4s comes before 3d.
- 3 Respect each capacity. Stop adding to a subshell once it is full: s holds 2, p holds 6, d holds 10, f holds 14.
- 4 Write it out. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ — the superscripts sum to 26.
- 5 Condense with a noble-gas core. Argon (Z = 18) covers 1s²…3p⁶, leaving [Ar] 4s² 3d⁶.
Subshell capacities and filling order
Each subshell type holds a fixed maximum; fill them in this sequence.
| Subshell type | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
The three rules behind it
Aufbau principle. Electrons occupy the lowest-energy orbital available before moving to a higher one — that is why the filling order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on.
Pauli exclusion principle. No two electrons share all four quantum numbers, so each orbital holds at most two electrons (with opposite spins). That sets the capacities: s = 2, p = 6, d = 10, f = 14.
Hund’s rule. Within a subshell, electrons spread out singly across the orbitals before pairing up, keeping spins parallel where possible.
Exceptions. A few elements deviate from the strict Aufbau prediction because a half-filled or fully filled d subshell is extra-stable. Chromium is [Ar] 4s¹ 3d⁵ (not 4s² 3d⁴), and copper is [Ar] 4s¹ 3d¹⁰ (not 4s² 3d⁹). This tool shows the Aufbau-predicted configuration, so treat Cr, Cu, and the other d- and f-block anomalies as known special cases.
Noble-gas shorthand. Replace the inner electrons that match the previous noble gas with its symbol in brackets, then list only the valence subshells — far quicker for heavy elements and the part that drives chemistry.