Electron Configuration Explained
By the Periodixy Editorial Team · Last reviewed July 10, 2026
An electron configuration is the address book of an atom's electrons — it records which energy levels and subshells every electron occupies. From it you can read off an element's valence electrons, its likely ions, and its place on the periodic table.
Configurations look cryptic (1s² 2s² 2p⁶…) but follow just three rules and one filling order.
Shells, subshells and orbitals
Electrons live in shells (numbered 1, 2, 3…) which contain subshells (s, p, d, f) made of orbitals. Each orbital holds at most two electrons.
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
The notation 2p⁴ reads: shell 2, subshell p, containing 4 electrons.
The three rules
- Aufbau principle — electrons fill the lowest-energy subshell available first.
- Pauli exclusion principle — an orbital holds at most two electrons, and they must have opposite spins.
- Hund's rule — within a subshell, electrons occupy empty orbitals singly before pairing up.
Aufbau filling order
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
Worked examples
Oxygen (Z = 8)
Write the electron configuration of oxygen.
- Fill 1s with 2 electrons (6 left).
- Fill 2s with 2 electrons (4 left).
- Put the last 4 into 2p.
Answer: 1s² 2s² 2p⁴ — shells: 2, 6 — valence electrons: 6
Iron (Z = 26)
Write the configuration of iron using noble gas shorthand.
- The last noble gas before iron is argon (Z = 18): [Ar] covers 1s² 2s² 2p⁶ 3s² 3p⁶.
- 8 electrons remain: 4s fills first (2), then 3d takes 6.
Answer: [Ar] 4s² 3d⁶ (often written [Ar] 3d⁶ 4s²)
Noble gas shorthand
Instead of writing every subshell, replace the inner core with the previous noble gas in brackets. Sodium's full configuration 1s² 2s² 2p⁶ 3s¹ becomes [Ne] 3s¹. The shorthand highlights exactly the electrons that matter for chemistry — the outer ones.
Famous exceptions
A handful of elements disobey the simple filling order because half-filled and fully-filled d subshells are extra stable. The two you may be asked about:
- Chromium: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) — a half-filled d subshell.
- Copper: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²) — a full d subshell.
The Electron Configuration Calculator flags every exception automatically and shows the measured configuration for all 118 elements.
Summary
- Configurations list electrons by shell and subshell: 1s² 2s² 2p⁶…
- Fill lowest energy first (Aufbau), two per orbital max (Pauli), spread out before pairing (Hund).
- Noble gas shorthand compresses the core: [Ne] 3s¹ for sodium.
- Chromium and copper are the classic exceptions, driven by d-subshell stability.