An atom's electron configuration is the inventory of its electrons, ordered by increasing energy. For carbon: 1s² 2s² 2p² — 2 electrons in the 1s shell, 2 in 2s, 2 in 2p. This compact notation carries a surprising amount of information — once you can decode it.
Notation structure
Each block is written n l ˣ, where: - n is the principal quantum number (1, 2, 3, …) — the "shell". - l is the subshell, labeled with a letter: s (l = 0), p (l = 1), d (l = 2), f (l = 3). - x is the number of electrons in that subshell, written as a superscript.
Maximum capacity per subshell: s = 2, p = 6, d = 10, f = 14. These numbers come from the fact that a subshell contains 2l + 1 orbitals, each holding up to 2 electrons (Pauli principle).
Filling order: the Madelung rule (Klechkowski)
Subshells are not filled in order of increasing n. They follow the Madelung rule (n + l rule): you fill first the subshell with the smallest n + l; for equal n + l, the one with the smallest n.
The order is:
1s · 2s · 2p · 3s · 3p · 4s · 3d · 4p · 5s · 4d · 5p · 6s · 4f · 5d · 6p · 7s · 5f · 6d · 7p
That's why potassium (Z = 19) puts its 19th electron in 4s, not 3d: 4 + 0 = 4 < 3 + 2 = 5.
The three filling rules
Within a subshell, electrons obey three principles:
1. Pauli: two electrons in the same atom cannot share all four quantum numbers. Consequence: maximum 2 electrons per orbital, with opposite spins. 2. Hund: electrons in the same subshell first occupy each orbital singly (parallel spins) before pairing up. Carbon is therefore 2p² with two singly-occupied p orbitals (not one doubly occupied). 3. Aufbau: you fill from lowest to highest energy, following the Madelung order.
Condensed notation
For heavy atoms, the full configuration becomes unwieldy. The condensed notation uses the previous noble gas as a root. Iron (Z = 26):
- Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
- Condensed: [Ar] 4s² 3d⁶
[Ar] stands for the argon configuration (Z = 18). You gain readability while highlighting valence electrons (those past the noble gas), which are the ones that drive chemistry.
The exceptions: Cr, Cu, Mo, Ag, Au, Pd…
The Madelung rule isn't universal. For a few elements, "anomalous" configurations are energetically favorable:
- Chromium (Z = 24): [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴. A half-filled d subshell is extra-stable.
- Copper (Z = 29): [Ar] 4s¹ 3d¹⁰ instead of [Ar] 4s² 3d⁹. A fully filled d subshell is extra-stable.
These exceptions stem from exchange energy between parallel-spin electrons, maximized at d⁵ and d¹⁰ configurations. The same anomalies recur lower in the table (Mo, Ag, Au).
Why it matters
The electron configuration lets you predict:
- The typical oxidation number (valence electrons leave first).
- The element's family (alkali = ns¹, noble gas = ns² np⁶, etc.).
- The reactivity (full shells are stable, shells one electron short are reactive).
- The bonds the element can form (orbitals available for covalency).
It's the most economical tool for understanding why chemistry repeats periodic patterns. Learning to read a configuration in 30 seconds is an investment that pays off for a chemist's whole life.