Electron Configuration

You’ll be able to

Order subshells by energy using the Aufbau principle
Write full and noble-gas electron configurations
Apply the Pauli exclusion principle and Hund’s rule

Electrons live in shells, subshells, and orbitals

Electrons occupy energy levels (n = 1, 2, 3…), each split into subshells (s, p, d, f). Each subshell holds a set number of electrons: s holds 2, p holds 6, d holds 10, f holds 14. Filling them in the right order tells you an atom’s chemistry.

Three rules that govern filling

Aufbau principle: fill the lowest-energy subshell first. Pauli exclusion: an orbital holds at most 2 electrons, with opposite spins. Hund’s rule: within a subshell, put one electron in each orbital before pairing any up — electrons avoid sharing an orbital until forced.

Aufbau filling order

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s

Note the swap: 4s fills before 3d because it is slightly lower in energy.

Worked example

Write the electron configuration of iron (Fe, Z = 26).

1.Iron has 26 electrons to place.
2.Fill in Aufbau order, tracking the running total: 1s² (2) 2s² (4) 2p⁶ (10) 3s² (12) 3p⁶ (18) 4s² (20) 3d⁶ (26).
3.Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
4.Noble-gas shorthand: replace 1s²…3p⁶ (which is argon) with [Ar]: [Ar] 4s² 3d⁶.

Answer: [Ar] 4s² 3d⁶

Checkpoint

What is the ground-state electron configuration of a sulfur atom (Z = 16)?

Watch out

Chromium and copper break the pattern: a half-full or full d-subshell is extra stable, so Cr is [Ar] 4s¹ 3d⁵ and Cu is [Ar] 4s¹ 3d¹⁰. These two exceptions are AP favorites.

Checkpoint

How many unpaired electrons does a nitrogen atom (1s² 2s² 2p³) have?

On the exam

AP loves connecting configuration to the periodic table: the s-block is groups 1–2, the d-block is the transition metals, and the p-block is groups 13–18. An element’s valence configuration is readable straight off its position.

Answer the 2 checkpoints as you read.