An electron configuration is the address of every electron in an atom — which shell, which subshell, how many. Learn the filling order and a couple of rules, and you can write the configuration of any atom or ion on the SL syllabus.
📘 What you need to know
A configuration lists electrons by shell, subshell and number, e.g. 1s² 2s² 2p⁶ …
Fill subshells in order of increasing energy (Aufbau) — remember 4s before 3d.
Hund’s rule: fill orbitals in a subshell singly first, with parallel spins, before pairing up.
Pauli: an orbital holds at most 2 electrons, with opposite spins.
Ions: anions gain electrons; cations lose them. Transition metals lose 4s before 3d.
Two exceptions: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹.
Reading the notation
Each part of a configuration term tells you something specific.
The three parts of a configuration term: the shell number, the subshell letter, and the electron count as a superscript.
Hund’s rule and box notation
Within a subshell, electrons spread out before they pair up. This is Hund’s rule: fill each orbital of a subshell singly first, all with the same spin, and only start pairing once every orbital has one electron. Pairing up too early would waste energy through repulsion between the two electrons.
Box notation: in a p subshell the three orbitals each take one electron before any pairing begins.
Two supporting rules: the Pauli exclusion principle says an orbital holds only 2 electrons, and they must have opposite spins (drawn as one up, one down arrow).
Full and shorthand configurations
There are two ways to write a configuration:
Full: list every electron from 1s onward — e.g. iron is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
Shorthand: replace the inner electrons with the nearest smaller noble gas in brackets — iron becomes [Ar] 4s² 3d⁶.
🧪 How to write one
Find the number of electrons (the atomic number for a neutral atom).
Fill subshells in energy order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p …
Respect the subshell limits (s=2, p=6, d=10).
For shorthand, swap the inner electrons for the previous noble gas symbol in brackets.
Configurations of ions
Ions have a different number of electrons from the neutral atom:
Anions (negative) — add electrons to the outer shell.
Cations (positive) — remove electrons from the outer shell.
Transition metals: fill 4s before 3d when neutral, but remove electrons from 4s first when forming ions.
⚠️ Common mix-up
When a transition metal forms an ion, take electrons from 4s before 3d — even though 4s was filled first. For example Fe²⁺ is [Ar] 3d⁶ (the two 4s electrons go).
The blocks of the periodic table
Elements are grouped into blocks by the subshell their outer (valence) electrons occupy: the s-block, p-block, d-block and f-block. The block tells you which subshell is filling last.
Exceptions: chromium and copper
Two elements break the expected pattern because a half-full or full d-subshell is extra stable:
Chromium is [Ar] 3d⁵ 4s¹, not [Ar] 3d⁴ 4s² — a half-full 3d.
Copper is [Ar] 3d¹⁰ 4s¹, not [Ar] 3d⁹ 4s² — a full 3d.
In both, an electron is promoted from 4s to 3d to reach that more stable arrangement.
WORKED EXAMPLE
Write the full and shorthand electron configurations of: (a) potassium (Z = 19), (b) calcium (Z = 20), (c) the Ca²⁺ ion.
(a) Potassium — 19 electrons1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹Shorthand: [Ar] 4s¹ (argon = 18 electrons).(b) Calcium — 20 electrons1s² 2s² 2p⁶ 3s² 3p⁶ 4s²Shorthand: [Ar] 4s². (4s fills before 3d.)(c) Ca²⁺ — lost 2 electrons → 181s² 2s² 2p⁶ 3s² 3p⁶ = [Ar]Ca²⁺ has the same configuration as argon
💡 Exam tip
Box (spin) diagrams can run left–right or up the page — either is fine. What matters: label the boxes, show the right number of electrons, and draw paired arrows in opposite directions.
Double-check transition-metal ions: remove 4s before 3d.
That completes the Electronic Configurations section. Next you’ll move on to Counting Particles by Mass — the Mole, the foundation of all quantitative chemistry.
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