Lesson 3 of 4

Writing electron configurations

Fill orbitals, account for exceptions and remove the correct electrons from ions.

A-Level 9476 (2026-2027)

I can write configurations of atoms and ions.Syllabus
Syllabus 9476, 1(h). Write configurations of atoms and ions.

Fill orbitals, then remove electrons from the outer shell

Use electron count, Pauli exclusion and Hund's rule; remember Cr, Cu and 4s removal.

Count electrons first. Fill lower-energy orbitals before higher-energy orbitals. Two electrons in one orbital have opposite spins (Pauli exclusion). In a set of equal-energy orbitals, put one electron into each with parallel spins before pairing (Hund's rule). Thus the three 2p electrons of nitrogen occupy three different p orbitals, rather than pairing in one immediately.

Configurations to reason from
Species	Electron count	Ground-state configuration
N	7	1s² 2s² 2p³
Cl	17	1s² 2s² 2p⁶ 3s² 3p⁵
Ca	20	[Ar]4s²
Cr	24	[Ar]3d⁵ 4s¹
Fe	26	[Ar]3d⁶ 4s²
Cu	29	[Ar]3d¹⁰ 4s¹
Fe²⁺	24	[Ar]3d⁶
Fe³⁺	23	[Ar]3d⁵
Cu²⁺	27	[Ar]3d⁹

Superscripts count electrons: 3d⁶ means six electrons distributed over five d orbitals. [Ar] abbreviates the 18-electron argon core. Chromium and copper have the ground-state exceptions shown above; a rigid filling diagram does not capture every balance of orbital energies and electron repulsions.

Worked exampleAn ion is not filled like a neutral atom with the same electron count

Write the configuration of Cr³⁺ (Z = 24).

Try it first. Then show the worked stepsHide the worked steps

Neutral chromium is [Ar]3d⁵ 4s¹.
Remove the 4s electron first, then two 3d electrons.
Check the total: 18 + 3 = 21 electrons, matching 24 - 3.

Answer

Cr³⁺ is [Ar]3d³. For these transition-metal cations, remove 4s electrons before 3d electrons.

Check your understanding

Fe²⁺ and Cr both have 24 electrons. Do they have the same configuration?

Yes. Both are [Ar]3d4 4s2, since they have the same number of electrons.
No. Fe²⁺ is [Ar]3d6, while Cr is [Ar]3d5 4s1.
No. Fe²⁺ is [Ar]3d4 4s2, while Cr is [Ar]3d5 4s1.

Show the answer

No. Fe²⁺ is [Ar]3d6, while Cr is [Ar]3d5 4s1.

Transition metal ions lose 4s electrons first, so Fe²⁺ keeps 3d6. Cr has the special 3d5 4s1 arrangement. The nuclear charges differ too.