Electronic Configuration of Elements and Ions: How to Write

Electronic configuration is the written address of every electron in an atom — which shell, subshell and orbital each one occupies. It is written as a string like 1s² 2s² 2p⁶, read left to right from lowest to highest energy, with superscripts counting the electrons in each subshell.

Writing configurations correctly is the gateway skill of inorganic chemistry. It tells you an element's group, its likely ions, its magnetic behaviour and even why the periodic table has its shape. Ions add one twist — electrons are added or removed in a specific order — and that is where most exam marks are won or lost.

Key Concepts

• Principal quantum number (n): the shell number: n = 1, 2, 3, … Higher n means higher energy and larger average distance from the nucleus.
• Azimuthal quantum number (l): the subshell shape: l = 0 (s), 1 (p), 2 (d), 3 (f).
• Subshell capacities: s holds 2, p holds 6, d holds 10, f holds 14 electrons.
• Noble-gas shorthand: [Ne] 3s¹ means the neon core (1s² 2s² 2p⁶) plus 3s¹ — sodium's configuration written compactly.
• Cations: electrons are removed from the subshell with the highest n first. For transition metals, ns electrons leave before (n−1)d electrons.
• Anions: electrons are added to the lowest-energy available subshell, following the normal filling order.
• Valence electrons: electrons in the outermost shell (plus d electrons for transition metals); they decide bonding and reactivity.

Formulas

• Electrons in shell n: maximum 2n² (n = 1: 2; n = 2: 8; n = 3: 18).
• Number of orbitals in subshell l: 2l + 1 (s: 1, p: 3, d: 5, f: 7), each holding at most 2 electrons.

Worked Example

Write the configurations of Zn (Z = 30) and the Zn²⁺ ion, and find how many unpaired electrons Zn²⁺ has.

1. Fill in order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰. Check the count: 2+2+6+2+6+2+10 = 30. Correct.
2. Shorthand: [Ar] 4s² 3d¹⁰.
3. Zn²⁺ loses 2 electrons from the highest-n subshell first: remove the 4s² electrons.
4. Zn²⁺ = [Ar] 3d¹⁰.
5. All five 3d orbitals are full (↑↓ in each), so unpaired electrons = 0.

Zn²⁺ is [Ar] 3d¹⁰ with 0 unpaired electrons (diamagnetic).

Common Misconceptions

• Zn²⁺ loses its 3d electrons because 3d is "outer". Removal order follows the principal quantum number n, not energy order: 4s (n = 4) empties before 3d (n = 3).
• Every element follows the filling order with no exceptions. Cr ([Ar] 3d⁵ 4s¹) and Cu ([Ar] 3d¹⁰ 4s¹) break the pattern because half-filled and fully filled d subshells are extra stable.
• Anions remove electrons like cations do. Anions add electrons — S (Z = 16, [Ne] 3s² 3p⁴) gains 2 to become S²⁻ = [Ne] 3s² 3p⁶, which is the argon configuration.

Real-World Applications

• Configurations fix each element's block and group in the periodic table.
• The number of unpaired electrons predicts whether a material is magnetic (used in data storage and MRI contrast agents).
• Transition-metal colours in pigments and gemstones come from d-electron arrangements.
• Ion configurations explain common oxidation states, like Fe²⁺/Fe³⁺ in rust and blood.
• Semiconductor doping choices depend on the valence configurations of dopant atoms.

Write configurations in energy order, use noble-gas shorthand, remove highest-n electrons first for cations and add electrons for anions. Master that routine — plus the Cr and Cu exceptions — and electronic structure questions become straightforward.