A level Chemistry · Edexcel 9CH0 · Topic 1B
Electronic configuration
Key points
- Electrons occupy orbitals, each holding up to two electrons with opposite spins. An s orbital is spherical; p orbitals are dumbbell-shaped (three per sub-shell).
- Sub-shells hold: s = 2, p = 6, d = 10 electrons. Shell n holds electrons.
- Fill in energy order: 1s 2s 2p 3s 3p 4s 3d 4p. Electrons occupy orbitals in a sub-shell singly before pairing up.
- Example: Fe is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s².
- Exceptions: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹.
- When transition metals form ions, 4s electrons are lost first, so Fe³⁺ is [Ar] 3d⁵.
- Use electrons-in-boxes diagrams to show spin, and the block (s, p, d) tells you which sub-shell is being filled.
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Electrons are not simply in circular shells: they occupy orbitals of different shapes and energies. Writing electron configurations correctly lets you predict an element's block, its ions and much of its chemistry.
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