Electron orbitals and configuration — CIE A-Level Chemistry
1. Quantum Structure: Shells, Subshells and Orbitals ★★☆☆☆ ⏱ 4 min
In the modern quantum model, electrons are arranged in principal shells (energy levels) labelled by the principal quantum number $n=1,2,3...$. Lower $n$ corresponds to lower energy and smaller distance from the nucleus. Each principal shell splits into subshells, which each contain a fixed number of orbitals.
2. Rules for Filling Orbitals ★★☆☆☆ ⏱ 5 min
- **Pauli Exclusion Principle**: Each orbital holds a maximum of 2 electrons with opposite spins.
- **Hund's Rule**: When filling degenerate (equal energy) orbitals (e.g. the three 2p orbitals), electrons fill each orbital singly first with parallel spins, before pairing up. This minimizes electron-electron repulsion.
For elements up to Z=36 (krypton), the order of filling orbitals by increasing energy is:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p
Exam tip: CIE examiners regularly test application of Hund's rule, so always check for single occupation before pairing electrons in orbitals.
3. Writing Electron Configurations ★★★☆☆ ⏱ 5 min
Electron configurations can be written in full, or abbreviated using noble gas core shorthand. For CIE exams, you are expected to write configurations for all atoms and ions up to Z=36.
4. Exceptions to the Aufbau Principle ★★★★☆ ⏱ 4 min
For CIE 9701, you only need to remember two common exceptions to the standard filling order for first-row transition metals: chromium (Z=24) and copper (Z=29). These exceptions occur because a half-filled (5 electrons) or fully filled (10 electrons) 3d subshell has extra stability from symmetry and exchange energy.
Exam tip: CIE almost always tests these two exceptions, so memorise: Cr = [Ar] 3d⁵ 4s¹, Cu = [Ar] 3d¹⁰ 4s¹.
Common Pitfalls
Why: Confuses the order of filling with the conventional writing order sorted by principal quantum number n
Why: Forgets the extra stability of a half-filled 3d subshell, the key exception tested by CIE
Why: Assumes 4s is always lower energy than 3d, which is only true before 3d starts filling
Why: Confuses the old Bohr model with the modern quantum model
Why: Violates Hund's rule of maximum multiplicity