Atomic Structure extends IGCSE knowledge into sub-shells, orbitals, and ionisation energies — providing the quantum mechanical foundation for understanding chemical bonding and periodicity.
Shells (n = 1, 2, 3...) contain sub-shells: s (1 orbital, max 2e⁻), p (3 orbitals, max 6e⁻), d (5 orbitals, max 10e⁻). Orbital: region of space where electron is likely found. Filling order: 1s 2s 2p 3s 3p 4s 3d 4p (Aufbau principle). Hund\'s rule: electrons fill orbitals singly first, same spin. Pauli exclusion: max 2 electrons per orbital, opposite spins. Examples: Fe [Ar] 3d⁶ 4s², Cu [Ar] 3d¹⁰ 4s¹ (exception for half/full d-shell stability).
First ionisation energy: energy to remove 1 mole of electrons from 1 mole of gaseous atoms. Factors: nuclear charge (higher → harder to remove), distance (further → easier), shielding (more inner electrons → easier). Trends: increases across a period (more protons, same shielding), decreases down a group (more shells, greater distance). Anomalies: drop from Group 2 to 3 (2p electron easier to remove than 2s), drop from Group 5 to 6 (paired electron in p orbital easier to remove). Successive ionisation energies: big jumps indicate when electrons from a new inner shell are being removed.
Excited electrons emit photons when they return to lower energy levels. Photon energy: ΔE = hf = hc/λ. Line spectrum: discrete lines (not continuous) → proves energy levels are quantised (electrons can only exist at specific energies). Each element has a unique spectrum (fingerprint). Convergence of lines → ionisation energy (electron removed completely). Mass spectrometry: used to determine isotopic composition and relative atomic mass.
Chromium is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) and copper is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²). The half-filled (3d⁵) and fully-filled (3d¹⁰) d sub-shells have extra stability due to: (1) symmetrical distribution of electron density, and (2) exchange energy — a quantum mechanical effect that stabilises configurations where electrons with parallel spins can exchange positions. The energy gained from this stability exceeds the energy cost of moving one electron from 4s to 3d. These are real observed configurations, confirmed by spectroscopic evidence.
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