Ch 2 covers atomic structure from Thomson to the quantum mechanical model — Bohr's model, dual nature of matter, Heisenberg uncertainty, quantum numbers, orbitals, and electron configuration rules.
Thomson → Rutherford → Bohr (quantized orbits, E = −13.6/n² eV for H). Dual nature: de Broglie λ = h/mv. Heisenberg uncertainty: Δx · Δp ≥ h/4π. Quantum mechanical model: electrons in orbitals (probability regions). Four quantum numbers: n (shell, 1,2,3…), l (subshell, 0 to n−1), mₗ (orientation, −l to +l), mₛ (spin, ±½).
Shapes: s (spherical), p (dumbbell, 3 orientations), d (cloverleaf, 5 orientations), f (7 orientations). Aufbau principle: fill in order of increasing energy (1s,2s,2p,3s,3p,4s,3d…). Pauli exclusion: max 2 electrons per orbital, opposite spins. Hund's rule: half-fill degenerate orbitals before pairing. Special: Cr = [Ar]3d⁵4s¹, Cu = [Ar]3d¹⁰4s¹ (extra stability of half/fully filled d).
Download: https://ncert.nic.in/textbook/pdf/kech102.pdf | Part I: https://ncert.nic.in/textbook/pdf/kech1ps.zip
Half-filled and fully filled subshells have extra stability due to symmetrical distribution of electrons and maximum exchange energy. [Ar]3d⁵4s¹ (half-filled d) is more stable than [Ar]3d⁴4s² because 5 unpaired d-electrons provide maximum exchange energy. Similarly, Cu is [Ar]3d¹⁰4s¹ (fully filled d) instead of [Ar]3d⁹4s².
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