Ch 12 covers atomic models — Rutherford's nuclear model, Bohr's quantised orbit model, hydrogen energy levels and spectral series, and limitations of the Bohr model.
Rutherford α-scattering: most passed through (atom mostly empty), few deflected back (positive nucleus). Bohr model (for hydrogen): electrons in quantised orbits, L = nh/2π. Radius r_n = 0.53n² Å. Energy E_n = −13.6/n² eV. Transition: electron jumps between levels emitting/absorbing photon of energy ΔE = hf.
Hydrogen spectrum: 1/λ = R(1/n₁² − 1/n₂²), R = 1.097 × 10⁷ m⁻¹. Lyman series: n₁=1 (UV). Balmer: n₁=2 (visible). Paschen: n₁=3 (IR). Brackett: n₁=4 (IR). Pfund: n₁=5 (IR). Bohr model limitations: works only for single-electron systems, doesn't explain fine structure or molecular bonding.
Download: https://ncert.nic.in/textbook/pdf/leph204.pdf | Part II: https://ncert.nic.in/textbook/pdf/leph2ps.zip
Bohr's model treats electrons as particles in well-defined orbits and ignores electron-electron interactions. Hydrogen has only one electron, so there is no inter-electron repulsion. Helium has two electrons that repel each other, and their interactions cannot be simply accounted for in Bohr's framework. Quantum mechanics (Schrödinger equation) is needed for multi-electron atoms.
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