Chapter 12 — Atoms

Chapter 12 of the Class 12 Physics NCERT textbook, "Atoms", covers atomic models — from Thomson's plum pudding model and Rutherford's nuclear model (derived from the gold foil alpha-particle scattering experiment) to Bohr's quantised orbit model, which successfully explains the hydrogen atom's discrete line spectrum and ionisation energy of 13.6 eV.

• Evolving pictures of the atom — The chapter follows the model's evolution from Thomson's uniform 'plum pudding' to Rutherford's nuclear atom, revealed when alpha particles scattered off a tiny, dense, positive nucleus.
• Why the classical atom fails — Rutherford's orbiting electrons should have spiralled into the nucleus by classical theory. This instability set the stage for a radically new, quantum description of the atom.
• Bohr's quantised orbits — Bohr's three postulates — stable orbits, quantised angular momentum, and photon emission on transitions — successfully explain hydrogen's energy levels, line spectrum, and 13.6 eV ionisation energy.
• Waves behind the orbits — De Broglie later justified Bohr's quantisation by picturing the electron as a standing wave, requiring the orbit to fit a whole number of wavelengths.

1. 01Rutherford's alpha-particle scattering experiment (Geiger-Marsden, 1911) used 5.5 MeV alpha-particles on a thin gold foil; only ~1 in 8000 deflected by more than 90°, proving the nucleus is tiny (~10⁻¹⁵ m) and dense
2. 02Bohr's first postulate: electrons revolve in stable stationary orbits without radiating energy, contradicting classical electromagnetic theory
3. 03Bohr's second postulate: angular momentum of the orbiting electron is quantised — L = nh/2π, where n is the principal quantum number
4. 04Bohr's third postulate: a photon of frequency ν is emitted when an electron transitions from a higher to a lower energy state, with hν = Ei − Ef
5. 05Energy of the nth orbit in hydrogen is En = −13.6/n² eV; ground state (n = 1) energy is −13.6 eV, giving an ionisation energy of 13.6 eV
6. 06De Broglie explained Bohr's quantisation by treating the orbiting electron as a standing wave: the orbit circumference must equal a whole number of de Broglie wavelengths (2πrn = nλ)