Physics · Class 12 · Chapter 12
Atoms
12 lessons 92 min 2 simulations
What is inside an atom, and why does hydrogen glow in a few sharp colours instead of a smooth rainbow? This chapter goes from Thomson's plum pudding to Rutherford's nucleus, found by firing alpha-particles at gold foil, and then to Bohr's hydrogen atom, whose three postulates give quantised orbits, energy levels En = −13.6 eV/n² and the line spectrum. De Broglie's standing waves explain Bohr's quantisation rule, and the chapter ends with what the Bohr model cannot do.
What the exam asks
NEET asks for the Geiger–Marsden observations and what they prove, the impact parameter and the distance of closest approach, the energy relations K = −E and U = 2E for a bound electron, Bohr's postulates, rn ∝ n², vn ∝ 1/n and En = −13.6/n² eV, excitation and ionisation energies, the photon from a transition (hν = Ei − Ef) and the standing-wave condition 2πrn = nλ. Marks are lost on signs (total energy is negative), on forgetting that the n = 1 → 2 gap is the largest, and on applying the Bohr model to helium.
Lessons
12 lessons · 92 min
1Early models of the atomNCERT §12.17 min2Alpha-particle scatteringNCERT §12.27 min3Alpha-particle trajectoryNCERT §12.2, §12.2.110 min 1 sim4Electron orbitsNCERT §12.2.27 min5Atomic spectraNCERT §12.36 min6Why Rutherford's atom failsNCERT §12.46 min7Bohr's postulatesNCERT §12.46 min8Energy levelsNCERT §12.4.16 min9Line spectra of hydrogenNCERT §12.510 min 1 sim10De Broglie's explanationNCERT §12.67 min11Limits of the Bohr modelNCERT §12.66 min12Chapter reviewMust-know facts, traps and formulas14 min