• summary_type: concept
• word_count: 170

Bohr (1913) resolved Rutherford's stability problem with three revolutionary postulates. First, electrons orbit the nucleus in specific allowed circular orbits called stationary states without radiating energy — this directly contradicts classical electrodynamics. Second, the angular momentum is quantized: L = mvr = nh/(2pi) = n-hbar, where n = 1, 2, 3, ... This is equivalent to saying the electron's de Broglie wave forms a standing wave around the orbit (2pir = nlambda). Third, when an electron jumps between orbits, a photon is emitted or absorbed with frequency f = (E_i - E_f)/h. The model successfully predicts hydrogen's spectrum with remarkable accuracy. It works for any hydrogen-like ion (one electron: He+, Li2+, Be3+) by including the nuclear charge Z. However, it fails completely for multi-electron atoms because it ignores electron-electron repulsion, cannot explain fine structure or transition intensities, and violates the uncertainty principle by assuming definite orbits.