• Tags: bohr, postulates, quantization
• Difficulty: Foundation

Bohr's three postulates: (1) Electrons orbit the nucleus in certain allowed circular orbits (stationary states) without radiating energy. (2) The angular momentum of the electron is quantized: L = mvr = nℏ = nh/(2π), n = 1, 2, 3, ... (3) When an electron jumps from a higher orbit (E_i) to a lower orbit (E_f), it emits a photon of frequency f = (E_i - E_f)/h. These postulates were revolutionary — they mixed classical mechanics (circular orbits) with quantum ideas (quantization, photon emission). The model works perfectly for hydrogen and hydrogen-like ions ($He^{+}$, $Li^{2+}$) but fails for multi-electron atoms due to electron-electron interactions.