Old quantum theory

Old quantum theory was the framework that applied quantization rules to classical mechanics and atomic models before modern quantum mechanics. It explained some atomic spectra but lacked a consistent general method.

Bohr model: An atomic model in which electrons occupy quantized orbits around a nucleus. Its allowed electron orbits were an early application of quantum rules to classical motion.
Planck's law: A formula describing the spectrum of radiation emitted by a black body at thermal equilibrium. Its derivation introduced discrete energy elements, the starting point for quantum theory.
Franck–Hertz experiment: An experiment showing that electrons transfer energy to mercury atoms in discrete amounts. It provided direct evidence for discrete atomic excitation energies assumed by early models.
Matrix mechanics: A formulation of quantum mechanics developed by Werner Heisenberg in 1925 using arrays of transition quantities. It abandoned classical electron orbits instead of refining their quantization.
Bohr–Sommerfeld quantization: A rule quantizing classical action integrals in units of Planck’s constant. It generalized Bohr’s circular orbits to more complex periodic motions.
Max Planck: A German theoretical physicist who introduced energy quantization in explaining blackbody radiation. Planck’s hypothesis supplied the quantum element that later atomic theories adopted.
Stark effect: The splitting or shifting of spectral lines in an electric field. Sommerfeld’s methods helped account for features of hydrogen spectra in external fields.
Wave mechanics: A formulation of quantum mechanics developed by Erwin Schrödinger using wave equations. It replaced prescribed orbits with mathematical states governed by a wave equation.
Correspondence principle: A principle requiring quantum predictions to approach classical predictions in the appropriate limit. It guided attempts to extend quantum rules while preserving classical behavior at large scales.
Niels Bohr: A Danish physicist whose atomic model assigned electrons discrete stationary states. Bohr turned quantum assumptions into a model that accounted for hydrogen’s spectrum.

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