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Molecular Physics and the Quantum Theory

by Owen Willans Richardson

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Summary

This 1914 book by Nobel laureate Owen Willans Richardson presents his early synthesis of quantum theory with molecular physics, arguing that the newly developed quantum hypotheses—particularly Planck’s radiation law and Bohr’s atomic model—could explain molecular phenomena such as specific heats, chemical bonding, and electron emission. Richardson systematically applies the quantum concept to molecular rotation and vibration, showing that classical statistical mechanics fails to account for observed heat capacities at low temperatures. He also extends the theory to thermionic emission, deriving his eponymous law (Richardson’s law) for the current density from a hot filament. The book’s central thesis is that quantum discontinuities are not merely atomic curiosities but fundamental to molecular behavior. Readers gain a clear, historically grounded understanding of how quantum theory first unified atomic and molecular physics, and why classical models were abandoned.

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Key concepts

  • Richardson’s law — The equation \( J = A T^2 e^{-W/kT} \) describing the thermionic emission current density from a heated metal surface, derived from quantum statistics.
  • Planck’s radiation law — The formula for blackbody radiation intensity that introduced the quantum of action \( h \), which Richardson applies to molecular energy exchanges.
  • Bohr’s atomic model — The planetary model of the atom with quantized electron orbits, used by Richardson to explain molecular spectra and stability.
  • Quantized molecular rotation — The idea that molecules can only rotate with discrete angular momentum values, resolving the discrepancy between classical and observed specific heats.
  • Einstein’s specific heat theory — The quantum explanation for the temperature dependence of solid heat capacities, which Richardson extends to molecular gases.
  • Thermionic emission — The release of electrons from a heated surface, which Richardson models using quantum statistics to derive his emission law.