Book: The Theory of Spectra and Atomic Constitution

Overview

Niels Bohr’s The Theory of Spectra and Atomic Constitution (1922) is a compact, programmatic presentation of the early quantum theory of atoms, written to clarify how discrete stationary states and quantum transitions account for observed spectral regularities and the periodic law. Gathering and refining insights from his 1913 trilogy and subsequent developments, Bohr develops a semiclassical framework that treats electrons in atoms as occupying quantized orbits and radiating only during transitions, and he uses this framework to interpret hydrogen spectra, extend to many-electron atoms, and relate atomic structure to chemical properties.

Quantum postulates and correspondence

Bohr restates two central postulates: atoms possess stationary states not described by classical radiation theory, and light of frequency ν is emitted or absorbed only when the atom makes a “quantum jump” between such states with hν equal to the energy difference. He adds the correspondence principle, asserting that at high quantum numbers the quantized motion approaches classical motion and that the Fourier components of classical orbits indicate which quantum transitions are allowed and how their intensities and polarizations behave.

From hydrogen to complex atoms

For hydrogen, quantization of angular momentum yields discrete energies that reproduce the Balmer and Rydberg series, explaining the Ritz combination principle as a direct consequence of level differences. Bohr discusses refinements such as elliptical orbits and additional quantum numbers, using the correspondence principle to organize multiplets and intensities, while stressing that orbital pictures are calculational devices rather than literal trajectories.

Periodic system and chemical properties

Extending to many-electron atoms, Bohr introduces shells and subshells (K, L, etc.) and an Aufbau scheme in which electrons fill the lowest available stationary states subject to screening. He uses characteristic X-ray lines and Moseley’s law to justify the shell structure and the primacy of atomic number. The arrangement of electrons explains periodic trends in valence and reactivity: closed-shell configurations underpin inert gases, single outer electrons account for alkali metals, and gradual filling of inner subshells correlates with transition-metal chemistry and spectra.

Spectral features and external fields

Bohr connects regularities in line spectra, term systems, series limits, and multiplet structure, to selection rules inferred via correspondence. The polarization of emitted light, and the effects of external electric and magnetic fields (Stark and Zeeman, including the Paschen, Back regime), are treated by perturbing stationary states and matching the multiplicity and polarization of observed components with classical harmonics at large quantum numbers. Anomalies and fine-structure details foreshadow the need for further quantum refinements.

Molecules and band spectra

The book sketches how rotational and vibrational quantization in molecules leads to band spectra, and how electronic transitions superposed on quantized nuclear motion produce characteristic band heads and envelopes. Bohr relates the stability of simple molecules to quantized electronic configurations and discusses the limited transfer of atomic selection rules to molecular cases, again guided by correspondence arguments.

Method, limits, and influence

Throughout, Bohr emphasizes a cautious, symbolic use of mechanical pictures: the stationary-state postulates restrict classical reasoning, while the correspondence principle anchors quantum rules in classical limits without claiming a full dynamical description. He acknowledges unresolved problems, especially in many-electron dynamics, the helium spectrum, and intensity calculations, and presents the theory as a scaffold that organizes data and points to deeper laws. The synthesis of spectra, atomic structure, and periodicity in this volume shaped the practice of spectroscopy and chemistry in the early 1920s and prepared the ground for matrix and wave mechanics, which would later supply a more complete quantum foundation for the phenomena Bohr organizes here.