Book: On the Constitution of Atoms and Molecules

Overview

Published as a three-part series in 1913, Niels Bohr’s On the Constitution of Atoms and Molecules fuses Rutherford’s nuclear atom with Planck’s quantum hypothesis to propose a new picture of atomic structure and radiation. Electrons move in certain permitted “stationary” orbits around a small, positively charged nucleus; they do not radiate while in these orbits. Light is emitted or absorbed only when an electron jumps between orbits, with the radiation frequency set by the difference in orbital energies divided by Planck’s constant. With a few bold postulates added to classical mechanics, Bohr accounted quantitatively for atomic spectra and sketched a new foundation for chemical structure.

Quantum postulates and atomic motion

Bohr’s central assumptions are: the existence of stationary states; a quantization rule for orbital motion (in circular orbits, angular momentum equals n times h/2π); and a radiation condition stating that a single quantum of energy hν is emitted or absorbed in a transition between states. These rules contradict classical electrodynamics’ prediction that an accelerating electron should continuously radiate and spiral into the nucleus, replacing it with a discrete, nonradiating set of orbits.

Hydrogen and the Rydberg law

Applying the postulates to an electron bound by Coulomb attraction, Bohr derived the allowed energies En ∝ −Z^2/n^2 and radii scaling as n^2, with n = 1, 2, 3, … and nuclear charge Z. Differences between these levels yield spectral lines that exactly fit the empirical Balmer, Lyman, and Paschen series. The well-known Rydberg formula emerges with a constant expressed in terms of fundamental constants, and it scales as Z^2 for hydrogenlike ions. Bohr identified a limiting frequency for each series and showed that for large n the emitted frequencies approach the classical orbital frequency, an early form of the correspondence idea that quantum results connect smoothly to classical physics in the appropriate limit.

Ionized helium and the Pickering lines

The theory required that features previously attributed to hydrogen, especially Fowler’s “Pickering” series, arise instead from singly ionized helium (He+, Z = 2). The predicted Z^2 displacement and pattern matched the observed lines, offering strong evidence for the nuclear atom and the quantum postulates.

Reduced mass and isotopic shifts

By replacing the electron mass with the electron, nucleus reduced mass, Bohr refined the Rydberg constant and predicted small isotope-dependent shifts in spectral lines. This made optical spectra a probe of nuclear masses and hinted that distinct isotopes would have measurably different wavelengths, a forecast later borne out by discoveries such as deuterium.

Molecules and the chemical bond

Part II extends the framework to molecules. For the simplest molecular ion, H2+, Bohr argued that a single electron can bind two protons by occupying stationary states in the combined field, explaining its stability and spectral features. He outlined how quantized electronic motion underlies molecular binding and how rotational and vibrational energies would be quantized, anticipating later quantum treatments of molecular structure.

Many-electron atoms and periodic regularities

Part III explores multi-electron atoms by arranging electrons in quantized rings or shells around the nucleus. Though approximate and soon superseded, this scheme connected valence and chemical periodicity to the filling of outer electron groups and offered qualitative explanations for alkali spectra and ionic sizes. Persistent difficulties with helium and heavier atoms highlighted the need for more elaborate quantum conditions and electron, electron interaction treatments.

Legacy and limitations

Bohr’s model explained absolute spectral positions, introduced the quantization of atomic motion, and tied atomic and molecular stability to discrete energy states. It did not account for line intensities, fine structure, or complex many-electron spectra, and its orbital picture was later replaced by wave mechanics and quantum numbers derived from the Schrödinger equation. Yet the 1913 papers marked a decisive turn: a workable quantum constitution of matter with predictive power, guiding a decade of advances in the old quantum theory and shaping the path to modern quantum mechanics.