Atomic Models, Nagaoka's Saturnian Model

In late 1903, Hantaro Nagaoka (1865–1950) developed the earliest published quasi-planetary model of the atom. This graduate of the University of Tokyo from 1887 spent his postdoctoral period in Vienna, Berlin and Munich before obtaining a professorship in Tokyo to become Japan’s foremost modern physicist. Nagaoka assumed that the atom is a large, massive, positively charged sphere, encircled by very many (in order of magnitude: hundreds) light-weight, negatively charged ► electrons, bound by electrostatic forces analogous to Saturn’s ring, which is stabilized and attracted to the heavy planet by gravitation and consists of a myriad of small fragments. Thus, Nagaoka’s model is also called a saturnian model. (Fig. 1) Even though its basic assumption foreshadowed later models of the atom, such as John William Nicholson’s (1881–1955) and Niels Bohr’s (1885–1962), it differed from ►; Bohr’s atomic model in crucial points. Unlike Bohr one decade later, Nagaoka thought that the observed atomic spectra should be directly correlated with the electron’s orbit frequency. Radioactivity was interpreted as an occasional breakdown of saturnian rings, with electrons then being ejected from the atoms as ß-rays. Consequently, Nagaoka and others tried to correlate spectral series, bands and other data observed in ► spectroscopy and early research on radioactivity with predictions derived from his model — in vain. Another problem of Nagaoka’s and Nicholson’s planetary models was a lack of stability of the electron orbits to oscillations orthogonal to the plane of rotation, as J J. Thomson pointed out, which ultimately led to Nagaoka himself abandoning the Saturnian model in 1908.