Between 1909 and 1911, British chemist Ernest Rutherford developed a new theoretical model of the atom which overturned existing hypotheses and became the dominant means of understanding these basic building blocks of all matter for most of the twentieth century. Although several components of the atom – especially the orbits of electrons – turned out to be substantially more complex than Rutherford envisioned, this model informed a century of chemical education, and will probably be immediately familiar to readers who took chemistry courses in high school.
PLUM PUDDING and the GOLD FOIL EXPERIMENT
That atoms existed, and were the building blocks of all larger matter, was not a new idea in Rutherford’s time; indeed, this also informed, just for example, the theoretical work of Michael Dalton, a century before. By the early twentieth century, chemists and physicists were beginning to piece together the constituent parts of atoms, as well, especially positively charged protons (the number of which determines what element an atom will be) and negatively charged electrons.
How these subatomic particles came together, however, was a matter of debate. Several years before Rutherford performed his work, J.J. Thomson had ascended to prominence through his so-called “plum pudding” theory of the atom. According to Thomson, the inner workings of the atom resembled a “pudding” or soup of free-floating protons, surrounding and sustaining various small structures made up of electrons which could be found throughout the body of the atom.
Rutherford was not convinced. In the gold foil experiment of 1909, Hans Geiger and Ernest Marsden, operating under Rutherford’s supervision, discovered that most radioactive alpha rays tended to pass directly through an atom, but a small number bounced back almost directly towards the source of the rays. This should not have happened: according to the plum pudding model, many of the rays should have been deflected somewhat as they collided with the electron structures, but the deflection should have been minimal. Instead, only some of the rays were deflected significantly – and that deflection was hardly minimal at all. There must be some unexpectedly large structure in the atom which was causing these large deflections.
THE NUCLEUS AND THE ORBITING ELECTRONS
In a 1911 publication analyzing the Geiger-Marsden experiments, Rutherford put forward a new model of the atom on the basis of this realization. Instead of a free-floating sea of protons, he argued that all of the protons in the atom – along with varying numbers of uncharged neutrons – were instead packed into an extremely dense “nucleus” at the centre of the atom. This structure resembled Thomson’s theory of the electron structures, except that it was reversed (the structures were made up of protons rather than electrons), and there was only one of them in every atom.
Moreover, Rutherford went on, the electrons were not sitting stably in solid structures: they were instead zipping about the core or nucleus of the atom at extremely high speeds. Inspired by charts of the solar system, Rutherford argued that the electrons probably circled the atomic nucleus in much the same way as planets circled the Sun: in other words, they followed regular, predictable paths, or “orbits.” This analogy to planets was not new – it also informed the concurrent work of Japanese physicist Hantaro Nagaoka – but Rutherford’s model seemed to be the most accurate.
A number of important hypotheses which have become wrapped up in the so-called “Rutherford model” of the atom or Rutherford atomic theory were not actually present in the original 1911 paper. For example, Rutherford did not initially note that the positive charge of the nucleus would be equivalent to its atomic number, or number of protons, instead briefly suggesting that the charge would instead be equal to its total mass (not the number of protons, but the number of protons and neutrons combined). Such gaps in the theory were relatively quickly filled in by other chemists, such as Henry Moseley and Antonius Broek.
Aside from later contributions of theoretical physics to the understanding of the makeup of subatomic particles, the most important error in Rutherford’s model was the suggestion that electrons could be found in regular orbits. Ironically, this component of the model is also the most prevalent in the popular imagination, and informs various logos and symbols of the atom, like the logos of American Atheists, the U.S. Department of Energy, and the International Atomic Energy Agency, among others. Dutch chemist Niels Bohr corrected this error several years later with the shell model of the atom, arguing that electrons followed effectively random paths through a three-dimensional “shell,” or layer, of the atom, rather than regular paths through a two-dimensional orbit.