Materials Science and Engineering/List of Topics/Rutherford Model of the Atom

The Rutherford model or planetary model was a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in (1909), which suggested to Rutherford's analysis (1911) that the Plum pudding model (of J. J. Thomson) of the atom was incorrect. Rutherford's new model for the atom, based on the experimental results, had a number of essential modern features, including a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom.

Experimental basis for the model

In the Geiger-Marsden experiment at Rutherford's laboratory, alpha particles were used as a probe into atomic structure by being allowed to pass through a thin piece of gold foil, then detected. Rutherford predicted that all of the particles would pass through the foil, or be deflected slightly. This is indeed what happened most of the time, but a few particles, 1 in 8000, bounced unexpectedly nearly straight back toward the source. This supported the hypothesis that atoms have a dense region containing most of their mass, and associated with a highly concentrated electric field (probably positive in nature), instead of spread-out positive or negative field. Rutherford thought it likely, on purely symmetric and aesthetic grounds, that such a region of dense charge and mass would be located in the atom's center. Such a region would then form a sort of atomic core.

In 1911, Rutherford came forth with his own physical model for subatomic structure, as an interpretation for the unexpected experimental results. In it, the atom is made up of a central charge (this is the modern atomic nucleus, though Rutherford did not use the term "nucleus" in his paper) surrounded by a cloud of orbiting electrons. In this 1911 paper, Rutherford only commits himself to a small central region of very high positive or negative charge in the atom, but uses the following language for pictorial purposes:

"For concreteness, consider the passage of a high speed a particle through an atom having a positive central charge N e, and surrounded by a compensating charge of N electrons."

From purely energetic considerations of how far alpha particles of known speed would be able to penetrate toward a central charge of 100 e, Rutherford was able to calculate that the radius of his gold central charge would need to be less (how much less could not be told) than 3.4 x 10^-14 metres (the modern value is only about a fifth of this). This was in a gold atom known to be 10^-8 metres or so in radius--- a very surprising finding, as it implied a strong central charge less than 1/3000th of the diameter of the atom.

The Rutherford model did not attribute any structure to the orbits of the electrons themselves, though it did mention the atomic model of Hantaro Nagaoka, in which the electrons are arranged in one or more rings (this is the only previous atomic model mentioned in Rutherford's 1911 paper).

The Rutherford paper suggested that the central charge of an atom might be "proportional" to its atomic mass in hydrogen mass units (roughly 1/2 of it, in Rutherford's model). For gold, this mass number is 197 (not then known to great accuracy) and was therefore modeled by Rutherford to be possibly 196. However, Rutherford did not attempt to make the direct connection of central charge to atomic number, since gold's place on the periodic table was known to be about 79, and Rutherford's more tentative model for the structure of the gold nucleus was 49 helium nuclei, which would have given it a mass of 196 and charge of 98. This differed enough from gold's "atomic number" (at that time merely its place number in the periodic table) that Rutherford did not formally suggest the two numbers might be exactly the same.

Successor model

The Rutherford model of the atom was soon superseded by the Bohr atom, which used some of the early quantum mechanical results to give locational structure to the behavior of the orbiting electrons, confining them to certain circular (and later eliptical) planar orbits. In the Bohr model, expanding on the work of Henry Moseley, the central charge was identified as being directly connected with the atomic number (that is, the element's place on the periodic table). Since the Bohr model is an improvement on the Rutherford model in this and other ways, some sources combine the two, referring to the Bohr model as the Rutherford-Bohr model. However, even an atom with a core containing an atomic number of charges was the work of a number of men, including those mentioned, and also lesser known workers such as Antonius Van den Broek.

The Rutherford model was important because it essentially proposed the concept of the nucleus, although this word is not used in the paper. What Rutherford notes as the (probable) concomitant of this results, is a "concentrated central charge" in the atom: "Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents." The central charge containing most of the atom's positive charge, invariably later become associated with a concrete structure, the atomic nucleus.

After the Rutherford model and its confirmation in the experiments of Henry Moseley and its theoretical description in the Bohr model of the atom, the study of the atom branched into two separate fields, nuclear physics, which studies the nucleus of the atom, and atomic physics which studies atom's electronic structure.