A stylised representation of the Rutherford model of a lithium atom (nuclear structure anachronistic)

The Rutherford model or planetary model is 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 and containing the bulk of the atomic mass (the nucleus of the atom), and a number of tiny electrons circling around the nucleus like planets around the sun.

[edit] Experimental basis for the model

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.

"For concreteness, consider the passage of a high speed α particle through an atom having a positive central charge N e, and surrounded by a compensating charge of N electrons." ^[1]

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^-10 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 didn't 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.

The Rutherford paper suggested that the central charge of an atom might be "proportional" to its atomic mass in hydrogen mass units u (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 u. 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 u, 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 u and charge of 98 e, which was much more in keeping with his experimentally-determined central charge for gold in this experiment of about 100 e. 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 (atomic number and nuclear charge) might be exactly the same. (A month after Rutherford's paper appeared, this proposal was made by Antonius van den Broek, and confirmed experimentally within two years, by Henry Moseley.

[edit] Key points

• The electron clouds of the atom do not influence alpha particle scattering.
• A large number of the atom's charges, up to a number equal to about half the atomic mass in hydrogen units, are concentrated in very small volume at the center of the atom. These are responsible for deflecting both alpha and beta particles.
• The mass of heavy atoms such as gold is mostly concentrated in the central charge region, since calculations show it is not deflected or moved by the high speed alpha particles, which have very high momentum in comparison to electrons, but not with regard to a heavy atom as a whole.

[edit] Contribution to modern science

After Rutherford's discovery, scientists started to realize that the atom is not ultimately a single particle, but is made up of far smaller subatomic particles. Following research was done to figure out the exact atomic structure which led to Rutherford’s gold foil experiment. They eventually discovered that atoms have a positively-charged nucleus (with an exact atomic number of charges) in the center, with a radius of about 1.2 x 10^-15 meters x [Atomic Mass Number]^1/3. Since electrons were found to be even smaller, this meant that the atom consists of mostly empty space.

Later on, scientists found the expected number of electrons (the same as the atomic number) in an atom by using X-rays. When an X-ray passes through an atom, some of it is scattered, while the rest passes through the atom. Since the X-ray loses its intensity primarily due to scattering at electrons, by noting the rate of decrease in X-ray intensity, the number of electrons contained in an atom can accurately be estimated.

[edit] Symbolism

Shield of the U.S. Atomic Energy Commission.

Despite its inaccuracy, the Rutherford model caught the imagination of the public in a way that the more correct Bohr model did not, and has continually been used as a symbol for atoms and atomic energy. Examples of its use over the past century include:

• The logo of the United States Atomic Energy Commission, which was in part responsible for its later usage in relation to nuclear fission technology in particular.
• The flag of the International Atomic Energy Agency is a Rutherford atom, enclosed in olive branches.
• The US minor league baseball Albuquerque Isotopes' logo is a Rutherford atom, with the electron orbits forming an A.
• The Unicode Miscellaneous Symbols codepoint U+269B (⚛) uses a Rutherford atom.
• On maps, it is generally used to indicate a nuclear power installation.
• The Atomic whirl, symbol of American Atheists, incorporates the design of a Rutherford atom.

[edit] References

1. ^ E. Rutherford. The Scattering of α and β Particles by Matter and the Structure of the Atom. April 1911, Philos. Mag, 21:669-688, pdf downloaded 2008-10-09

[edit] External links

• Rutherford's Model by Raymond College
• Rutherford's Model by Kyushu University