Geiger-Marsden experiment

The Geiger-Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was an experiment done by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester which led to the downfall of the plum pudding model of the atom.

They measured the deflection of alpha particles (helium ions with a positive charge) directed normally onto a sheet of very thin gold foil. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees; some were even scattered back toward the source. From this observation Rutherford concluded that the atom contained a very physically-small (as compared with the size of the atom) positive charge, which could repel the alpha particles if they came close enough, subsequently developed into the Bohr model.

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Methodology

Geiger and Marsden bombarded a number of different metal foils with alpha particles generated from a tube of "radium emanation" (radon) gas. A low power microscope was used to count the scattering of these particles, a procedure requiring many hours in a darkened room watching for tiny flashes of light as the scattered particles struck a zinc sulfide scintillant screen.

A variety of different foils were used such as aluminium, iron, gold and lead along with different thicknesses of gold foil made by packing several pieces of very thin foil together. Given the very high mass and momentum of an alpha particle, the expectation was that the particles would pass through having been deflected by a tiny angle at most, with the number of particles penetrating falling off as the thickness of foil (and the atomic weight of its material) was increased; the remainder being absorbed.

However they were astonished to find that although this was generally true, around 1 in 8000 particles were reflected through more than 90 degrees even with a single sheet of extremely thin, 6x10^-8 metre (or about 200 atoms) thick, gold foil, an observation completely at odds with the predictions of the plum pudding model.

Conclusions: the atomic theory by Rutherford

The result was completely unpredicted, prompting Rutherford to later comment "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you".

Early in 1911, Rutherford came forth with his own model for subatomic structure, known as the Rutherford model. The observations indicated that a model of the atom with a diffuse charge was incorrect and that a large amount of atomic charge was instead concentrated at some point, giving it a very high electric field. He concluded that the atom might be mostly empty space, with most of the atom's mass and a large fraction of one of its two kinds of charge concentrated in a tiny center, the "central charge" (later termed universally the nucleus, though not by Rutherford). Rutherford was unable to say from his experiment whether or not the nuclear charge was positive or negative, but considered the possibility that the high electric field seen deflecting the alphas, was positive: "For concreteness, consider the passage of a high speed a particle through an atom having a positive central charge Ne, and surrounded by a compensating charge of N electrons."

He calculated the charge from two experiments to be 97 (in one case) and 114 (in the other case) for gold. Thus, whether the concentrated charge was positive or negative, it was on the order of about 100 units. Rutherford considered the magnitude of the concentrated charge therefore (along with beta scattering experiments done by others on other elements, including platinum, which gave about the same result for the charge) to be "proportional" to atomic mass. This mass he took for gold, as 197. Rutherford does not mention the idea of atomic number in the paper, and does leave open the possibility that small fraction of the particular charge that was concentrated in the atom (positive or negative), might in part be diffusely distributed elsewhere.

Rutherford's results were enough to allow him to definitively reject Thomson's plum pudding model of the atom, since none of Thomson's negative "corpuscles" or electrons contained enough charge or mass to deflect alphas strongly, nor did the diffuse positive "pudding" or cloudlike positive charge, in this model. The only other atomic model which Rutherford considered, was the Saturnian model of Nagaoka, in which the electrons orbited a central positive charge in a stable flat ring, much like Clerk Maxwell's model of the particulate rings of the planet Saturn, with electrons being held in orbit around it by electrostatic attraction. Rutherford noted that this model would be consistent with his results, and Nagaoka's 1904 paper is the only one which Rutherford's 1911 paper cites.

Rutherford found the nuclear diameter for a charge of 100 units must be concentrated in an area of less than about 3.4 x 10^-14 metres in radius (this distance is the calculated closest approach for a straight-on projected path of an alpha of the known energies being used, toward a nucleus with a charge of 100 e), in the centre of a 10^-10 metre diameter atom. Those alpha particles that had come into proximity with the nucleus had been strongly deflected, whereas the majority had passed at a relatively great distance to it. As for the structure of the nucleus itself, Rutherford put forward the suggestion that the nucleus of gold might be made of 49 helium nuclei (alpha particles), giving it a mass of 196 (as opposed to the known mass of 197), and a charge of 98 (gold's place on the periodic table was 79, probably explaining why Rutherford did not immediately propose that his measured charge and atomic number were the same). Rutherford noted that the presence of helium nuclei in the nucleus would explain the high speeds of alpha particles ejected from a positive nucleus by the mechanism of mere electrostatic repulsion, without having to postulate that these particles were in rapid motion inside the nucleus already. Such a model implies a binding mechanism for the nucleus, but Rutherford did not speculate as to the nature of such a force.

Rutherford's model was later developed by Niels Bohr into the Bohr model proposed in 1913. The Rutherford/Nagaoka atom had a number of problems, in particular that electrons in any kind of orbit about a central charge should radiate electromagnetic energy, and rapidly spiral into the nucleus. These problems were dealt with by Bohr by simply postulating a new kind of mechanics which forbade electron orbits smaller than a certain minimal size (by forbidding electrons less than a certain minimal angular momentum).

See also

• List of famous experiments
• 1909 in science

References

• Geiger H. & Marsden E. (1909). "On a Diffuse Reflection of the α-Particles". Proceedings of the Royal Society, Series A 82: 495–500.
• Rutherford E. (1911). "The Scattering of α and β Particles by Matter and the Structure of the Atom". Philosophical Magazine, Series 6 21: 669–688.
• JPEG images of Rutherford's 1911 paper
• Description of the experiment, from the New Mexico Institute of Mining and Technology
• Short biography of Ernest Rutherford